Showing papers in "Geological Society, London, Special Publications in 1998"

The Cambrian-Silurian tectonic evolution of the northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus

Abstract: This paper is included in the Special Publication entitled 'Lyell: the past is the key to the present', edited by D.J. Blundell and A.C. Scott. This paper presents new ideas on the Early Palaeozoic geography and tectonic history of the Iapetus Ocean involved in the formation of the northern Appalachian-British Caledonide Orogen. Based on an extensive compilation of data along the length of the orogen, particularly using well-preserved relationships in Newfoundland as a template, we show that this orogen may have experienced a very complicated tectonic evolution that resembles parts of the present west and southwest Pacific Ocean in its tectonic complexities. Closure of the west and southwest Pacific Ocean by forward modelling of the oblique collision between Australia and Asia shows that transpressional flattening and non-coaxial strain during terminal collision may impose a deceptively simple linearity and zonation to the resultant orogen and, hence, may produce a linear orogen like the Appalachian-Caledonian Belt. Oceanic elements may preserve along-strike coherency for up to several thousands of kilometres, but excision and strike-slip duplication, as a result of oblique convergence and terminal collisional processes, is expected to obscure elucidation of the intricacies of their accretion and collisional processes. Applying these lessons to the northern Appalachian-Caledonian belt, we rely principally on critical relationships preserved in different parts of the orogen to constrain tectonic models of kinematically-related rock assemblages. The rift-drift transition, and opening of the Iapetus Ocean took place between c.590-550 Ma. Opening of Iapetus was temporally and spatially related to final closure of the Brazilide Ocean and amalgamation of Gondwanaland. During the Early Ordovician, the Laurentian margin experienced obduction of young, supra-subduction-zone oceanic lithosphere along the length of the northern Appalachian-British Caledonian Belt. Remnants of this lithosphere are best preserved in western Newfoundland and are referred to as the Baie Verte Oceanic Tract. Convergence between Laurentia and the Baie Verte Oceanic Tract was probably dextrally oblique. Slab break-off and a subsequent subduction polarity reversal produced a continental magmatic arc, the Notre Dame Arc, on the edge of the composite Laurentian margin. The Notre Dame Arc was mainly active during the late Tremadoc-Caradoc interval and was flanked by a southeast- or south-facing accretionary complex, the Annieopsquotch Accretionary Tract. Southerly drift of Laurentia to intermediate latitudes of c.20-25°S was associated with the compressive (Andean) nature of the arc and the accompanying backthrusting of the already-accreted Baie Verte Oceanic Tract further onto the Laurentian foreland. Equivalents of the Notre Dame Arc and its forearc elements in the British Isles have been preserved as independent slices in the Midland Valley and possibly the Northern Belt of the Southern Uplands. During the late Tremadoc (c.485 Ma), the passive margin on the eastern side of Iapetus also experienced obduction of primitive oceanic arc lithosphere. This arc is referred to as the Penobscot Arc. The eastern passive margin was built upon a Gondwanan fragment (Ganderia) that rifted off Amazonia during the Early Ordovician and probably travelled together with the Avalonian terranes as one microcontinent. The departure of Ganderia and Avalonia from Gondwana opened the Rheic Ocean. Equivalents of the Penobscot Arc may be preserved in New Brunswick and Maine, Leinster in eastern Ireland, and Anglesey in Wales. An arc-polarity reversal along the Ganderian margin after the soft Penobscot collision produced a new arc: the west-facing Popelogan-Victoria Arc, which probably formed a continuous arc system with the Bronson Hill Arc in New England. The Popelogan-Victoria Arc transgressed from a continental to an oceanic substrate from southern to northeastern Newfoundland. Rapid roll-back rifted the Popelogan-Victoria Arc away from Ganderia during the late Arenig (c.473 Ma) and opened a wide back-arc basin; the Tetagouche-Exploits back-arc basin. The Popelogan-Victoria Arc was accreted sinistrally oblique to the Notre Dame Arc and, by implication, Laurentia during the Late Ordovician. After accretion, the northwestward-dipping subduction zone stepped eastwards into the Tetagouche-Exploits back-arc basin.

Transpression and transtension zones

Abstract: Transpression and transtension are strike-slip deformations that deviate from simple shear because of a component of, respectively, shortening or extension orthogonal to the deformation zone. These three-dimensional non-coaxial strains develop principally in response to obliquely convergent or divergent relative motions across plate boundary and other crustal deformation zones at various scales. The basic constant-volume strain model with a vertical stretch can be modified to allow for volume change, lateral stretch, an oblique simple shear component, heterogeneous strain and steady-state transpression and transtension. The more sophisticated triclinic models may be more realistic but their mathematical complexity may limit their general application when interpreting geological examples. Most transpression zones generate flattening (k 1) finite strains, although exceptions can occur in certain situations. Rela- tive plate motion vectors, instantaneous strain (or stress) axes and finite strain axes are all oblique to one another in transpression and transtension zones. Kinematic partitioning of non-coaxial strike-slip and coaxial strains appears to be a characteristic feature of many such zones, especially where the far-field (plate) displacement direction is markedly oblique (<20 ~ to the plate or deformation zone boundary. Complex foliation, lineation and other structural patterns are also expected in such settings, resulting from switching or pro- gressive rotation of finite strain axes. The variation in style and kinematic linkage of trans- pressional and transtensional structures at different crustal depths is poorly understood at present but may be of central importance to understanding the relationship between defor- mation in the lithospheric mantle and crust. Existing analyses of obliquely convergent and divergent zones highlight the importance of kinematic boundary conditions and imply that stress may be of secondary importance in controlling the dynamics of deformation in the crust and lithosphere.

The Pampean Orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Cordoba

Abstract: A detailed study of the pre-Silurian geology of the Sierras de Cordoba, Eastern Sierras Pampeanas, is used to define the sequence of magmatic and metamorphic events during the Pampean orogeny. This primarily involved Early to Mid-Cambrian subduction and terrane collision at the western margin of Gondwana during the amalgamation of the super-continent. Evidence for this is based principally on new information concerning (a) regional mapping and field relations, (b) analysis of the structures, deformational history and meta-morphic evolution and (c) geochronology and geochemistry of the igneous and metamorphic rocks. The main events recognized are (1) Late Proterozoic break-up of Rodinia (Nd model ages of 1500 ± 200 Ma, inherited zircons 800–1400 Ma), (2) development of an Early Cam-brian passive margin sequence (Puncoviscana Formation and equivalents), (3) emplacement of metaluminous calc-alkaline granitoids (G1a, dated at 530 ± 3 Ma) as a result of NE-directed subduction, (4) crustal thickening, ophiolite obduction, compression and high-grade metamorphism (M2: 8.6±0.8 kbar, 810 ± 50°C, c.525 Ma) related to collision, and culmina-ting in (5) isothermal uplift and widespread low-P anatexis (M3, 4.0±0.5 kbar, 715 ± 15°C, c.520 Ma). The last event is responsible for the linked generation of highly peraluminous granites (G1b) and cordieritites. Subsequent emplacement into the accreted terrane of Ordovician trondhjemite-tonalites (500-470 Ma) and dextral wrench shear are interpreted as inner cordilleran counterparts of the Famatinian arc, which developed to the west along the newly-formed proto-Andean margin.

A primer on the geological occurrence of gas hydrate

Abstract: Abstract Natural gas hydrates occur world-wide in polar regions, usually associated with onshore and offshore permafrost, and in sediment of outer continental and insular margins. The total amount of methane in gas hydrates probably exceeds 1019 g of methane carbon. Three aspects of gas hydrates are important: their fossil fuel resource potential; their role as a submarine geohazard; and their effects on global climate change. Because gas hydrates represent a large amount of methane within 2000 m of the Earth’s surface, they are considered to be an unconventional, unproven source of fossil fuel. Because gas hydrates are metastable, changes of pressure and temperature affect their stability. Destabilized gas hydrates beneath the sea floor lead to geological hazards such as submarine slumps and slides, examples of which are found world-wide. Destabilized gas hydrates may also affect climate through the release of methane, a ‘greenhouse’ gas, which may enhance global warming and be a factor in global climate change.

Pyroclastic density currents

Abstract: Abstract High-speed, gravity-driven flows of hot particles and gas are a common and highly destructive product of explosive volcanism. They range widely in nature from expanded, turbulent suspension currents formed by lateral blasts or by the fountaining of vertical eruption columns, to highly concentrated granular avalanches formed by lava dome col-lapse or as dense underflows beneath suspension currents. The deposits from these flows, here called pyroclastic density currents, range in volume from much less than 1 km3 to thousands of cubic kilometres, and may extend over 100 km from their source. This chapter reviews the eruption, transport and deposition of pyroclastic density currents from both geological and physical perspectives, focussing on some recent advances.

On the occurrence and characterization of ultrahigh-temperature crustal metamorphism

Abstract: Abstract Ultrahigh-temperature (UHT) crustal metamorphism is a division of medium-pressure granulite facies metamorphism where peak temperatures of 900–1100°C have been attained at pressures in the range 7–13 kbar. The key indicators of UHT conditions are mineral assemblages involving combinations of sapphirine, garnet, aluminous orthopyroxene, cordierite, sillimanite, spinel and quartz in pelites and quartzites. Experimentally constrained and calculated FMAS and KFMASH petrogenetic grids involving these phases and additional osumilite and melt indicate that sapphirine + quartz is stable only at >1040°C in reduced rocks, that osumilite is restricted to >900°C for pressures greater than 6 kbar and has an ultimate stability limit of 9 kbar in FMAS, and that the orthopyroxene + sillimanite + quartz assemblage is restricted to pressures greater than 8 kbar in KFMASH. These criteria, coupled with the grids isoplethed for the Mg/(Mg + Fe) of garnet and Al-content of orthopyroxene allow the peak pressure-temperature (P-T) conditions of several UHT occurrences to be defined and the post-peak P-T paths delineated. UHT conditions are seldom determined from slowly cooled granulites using conventional geothermometry principally because of the propensity of Fe-Mg exchange thermometry to only record closure temperatures of 700–850°C. However, pressure-convergence calculations for several granulites with UHT mineral assemblages yield back-calculated mineral compositions that are consistent with temperatures of 950–1000°C prior to post-peak Fe-Mg re-equilibration. The best compositional indicator of UHT conditions remains the preservation of high Al2O3 contents (8–12 wt%) in orthopyroxene coexisting with garnet, sillimanite or sapphirine. The P-T conditions and records preserved in the currently documented UHT localities and terranes are varied. Both types of post-peak P-T path isobaric cooling and isothermal decompression (ITD) are recorded from reaction textures in different UHT terranes, and several preserve very similar ITD histories that may reflect the final stage of collisional orogenesis. Although counter-clockwise and clockwise P-T paths have been proposed on the basis of textural observation for some terranes, the prograde P-T histories of most UHT areas are not known. Such information, and further experimental constraints on quartz-absent assemblages at UHT conditions, are of prime importance to interpret further this extreme form of crustal metamorphism.

Extended models of transpression and transtension, and application to tectonic settings

Abstract: Abstract We introduce a spectrum of transpressional and transtensional deformations that potentially result from oblique plate interaction. Five separate types of deformation are designated, in which a simple shear deformation is combined with an orthogonal coaxial deformation. The types vary in the amount of extension v. contraction, both parallel to the margin and vertically. The interaction between the angle of convergence, kinematic vorticity, infinitesimal strain axes, finite strain, and rotation of material lines and planes is investigated. Quantification of the finite strain indicates that the orientation, magnitude, and geometry (flattening, constriction, etc.) change continually during steady-state transpression. These results are then applied to the cases of transpression, particularly resulting from oblique plate convergence of terranes. The obliquity of plate motion and the geometry of the plate margin determine which of the types of transpression or transtension is favoured. A component of margin-parallel stretching also potentially causes terrane motion to locally exceed oblique plate motion, or move opposite to the general direction of movement between the converging plate boundaries. The kinematic models also suggest that the boundaries between converging terranes are likely to exhibit vertical foliation, but either vertical or horizontal lineation. Finally, narrow transpressional zones between colliding blocks may have very high uplift rates, resulting in exhumation of high-grade metamorphic fabrics.

The Famatinian magmatic arc in the central Sierras Pampeanas: an Early to Mid-Ordovician continental arc on the Gondwana margin

Abstract: A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Cordoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513 ± 31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos-Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470–450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial 87Sr/86Sr (0.7075–0.7105) and low ɛNdt (−4.6 to −6.3), inherited from lower crust. Sm-Nd model ages of 1600–1700 Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.

U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin

Abstract: Abstract New SHRIMP zircon and monazite 206Pb/238U and 208Pb/232Th ages on structurally controlled units and 40Ar-39Ar step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by c. 530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at c. 480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450–460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian.

Palaeozoic petroleum systems of North Africa

Abstract: Abstract The Palaeozoic petroleum systems of North Africa contain five large giant (> 1 billion barrels of oil equivalent) and 24 giant (> 250 million barrels of oil equivalent) oil and gas fields with total recoverable reserves discovered to date of more than 46 billion barrels of oil equivalent. This article presents a classification of these petroleum systems based upon their productivity and maturity. Productivity of each system has been estimated from the associated hydrocarbon reserves and maturity from an analysis of their geological history ranging from initial genesis to maturity, destruction and final extinction. Key factors controlling both productivity and maturity include hydrocarbon charge, style of drainage and entrapment, and intensity of post-entrapment tectonic, thermal and hydrodynamic destructive processes. The regionally extensive Lower Silurian Tanezzuft Formation is the origin of 80–90% of Palaeozoic sourced hydrocarbons, with a further 10% from the Upper Devonian Frasnian shales, charging a number of intra-Palaeozoic and basal Triassic reservoirs. Triassic fluvial sands are the most important of these, hosting just over half of the total reserves, while Cambro-Ordovician and Lower Devonian F6 sandstone reservoirs are the second and third most significant, respectively. Three categories of Palaeozoic petroleum systems have been identified: (1) Mesozoic to early Tertiary charged systems with Triassic-Liassic shale and evaporite seals in the Mesozoic sag or ‘Triassic’ Basin of the northern Sahara Platform. These include > 78% of the total discovered reserves, with > 56% in the supergiants, Hassi R’Mel and Hassi Messaoud fields (2) Mesozoic to early Tertiary charged systems with intra-Palaeozoic shale seals in basins of south and east of the Triassic Basin. These include > 18% of the total discovered reserves, mostly in the prolific Illizi Basin. (3) Now largely extinct Palaeozoic charged systems with intra-Palaeozoic seals in basins of southwest Algeria and Morocco with 3% of discovered reserves. The productivity of these systems varies considerably. Hassi R’Mel and Hassi Messaoud are classified as super-productive, located on the crests of broad Palaeozoic arches which encouraged extremely efficient lateral migration focusing, and a very high impedance entrapment style. Other petroleum systems within the Triassic Basin are of high productivity with somewhat less effective migration focusing and impedance characteristics. Because of a regional evaporite seal and minimal late stage modification, these systems are all preserved in a mature phase of evolution. Basins south and east of the Triassic Basin are in various stages of destruction with variable productivities reflecting both less robust seals and post-entrapment modification by Austrian and mid-Tertiary uplift, tilting, remigration, spillage and freshwater flushing. The Illizi Basin is the least affected by these late stage destructive processes with some 15% of total discovered reserves still remaining. The Palaeozoic charged systems of southwest Algeria and Morocco were largely destroyed by Hercynian, Austrian and mid-Tertiary deformation. Only the high relief Hercynian anticlines of the Ahnet-Gourara Basin retained their trapping integrity and still reservoir a very significant amount of gas. Apart from scattered hydrocarbon shows and a few small residual accumulations the other basins in this region now all appear to be extinct.

Mesozoic rifting and basin inversion along the northern African Tethyan margin: an overview

Abstract: Abstract The northern African Tethyan margin registered three major rifting episodes from the latest Palaeozoic-earliest Mesozoic to the earliest Cenozoic. Break-up of Gondwana was initiated in the late Carboniferous. Along the northern African-Arabian plate margin rifting propagated westward from the northeastern Arabian margin to Morocco during the Permian and Triassic, and was accompanied by Mid-Late Triassic-earliest Liassic extensive alkaline flow basalts. Rifting continued during the Liassic, e.g. in the Moghrebian Atlas troughs. A second stage of rifting occurred in the Late Jurassic and continued into, or was rejuvenated during, the Early Cretaceous. Along the east Mediterranean margin, some large E-W trending rifts formed, often with associated volcanism, e.g. southern Sirt and Abu Gharadig. Most researchers believe the oceanization of the eastern Mediterranean basin occurred at this time. Flexural subsidence affected the Moghrebian Atlas, including the Riffian-Tellian domain, where a thick flysch series was deposited. Two short-lived, but widespread and tectonically important, compressional or wrench-dominated events occurred during the Santonian and the latest Maastrichtian. From Morocco to Oman, most sedimentary basins were folded and inverted at these times. The Santonian event resulted from onset of the collision between the African-Arabian and Eurasian plates. New stress-fields favoured NE-SW extension along the African plate margin, generating or rejuvenating some rifts, e.g. in the Euphrates trough and in the Libya-Tunisia region (northwestern Sirt, Pelagian Sea). Rifting and magmatism continued in these regions into the Paleocene. During the Mesozoic, therefore, the northern margin of the African-Arabian plate registered both rifting resulting in the oceanization of the Tethys and rifting resulting from the initiation of the closure of the Tethys. The intraplate domain exhibited echoes of the tectonic events affecting the margin.

Fault sealing processes in siliciclastic sediments

Abstract: Abstract The microstructure and petrophysical properties of fault rocks from siliciclastic hydrocarbon reservoirs of the North Sea are closely related to the effective stress, temperature and sediment composition at the time of deformation, as well as their post-deformation stress and temperature history. Low permeability fault rocks may develop due to a combination of processes including: the deformation induced mixing of heterogeneously distributed fine-grained material (principally clays) with framework grains, pressure solution, cataclasis, clay smear, and cementation. Fault rocks can be classified into various types (disaggregation zones, phyllosilicate-framework fault rocks, cataclasites, clay smears, and cemented faults/fractures) based upon their clay and cement content as well as the amount of cataclasis experienced. In the absence of extensive cementation, the distribution of fault rock types along a fault plane can often be predicted from a detailed knowledge of the reservoir sedimentology. The permeability of fault rocks can vary by over six orders of magnitude, depending on the extent to which the porosity reduction processes have operated. Utilizing the strong link between the petrophysical properties of fault rocks and their geohistory allows the risks associated with fault seal evaluation to be reduced.

Transpression (or transtension) zones of triclinic symmetry: natural example and theoretical modelling

Abstract: We describe a natural shear zone with triclinic symmetry, present a general model for triclinic shear zones based on natural examples, and investigate the kinematics and strain geometry within such zones. In the Roper Lake shear zone in the Canadian Appalachians, the orientation of a stretching lineation is oriented approximately down-dip near the shear zone boundary and becomes gradually shallower towards the centre. The structures in the central portion of the shear zone exhibit approximately monoclinic symmetry where the poles to both the S- and C-surfaces, the stretching lineation on the S-surfaces and the striations on the C-surfaces all plot in a great circle girdle. However, the lineations from the marginal portion do not plot in the same girdle, and the bulk symmetry of the shear zone is triclinic. Theoretical modelling shows that the observed strain geometry can be interpreted by an oblique transpression with a larger ratio of simple shear to pure shear in the centre of the shear zone than in the margin. The latter suggests a higher degree of localization of the zone boundary-parallel movement component relative to the boundary-normal compression component. We emphasize that since the imposed boundary displacements for most natural shear zones lie between dip slip and strike slip, their movement pictures are generally triclinic; monoclinic shear zones are special end members. Structural data that exhibit monoclinic symmetry do not necessarily mean that they resulted from a monoclinic movement picture; the present modelling demonstrates that a triclinic movement picture with a high ratio of boundary-parallel movement to boundary-normal movement can result in apparent monoclinic structural geometry. The results of the modelling also show that the simple statement made for simple shear zones that stretching lineations will align with, and therefore indicate, the shear direction cannot be extrapolated to three dimensional transpressional(-transtensional) shear zones.

Faulting, fault sealing and fluid flow in hydrocarbon reservoirs: an introduction

Abstract: Abstract A predictive knowledge of fault zone structure and transmissibility can have an enormous impact on the economic viability of exploration targets and generate considerable benefits during reservoir management. Understanding the effects of faults and fractures on fluid flow behaviour and distribution within hydrocarbon provinces has therefore become a priority. To model fluid flow in hydrocarbon reservoirs, it is essential to gain a detailed insight into the evolution, structure and properties of faults and fractures. Generation of realistic flow models also requires calibration with data on the fluid distributions and flow rates from hydrocarbon fields. Most hydrocarbon geologists at one time or another have asked the question ‘What is the behaviour of this fault?’. This question, as emphasized by the contributions to this volume, should more fundamentally be phrased; ‘What is the geometry of this fault zone, what are the nature and petrophysical properties of any fault rocks developed and how are they distributed in the subsurface?’. An additional important question is ‘What impact could the fault zone have on fluid flow through time?’. The properties and evolution of fault zones can be evaluated using the combined results of structural core and down-hole logging, microstructural and physical property characterization, together with analysis of faults from seismic and outcrop studies and well test data. Successful fault analysis depends upon the amalgamation of these data and incorporation into robust numerical flow models.

Evolution of the western Swiss Molasse basin: structural relations with the Alps and the Jura belt

Abstract: Abstract The tectonic evolution of the NW alpine front and foreland basin is reviewed in the light of new structural and chrono-stratigraphical data. Seismic-reflection profiles from the Jura fold thrust belt and Molasse basin, surface-geology and thrust-system considerations lead to a complete cross-section of the NW Alpine front including the Helvetic domain. Restoration of this section places individual Cenozoic formations in their approximate palaeogeographic position. ‘Geohistory’ plots are constructed for five profiles along a SE-NW transect. Thrust front, onlap and forebulge advanced at high rates of 10–20 km/Ma−1 at the onset of foreland basin formation in the late Eocene/ early Oligocene (40–30 Ma). In these early stages, the foreland basin is an underfilled flexural trough with about 100 km width, less than 600 m water depth at the deepest point and less than 200 m of total accumulated sediments. From 30 to 22 Ma, thrust front and ‘pinch-out’ migrate at a decreased rate of about 5 km/Ma−1 northwestward. The basin width remains constant at around 100 km; an increased total subsidence (c. 2.7 km) is compensated by sedimentation. At around 22 Ma, the thrust front seems to come to a halt southeast of Lausanne, whereas a strong subsidence trend prevails. After the Serravallian (c. 12 Ma) the Alpine thrust front jumps by about 100 km northwestward from a position southeast of Lausanne to the external Jura leading to thrust related uplift, deformation and concommitant erosion of the entire basin fill. No new flexural foreland basin in response to the modified thrust- and load-geometry has yet been developed. The present-day Molasse basin is only a small remnant of a much larger foreland basin in a very advanced stage of its evolution.

Barrovian regional metamorphism: where's the heat?

Abstract: Abstract Coupled thermal-mechanical models of convergent orogens offer a novel way to investigate the interactions between heat and tectonics that lead to regional metamorphism. In this study, the effects of different distributions of heat-producing material in the crust and upper mantle on crustal thermal histories and deformation fields are investigated. The models involve subduction-driven collision with moderate convergence and erosion rates. For models involving standard continental crust, where heat production is initially concentrated in the upper crust, P-T-t paths do not intersect the field of typical Barrovian P-T conditions. However, heat-producing material can be tectonically redistributed, for example, by subduction of crustal rocks to upper mantle depths, or by formation of thick accretionary wedges or continental margin sequences during convergence. Models that include a wedge of heat-producing material in the upper mantle generate high temperatures in the lower crust and upper mantle that lead to a change in orogenic style; radioactive heating of partially subducted crustal material on time scales of 10–30 Ma yields temperatures high enough for partial melting. However, crustal P-T-t paths are unlikely to intersect the Barrovian field unless erosion or convergence rates change. Models that include a crustal-scale region with moderate, uniform heat production, simulating a large accretionary wedge or tectonically thickened continental margin sequence, generate P-T-t paths that intersect the Barrovian field. However, as convergence proceeds, the heat-producing region is deformed, eroded, and reduced in volume, so that the model orogen begins to cool down after about 20 Ma. The model results provide an explanation for many first-order tectonic and metamorphic features of small orogens, including metamorphic styles ranging from blueschists to the Barrovian series to granulites, late-orogenic granitoid magmatism, and the crustal-scale tectonic features associated with regional metamorphic belts. We conclude that the thermal state of an orogen is controlled by the evolving competition between cooling by subduction and radioactive heating within the deforming orogen.

Transpressional kinematics and magmatic arcs

Abstract: Abstract Most continental magmatic arcs occur in obliquely convergent settings and display strike-slip movement within, or adjacent to, the magmatic arc, and contractional structures in the forearc and backarc regions. Thus, three-dimensional transpressional kinematics typifies many arc settings, both modern and ancient. Intrusions cause magma-facilitated strike-slip partitioning, even in cases where the relative angle of plate convergence is almost normal to the plate boundary. Transpressional systems are preferentially intruded by magmas because of the steep pressure gradients in vertical strike-slip shear zones and their ability to force magma upward. Both buoyancy and transpressional dynamics cause a component of magma overpressuring, which in turn expels granitic magma upward following the vertical pressure gradient. The tectonic and magmatic processes are linked in a positive feedback loop which facilitates the upward movement of magma. We propose a lithospheric-scale, three-dimensional model of transpressional arc settings. Strike-slip motion is partitioned into the magmatic arc settings because of the linear and margin-parallel trend of the vertical, lithospheric-scale weakness caused by ascending magma. The parallelism of contraction structures in the forearc and backarc regions is caused by mechanical coupling through the lower crust and upper lithospheric mantle. The displacement field of the basal layer of the arc system provides the boundary condition for the upper-crustal, strike-slip partitioned deformation.

Transpressional tectonics along the Karakoram fault zone, northern Ladakh: constraints on Tibetan extrusion

Abstract: Abstract The Tibetan plateau north of the Himalaya has approximately double normal crustal thickness (60–75 km) and has been homogeneously shortened since the India-Asia collision at 60–50 Ma ago, yet, with minimal erosion rates, has almost no middle or deep crustal rocks exposed at the surface. In the Karakoram range, west of Tibet, early Tertiary crustal thickening and regional metamorphism resulted in Miocene crustal melting producing the Baltoro monzogranite-leucogranite batholith. Late Tertiary transpression along the western margin of the Tibetan plateau, caused by the continued northward penetration of India into Asia, led to exhumation of migmatites in the Pangong range and Baltoro-type granites along the Karakoram fault. Detailed studies of the Karakoram fault zone in northern Ladakh, India, show that Baltoro-type two-mica ± garnet leucogranites, intruded 21–18.0 Ma ago, have been offset a maximum of 150 km right-laterally. Average slip rates since 18.0 ± 0.6 Ma (2σ) are 8.3 mm/a. 40Ar/39Ar mica cooling ages are 11.3 Ma on both sides of the main (southwestern) strand of the fault, suggesting that most of the exhumation of the Pangong migmatites and leucogranites must have occurred between 18.0 and 11.3 Ma. During this time, at slip rates of 8.3 mm/a the rocks would have moved horizontally right-laterally for c. 56 km and been exhumed by c. 20 km vertically during transpression, using the measured 20° plunge of lineations. The high exhumation rate (3.0 mm/a) and amount of erosion (20 km) inferred between 18.0 and 11.3 Ma may also reflect the partitioning between an early transpressional strain associated with crustal thickening and exhumation of the Pangong deep crustal migmatites and leucogranites, and a later dominantly dextral strike-slip phase of fault motion along the central part of the Karakoram fault from c. 11 to 0 Ma. This timing may also coincide with the initiation of the N-S aligned normal faults and E-W extension in southern Tibet. We suggest that the relatively minor dextral offset (150 km) and the young age of initiation on this bounding fault do not support the model of large-scale extrusion of Tibetan crust, but they suggest instead that deformation of Tibet was taken up predominantly by crustal thickening.

The proto-Andean margin of Gondwana: an introduction

Abstract: A basic background is presented for the discussion of the Early Palaeozoic geology of western Argentina covered by this book. This includes the definition and terminology of orogenic cycles on this part of the Gondwana margin, represented by the Eastern Sierras Pampeanas. The Pampean orogeny (Early Cambrian) relates to an intense but short-lived period of terrane collision predating the rifting of the Precordillera terrane from Laurentia. The Famatinian cycle is predominantly represented by intense subductionrelated magmatism of Early-Middle Ordovician age, developed on the continental margin of Gondwana during the rifting and drifting of the Precordillera terrane. The Grenvillian basement of the latter is further exemplified by a new Rb-Sr whole-rock isochron age of 1021 ± 12 Ma for orthogneisses from the Western Sierras Pampeanas. A mid-Ordovician granite in this area (dated at 481 ± 6 Ma by U-Pb ion microprobe data) may be related to rifting while the Precordillera terrane was still attached to Laurentia. A divergence of opinion is pointed out between some authors in this book who favour mid-Ordovician collision of the Precordillera with Gondwana, and others who place it much latter, in Silurian or Devonian times.

Exhumation of UHP rocks by transtension in the Western Gneiss Region, Scandinavian Caledonides

Abstract: Abstract In the Western Gneiss Region (WGR) in the Scandinavian Caledonides, Scandian ecologites (P = 16 to >28 kbar) occur in a large area of reworked Proterozoic gneisses, structurally below a series of Scandian nappes. The top-to-the-west, extensional Nordfjord-Sogn Detachment (NSD) separates the WGR from allochthonous units, which include several late-orogenic Devonian basins. The allochthon has not experienced Scandian high-pressure (HP) metamorphism. Below the NSD, the WGR is intensely deformed under late-orogenic amphibolite-facies conditions. This deformation is bulk-constrictional, as indicated by a linear feldspar fabric within augen gneisses and tight to isoclinal, lineation-parallel folds within layered gneisses. In a later stage, the NSD, the WGR and the Devonian basins were folded by east-west trending folds, coeval with continuing movement along detachments. To explain these features we propose a transtensional model for the late-orogenic evolution of the WGR. Transtension in West Norway had a sinistral sense and was partially partitioned with increasing transtensional angle towards the NE-SW trending Møre-Trøndelag Fault Zone in the NW. During transtension, there is a strong tendency for rejuvenation of detachments, because detachments fold and may lock as they move. In the WGR, the younger Hornelen Detachment developed above the older NSD. Transtension was the principal exhumation mechanism of the HP and ultra-high-pressure (UHP) rocks in the WGR and involved oblique plate divergence of Laurentia and Baltica during the Early Devonian.

Vesiculation processes in silicic magmas

Abstract: Abstract The physics of vesiculation, i.e. the process of bubble formation and evolution, controls the manner of volcanic eruptions. Vesiculation may lead to extreme rates of magma expansion and to explosive eruptions or, at the other extreme, to low rates and calm effusion of lava domes and flows. In this paper, we discuss the theory of the different stages of vesiculation and examine the results of relevant experimental studies. The following stages are discussed: (1) The development of supersaturation of volatiles in melts. Supersaturation may develop due to a decrease in equilibrium solubility following changes in ambient pressure or temperature or due to an increase in the magma volatile content (e.g. in response to crystallization of water-free or waterpoor mineral assemblages). (2) Bubble nucleation. The classical theory of homogeneous nucleation and some modern modifications, heterogeneous nucleation with emphasis on the nucleation of water bubbles in rhyolitic melts and the role of specific crystals as heterogeneous sites. (3) Bubble growth. The effect of diffusion, viscosity, surface tension, ambient pressure and inter-bubble separation on the dynamics of growth. (4) Bubble coalescence. The theory of coalescence of static foams. factors that may effect coalescence in expanding foams and shape relaxation following bubble coalescence.

The Carboniferous evolution of Nova Scotia

Abstract: Abstract Nova Scotia during the Carboniferous lay at the heart of palaeoequatorial Euramerica in a broadly intermontane palaeoequatorial setting, the Maritimes-West-European province; to the west rose the orographic barrier imposed by the Appalachian Mountains, and to the south and east the Mauritanide-Hercynide belt. The geological affinity of Nova Scotia to Europe, reflected in elements of the Carboniferous flora and fauna, was mirrored in the evolution of geological thought even before the epochal visits of Sir Charles Lyell. The Maritimes Basin of eastern Canada, born of the Acadian-Caledonian orogeny that witnessed the suture of Iapetus in the Devonian, and shaped thereafter by the inexorable closing of Gondwana and Laurasia, comprises a near complete stratal sequence as great as 12 km thick which spans the Middle Devonian to the Lower Permian. Across the southern Maritimes Basin, in northern Nova Scotia, deep depocentres developed en echelon adjacent to a transform platelet boundary between terranes of Avalon and Gondwanan affinity. The subsequent history of the basins can be summarized as distension and rifting attended by bimodal volcanism waning through the Dinantian, with marked transpression in the Namurian and subsequent persistence of transcurrent movement linking Variscan deformation with Mauritainide-Appalachian convergence and Alleghenian thrusting. This Mid-Carboniferous event is pivotal in the Carboniferous evolution of Nova Scotia. Rapid subsidence adjacent to transcurrent faults in the early Westphalian was succeeded by thermal sag in the later Westphalian and ultimately by basin inversion and unroofing after the early Permian as equatorial Pangaea finally assembled and subsequently rifted again in the Triassic. The component Carboniferous basins have provided Nova Scotia with its most important source of mineral and energy resources for three centuries. Their combined basin-fill sequence preserves an exceptional record of the Carboniferous terrestrial ecosystems of palaeoequatorial Euramerica, interrupted only in the mid-late Viséan by the widespread marine deposits of the hypersaline Windsor gulf; their fossil record is here compiled for the first time. Stratal cycles in the marine Windsor, schizohaline Mabou and coastal plain to piedmont coal measures ‘cyclothems’ record Nova Scotia’s palaeogeographic evolution and progressively waning marine influence. The semiarid palaeoclimate of the late Dinantian grew abruptly more seasonally humid after the Namurian and gradually recurred by the Lower Permian, mimicking a general Euramerican trend. Generally more continental and seasonal conditions prevailed than in contemporary basins to the west of the Appalachians and, until the mid-Westphalian, to the east in Europe. Palaeogeographic, paleoflow and faunal trends point to the existence of a Mid-Euramerican Sea between the Maritimes and Europe which persisted through the Carboniferous. The faunal record suggests that cryptic expressions of its most landward transgressions can be recognized within the predominantly continental strata of Nova Scotia.

Strike-slip partitioned transpression of the San Andreas fault system: a lithospheric-scale approach

Abstract: Abstract Physical and numerical experiments on transpression indicate that strike-slip partitioning is facilitated by low angle of convergence, such as occurs on the modern San Andreas fault system. Cross-sections across the San Andreas fault overestimate the amount of margin-normal contraction, because they do not include the effect of horizontal stretching caused by the wrench component of deformation. When this correction is made assuming a strike-slip partitioned transpression model, contraction estimated from cross-sections is consistent with both the normal and tangential movements imposed by plate motion: there is no San Andreas discrepancy. Seismic anisotropy (shear-wave splitting) and teleseismic analyses suggest that the upper mantle in the San Andreas region is thick and largely undeformed to the SW of the San Andreas fault, but thin and intensely sheared in a zone 50–100 km wide beneath the NE region of the San Andreas fault system. Seismic anisotropy is best explained by a transpression zone of vertical foliation (and possibly horizontal lineation) which penetrates the asthenospheric mantle. Using a lithospheric-scale approach, the transfer of the displacement field from the penetratively deformed mantle to the strike-slip partitioned upper crust necessitates zones of accommodation in the mid- to lower crust, corresponding to the flatlying detachments that underlie the San Andreas fault system. The mechanics of the San Andreas fault are best considered in connection with the strength of the various lithospheric layers involved in this system.

Gas hydrates along the northeastern Atlantic margin: possible hydrate-bound margin instabilities and possible release of methane

Abstract: Abstract The presence of gas hydrates and free gas in oceanic sediments along the northeastern European Margin is documented in high-frequency near-vertical and wide-angle seismic reflection data. Shallow-water and deep-water gas hydrate instabilities can cause free gas to escape from oceanic sediments. Particularly, methane from shallow-water gas hydrate destabilization may then get transferred from the sediments into the water column, and eventually into the atmosphere. Deep-water gas hydrates are coincident with areas and depths of slope failures in continental margin sediments. Comparisons between seismicity and the potential hydrate distributions suggest a correlation between hydrate instability and margin instabilities along the north-eastern Atlantic Margin.

Structure and content of the Moab Fault Zone, Utah, USA, and its implications for fault seal prediction

Abstract: Abstract The structure and content of the Moab Fault zone are described for 37 transects across the fault zone where throws range from less than 100 m to c. 960 m. The 45 km long fault trace intersects a sedimentary sequence containing a high proportion of sandstones with good reservoir properties, interspersed with numerous mudstone layers. Typically, the fault zone is bounded by two external slip zones with the fault zone components separated by up to nine internal slip zones. Fault zone components are tabular lenses of variably deformed sandstones and sandstone cataclasites and breccia, with a wide size range, usually enclosed in a matrix of shaley fault gouge containing mm to m scale entrained sandstone fragments. Neither fault zone structure nor content can be predicted by extrapolation over distances as little as 10 m. Although variable in thickness, shaley gouge is always present except where the mudstone is

Physical character and fluid-flow properties of sandstone-derived fault zones

Abstract: Abstract Evaluation of hydrocarbon entrapment and production patterns in faulted sandstone reservoirs requires understanding of the nature and fluid-flow properties of sandstone-derived fault zones. This study documents the interrelationships between sandstone composition, deformation mechanisms, fault-zone character, and fluid-flow properties (permeability and capillary properties) using a global sandstone dataset. Quartz-rich sandstones deform by cataclasis (most commonly), diffusive mass transfer, or a combination of these processes to form deformation bands. The fluid-flow properties of these zones depend on deformation mechanism(s). Faulting of mineralogically immature sandstones results in the formation of clay-matrix gouge zones by a combination of processes, including cataclasis, intergranular sliding in clay-rich materials, and diffusive mass transfer. Clay-matrix gouge zones generally have lower permeabilities and higher capillary displacement pressures than deformation bands. Most deformation bands have capillary properties sufficient to maintain hydrocarbon column-height differences of less than 75m across them, whereas clay-matrix gouge zones can potentially seal hydrocarbon columns with heights of several hundred metres. Both low-permeability deformation bands and clay-matrix gouge zones are likely to influence production patterns, although the magnitude of these effects will depend on the spatial distribution and abundance of faults and the permeabilities of the fault zones and undeformed sandstone.

Time constraints on the Early Palaeozoic docking of the Precordillera, central Argentina

Abstract: Abstract 40Ar/39Ar incremental-release ages have been determined for muscovite and hornblende concentrates prepared from basement rocks of the Sierra de Pie de Palo, in the western Sierras Pampeanas, east of the Early Palaeozoic Precordillera. The basement is mainly represented by variably metamorphosed units, including ophiolites, orthogneisses and schists. Previous U-Pb, Rb-Sr, K-Ar, and 40Ar/39Ar dating of magmatic and metamorphic zircons from the basement has indicated a Middle Proterozoic age. The 40Ar/39Ar plateau ages constrain a series of ductile deformational events that are correlated with development of structural discontinuities in adjacent foreland basins. The following deformational events are postulated: (1) initiation of collision and drowning of the Precordillera platform against Gondwana (470–460 Ma); (2) flexural extension associated with normal faulting due to tectonic loading of the Gondwana margin; and (3) development of a foreland basin (450–430 Ma). These Mid-Ordovician-Silurian events were related to collision of several exotic Early Palaeozoic terranes along the Gondwana margin. The Punta Negra foreland basin developed further west during Early to Mid-Devonian (410–380 Ma) times, and was linked to the beginning of the docking of the Chilenia terrane along the western margin of the Precordillera.

Gas loss from magmas through conduit walls during eruption

Abstract: Abstract There is ample evidence that the transition from an explosive eruption regime to an effusive regime can be due to magma losing gas to fractured country rock during ascent towards the surface. This is shown by the deuterium: hydrogen isotopic ratios and dissolved water contents of erupted samples, and by various petrological observations. Field studies demonstrate that the walls of an eruption conduit are fractured and penetrated by veins infilled with vesicular pyroclasts and ash. At Mule Creek, New Mexico, USA, a fossil eruption conduit filled with lava can be studied over a height of 300 m. The gas volume fraction exhibits complex variations with height and with horizontal distance from the walls, which is not compatible with closed system degassing. Lava close to the conduit walls is almost devoid of vesicles, showing that gas escape has been efficient. Gas flow through liquid magma may be achieved through fractures or through connected bubbles. Theoretical flow models which account for gas loss through conduit walls show that eruptive behaviour is very sensitive to the eruption rate and to the chamber pressure. A gradual decrease of chamber pressure, due to withdrawal of magma, leads to a transition from explosive to effusive conditions. Conversely, a gradual increase of chamber pressure, due to reinjection from a deeper source of magma, leads to a transition from effusive to explosive conditions. The initiation and propagation of gas-filled fractures during an ongoing eruption may be detected seismically. This chapter puts together several independent pieces of evidence in a coherent framework and includes a discussion of unresolved questions.

Quantified vertical motions and tectonic evolution of the SE Pyrenean foreland basin

Abstract: This work was founded by IBS Project, Joule 11 Programme (JOU2-CT92-110), the 'Comissionat per Universitats i Recerca de la Generalitat de Catalunya', Quality Group GRQ94-1048, Total Exploration Production France and by grants from the National Science Foundation (EAR8816181, 9018951) and the Petroleum Research Fund (ACS-PRF 20591, 17625, 23881) to D.W.B.

Introduction to the development, evolution and petroleum geology of the Wessex Basin

Abstract: Abstract Despite containing the largest known onshore oilfield in western Europe, the Wessex Basin hydrocarbon province appears to be extremely limited spatially and it currently only consists of three producing oilfields: Wytch Farm, Wareham and Kimmeridge. The main factor which controls hydrocarbon prospectivity in the area appears to be preservation of oil accumulations originally sited in Mesozoic tilted faultblocks. The extensional palaeostructures of Wytch Farm and Wareham are interpreted to have been charged by upwards migration of oil from mature Liassic source rocks situated across the Purbeck-Isle of Wight fault system in the Channel (Portland-Wight) sub-basin prior to, and unaffected by, either significant effects of intra-Cretaceous (Albian-Aptian) easterly tilting or by Tertiary tectonic inversion. To date, only the small Kimmeridge oilfield, which is situated in the core of a periclinal fold created in response to structural inversion, suggests that any hydrocarbon remigration into younger structural inversion structures has taken place.