Showing papers in "Geological Society, London, Special Publications in 2005"

The Tasmanides of eastern Australia

Abstract: Abstract The Tasmanides of eastern Australia record the break-up of Rodinia, followed by the growth of orogenic belts along the eastern margin of Gondwana. Spatially, the Tasmanides comprise five orogenic belts, with an internal Permian-Triassic rift-foreland basin system. Temporally, the Tasmanides comprise three (super)cycles, each encompassing relatively long periods of sedimentation and igneous activity, terminated by short deformational events. The Neoproterozoic-earliest Ordovician Delamerian cycle began by rifting, followed by convergent margin tectonism and accretion of island-arc forearc crust and ?island arcs in the Middle-Late Cambrian. The Ordovician-Carboniferous convergent margin Lachlan supercycle consists of three separate cycles, each ending in major deformation. The Ordovician Benambran cycle includes convergent (island-arc) and transform margin activity terminated by terrane accretion in the latest Ordovician-earliest Silurian. The Silurian-Middle Devonian Tabberabberan cycle reflects development of a large back-arc basin system, marked by rift basins and granite batholiths, behind intra-oceanic arcs and an Ordovician-Early Devonian terrane that were accreted in the Middle Devonian. The Middle Devonian to Carboniferous Kanimblan cycle began by rifting, followed by continental sedimentation inboard of a major convergent margin system that forms the early part of the Late Devonian-Traissic Hunter-Bowen supercycle. This supercycle comprises a Late Devonian-Carboniferous continental arc, forearc basin and outboard accreted terranes and subduction complexes intruded by the roots of a Permian-Triassic continental margin arc. Complex deformation ended with accretion of an intra-oceanic arc in the Early Triassic. Key features of the Tasmanides are: continuity of cycles across and along its length, precluding growth by simple eastwards accretion; development of a segmented plate margin in the Late Cambrian, reflected by major rollback of the proto-Pacific plate opposite the southern part of the Tasmanides; rifting of parts of the Delamerian margin oceanwards, to form substrate to outboard parts of the Tasmanides; the presence of five major Ordovician terranes in the Lachlan Orogen; and the generation of deformations either by the accretion of arcs, the largely orogen-parallel ‘transpressive’ accretion of Ordovician turbidite terranes (in the Lachlan Orogen), or by changes in plate coupling.

The Neuquén Basin: an overview

Abstract: The Neuquen Basin of Argentina and central Chile contains a near-continuous Late Triassic–Early Cenozoic succession deposited on the eastern side of the evolving Andean mountain chain. It is a polyphase basin characterized by three main stages of evolution: initial rift stage; subduction-related thermal sag; and foreland stage. The fill of the basin records the tectonic evolution of the central Andes with dramatic evidence for baselevel changes that occurred both within the basin and along its margins. The record of these changes within the mixed siliclastic–carbonate succession makes the basin an excellent field laboratory for sequence stratigraphy and basin evolution. The 4000 m-thick fill of the basin also contains one of the most complete Jurassic–Early Cretaceous marine fossil records, with spectacular finds of both marine and continental vertebrates. The basin is also the most important hydrocarbon-producing province in southern South America, with 280.4 10 m of oil produced and an estimated 161.9 10 m remaining. The principal components of the hydrocarbon system (source and reservoir) crop out at the surface close to the fields. The deposits of the basin also serve as excellent analogues to reservoir intervals worldwide. This paper aims to provide a brief introduction to the Neuquen Basin. It should provide a stepping stone for further reading and also for further studies. This paper also serves as an introduction to this Special Publication, which details the most recent work within the basin. The proposed goals of the Special Publication are as follows. . To present the Neuquen Basin as an integrated case study in sequence stratigraphy and basin analysis. . To document the latest developments in vertebrate and invertebrate palaeontology. . To consider the basin in the context of the structural evolution of the central Andes. . To document the latest studies on specific stratigraphic intervals in a way that allows the reader to build up a complete picture of the basin fill and the way in which the various depositional systems have evolved through time. . To present specific studies from the basin that highlights concepts and models in sequence stratigraphy that are exportable to other systems. Introduction to the Neuquen Basin The Neuquen Basin is located on the eastern side of the Andes in Argentina and central Chile, between 328 and 408S latitude (Figs 1 & 2). It covers an area of over 120 000 km (Yrigoyen 1991) and comprises a continuous record of up to 4000 m of stratigraphy. This Late Triassic– Early Cenozoic succession includes continental and marine siliciclastics, carbonates and evaporites that accumulated under a variety of basin styles (Fig. 3). The basin has a broadly triangular shape (Fig. 1) and two main regions are commonly recognized: the Neuquen Andes to the west, From: VEIGA, G. D., SPALLETTI, L. A., HOWELL, J. A. & SCHWARZ, E. (eds) 2005. The Neuquen Basin, Argentina: A Case Study in Sequence Stratigraphy and Basin Dynamics. Geological Society, London, Special Publications, 252, 1–14. 0305-8719/05/$15.00 # The Geological Society of London 2005. and the Neuquen Embayment to the east and SE. The majority of the Basin’s hydrocarbon fields are located in the Neuquen Embayment where most of the Mesozoic sedimentary record is in the subsurface and the strata are relatively undeformed. This is in contrast to the Andean region where Late Cretaceous–Cenozoic deformation has resulted in the development of a series of N–S-oriented fold and thrust belts (Aconcagua, Marlargue and Agrio fold and thrust belts, Fig. 2) that provide excellent outcrops of the Mesozoic successions. During present times and throughout much of its history the triangular Neuquen Basin has been limited on its NE and southern margins by wide cratonic areas of the Sierra Pintada Massif and the North Patagonian Massif, respectively (Fig. 1). The western margin of the basin is the Andean magmatic arc on the active western margin of the Gondwanan–South American Plate. This geotectonic framework and the highly complex history of the basin are largely controlled by changes in the tectonics on the western margin of Gondwana. The evolution and development of the basin can be considered in three stages (Fig. 3). 1. Late Triassic–Early Jurassic: prior to the onset of subduction on its western margin, Fig. 1. Sketch map of the Neuquen Basin showing the approximate location (boxes and stars) of the contributions included in this publication. 1, Ramos & Folguera; 2, Zapata & Folguera; 3, Aguirre-Urreta et al.; 4, McIlroy et al.; 5, Schwarz & Howell; 6, Veiga et al.; 7, Stromback et al.; 8, Doyle et al.; 9, Scasso et al.; 10, Sagasti; 11, Tyson et al.; 12, Morgans-Bells & McIlroy; 13, Gasparini & Fernandez; 14, Lazo et al.; 15, Coria & Salgado. J. A. HOWELL ET AL. 2

Early-Middle Pleistocene transitions: an overview and recommendation for the defining boundary

Abstract: The Early-Middle Pleistocene transition (c. 1.2-0.5 Ma), sometimes known as the 'mid- Pleistocene revolution', represents a major episode in Earth history. Low-amplitude 41-ka obliq- uity-forced climate cycles of the earlier Pleistocene were replaced progressively in the later Pleistocene by high-amplitude 100-ka cycles. These later cycles are indicative of slow ice build-up and subsequent rapid melting, and imply a transition to a strongly non-linear forced climate system. Changes were accompanied by substantially increased global ice volume at 940 ka. These climate transformations, particularly the increasing severity and duration of cold stages, have had a pro- found effect on the biota and the physical landscape, especially in the northern hemisphere. This review assesses and integrates the marine and terrestrial evidence for change across this transition, based on the literature and especially the following 17 chapters in the present volume. Orbital and non-orbital climate forcing, palaeoceanography, stable isotopes, organic geochemistry, marine micropalaeontology, glacial history, loess-palaeosol sequences, pollen analysis, large and small mammal palaeoecology and stratigraphy, and human evolution and dispersal are all considered, and a series of discrete events is identified from Marine Isotope Stage (MIS) 36 (c. 1.2 Ma) to MIS 13 (c. 540-460 Ma). Of these, the cold MIS 22 (c. 880-870 ka) is perhaps the most profound. However, we here endorse earlier views that on practical grounds the Matuyama-Brunhes palaeomagnetic Chron boundary (mid-point at 773 ka, with an estimated duration of 7 ka) would serve as the best overall guide for establishing the Early-Middle Pleistocene Subseries boundary.

Tectonic evolution of the Andes of Neuquén: constraints derived from the magmatic arc and foreland deformation

Abstract: Fil: Ramos, Victor Alberto. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geologia. Laboratorio de Tectonica Andina; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina

Mid-Pleistocene revolution and the ‘eccentricity myth’

Abstract: The mid-Pleistocene revolution (MPR) is the term used to describe the transition between 41 ka and 100 ka glacial-interglacial cycles which occurred about one million years ago. Despite eccentricity having by far the weakest influence on insolation received at the Earth's surface of any of the orbital parameters, it is often assumed to be the primary driver of the post-MPR 100 ka climate cycles. The traditional solution to this is to call for a highly nonlinear response by the global climate system to eccentricity. This 'eccentricity myth' is a simplified view of the relationship between global climate and orbital forcing and is in part due to an artefact of spectral analysis. Our aim here is to clarify the often confused role of eccentricity and review current theories of the MPR. We suggest that the post-MPR '100 ka' glacial-interglacial cycles are more closely linked to precession, with the saw-toothed climate cycles being defined by every four or five precessional cycle. Because control over the number of precessional cycles involved is determined by eccentricity, eccentricity at most paces rather than drives the system. If true, then one must also question whether the MPR, itself defined by an abrupt change in spectral characteristics, is not also somewhat misconceived.

The emperor’s new clothes: sustainable mining?

Abstract: Over the last few years, the idea of ‘sustainable mining’ has, thanks to industry sponsorship, been working its way into the agenda of many international processes. There is now a push in many countries to invite in multinational mining companies with the idea that there is a “new, sustainable mining” which is different from the old, bad practices of the past. Yet, what has actually changed in the industry to match this shift in rhetoric? From the perspective of mine-affected communities nothing seems to have changed. Their land is still being taken from them without giving their free, prior and informed consent, and they are suffering the same ill effects on their ways of life, health and environment. This paper will illustrate how under this rhetoric, the mining industry ‘emperor’ has the same old naked ambitions.

Alluvial fans: geomorphology, sedimentology, dynamics — introduction. A review of alluvial-fan research

Abstract: This volume presents a series of papers on the geomorphology, sedimentology and dynamics of alluvial fans, selected from those presented at the ‘Alluvial Fans’ Conference held in Sorbas, SE Spain in June 2003. The conference was sponsored primarily by the British Geomorphological Research Group and the British Sedimentological Research Group, both organizations affiliated to the Geological Society of London. It is some time since an international conference has been held that was exclusively devoted to the geomorphology and sedimentology of alluvial fans. The previous such conference was that organized by Terry Blair and John McPherson in 1995, and held in Death Valley, a classic setting for alluvial fans (Denny 1965; Blair & McPherson 1994a). Although many of the papers presented there have since been published, no dedicated volume on alluvial fans as a whole resulted from that meeting, so even longer has elapsed since there has been a specific publication devoted wholly to a series of papers on the geomorphology and sedimentology of alluvial fans (Rachocki & Church 1990). South-east Spain was chosen as the venue for this conference, partly for logistic reasons and partly because it is a tectonically active dry region within which there is a wide range of Quaternary alluvial fans. These fans exhibit differing relationships between tectonic, climatic and base-level controls (Harvey 1990, 2002a, 2003; Mather & Stokes 2003; Mather et al. 2003), core themes in consideration of the dynamics of alluvial fans. An emphasis within the previous alluvial fan literature has been on fans within

Influence of phyllosilicates on fault strength in the brittle-ductile transition: insights from rock analogue experiments

Abstract: Despite the fact that phyllosilicates are ubiquitous in mature fault and shear zones, little is known about the strength of phyllosilicate-bearing fault rocks under brittle–ductile transitional conditions where cataclasis and solution-transfer processes are active. In this study we explored steady-state strength behaviour of a simulated fault rock, consisting of muscovite and halite, using brine as pore fluid. Samples were deformed in a rotary shear apparatus under conditions where cataclasis and solution transfer are known to dominate the deformation behaviour of the halite. It was found that the steady-state strength of these mixtures is dependent on normal stress and sliding velocity. At low velocities (,0.5 mm s) the strength increases with velocity and normal stress, and a strong foliation develops. Comparison with previous microphysical models shows that this is a result of the serial operation of pressure solution in the halite grains accommodating frictional sliding over the phyllosilicate foliation. At high velocities (.1 mm s), velocityweakening frictional behaviour occurs along with the development of a structureless cataclastic microstructure. Revision of previous models for the low-velocity behaviour results in a physically realistic description that fits our data well. This is extended to include the possibility of plastic flow in the phyllosilicates and applied to predict steady-state strength profiles for continental fault zones containing foliated quartz–mica fault rocks. The results predict a significant reduction of strength at mid-crustal depths and may have important implications for crustal dynamics and seismogenesis. Classical models for the steady-state strength of the crust consist of a two-mechanism brittle– ductile strength profile, based on Byerlee’s law plus a dislocation creep law for quartz (e.g. Sibson 1977; Schmid & Handy 1991; Scholz 2002). However, grain size sensitive processes, such as pressure solution, and the production of weak phyllosilicates known to be important under mid-crustal, brittle–ductile transitional conditions (e.g. Rutter & Mainprice 1979; Passchier & Trouw 1996; Imber et al. 2001; Holdsworth et al. 2001) are neglected, as are the effects of phyllosilicate foliation development. Such processes have long been anticipated to lead to some form of hybrid frictional–viscous rheological behaviour in the brittle–ductile transition (Sibson 1977; Rutter & Mainprice 1979; Lehner & Bataille 1984/85; Schmid & Handy 1991; Wintsch et al. 1995; Handy et al. 1999). The steady-state stress levels at which midcrustal fault rocks deform may, therefore, be much lower than those predicted using a classical two-mechanism strength profile (see Fig. 1) (Sibson 1977; Byerlee 1978; Schmid & Handy 1991; Scholz 2002). Moreover, phyllosilicate foliation development and processes such as pressure solution can be expected to play an important role in controlling transient healing, cementation and strength recovery of fault rocks, thus influencing the rateand statedependent frictional and seismogenic behaviour (Fredrich & Evans 1992; Beeler et al. 1994; Karner et al. 1997; Bos & Spiers 2000, 2002a; Beeler & Hickman 2001; Saffer & Marone, 2003). Numerous authors have considered the possible weakening effects of phyllosilicates, foliation development, pressure solution and cataclasis within faults and shear zones in the brittle–ductile transition, using both theoretical and experimental approaches (Rutter & Mainprice 1979; Lehner & Bataille 1984/85; Logan & Rauenzahn 1987; Kronenberg et al. 1990; Shea & Kronenberg 1992, 1993; Mares & Kronenberg, 1993; Chester 1995; Blanpied et al. 1998; Gueydan et al. 2001, 2003). Such studies have produced a consensus that fault rocks containing a contiguous and From: BRUHN, D. & BURLINI, L. (eds) 2005. High-Strain Zones: Structure and Physical Properties. Geological Society, London, Special Publications, 245, 303–327. 0305-8719/05/$15.00 # The Geological Society of London 2005. well-developed phyllosilicate (mica) foliation can potentially be as weak as the frictional strength of the phyllosilicate, at high crustal levels (Logan & Rauenzahn 1987; Shea & Kronenberg 1992), or as weak as the crystalplastic flow strength of the phyllosilicate basal plane, at deeper levels (Hickman et al. 1995; Wintsch et al. 1995). However, the time and technical limitations of experiments on silicate fault rocks (e.g. Blanpied et al. 1991, 1995; Chester & Higgs 1992; Chester 1994; Kanagawa et al. 2000) have precluded systematic studies of large strain sliding behaviour, with associated foliation development, under hydrothermal, brittle–ductile conditions where the relevant processes are active. Large strain, steady-state rheological laws for faults incorporating the effects of cataclasis, pressure solution and foliation development in the brittle–ductile transition are therefore not available. Recently, Bos and co-workers (Bos et al. 2000a, b; Bos & Spiers 2001) performed ultrahigh strain rotary shear experiments on simulated fault gouges consisting of mixtures of halite (rock salt) and kaolinite. The experiments were carried out under room temperature conditions where pressure solution and cataclasis are known to dominate over dislocation creep in the halite, thus modelling the brittle–ductile transition. In wet samples with .10 wt% clay brittle failure was followed by strain weakening towards a steady-state shear strength that was dependent on both sliding rate and normal stress (frictional–viscous flow). Strongly foliated microstructures were produced in these experiments, closely ressembling natural mylonite or phyllonite microstructures, without the operation of dislocation creep. Bos & Spiers (2002b) developed a microphysical model to explain the observed steady-state behaviour, based on the steady-state microstructure and corresponding mechanical analogue diagram of Figure 2. In this model, the shear strength of the gouge is determined by the combined resistance to shear offered by frictional sliding on the phyllosilicate foliae, pressure solution in the halite and dilatation on the foliation (work against the normal stress). The model accordingly predicts three velocity regimes, namely: (1) a low-velocity regime, where pressure solution is so easy that the strength of the gouge is determined by sliding friction on the foliation; (2) an intermediate velocity regime, where the strength of the gouge is determined by accommodation through pressure solution; and (3) a highvelocity regime, where pressure solution is too slow to accommodate geometric incompatibilities, so that dilatation occurs. Bos & Spiers (2002b) reported good agreement between their experimental data and model, and went on to apply the model to predict crustal strength profiles for quartz–mica fault rocks. These showed a major weakening (2–5 times) of crustal fault zones around the brittle–ductile transition (5–15 km, see Bos & Spiers 2002b), in qualitative agreement with inferences drawn from numerous geological and geophysical studies (Lachenbruch & Sass 1980; Schwarz & Stockhert 1996; Imber et al. 2001; Stewart et al. 2000; Zoback 2000; Townend & Zoback 2001). However, there are several aspects of the Bos–Spiers model that have not been tested or are not physically realistic. First, the model is based on experiments in which the phyllosilicate phase (ultrafine kaolinite) was unrealistically fine in relation to the halite grains and to natural faultrock microstructures (e.g Imber et al. 2001). Second, the model has not been tested in the low-velocity regime (Regime 1), nor adequately at high velocities (Regime 3) where it must eventually break down due to fault-rock failure. Third, the model employs an unnecessary and physically unrealistic approximation to couple the mechanical effects of pressure solution and dilatation, and deals only with a single-valued grain size. Finally, the model is restricted to frictional behaviour of the phyllosilicate foliae, whereas under the conditions of the Fig. 1. Schematic of classical crustal strength profile, showing brittle–frictional behaviour dominating at upper crustal levels, and dislocation creep determining crustal strength at deeper levels (solid lines). The dashed line represents the widely accepted effects of fluidassisted deformation mechanisms on crustal strength (After Bos et al. 2000b.) A. R. NIEMEIJER & C. J. SPIERS 304

Terrane processes at the margins of Gondwana

Abstract: The process of terrane accretion is vital to the understanding of the formation of continental crust. Accretionary orogens affect over half of the globe and have a distinctively different evolution to Wilson-type orogens. It is increasingly evident that accretionary orogenesis has played a significant role in the formation of the continents. The Pacific-margin of Gondwana preserves a major orogenic belt, termed here the ‘Australides’, which was an active site of terrane accretion from Neoproterozoic to Late Mesozoic times, and comparable in scale to the Rockies from Mexico to Alaska, or the Variscan-Appalachian orogeny. The New Zealand sector of this orogenic belt was one of the birthplaces of terrane theory and the Australide orogeny overall continues to be an important testing ground for terrane studies. This volume summarizes the history and principles of terrane theory and presents 16 new works that review and synthesize the current state of knowledge for the Gondwana margin, from Australia through New Zealand and Antarctica to South America, examining the evolution of the whole Gondwana margin through time.

Geochronology of Proterozoic basement inliers in the Colombian Andes: tectonic history of remnants of a fragmented Grenville belt

Abstract: Basement inliers of high-grade metamorphic rocks within the eastern Colombian Andes record a Grenvillian history. Among them, the Garzon Complex and the Dibulla, Bucaramanga and Jojoncito gneisses were studied using different geochronological methods to produce better correlations in the context of the reconstruction of the Grenville belt and of the supercontinent of Rodinia. The dynamic evolution of all of these units includes a final collisional event with exhumation of high-grade rocks. Such a tectonic history bears strong similarities with the Grenville Province in Canada and seems to confirm that these domains took part in the aggregation of Rodinia. Mesoproterozoic U-Pb zircon ages indicate heritage from magmatic protoliths, and the Sm-Nd model ages, as well as the e Nd values, suggest derivation from an evolved continental domain, such as the Amazonian craton, with some mixing with juvenile Neoproterozoic material. When these continental fragments are correlated with similar terrains in Mexico and the Central Andes, a large crustal fragment is implied; very probably it made up the southern portion of the Grenville belt within Rodinia, which was disrupted when Laurentia separated from Gondwana forming the Iapetus Ocean, leaving behind cratonic fragments that were later accreted to the South American Platform.

Controls on the heterogeneous distribution of mineral deposits through time

Abstract: Mineral deposits exhibit heterogeneous distributions, with each major deposit type showing distinctive, commonly unique, temporal patterns. These reflect a complex interplay between formational and preservational forces that, in turn, largely reflect changes in tectonic processes and environmental conditions in an evolving Earth. The major drivers were the supercontinent cycle and evolution from plume-dominated to modern-style plate tectonics in a cooling Earth. Consequent decrease in the growth rate of continental crust, and change from thick, buoyant sub-continental lithospheric mantle (SCLM) in the Precambrian to thinner, negatively buoyant SCLM in the Phanerozoic, led to progressive decoupling of formational and preservational processes through time. This affected the temporal patterns of deposit types including orogenic gold, porphyry and epithermal deposits, volcanic hosted massive sulphide (VHMS), palaeoplacer Au, iron oxide, copper gold (IOCG), platinum group elements (PGE), diamond and probably massive sulphide SEDEX deposits. Sedimentary mineral deposits mined for redox- sensitive metals show highly anomalous temporal patterns in which specific deposit types are restricted to particular times in Earth history. In particular, palaeoplacer uranium, banded iron formation (BIF) and BIF-associated manganese carbonates that formed in the early Precambrian do not reappear in younger basins. The most obvious driver is progress- ive oxidation of the atmosphere, with consequent long-term changes in the hydrosphere and biosphere, the latter influencing the temporal distribution and peak development of deposits such as Mississippi Valley types (MVT), hosted in biogenic sedimentary rocks.

Pacific subduction coeval with the Karoo mantle plume: the Early Jurasssic Subcordilleran belt of northwestern Patagonia

Abstract: The Early Mesozoic magmatism of southwestern Gondwana is reviewed in the light of new U-Pb SHRIMP zircon ages (181 ± 2 Ma, 181 ± 3 Ma, 185 ± 2 Ma, and 182 ± 2 Ma) that establish an Early Jurassic age for the granites of the Subcordilleran plutonic belt in northwestern Argentine Patagonia. New geochemical and isotopic data confirm that this belt represents an early subduction-related magmatic arc along the proto-Pacific margin of Gondwana. Thus, subduction was synchronous with the initial phase of Chon Aike rhyolite volcanism ascribed to the thermal effects of the Karoo mantle plume and heralding rifting of this part of the supercontinent. Overall, there is clear evidence that successive episodes of calc-alkaline arc magmatism from Late Triassic times until establishment of the Andean Patagonian batholith in the Late Jurassic involved westerly migration and clockwise rotation of the arc. This indicates a changing geodynamic regime during Gondwana break-up and suggests differential rollback of the subducted slab, with accretion of new crustal material and/or asymmetrical ‘scissor-like’ opening of back-arc basins. This almost certainly entailed dextral displacement of continental domains in Patagonia.

Glacier-permafrost interaction in Arctic and alpine mountain environments with examples from southern Norway and Svalbard

Abstract: Abstract The interaction between glaciers and permafrost was long ago addressed for glaciers in Arctic regions. Analogies from modern environments have been used to understand landform development at the margins of Pleistocene ice sheets. During more recent decades many systematic measurements of permafrost in boreholes, geophysical soundings and temperature monitoring have revealed permafrost to be more abundant in many more high-mountain areas than previously thought. This suggests that permafrost may be a governing factor not only for periglacial landform evolution in these areas, but also, given the potential for glacier-permafrost interaction, for glacial landform generation. This paper presents and discusses observation and study results on the geomorphological significance of the interrelationship between glaciers and permafrost, in relation to geomorphological processes, landform generation and response of the system to climate fluctuations.

Pollen records and climatic cycles in the North Mediterranean region since 2.7 Ma

Abstract: Abstract This synthesis incorporates the 16 most important pollen records available across the North Mediterranean region sensu lato for the last 2.7 Ma. Their location is discussed with respect to the present-day bioclimatic Mediterranean realm. A special effort has been made to redraw, where necessary, the pollen records in terms of modern cyclostratigraphy. The complexity of the evolution of the Mediterranean flora and vegetation as forced by the climatic cycles is evident. The influence of the latitudinal thermic (and xeric) gradient is confirmed, and the superimposition of a longitudinal gradient, forced by the Asian monsoon, is considered. The Mediterranean flora and vegetation were not altered by any important event during the Early-Middle Pleistocene transition between 1.2 and 0.7 Ma.

Flow of partially molten crust and origin of detachments during collapse of the Cordilleran Orogen

Abstract: In metamorphic core complexes two types of detachments develop, coupled by flow of partially molten crust: a channel detachment and a rolling-hinge detachment. The channel detachment, on the hinterland side of the orogen, represents the long-lived interface that separates the partially molten crust flowing in a channel from the rigid upper crustal lid. On the foreland side of the core complex, a rolling-hinge detachment develops. This detachment dips toward the foreland, probably affects the whole crust, and its geometry is governed by strain localization at the critical interface between cold foreland and hot hinterland. Activation of the rolling-hinge detachment drives rapid decompression and melting, leading to the diapiric rise of migmatite domes in the footwall of the detachment. A kinematic hinge (switch in sense of shear) separates the two types of detachments. Structural, metamorphic and geo/thermochronological studies in the Shuswap core complex (North American Cordillera), combined with an anisotropy of magnetic susceptibility study of leucogranites concentrated in the detachments, suggest that this orogen collapsed rapidly through the development of channel and rolling-hinge detachments in the early Eocene. The kinematic hinge is currently located approximately 40 km west of the footwall in which it originated, corresponding to a mean exhumation rate of .5 km Ma, which explains the near-isothermal decompression recorded within the migmatite dome. A new paradigm for orogeny involves the development of a topographic plateau and a thermally and rheologically layered crust, with a relatively thin, rigid lid of upper crust overlying a partially molten layer that makes up a significant fraction of the crust (Nelson et al. 1996; Schilling & Partzsch 2001). This low-viscosity layer may flow in a channel (Royden 1996; Royden et al. 1997) that decouples the upper crust from the lower crust–upper mantle (Nelson et al. 1996; McKenzie et al. 2000; Beaumont et al. 2001; Rey et al. 2001; Vanderhaeghe & Teyssier 2001a, b; Vanderhaeghe et al. 2003a). The behaviour of partially molten crust has significant implications for the mechanics of orogens, and in particular for the origin and evolution of the interface between the partially molten layer and the rigid upper crust. In exhumed orogens, anatectic migmatite domes are commonly exposed, suggesting upward flow of partially molten crust, and the interface between upper crust and migmatites is commonly represented by a shallowly dipping detachment. Structural, metamorphic and geo/ thermochronological data show that melt crystallization in migmatite domes and leucogranites, activation of detachments, and cooling/exhumation of the footwall rocks are largely coeval (Vanderhaeghe & Teyssier 2001b). Therefore, the formation and evolution of detachments may be fundamentally tied to the dynamics of partially molten crust, including both lateral From: BRUHN, D. & BURLINI, L. (eds) 2005. High-Strain Zones: Structure and Physical Properties. Geological Society, London, Special Publications, 245, 39–64. 0305-8719/05/$15.00 # The Geological Society of London 2005. (channel) and vertical (diapiric) flow, as the orogen evolves from an orogenic plateau phase to a collapse phase (Fig. 1) (Rey et al. 2001). The kinematic relationship between upper crustal extension and lower crustal flow has been debated vigorously with regard to the origin of metamorphic core complexes (MCC) (Crittenden et al. 1980; Lister & Davis 1989; Malavieille 1993; Wills & Buck 1997; Zheng et al. 2004). The prevailing view is that the ductile crust flows passively to accommodate extension of the upper crust (Bird 1991; Axen et al. 1998). The lower crust flows toward the thinned zone (Buck 1988, 1991; Brun et al. 1994) to fill the gap beneath the stretched upper crust and to spread the thinning (Bertotti et al. 2000). The progressive rotation of early formed normal faults in the footwall of active highangle normal faults, as in a ‘rolling-hinge’ model, explains the existence of shallowly dipping detachments. In the Shuswap MCC, North American Cordillera, the interface between upper crust and partially molten crust has been exhumed. We present new metamorphic and geochronological data, and synthesize existing structural, metamorphic and age data from this complex to evaluate the evolution of high strain detachment zones. In addition, we use the anisotropy of magnetic susceptibility to study the deformation of well-dated leucogranite laccoliths concentrated in the detachments. Conceptual models for flow of partially molten crust Two types of models describe the relative contribution of the upper crust and lower crust to the collapse process: one in which collapse of thickened crust occurs without extension at the boundaries (fixed-boundary collapse); and another in which the boundaries of the thickened crust extend (free-boundary collapse) (Rey et al. 2001) (Fig. 1). Channel flow (Royden 1996) beneath a rigid upper crustal lid (blind collapse of Rey et al. 2001) (Fig. 1b) displays a sense of flow from the centre of the orogen towards the foreland, driven by the gravitational potential of the thick crust. Three elements of this model are important for kinematics: (1) the lower– middle crust flows relative to the rigid lid, imposing centripetal (top to the hinterland) sense of shear relative to the orogen at the interface between the channel and the lid; (2) the velocity gradient in the channel imposes a reversal of sense of shear across the channel from centripetal at the top to centrifugal at the bottom (Fig. 1b); and (3) a gradient in the component of simple shear occurs from coaxial flow in the centre of the orogen to non-coaxial flow toward the margin. In the centre of the channel, velocity gradients are minimal and coaxial flow dominates. An alternative model is one in which sliding of upper crust away from the kinematic axis of the orogen translates into thrusting in the foreland (Fig. 1c) (Axen et al. 1998). In this case, the sense of shear at the base of crust is centrifugal relative to the orogen and, significantly, opposite to the case of channel flow. Therefore, in the history of the orogen, if detachment tectonics followed a period of channel flow, the sense of shear would reverse. If the orogen extends at the boundaries according to the free-boundary collapse of Rey et al. (2001), the compatibility of deformation between upper and lower crust and within the lower crust dictates the kinematics of the system. During symmetric extension (Fig. 1d), sense of shear beneath the upper crust is either centripetal or centrifugal dependent on local differential velocities between the upper and lower crust. The lower crust is characterized by FREE BOUNDARY COLLAPSE Channel Flow Symmetric Detachments Initial Condition Thickened Crust FIXED BOUNDARY COLLAPSE

Reactive iron enrichment in sediments deposited beneath euxinic bottom waters: constraints on supply by shelf recycling

Abstract: Abstract Modern and ancient euxinic sediments are often enriched in iron that is highly reactive towards dissolved sulphide, compared to continental margin and deep-sea sediments. It is proposed that iron enrichment results from the mobilization of dissolved iron from anoxic porewaters into overlying seawater, followed by transport into deep-basin environments, precipitation as iron sulphides, and deposition into sediments. A diagenetic model shows that diffusive iron fluxes are controlled mainly by porewater dissolved iron concentrations, the thickness of the surface oxygenated layer of sediment and to a lesser extent by pH and temperature. Under typical diagenetic conditions (pH < 8, porewater Fe2+ = 10−6 g cm−3) iron can diffuse from the porewaters in continental margin sediments to the oxygenated overlying seawater at fluxes of 100–1000 μg cm−2 a−1. The addition of reactive iron to deep-basin sediments is determined by the magnitude of this diffusive flux, the export efficiency (ɛ) of recycled iron from the shelf, the ratio of source area (S) to basin sink area (B) and the trapping of reactive iron in the deep basin. Values of ɛ are poorly constrained but modern enclosed or semi-enclosed sedimentary basins show a wide variation in S/B ratios (0.25–13) where the shelf source area is defined as sediments at less than 200 m water depth. Diffusive fluxes in the range 100–1000 μg cm−2 a−1 are able to produce the observed reactive iron enrichments in the Black Sea, the Cariaco Basin and the Gotland Deep for values of ɛ × S/B from 0.1–5. Transported reactive iron can be trapped physically and/or chemically in deep basins. Physical trapping is controlled by basin geometry and chemical capture by the presence of euxinic bottom water. The S/B ratios in modern basins may not be representative of those in ancient euxinic/semi-euxinic sediments but preliminary data suggest that ɛ × S/B in ancient euxinic sediments has a similar range as in modern euxinic sediments.

The accretionary history of southern South America from the latest Proterozoic to the Late Palaeozoic: some palaeomagnetic constraints

Abstract: It is now accepted that southern South America was formed from several terranes of diverse origin and evolution. However, a detailed history of the accretionary processes has not been unravelled yet. Palaeomagnetism can play an important role in such an endeavour. Palaeomagnetic constraints on the tectonic evolution of this region in the Proterozoic and Palaeozoic are reviewed and discussed. Data from the Rio de la Plata craton suggest that this block was already attached to most major Gondwana blocks by the end of the Proterozoic and may have formed a single continental mass with Congo-Sao Francisco, West Nile and Arabia throughout most of the Vendian. A large ocean separating these cratons from Amazonia and West Africa, prior to Gondwana assembly, is supported by available palaeomagnetic data. To the west of the Rio de la Plata craton is the Pampia terrane. Despite lack of palaeomagnetic data, geological evidence supports a model of Early Cambrian collision between these blocks. An Early Ordovician magmatic arc, the Famatina-Eastern Puna belt, which had developed on the western margin of the already accreted Pampia terrane, shows a systematic pattern of large clockwise rotation that has been interpreted as representative of the whole terrane. The favoured tectonic model portrays a continental magmatic arc with a back-arc basin to the east that was closed when the terrane rotated. There is little doubt of a Laurentian origin for the Cuyania (Precordillera) terrane, given the amount and diversity of evidence, including palaeomagnetism. The tectonic mechanism for accretion and its timing are still controversial. New palaeomagnetic data from Late Ordovician rocks of Cuyania support the 'Laurentian plateau' hypothesis, which suggests that Cuyania was still linked to Laurentia well into the Ordovician. Nevertheless, these new data do not rule out the more generally favoured 'microcontinent model'. To the west of Cuyania is the Chilenia terrane, separated by a belt of ophiolites of Late Ordovician age. Very little is known about this terrane, although some U-Pb ages and Nd model ages point to a Laurentian origin for its basement. Lack of palaeomagnetic data precludes determining its kinematic evolution. The Arequipa-Antofalla block may actually be a composite terrane. Palaeomagnetic data obtained so far come exclusively from the southern Antofalla block. Recently acquired data in the western Puna of Argentina confirm the originally proposed distribution of Early Palaeozoic palaeomagnetic poles, which, despite several uncertainties, delineate a pattern of significant counterclockwise rotations with a possible anomaly in palaeolatitude for the late Cambrian. The data suggest a major tectonic discontinuity between the Eastern and Western Puna of Argentina in the Early Palaeozoic. Four palaeomagnetic poles of Devonian to Permian age from the North Patagonian Massif are consistent in position and age with the Gondwana apparent polar wander path, suggesting that both continental masses have not experienced major relative displacement since the Devonian. The data do not, however, rule out a restricted separation of Patagonia orthogonal to its northern boundary in the Early or Middle Palaeozoic and subsequent collision in the Late Palaeozoic. © The Geological Society of London 2005.

Provenance studies of very low- to low-grade metasedimentary rocks of the Puncoviscana complex, northwest Argentina

Abstract: Abstract A provenance study of Neoproterozoic to Lower Cambrian rocks for the entire Puncoviscana Basin was conducted, using 119 samples from 15 different outcrops. Petrographic data (Qt60–80, F15–35, L5–20, P/F 0.2–0.4, Lv/L = 0) show a composition comparable to foreland-basin successions. Lithoclasts are of metamorphic and metasedimentary origin. Volcanic debris is detected only in the form of sanidine, and volcanic lithoclasts were probably decomposed to form pseudo-matrix. Framework clasts are sub-angular to sub-rounded, and the rocks are poorly sorted. Major element geochemistry shows a moderate to high Chemical Index of Alteration (56–77) and failed to provide coherent provenance and rock classification. Trace element geochemistry suggests a rhyodacitic composition overall. Rare earth element patterns are comparable to those of model upper continental crust (UCC), as are concentrations of Nb, Ta, Ti, Th-Sc and Eu/Eu* (0.45–0.87; 95% between 0.4 and 0.7); reworking signatures are not detected. The uniform mineralogical and geochemical composition reflects supra-crustal source(s) for the entire basin, including significant metamorphic rock debris. The Puncoviscana complex is interpreted as a peripheral Pampean foreland basin, fed mainly from an eastern fold-thrust belt, but includes relicts of pre- and syn-collisional magmatic activity as well. A source area of UCC composition to the west is represented by the Arequipa block.

Electrical conductivity images of active and fossil fault zones

Abstract: Abstract We compare recent magnetotelluric investigations of four large fault systems: (i) the actively deforming, ocean-continent interplate San Andreas Fault (SAF); (ii) the actively deforming, continent-continent interplate Dead Sea Transform (DST); (iii) the currently inactive, trench-linked intraplate West Fault (WF) in northern Chile; and (iv) the Waterberg Fault/Omaruru Lineament (WF/OL) in Namibia, a fossilized intraplate shear zone formed during early Proterozoic continental collision. These fault zones show both similarities and marked differences in their electrical subsurface structure. The central segment of the SAF is characterized by a zone of high conductivity extending to a depth of several kilometres and attributed to fluids within a highly fractured damage zone. The WF exhibits a less pronounced but similar fault-zone conductor (FZC) that can be explained by meteoric waters entering the fault zone. The DST appears different as it shows a distinct lack of a FZC and seems to act primarily as an impermeable barrier to cross-fault fluid transport. Differences in the electrical structure of these faults within the upper crust may be linked to the degree of deformation localization within the fault zone. At the DST, with no observable fault-zone conductor, strain may have been localized for a considerable time span along a narrow, metre-scale damage zone with a sustained strength difference between the shear plane and the surrounding host rock. In the case of the SAF, a positive correlation of conductance and fault activity is observed, with more active fault segments associated with wider, deeper and more conductive fault-zone anomalies. Fault-zone conductors, however, do not uniquely identify specific architectural or hydrological units of a fault. A more comprehensive whole-fault picture for the brittle crust can be developed in combination with seismicity and structural information. Giving a window into lower-crustal shear zones, the fossil WF/OL in Namibia is imaged as a subvertical, 14 km-deep, 10 km-wide zone of high and anisotropic conductivity. The present level of exhumation suggests that the WF/OL penetrated the entire crust as a relatively narrow shear zone. Contrary to the fluid-driven conductivity anomalies of active faults, the anomaly here is attributed to graphitic enrichment along former shear planes. Once created, graphite is stable over very long time spans and thus fault/shear zones may remain conductive long after activity ceases.

Climatic patterns revealed by pollen and oxygen isotope records across the Matuyama–Brunhes Boundary in the central Mediterranean (southern Italy)

Abstract: A c. 50 m thick section located in the Crotone Basin (southern Italy) was investigated using oxygen isotopes, pollen and planktonic foraminifera. The section records two complete trans- gressive-regressive cycles mainly driven by glacio-eustasy. Biostratigraphy and oxygen isotope chronology indicate that the section spans from Marine Isotope Stage (MIS) 22 (c. 0.87 Ma) to MIS 18.3 (c. 0.73 Ma), thus straddling the Matuyama-Brunhes (M-B) boundary which occurs in the middle of MIS 19. The rich pollen assemblages provide a unique record of the vegetation in the central Mediterranean during the Early-Middle Pleistocene climatic transition. Interglacials are characterized by a mesothermic vegetation similar to the present day, whereas a rain-demanding conifer forest dominates the glacials of MIS 20 and MIS 18. This is unexpected because it is gener- ally considered that during the Pleistocene, glacials in central Mediterranean were characterized by steppe (arid) conditions. By contrast, arid conditions occur during the deglaciations. These results are inconsistent with the widespread practice of linking glacials with arid conditions in the central Mediterranean during Pliocene and Early Pleistocene times. This study emphasizes the need to establish more accurate land-sea correlation.

Investigating glacier-permafrost relationships in high-mountain areas: historical background, selected examples and research needs

Abstract: Abstract Investigations on the relationships and interactions between glaciers and permafrost in high-mountain regions have long been neglected. As a consequence, numerous fascinating questions remain open and offer possibilities for highly relevant, innovative and integrative research concerning materials, processes, landforms, environmental aspects and natural hazards. The historical background to this situation is first reviewed, examples are given of some key unanswered questions and two case studies are presented to illustrate the importance of considering the combined effects of glaciers and permafrost, particularly in the context of hazard assessments in high mountains.

Dynamic recrystallization and strain softening of olivine aggregates in the laboratory and the lithosphere

Abstract: Abstract The effects of dynamic recrystallization on the deformation mechanisms and rheology of olivine aggregates in the laboratory and the lithosphere are reviewed in this paper. The low-strain rheology of olivine is well documented; however, deformation in the lithosphere often involves large strains. Large strain experiments show that recrystallization can result in both hardening and softening during deformation. Moderate strain softening in experimental shear and torsion can be explained by the operation of dislocation-accommodated grain boundary sliding in bands of fine recrystallized grains. Data on the temperature dependence of recrystallized grain size are needed to extrapolate the effects of dynamic recrystallization to the lithosphere. Theories of dynamic recrystallization suggest that grain size is strongly stress dependent and moderately temperature dependent. A re-analysis of experimental grain size data indicates that the recrystallized grain size is temperature independent for olivine aggregates with low water content (<300 ppm H/Si). Rheological regime maps have been constructed for the lithospheric mantle. The maps suggest that grain size sensitive power law creep, involving both grain boundary sliding and dislocation creep, will produce strong strain softening, greater than found so far in experimental studies, in dry and wet lithosphere shear zones.

Tectonic evolution of the Andean Fold and Thrust Belt of the southern Neuquén Basin, Argentina

Abstract: The Andean Fold and Thrust belt between 368 and 398S can be divided in two sectors. The Eastern Sector corresponds to the Agrio Fold and Thrust Belt (FTB) characterized by a major exhumation during the Late Cretaceous, and minor deformation during the late Eocene and Late Miocene. The Western Sector corresponds to the main cordillera and is characterized by a complex evolution that involves periods of out-of-sequence thrusting with respect to the previously deformed outer sector, and pulses of relaxation of the compressive structure. Cretaceous uplift constituted an orogenic wedge that extended to the inner sectors of the Agrio FTB. Eocene compression was mainly concentrated within the Western Sector but may have reactivated the pre-existing structures of the Agrio FTB, such as the Cordillera del Viento. Late Miocene minor compressional deformation occurred in the retro-arc area and extended into the foreland area. This deformation event produced the closure of a short-lived intra-arc basin (Cura Mallin Basin, 25–15 Ma) at the innermost sector of the FTB. The Pliocene and Quaternary, between 378300and 398S, have been periods of relaxation of the inner part of the FTB and fossilization of the Agrio Fold and Thrust Belt. Localization of episodic late Oligocene–Early Miocene and Pliocene to the present extensional structures in the intraand inner retro-arc is controlled by pre-existing Jurassic half-grabens related to the formation of the Neuquen Basin. The Jurassic rift seems to be controlled by deep crustal–lithospheric discontinuities derived from a Proterozoic– Palaeozoic history of amalgamation in the area, now deeply buried under multiple episodes of Mesozoic–Tertiary synorogenic and synextensional sedimentation. Geophysical studies have revealed that the Andes mountain belt is extremely variable in crustal thickness and topography (Introcaso et al. 2000). The topography varies between broad amplitudes greater than 700 km measured from the trench, and narrow belts restricted to the inner sectors of the fold and thrust belt. These variations are mainly related to shortening within the Andes (Ramos et al. 2004). However, the causes of variable shortening and relief remain open to discussion and include several key factors: (1) shortening of the mantle lithosphere related to overthrusting of the Andes over old cratonic shields (Lyon-Caen et al. 1985; Lamb & Hoke 1997; Kley et al. 1999); (2) pre-existing anisotropies in the foreland of the orogenpre-dating theAndeanorogeny, which differentially deformed under compression (Allmendinger & Gubbels 1996); (3) changes in the lithospheric thermal structure and the consequent development of brittle–ductile transitions that become new detachments where the upper crust yields and is stacked over the foreland (James & Sacks 1999; Ramos et al. 2002); and (4) climate (Beaumont et al. 1992; Thomson 2002). At this latitude (378–398S), the Andean mountain belt deforms the Mesozoic Neuquen Basin (Fig. 1). The maximum topographic heights are restricted to a narrow band next to the volcanic arc, and the external zone represents a smooth surface where older deformations have taken place during the Late Cretaceous and Palaeogene (Fig. 1) (Zapata et al. 1999, 2002). The lack of amplitude and height of the orogenic system, in comparison with neighbour segments to the north, are in accordance with the minimum shortening computed from surface structures and crustal roots (Zapata et al. 1999; Ramos et al. 2004). The tectonic evolution of the Andean mountains in the southern portion of the Neuquen Basin reveals certain anomalies to the general From: VEIGA, G. D., SPALLETTI, L. A., HOWELL, J. A. & SCHWARZ, E. (eds) 2005. The Neuquen Basin, Argentina: A Case Study in Sequence Stratigraphy and Basin Dynamics. Geological Society, London, Special Publications, 252, 37–56. 0305-8719/05/$15.00 # The Geological Society of London 2005. picture of progressive foreland-propagating deformation in the Andes. These anomalies are characterized by a positive roll-back velocity, since the break-up of southern Gondwana (Ramos 1999b), and periods of foreland propagation of thrust sheets alternating with periods of tectonic relaxation, possibly originated from changes in the Wadati–Benioff geometry. However, are these real anomalies from the Andean orogeny point of view or do they exemplify a long-standing process in many segments along the Andean chain that elsewhere have been obscured by younger tectonic imprints? Other subduction-related orogens around the Fig. 1. Regional location map, where main morphostructural units of the Andes between 368 and 408S are displayed. (A) Arc and retro-arc morphostructural units. (B) Fore arc to retro-arc systems mentioned throughout the paper. (C) Thematic mapper scan of the area occupied by the Neuquen Embayment during the Mesozoic from the western to the eastern side of the present Andean belt. The square represents the area of Figure 5 and the black line indicates the position of the profile in Figure 10. T. ZAPATA & A. FOLGUERA 38

Factors controlling sequence development on Quaternary fluvial fans, San Joaquin Basin, California, USA

Abstract: Abstract Variable geometry and distribution of stratigraphic sequences of fluvial fans in the eastern San Joaquin Basin, California, were controlled by tectonics, through basin subsidence and basin width, and response to Quaternary climate change, related to the degree of change in sediment supply to stream discharge ratios and local base-level elevation changes. Three fluvial fans — the Kings River, Tuolumne River and Chowchilla River fans — illustrate the influence of these factors on ultimate sequence geometry. In areas with high subsidence rates (e.g. the Kings River fluvial fan) sequences are relatively thick and apices of subsequent sequences are vertically stacked. Areas with relatively low subsidence rates (e.g. the Tuolumne River fan) produced laterally stacked sequences. Rivers that experienced a significant increase in sediment supply to stream discharge ratios due to direct connection to outwash from glaciated portions of the Sierra Nevada developed high accommodation space and relatively thick sequences with deep incised valleys. Conversely, rivers that were not connected to glaciated regions (e.g. the Chowchilla River fan) and, thus, experienced a relatively minor change in sediment supply to discharge ratios during climate change events, produced thinner sequences that lack deep incised valleys. Local base-level connection to sea level, via the axial San Joaquin River, produced deeper incised valleys than those of internally drained rivers. Finally, narrow basin width allowed glacially connected fans to completely fill available accommodation space, thus producing smaller fans that lack preservation of distal, interglacial deposits. Evaluation of these controls allows prediction of sequence geometries and facies distributions for other San Joaquin Basin fans for input into future hydrogeological models.

New Zealand tectonostratigraphy and implications from conglomeratic rocks for the configuration of the SW Pacific margin of Gondwana

Abstract: Abstract The active margin of Gondwana is presently preserved in the southwest Pacific region in the formerly continuous Gondwana fragments of Australia, Antarctica and New Zealand. The Phanerozoic tectonic history of New Zealand is interpreted in terms of progressive Pacific-ward growth by accretion of arc-trench systems and the basement rocks are described in terms of a number of volcano-sedimentary accreted terranes, suites and batholiths that intrude the terranes. The age of these basement rocks ranges from Early Cambrian to late Early Cretaceous. The origin of the magmatic and sedimentary rocks and the time of accretion of the New Zealand terranes to the Gondwana margin are important for the understanding of Phanerozoic Pacific tectonics. Geochronological research over the last decade on igneous rocks and conglomeratic units shows that the Tutoko Complex/Amundsen Province plutons are major contributors of detritus to the Pahau depositional basin and that the Antarctic sector of the Panthalassan Gondwana margin has to be (re)considered as the likely source for the Permo-Triassic Rakaia sediments. Igneous clast data have greatly improved understanding of the evolution of the New Zealand microcontinent and have put tighter constraints on its Mesozoic tectonic setting within the southwest Pacific margin of Gondwana.

Decoupling and its relation to strain partitioning in continental lithosphere: insight from the Periadriatic fault system (European Alps)

Abstract: Abstract The Periadriatic fault system (PFS) is an array of late orogenic faults (35-15 Ma) in the retro-wedge of the Alpine orogen that accommodated dextral transpression during oblique indentation by the southern Alpine crust. Decoupling along the leading edges of the southern Alpine indenter occurred where inherited lithological and rheological contrasts were accentuated by lateral thermal gradients during emplacement of the warm orogenic retro-wedge next to the cold indenter. In contrast, decoupling within the core and retro-wedge of the orogen occurred in a network of folds and mylonitic faults. In the Eastern Alps, this network comprises conjugate sets of upright, constrictional folds, strike-slip faults and low-angle normal faults that accommodated nearly coaxial NNE-SSW shortening and E-W extensional exhumation of the Tauern thermal dome. The dextral shear component of oblique convergence was taken up by a discrete, brittle fault parallel to the indenter surface. In the Central and Western Alps, a steep mylonitic backthrust, upright folds, and low-angle normal faults effected transpressional exhumation of the Lepontine thermal dome. Mylonitic thrusting and dextral strike-slip shearing along the steep indenter surface are transitional along strike to low-angle normal faults that accommodated extension at the western termination of the PFS. The areal distribution of poles to mylonitic foliation and stretching lineation of these networked structures is related to the local shape and orientation of the southern Alpine indenter surface, supporting the interpretation of this surface as the macroscopic shearing plane for all mylonitic segments of the PFS. We propose that mylonitic faults nucleate as viscous instabilities induced by cooling, or more often, by folding and progressive rotation of pre-existing foliations into orientations that are optimal for simple shearing parallel to the eigenvectors of flow. The mechanical anisotropy of the viscous continental crust makes it a preferred site of decoupling and weakening. Networking of folds and mylonitic fault zones allow the viscous crust to maintain strain compatibility between the stronger brittle crust and upper mantle, while transmitting plate forces through the lithosphere. Decoupling within the continental lithosphere is therefore governed by the symmetry and kinematics of strain partitioning at, and below, the brittle-to-viscous transition.

Deposition and stratigraphic architecture of an outcropping ancient slope system: Tres Pasos Formation, Magallanes Basin, southern Chile

Abstract: Abstract The Tres Pasos Formation, Magallanes Basin, Chile, represents the deposit of a submarine slope depositional system. The formation is approximately 1500 m thick where exposed in the Ultima Esperanza district of southernmost Chile. It is characterized by a basal turbiditic sandstone unit up to 200 m thick that shows a north-to-south, proximal-to-distal facies evolution from turbidite channel-fill complexes to sheet-like sandstone units. This unit is interpreted as having been deposited at or near the base of slope. Overlying the basal sandstone unit is approximately 500 m of amalgamated mass transport complexes, fine-grained strata, and channelized and non-channelized turbidity current deposits, collectively comprising the middle part of the formation. Mass transport complexes exert a primary control on the character and grain size of turbidite sandstone bodies in the basal and middle part of the formation. In the southern part of the study area, a 300 m thick coarse-grained unit interpreted as a turbidite channel-fill complex partially replaces the middle part. The upper part of the formation is approximately 500 m thick and consists primarily of fine-grained strata. Failure scarps and thin turbidite channel-fill units are present in this upper part, interpreted as upper slope deposits.

Sedimentology of the tide-dominated Jurassic Lajas Formation, Neuquén Basin, Argentina

Abstract: Abstract Tidal depositional systems are often interpreted as lowstand/transgressive estuarine deposits within sequences that are either wave or river dominated during highstand times. The Middle Jurassic Lajas Formation of the Neuquén Basin, Argentina, comprises 600 m of well-exposed tide-dominated facies deposited within four unconformitybounded sequences, spanning approximately 4.5 Ma. Facies associations include tidedominated deltas, sandy-heterolithic tidal channel fills and extensive progradational tidal-flat successions, which are locally cut by heterolithic tidal channel fills. Despite the narrow bathymetric depositional range and the complex facies variability, flooding surfaces can be defined and mapped along a 48 km-long outcrop belt. These flooding surfaces allow definition of three distinct types of parasequence that exhibit coarsening-upwards, finingupwards and coarsening- to fining-upwards motifs. Sequence boundaries are marked by widespread, but shallow, incision, and the juxtaposition of stacked fluvial/tidal channel fills on a variety of subtidal and intertidal facies. Unconventional grain-size changes at sequence boundaries can occur where basinward facies shifts are marked by juxtaposition of heterolithic-argillaceous intertidal/supratidal mudflat deposits on subtidal sandflat facies. The maintenance of macrotidal conditions through complete base-level cycles is interpreted as being due to the structural topography inherited from rifting, causing the whole sub-basin to behave as a structurally controlled embayment.

Lower Cretaceous (Berriasian–Aptian) biostratigraphy of the Neuquén Basin

Abstract: The Berriasian-Aptian succession in the Neuquen Basin is mainly marine in the lower part and non-marine in the upper portion. A detailed ammonite zonation is presented for the Berriasian-?Early Barremian interval. While some ammonite taxa are endemic, others are widely distributed and there are several levels where correlation can be suggested with the 'standard' stages and zones of the Tethyan Mediterranean area. Several nannofossil bioevents are recognized, and these provide evidence for correlation with Tethyan areas. Correlations suggested by both groups are reasonably consistent. Berriasian-Aptian palynomorphs include both terrestrial and marine forms. Several terrestrial assemblages can be recognized, but the marine forms are mainly long-ranging taxa, especially in the Agrio Formation. © The Geological Society of London 2005.

Holocene permafrost aggradation in Svalbard

Abstract: Abstract The distribution and dynamics of permafrost represent a complex problem, confounded by a short research history and a limited number of deep vertical temperature profiles. This lack of knowledge is pronounced for the High Arctic, where most permafrost is found and where amplified responses to various climatic forcing mechanisms are expected. Within the High Arctic, the Svalbard region displays a unique climatic sensitivity and knowledge of Holocene, and modern permafrost dynamics in this region therefore have special interest. This paper reviews knowledge on Holocene permafrost development in Svalbard and the climatic background for this. In Svalbard, modern permafrost thickness ranges from less than 100 m near the coasts to more than 500 m in the highlands. Ground ice is present as rock glaciers, as ice-cored moraines, buried glacial ice, and in pingos and ice wedges in major valleys. Svalbard is characterized by ongoing local-scale twentieth-century permafrost aggradation, even though a distinct temperature increase around 1920 introduced relatively unfavourable climatic conditions for permafrost in Svalbard. Modern permafrost aggradation is to a large extent controlled by wind, solid precipitation and avalanche activity, and exemplifies the complexity of relating climate and permafrost dynamics.