Showing papers in "Geosphere in 2016"

Lateral magma flow in mafic sill complexes

Abstract: The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field- and seismic reflection–based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources.

Quantifying comparison of large detrital geochronology data sets

Abstract: The increase in detrital geochronological data presents challenges to existing approaches to data visualization and comparison, and highlights the need for quantitative techniques able to evaluate and compare multiple large data sets. We test five metrics commonly used as quantitative descriptors of sample similarity in detrital geochronology: the Kolmogorov-Smirnov (K-S) and Kuiper tests, as well as Cross-correlation, Likeness, and Similarity coefficients of probability density plots (PDPs), kernel density estimates (KDEs), and locally adaptive, variable-bandwidth KDEs (LA-KDEs). We assess these metrics by applying them to 20 large synthetic data sets and one large empirical data set, and evaluate their utility in terms of sample similarity based on the following three criteria. (1) Similarity of samples from the same population should systematically increase with increasing sample size. (2) Metrics should maximize sensitivity by using the full range of possible coefficients. (3) Metrics should minimize artifacts resulting from sample-specific complexity. K-S and Kuiper test p-values passed only one criterion, indicating that they are poorly suited as quantitative descriptors of sample similarity. Likeness and Similarity coefficients of PDPs, as well as K-S and Kuiper test D and V values, performed better by passing two of the criteria. Cross-correlation of PDPs passed all three criteria. All coefficients calculated from KDEs and LA-KDEs failed at least two of the criteria. As hypothesis tests of derivation from a common source, individual K-S and Kuiper p-values too frequently reject the null hypothesis that samples come from a common source when they are identical. However, mean p-values calculated by repeated subsampling and comparison (minimum of 4 trials) consistently yield a binary discrimination of identical versus different source populations. Cross-correlation and Likeness of PDPs and Cross-correlation of KDEs yield the widest divergence in coefficients and thus a consistent discrimination between identical and different source populations, with Cross-correlation of PDPs requiring the smallest sample size. In light of this, we recommend acquisition of large detrital geochronology data sets for quantitative comparison. We also recommend repeated subsampling of detrital geochronology data sets and calculation of the mean and standard deviation of the comparison metric in order to capture the variability inherent in sampling a multimodal population. These statistical tools are implemented using DZstats, a MATLAB-based code that can be accessed via an executable file graphical user interface. It implements all of the statistical tests discussed in this paper, and exports the results both as spreadsheets and as graphic files.

Testing models of Tibetan Plateau formation with Cenozoic shortening estimates across the Qilian Shan–Nan Shan thrust belt

Abstract: Competing models that account for the construction of the Tibetan Plateau include continental subduction, underthrusting, distributed shortening, channel flow, and older crustal-structure inheritance. Well-constrained estimates of crustal shortening strain serve as a diagnostic test of these plateau formation models and are critical to elucidate the dominant mechanism of plateau development. In this work we estimate the magnitude of Cenozoic shortening across the northern Qilian Shan–Nan Shan thrust belt, along the northeastern plateau margin, based on detailed analysis and reconstruction of three high-resolution seismic reflection profiles. By integrating surface geology, seismic data, and the regional tectonic history, we demonstrate that this thrust system has accumulated >53% Cenozoic strain (∼50 km shortening), accommodated by several south-dipping thrust faults. Based on the observed strain distribution across northern Tibet, including lower strain (30%–45%) within the interior of the Qilian Shan–Nan Shan thrust belt, we suggest that a combination of distributed crustal shortening and minor (

Slab-rollback ignimbrite flareups in the southern Great Basin and other Cenozoic American arcs: A distinct style of arc volcanism

Abstract: In continental-margin subduction zones, basalt magmas spawned in the mantle interact with the crust to produce a broad spectrum of volcanic arc associations. A distinct style of very voluminous arc volcanism develops far inland on thick crust over periods of 10–20 m.y. and involves relatively infrequent caldera-forming explosive eruptions of dominantly calc-alkaline rhyolite, dacite, and trachydacite with repose times of 104–106 yr. Volumes of individual eruptions are large (102–103 km3), and nested super-eruptions of thousands of cubic kilometers are common. Calderas are as much as 60–75 km in diameter, and surrounding individual ignimbrite outflow sheets extend outward as much as 150 km, blanketing upwards of 105 km2. Little or no basalt is extruded, whereas andesitic differentiates coeval with silicic ignimbrites range from ­minor to dominant in relative volume. A common feature in these flareups is essentially nonextending, thick, inland crust overlying a subducting oceanic plate with transverse tears that rolled back to a steeper dip from a previously flat configuration. Lithospheric delamination is locally possible. Large volumes of basalt that provide heat and mass for silicic magma generation in the crust form by fluid fluxing of the growing mantle wedge overlying the steepening dehydrating slab and from asthenospheric decompression. Variations in the mantle input, together with variations in crustal thickness, temperature, and composition, modulate the expression of the flareups. As a consequence of the high flux of mantle-derived magma into the thick crust, geotherms become elevated, and the brittle-ductile transition can rise to depths as shallow as 7 km. At this transition, diapirically rising magmas from a melting, assimilation, storage, and homogenization (MASH) zone are blocked and spread laterally into discoid chambers that grow until a thermomechanical threshold is attained, triggering climactic eruption and caldera collapse. This ignimbrite flareup style of continental arc volcanism is exemplified by the mid-Cenozoic southern Great Basin ignimbrite province; other examples include the contemporaneous Southern Rocky Mountain, Mogollon-Datil, vast Sierra Madre Occidental volcanic fields, and the late Cenozoic Altiplano-­Puna volcanic complex in the Central Andes. Rhyolitic and trachydacitic ignimbrites typically have erupted, but where the crust was predominantly felsic, prewarmed, and orogenically thickened, well-developed MASH zones have spawned multiple super-eruptions of phenocryst-rich dacite, or monotonous intermediates, and smaller volumes of calc-alkaline rhyolite ignimbrite. In the Great Basin, eruptions of dry, hot trachydacite magma followed the monotonous intermediates. Partial melting in thinner crust with a major mafic component yielded more alkalic rhyolite and related trachydacite.

Petrogenesis and provenance of distal volcanic tuffs from the Permian–Triassic Karoo Basin, South Africa: A window into a dissected magmatic province

Abstract: We present zircon rare earth element (REE) compositions integrated with U-Pb ages of zircon and whole-rock geochemistry from 29 volcanic tuffs preserved in the Karoo Supergroup, South Africa, to investigate the history of magmatism in southern Gondwana. Whole-rock compositions suggest a subduction-driven magmatic arc source for early (before 270 Ma) to middle Permian (270–260 Ma) Karoo tuffs. After ca. 265 Ma, the magmatic source of the volcanic deposits transitioned toward intraplate shallow-sourced magmatism. Zircon U-Pb ages and REE chemistry suggest that early to middle Permian magmas were oxidizing, U- and heavy (H) REE–enriched, melts; middle Permian to Triassic zircons record HREE-depleted, more reduced magmatism. Middle Permian to Triassic tuffs contain increasingly large volumes of zircon cargo derived from assimilated crustal material; therefore magmas may have been zircon undersaturated, resulting in less zircon growth and increased inheritance in late Permian to Triassic Gondwanan volcanics. Zircon U-Pb ages and zircon REE chemistry suggest a shift from arc magmatism in the early Permian to extensional magmatism by the late Permian, which may be associated with development of a backarc magmatic system adjacent to western Antarctica that predates known extensional volcanism elsewhere in Gondwana. Opening of the Southern Ocean in the Jurassic–Cretaceous paralleled this extensional feature, which may be related to reactivation of this Permian–Triassic backarc. This study demonstrates the potential of zircon U-Pb age and REE compositions from volcanic tuffs preserved in sedimentary strata to provide a more complete record of magmatism, when the magmatic province has been largely lost to active tectonism.

Magmatic history and crustal genesis of western South America: Constraints from U-Pb ages and Hf isotopes of detrital zircons in modern rivers

Abstract: Western South America provides an outstanding laboratory for studies of magmatism and crustal evolution because it contains Archean–Paleoproterozoic cratons that amalgamated during Neoproterozoic supercontinent assembly, as well as a long history of Andean magmatism that records crustal growth and reworking in an accretionary orogen. We have attempted to reconstruct the growth and evolution of western South America through U-Pb geochronologic and Hf isotopic analyses of detrital zircons from 59 samples of sand mainly from modern rivers. Results from 5524 new U-Pb ages and 1199 new Hf isotope determinations are reported. Our data are integrated with previously published geochronologic and Hf isotopic information, yielding a combined record that includes >42,000 ages and >1900 Hf isotope analyses. These large data sets yield five main conclusions: (1) South America has an age distribution that is similar to most other continents, presumably reflecting processes of crustal generation and/or preservation related to the supercontinent cycle, with age maxima at 2.2–1.8 Ga, 1.6–0.9 Ga, 700–400 Ma, and 360–200 Ma; (2) <200 Ma magmatism in western South America has age maxima at ca. 183, 166, 149, 125, 110, 88, 65, 35, 21, and 4 Ma (with significant north-south and east-west variations), yielding an average cyclicity of ∼33 m.y.; (3) for the past 200 m.y., no correlation exists between magmatism and the velocity of convergence between central South America and Pacific oceanic plates, the age of the downgoing plate, or the absolute motion of South America; (4) Hf isotopes record reworking of older crustal materials during most time periods, with incorporation of juvenile crust at ca. 1.6–1.0 Ga, 500–300 Ma, and ca. 175–35 Ma; and (5) the Hf isotopic signature of <200 Ma magmatism is apparently controlled by the generation of evolved crust during crustal thickening and eastward arc migration, versus juvenile magmas during extensional tectonism and westward and/or outboard migration of arc magmatism.

New constraints on the magma distribution and composition beneath Volcán Uturuncu and the southern Bolivian Altiplano from magnetotelluric data

Abstract: Magnetotelluric (MT) data were used to create a three-dimensional electrical resistivity model of the Altiplano-Puna magma body (APMB) in the area surrounding Volcan Uturuncu in southern Bolivia. This volcano is at the center of a zone of surface deformation with a diameter of 150 km and persistent inflation of ∼10 mm/yr. Low electrical resistivities (<3 Ωm) at a depth of 14 + 1/–3 km below sea level (16–20 km below surface) are interpreted as being due to the presence of andesite melts in the APMB, and require a minimum melt fraction of 15%. The upper crustal resistivity structure is characterized by finite-length, dike-shaped conductors, oriented approximately east-west near sea level. A combination of dacite partial melts and aqueous fluids is required to explain the observed low-resistivity values. Geodetic data do not require any deformation in these shallow regions. The geometry of the upper surface of the APMB beneath Volcan Uturuncu is consistent with that predicted by geodynamic models that suggest that the APMB bulges upward directly beneath Volcan Uturuncu, near the measured inflation center (∼3 km west of Volcan Uturuncu). Viscosity estimates from the MT-derived resistivity model gives a maximum value of 1016 Pa·s and is consistent with models that propose diapir-like ascent of magma above the APMB. Resistivity models are compared and quantitatively correlated to seismic velocity models, showing good agreement on the spatial extent and depth of the APMB. A forward modeling study shows that the small differences in the depth to the top of the APMB between the different geophysical methods could be explained by variations in the composition of the magma body.

Evolution of upper crustal faulting assisted by magmatic volatile release during early-stage continental rift development in the East African Rift

Abstract: During the development of continental rifts, strain accommodation shifts from border faults to intra-rift faults. This transition represents a critical process in the evolution of rift basins in the East African Rift, resulting in the focusing of strain and, ultimately, continental breakup. An analysis of fault and fluid systems in the younger than 7 Ma Natron and Magadi basins (Kenya-Tanzania border) reveals the transition as a complex interaction between plate flexure, magma emplacement, and magmatic volatile release. Rift basin development was investigated by analyzing fault systems, lava chronology, and geochemistry of spring systems. Results show that extensional strain in the 3 Ma Natron basin is primarily accommodated along the border fault, whereas results from the 7 Ma Magadi basin reveal a transition to intra-rift fault–dominated strain accommodation. The focusing of strain into a system of intra-rift faults in Magadi also occurred without oblique-style rifting, as is observed in Ethiopia, and border fault hanging-wall flexure can account for only a minor portion of faulting along the central rift axis (∼12% or less). Instead, areas of high upper crustal strain coincide with the presence of hydrothermal springs that exhibit carbon isotopes and N2-He-Ar abundances indicating mixing between mantle-derived (magmatic) fluids and air saturated water. By comparing the distribution of fault-related strain and zones of magmatic fluid release in the 3 Ma Natron and 7 Ma Magadi basins, we present a conceptual model for the evolution of early-stage rifting. In the first 3 m.y., border faults accommodate the majority of regional extension (1.24–1.78 mm yr–1 in Natron at a slip rate ranging 1.93–3.56 mm yr–1), with a significant portion of intra-rift faulting (38%–96%) driven by flexure of the border fault hanging wall. Fluids released from magma bodies ascend along the border fault and then outward into nearby faults forming in the flexing hanging wall. By 7 m.y., there is a reduction in the amount of extension accommodated along the border fault (0.40–0.66 mm yr–1 in Magadi at a slip rate ranging from 0.62 to 1.32 mm yr–1), and regional extension is primarily accommodated in the intra-rift fault population (1.34–1.60 mm yr–1), with an accompanying transition of magmatic volatile release into the rift center. The focusing of magma toward the rift center and concomitant release of magmatic fluids into the flexing hanging wall provides a previously unrecognized mechanism that may help to weaken crust and assist the transition to intra-rift dominated strain accommodation. We conclude that the flow of magmatic fluids within fault systems plays an important role in weakening lithosphere and focusing upper crustal strain in early-stage continental rift basins prior to the establishment of magmatic segments.

Magmatic versus phreatomagmatic fragmentation: Absence of evidence is not evidence of absence

Abstract: Fragmentation processes in eruptions are commonly contrasted as phreatomagmatic or magmatic; the latter requires only fragmentation of magma without external intervention, but often carries the connotation of disruption by bubbles of magmatic gas. Phreatomagmatic fragmentation involves vaporization and expansion of water as steam with rapid cooling and/or quenching of the magma. It is common to assess whether a pyroclast formed by magmatic or phreatomagmatic fragmentation using particle vesicularity, shape of particles, and degree of quenching. It is widely known that none of these criteria is entirely diagnostic, so deposit features are also considered; welding and/or agglomeration, particle aggregation, lithic fragment abundance, and proportion of fines. Magmatic fragmentation yields from rhyolite pumice to obsidian to basaltic achneliths or carbonatitic globules, making direct argument for magmatic fragmentation difficult, so many have taken an alternative approach. They have tested for phreatomagmatism using the fingerprints listed above, and if the fingerprint is lacking, magmatic fragmentation is considered proven. We argue that this approach is invalid, and that the criteria used are typically incorrect or incorrectly applied. Instead, we must consider the balance of probabilities based on positive evidence only, and accept that for many deposits it may not be possible with present knowledge to make a conclusive determination.

Andean topographic growth and basement uplift in southern Colombia: Implications for the evolution of the Magdalena, Orinoco, and Amazon river systems

Abstract: Surface uplift of the Garzon Massif in the northern Andes formed a critical orographic barrier (2500–3000 m elevation) that generated a deep rain shadow and strongly influenced the evolution of the largest river systems draining northern South America. This basement massif and its corresponding foreland basement high define the headwaters and drainage divides of the Amazon, Orinoco, and Magdalena Rivers. Despite its pivotal role, the exhumation history of the Garzon Massif and its relationships to the structural evolution of the broader Eastern Cordillera fold-thrust belt remain unclear. The northern Andes underwent major Cenozoic shortening, with considerable thin-skinned and thick-skinned deformation and topographic development in the Eastern Cordillera focused during late Miocene time. On the basis of widespread coarse-grained nonmarine sedimentation, previous studies have inferred that uplift of the Garzon Massif began during the late Miocene, coincident with rapid elevation gain elsewhere in the Eastern Cordillera. We take an integrated, multiproxy approach to better reconstruct Andean topographic growth and distinguish between exhumation and surface uplift of the Garzon Massif. We present new U-Pb detrital zircon provenance data, sandstone petrographic data, and paleoprecipitation data from upper Miocene clastic fill of the Neiva Basin within the adjacent Upper Magdalena Valley of the modern hinterland. In addition, six new apatite fission track (AFT) ages from the central segment of the northeast-trending Garzon Massif (Jurassic granite and Proterozoic gneiss and schist) directly constrain its Neogene exhumation history. The results indicate that early exhumation may have initiated by ca. 12.5 Ma, but a substantial orographic barrier was not fully established until ca. 6–3 Ma, when >1 km/m.y. of material was exhumed. Thermal history modeling of the AFT data suggests diminished exhumation thereafter (3–0 Ma), during latest Cenozoic oblique Nazca–South America convergence. This exhumation history is consistent with paleontological data suggesting late Miocene divergence of the three river systems, with associated transcontinental drainage of the Amazon River.

A comparison of terrestrial laser scanning and structure-from-motion photogrammetry as methods for digital outcrop acquisition

Abstract: Terrestrial laser scanning (TLS) has been used extensively in Earth Science for acquisition of digital outcrop data over the past decade. Structure-from-­motion (SfM) photogrammetry has recently emerged as an alternative and competing technology. The real-world performance of these technologies for ground-based digital outcrop acquisition is assessed using outcrops from North East England and the United Arab Emirates. Both TLS and SfM are via­ble methods, although no single technology is universally best suited to all situations. There are a range of practical considerations and operating conditions where each method has clear advantages. In comparison to TLS, SfM benefits from being lighter, more compact, cheaper, more easily replaced and repaired, with lower power requirements. TLS in comparison to SfM provides intrinsically validated data and more robust data acquisition in a wide range of operating conditions. Data post-processing is also swifter. The SfM data sets were found to contain systematic inaccuracies when compared to their TLS counterparts. These inaccuracies are related to the triangulation approach of the SfM, which is distinct from the time-of-flight principle employed by TLS. An elaborate approach is required for SfM to produce comparable results to TLS under most circumstances.

Provenance evolution during progressive rifting and hyperextension using bedrock and detrital zircon U-Pb geochronology, Mauléon Basin, western Pyrenees

Abstract: The responses of sedimentary systems to rifting at continental margins are three-dimensional and involve the mixing of various sediment sources through tectonic drivers and sediment response. Such sedimentary responses have not been well studied along magma-poor, hyperextended margins where the crust is stretched and thinned to ≤10 km. The asymmetric Mauleon Basin of the western Pyrenees is the product of such magma-poor hyperextension resulting from lateral rift propagation from the Bay of Biscay during Cretaceous time. After rifting, limited shortening during Cenozoic Pyrenean inversion uplifted the basin, resulting in preservation of outcrops of rift basin fill, upper and lower crustal sections, serpentinized lithospheric mantle, and basic rift-fault relationships. In this study ~5800 new zircon U-Pb ages were obtained from prerift, synrift, and postrift strata; the ages constrain the proximal to distal evolution of the Mauleon Basin and define a general model for sediment routing during rifting. Zircon U-Pb analyses from lower crustal granulites indicate that granulite plutons crystallized at 279 ± 2 and 274 ± 2 Ma, and paragneissic granulites yielded zircon rim ages of ca. 295 Ma. Detrital zircon U-Pb ages from western Pyrenean prerift strata show age modes of ca. 615 and ca. 1000 Ma, suggesting continual recycling and/or well-mixed Gondwanan-sourced sediments throughout the Paleozoic and early Mesozoic; additional Paleozoic age components (ca. 300 and ca. 480 Ma) are also observed. The variation of detrital zircon U-Pb ages in synrift and postrift strata illustrates that during rifting, provenance varied spatially and temporally, and sediment routing switched from being regionally, to locally, and then back to regionally derived within individual structurally controlled subbasins.

Rock-avalanche dynamics revealed by large-scale field mapping and seismic signals at a highly mobile avalanche in the West Salt Creek valley, western Colorado

Abstract: On 25 May 2014, a rain-on-snow–induced rock avalanche occurred in the West Salt Creek valley on the northern flank of Grand Mesa in western Colorado (United States). The avalanche mobilized from a preexisting rock slide in the Green River Formation and traveled 4.6 km down the confined valley, killing three people. The avalanche was rare for the contiguous United States because of its large size (54.5 Mm 3 ) and high mobility (height/length = 0.14). To understand the avalanche failure sequence, mechanisms, and mobility, we conducted a forensic analysis using large-scale (1:1000) structural mapping and seismic data. We used high-resolution, unmanned aircraft system imagery as a base for field mapping, and analyzed seismic data from 22 broadband stations (distances

Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the evolution of the Midcontinent Rift

Abstract: A crucial constraint on the evolution of the ca. 1100 Ma Midcontinent Rift (MCR) in North America comes from the Jacobsville Sandstone, Bayfield Group, and other equivalent sedimentary rocks (JBE) that overlie the volcanics and sediments deposited in the MCR basin near Lake Superior. The MCR began extending ca. 1120 Ma and failed—ceased extending—at ca. 1096 Ma, although volcanism continued for ∼10 m.y. The JBE’s age is poorly constrained, with proposed ages ranging from ca. 1100 to ca. 542 Ma (i.e., youngest Precambrian). It has been proposed that the JBE were deposited shortly prior to or during the time when the MCR failed due to regional compression occurring ca. 1060 Ma as part of the Grenville orogeny (1300–980 Ma). However, the JBE are not conformable with the youngest rift-filling strata and differ compositionally from them. We present an analysis of 2050 new detrital-zircon ages showing that the JBE are younger than 959 ± 19 Ma. Thus, the JBE and the compression recorded in them that inverted the basin postdate the Grenville orogeny and are unrelated to the rift’s failure. The JBE may be significantly, perhaps ∼200–300 m.y., younger than the maximum age from zircons and may have been deposited shortly after a Snowball Earth event.

Along-strike diachroneity in deposition of the Kailas Formation in central southern Tibet: Implications for Indian slab dynamics

Abstract: The Oligocene–Miocene Kailas Formation is exposed along strike for ∼1300 km within the southernmost Lhasa terrane. In this study, we documented the sedimentology, structure, and age of this unit exposed between 87°E and 90°E. Within this region, the Kailas Formation is composed of continental deposits dominated by conglomerate and sandstone, with lesser volumes of siltstone and paleosols. These rocks were deposited nonconformably on Gangdese Batholith and related volcanic rocks along their northern boundary, whereas to the south, the south-dipping Great Counter Thrust places them in contact with Xigaze forearc and melange units. We interpret the Kailas Formation to have been deposited in alluvial-fan and fluvial environments with sediment principally derived from the north. Based on sedimentology and structural relationships, we interpret these rocks to have formed in a north-south extensional setting. New zircon U-Pb ages from volcanic tuffs and flows show that Kailas Formation deposition is younger to the east: Deposition occurred between 26 Ma and 24 Ma in western Tibet (81°E), at 25–23 Ma north of Lazi (87.8°E), at 23–22 Ma near Dazhuka (89.8°E), and as late as 18 Ma southwest of Lhasa (92°E). Overall, basin development propagated eastward at a rate of ∼300 mm/yr. This pattern and rate of propagation are similar to that of the temporal-spatial distribution of adakitic and ultrapotassic magmatism within the Lhasa terrane to the north, which has been interpreted as a record of slab breakoff. Magmatism lags several million years behind Kailas basin development at most locations. We interpret the Kailas basin to have formed as the result of Indian slab shearing and breakoff, which began in western Tibet around 26 Ma and reached eastern Tibet by ca. 18 Ma.

The stratigraphic expression of decreasing confinement along a deep-water sediment routing system: Outcrop example from southern Chile

Abstract: The products of sediment-laden turbidity currents that traverse areas of decreasing confinement on submarine slopes include erosional and depositional features that record the inception and propagation of deep-sea channels. The cumulative stratigraphic expression and deposits of such transitions, however, are poorly constrained relative to depositional settings dominated by end-member confined (i.e., submarine channel fill) and unconfined (i.e., lobe) deposits. Upper Cretaceous strata of the Magallanes foreland basin in southern Chile are characterized by a variety of stratigraphic architectural elements in close juxtaposition both laterally and vertically, including: (1) low-aspect-ratio channelform bodies attributed to slope channel fills; (2) high-aspect-ratio channelform bodies interpreted as the deposits of weakly confined submarine channels; (3) lenticular sedimentary bodies considered to represent the infill of laterally coalesced scours; (4) discontinuous channelform bodies representing isolated scour fills; and (5) a cross-stratified, positive-relief sedimentary body, which is interpreted to record an upslope-migrating depositional bedform. These elements are interpreted to have formed at a submarine sediment routing system segment characterized by a break in slope, and an accompanying decrease in confinement. The various architectural elements examined are interpreted to record a unique stratigraphic perspective of turbidite channels at various stages of development, from early-stage discontinuous and isolated scour fills to low-aspect-ratio channel units.

Geochronological imaging of an episodically constructed subvolcanic batholith: U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex of the Central Andes

Abstract: Zircons from 15 crystal-rich monotonous intermediate ignimbrites and 1 crystal-poor rhyolite ignimbrite erupted during the 11–1 Ma Altiplano-Puna Volcanic Complex (APVC) ignimbrite flare-up record multiscale episodicity in the magmatic history of the shallowest levels (5–10 km beneath the surface) of the Altiplano-Puna Magma Body (APMB) This record reveals the construction of a subvolcanic batholith and its magmatic and eruptive tempo More than 750 U-Pb ages of zircon rims and interiors of polished grains determined by secondary ion mass spectrometry define complex age spectra for each ignimbrite with a dominant peak of autocrysts and subsidiary antecryst peaks Xenocrysts are rare Weighted averages obtained by pooling the youngest analytically indistinguishable zircon ages mostly correspond to the dominant crystallization ages for zircons in the magma These magmatic ages are consistent with eruptive stratigraphy, and fall into four groups defining distinct pulses (from older to younger, pulses 1 through 4) of magmatism that correlate with eruptive pulses, but indicate that magmatic construction in each pulse initiated at least 1 my before eruptions began Magmatism was initially distributed diffusely on the eastern and western flanks of the APVC, but spread out over much of the APVC as activity waxed before focusing in the central part during the peak of the flare-up Each pulse consists of spatially distinct but temporally sequenced subpulses of magma that represent the construction of pre-eruptive magma reservoirs Three nested calderas were the main eruptive loci during the peak of the flare-up from ca 6 to 25 Ma These show broadly synchronous magmatic development but some discordance in their later eruptive histories These relations are interpreted to indicate that eruptive tempo is controlled locally from the top down, while magmatic tempo is a more systemic, deeper, bottom-up feature Synchroneity in magmatic history at distinct upper crustal magmatic foci implicates a shared connection deeper within the APMB Each ignimbrite records the development of a discrete magma Zircon age distributions of individual ignimbrites become more complex with time, reflecting the carryover of antecrysts in successively younger magmas and attesting to upper crustal assimilation in the APVC Although present, xeno­crysts are rare, suggesting that inheritance is limited This is attributed to basement assimilation under zircon-undersaturated conditions deeper in the APMB than the pre-eruptive levels, where antecrysts were incorporated in zircon-saturated conditions Magmatic ages for individual ignimbrites are older than the 40 Ar/ 39 Ar eruption ages This difference is interpreted as the average minimum Zr-saturated melt-present lifetime for APVC magmas, the magmatic duration or Δ age The average Δ age of ca 04 Ma indicates that thermochemical conditions for zircon saturation were maintained for several hundreds of thousands of years prior to eruption of APVC magmas This is consistent with a narrow range of zircon saturation temperatures of 730–815 °C that record upper crustal conditions and Zr/Hf, Th/U, Eu/Eu*, and Ti that reveal protracted magma differentiation under secular cooling rates an order of magnitude slower than typical pluton cooling rates In concert, these data all suggest that the pre-eruptive magma reservoirs were perched in a thermally and chemically buffered state during their long pre-eruptive lifetimes Trace element variations suggest subtle differences in crystallinity, melt fraction, and melt composition within different zones of individual magma reservoirs Significant volumes of plutonic rocks associated with ignimbrites are supported by geophysical data, the limited compositional range over 10 my, the thermal inertia of the magmatic systems, and the evidence of resurgent magmatism and uplift at the calderas and eruptive centers, the distribution of which defines a composite, episodically constructed subvolcanic batholith The multiscale episodicity revealed by the zircon U-Pb ages of the APVC flare-up can be interpreted in the context of continental arc magmatic systems in general The APVC ignimbrite flare-up as a whole is a secondary pulse of ∼10 my, with magmatic pulses 1 through 4 reflecting tertiary pulses of ∼2 my, and the individual ignimbrite zircon spectra defining quaternary pulses of

Assembling the world’s type shallow subduction complex: Detrital zircon geochronologic constraints on the origin of the Nacimiento block, central California Coast Ranges

Abstract: Temporal and spatial patterns in the architecture of the Franciscan Complex provide valuable insights into the subduction processes through which such patterns arise. The Nacimiento Franciscan belt is an allochthonous sliver of subduction assemblages in the central California Coast Ranges displaced either: (1) from southern California by >300 km of Neogene dextral slip along the San Andreas fault system or (2) from central California to southern California and back again, by >500 km of Late Cretaceous–Paleocene sinistral slip along the Sur-Nacimiento fault followed by San Andreas–related motion. New U-Pb detrital zircon data from 20 (meta)clastic samples indicate that the Nacimiento Franciscan section was assembled between ca. 95 and 80 Ma. Abundant Cretaceous (particularly Late Cretaceous) and diminishing amounts of Jurassic and Proterozoic zircon grains point to a southern California origin for Nacimiento Franciscan protoliths, precluding significant sinistral strike-slip along the Sur-Nacimiento fault. Furthermore, the suite of detrital zircon ages reported here bears a strong resemblance to new and existing data from subduction complexes in southern California that were emplaced during Laramide shallow subduction (i.e., Sierra de Salinas, Portal Ridge, Quartz Hill, Rand, San Emigdio, and Tehachapi schists). Hence, the Nacimiento Franciscan is distinct from Franciscan rocks in central and northern California and more likely represents an outboard element of the Late Cretaceous southern California low-angle subduction system. Upon restoring the Nacimiento block to its Late Cretaceous position, an inboard-younging trend is apparent in the composite Nacimiento–southern California schist belt, suggesting that progressively younger accretionary materials were underplated farther inboard by tectonic erosion. We posit that arc and forearc elements absent from southern California were removed by a combination of physical and tectonic erosion attending shallow subduction, interleaved in the subduction complex, and recycled into the mantle. Steepening of the Laramide slab was marked by a phase of crustal extension in the overriding plate. During this phase, the Sur-Nacimiento fault likely functioned as a segment of a low-angle normal fault system spanning the southern Sierra Nevada batholith to the Nacimiento accretionary system.

Multifluid geo-energy systems: Using geologic CO2 storage for geothermal energy production and grid-scale energy storage in sedimentary basins

Abstract: We present an approach that uses the huge fluid and thermal storage capac­ity of the subsurface, together with geologic carbon dioxide (CO 2 ) ­storage, to harvest, store, and dispatch energy from subsurface (geothermal) and surface (solar, nuclear, fossil) thermal resources, as well as excess energy on electric grids. Captured CO 2 is injected into saline aquifers to store pressure, generate artesian flow of brine, and provide a supplemental working fluid for efficient heat extraction and power conversion. Concentric rings of injection and production wells create a hydraulic mound to store pressure, CO 2 , and thermal energy. This energy storage can take excess power from the grid and excess and/or waste thermal energy and dispatch that energy when it is demanded, and thus enable higher penetration of variable renewable energy technologies (e.g., wind and solar). CO 2 stored in the subsurface functions as a cushion gas to provide enormous pressure storage capacity and displace large quantities of brine, some of which can be treated for a variety of beneficial uses. Geo­thermal power and energy-storage applications may generate enough revenues to compensate for CO 2 capture costs. While our approach can use nitrogen (N 2 ), in addition to CO 2 , as a supplemental fluid, and store thermal energy, this study focuses on using CO 2 for geothermal energy production and grid-scale energy storage. We conduct a techno-economic assess­ment to determine the levelized cost of electricity using this approach to generate geothermal power. We present a reservoir pressure management strategy that diverts a small portion of the produced brine for beneficial consumptive use to reduce the pumping cost of fluid recirculation, while reducing the risk of seismicity, caprock fracture, and CO 2 leakage.

Mid-crustal deformation of the Annapurna-Dhaulagiri Himalaya, central Nepal: An atypical example of channel flow during the Himalayan orogeny

Abstract: The channel-flow model for the Greater Himalayan Sequence (GHS) of the Himalayan orogen involves a partially molten, rheologically weak, mid-crustal layer “flowing” southward relative to the upper and lower crust during late Oligocene–Miocene. Flow was driven by topographic overburden, underthrusting, and focused erosion. We present new structural and thermobarometric analyses from the GHS in the Annapurna-Dhaulagiri Hima­laya, central Nepal; these data suggest that during exhumation, the GHS cooled, strengthened, and transformed from a weak “active channel” to a strong “channel plug” at greater depths than elsewhere in the Himalaya. After strengthening, continued convergence resulted in localized top-southwest (top-SW) shortening on the South Tibetan detachment system (STDS). The GHS in the Annapurna-Dhaulagiri Himalaya displays several geological features that distinguish it from other Himalayan regions. These include reduced volumes of leucogranite and migmatite, no evidence for partial melting within the sillimanite stability field, reduced structural thickness, and late-stage top-southwest shortening in the STDS. New and previously published structural and thermobarometric constraints suggest that the channel-flow model can be applied to mid-Eocene–early Miocene mid-crustal evolution of the GHS in the Annapurna-Dhaulagiri Himalaya. However, pressure-temperature-time (PTt) constraints indicate that following peak conditions, the GHS in this region did not undergo rapid isothermal exhumation and widespread sillima­nite-grade decompression melting, as commonly recorded elsewhere in the Hima­laya. Instead, lower-than-typical structural thickness and melt volumes suggest that the upper part of the GHS (Upper Greater Himalayan Sequence [UGHS]—the proposed channel) had a greater viscosity than in other Hima­layan regions. We suggest that viscosity-limited, subdued channel flow prevented exhumation on an isothermal trajectory and forced the UGHS to exhume slowly. These findings are distinct from other regions in the Himalaya. As such, we describe the mid-crustal evolution of the GHS in the Annapurna-­Dhaulagiri Himalaya as an atypical example of channel flow during the Himalayan orogeny.

Thermochronology of extensional orogenic collapse in the deep crust of Zealandia

Abstract: The exhumed Fiordland sector of Zealandia offers a deep-crustal view into the life cycle of a Cordilleran-type orogen from final magmatic construction to extensional orogenic collapse. We integrate U-Pb thermochronologic data from metamorphic zircon and titanite with structural observations from >2000 km 2 of central Fiordland to document the tempo and thermal evolution of the lower crust during the tectonic transition from arc construction and crustal thickening to crustal thinning and extensional collapse. Data reveal that garnet granulite facies metamorphism and partial melting in the lower crust partially overlapped with crustal thickening and batholith construction during emplacement of the Western Fiordland Orthogneiss (WFO) from 118 to 115 Ma. Metamorphic zircons in metasedimentary rocks yield 206 Pb/ 238 U (sensitive high-resolution ion microprobe–reverse geometry) dates of 116.3–112.0 Ma. Titanite laser ablation split stream inductively coupled plasma–mass spectrometry chronology from the same rocks yielded complex results, with relict Paleozoic 206 Pb/ 238 U dates preserved at the margins of the WFO. Within extensional shear zones that developed in the thermal aureole of the WFO, titanite dates range from 116.2 to 107.6 Ma and have zirconium-in-titanite temperatures of ∼900–750 °C. A minor population of metamorphic zircon rims and titanites in the Doubtful Sound region yield younger dates of 105.6–102.3 Ma with corresponding temperatures of 740–730 °C. Many samples record Cretaceous overdispersed dates with 5–10 m.y. ranges. Core-rim traverses and grain maps show complex chemical and temporal variations that cannot easily be attributed to thermally activated volume diffusion or simple core-rim crystallization. We interpret these Cretaceous titanites not as cooling ages, but rather as recording protracted growth and/or crystallization or recrystallization in response to fluid flow, deformation, and/or metamorphic reactions during the transition from garnet granulite to upper amphibolite facies metamorphism. We propose a thermotectonic model that integrates our results with structural observations. Our data reveal a clear tectonic break at 108–106 Ma that marks a change in processes deep within the arc. Prior to this break, arc construction processes dominated and involved (1) emplacement of mafic to intermediate magmas of the Malaspina and Misty plutons from 118 to 115 Ma, (2) contractional deformation at the roof of the Misty pluton in the Caswell Sound fold-thrust belt from 117 to 113 Ma, and (3) eclogite to garnet granulite facies metamorphism and partial melting over >8 m.y. from 116 to 108 Ma. These processes were accompanied by complex patterns of lower crustal flow involving both horizontal and vertical displacements. After this interval, extensional orogenic collapse initiated along upper amphibolite facies shear zones in the Doubtful Sound shear zone at 108–106 Ma. Zircon and titanite growth and/or crystallization or recrystallization at this time clearly link upper amphibolite facies metamorphism to mylonitic fabrics in shear zones. Our observations are significant in that they reveal the persistence of a hot and weak lower crust for ≥15 m.y. following arc magmatism in central Fiordland. We propose that the existence of a thermally weakened lower crust within the Median Batholith was a key factor in controlling the transition from crustal thickening to crustal thinning and extensional orogenic collapse of the Zealandia Cordillera.

Age and anatomy of the Gongga Shan batholith, eastern Tibetan Plateau, and its relationship to the active Xianshui-he fault

Abstract: The Gongga Shan batholith of eastern Tibet, previously documented as a ca. 32–12.8 Ma granite pluton, shows some of the youngest U-Pb granite crystallization ages recorded from the Tibetan Plateau, with major implications for the tectonothermal history of the region. Field observations indicate that the batholith is composite; some localities show at least seven crosscutting phases of granitoids that range in composition from diorite to leucocratic monzogranite. In this study we present U-Pb ages of zircon and allanite dated by laser ablation–inductively coupled plasma–mass spectrometry on seven samples, to further investigate the chronology of the batholith. The age data constrain two striking tectonic-plutonic events: a complex Triassic–Jurassic (ca. 215–159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14–5 Ma) record of monzonite-leucogranite emplacement. The former age range is attributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduction zone magmatism along the western Yangtze block of south China. The younger component may be related to localized partial melting (muscovite dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, and are among the youngest crustal melt granites exposed on the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of zircons and growth and/or resetting of allanites. The young garnet, muscovite, and biotite leucogranites occur mainly in the central part of the batholith and adjacent to the eastern margin of the batholith at Kangding, where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seismically active strike-slip fault in Tibet and is thought to record the eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites of the Gongga Shan massif and has a major transpressional component in the Kangding-Moxi region. The course of the Xianshui Jiang river is offset by ∼62 km along the Xianshui-he fault and in the Kangding area granites as young as ca. 5 Ma are cut by the fault. Our new geochronological data show that only a part of the Gongga Shan granite batholith is composed of young (Miocene) melt, and we surmise that as most of eastern Tibet is composed of Precambrian–Triassic Indosinian rocks, there is no geological evidence to support regional Cenozoic internal thickening or metamorphism and no evidence for eastward-directed lower crustal flow away from Tibet. We suggest that underthrusting of Indian lower crust north as far as the Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Yukon-Tanana terrane, Coast Mountains, southeast Alaska

Abstract: Rocks of the SE Alaska subterrane of the Yukon-Tanana terrane (YTTs) consist of regionally metamorphosed marine clastic strata and mafic to felsic volcanic-plutonic rocks that have been divided into the pre-Devonian Tracy Arm assemblage, Silurian–Devonian Endicott Arm assemblage, and Mississippian–Pennsylvanian Port Houghton assemblage. U-Pb geochronologic and Hf isotopic analyses were conducted on zircons separated from 23 igneous and detrital samples in an effort to reconstruct the geologic and tectonic evolution of this portion of YTT. Tracy Arm assemblage samples are dominated by Proterozoic (ca. 2.0–1.6, 1.2–0.9 Ga) and Archean (2.7–2.5 Ga) zircons that yield typical cratonal eHf( t ) values. Endicott Arm assemblage samples yield U-Pb ages that range from Late Ordovician to Early Devonian and eHf( t ) values that range from highly juvenile to moderately evolved. Port Houghton assemblage samples yield similar Ordovician–Devonian ages and eHf( t ) values, and also include early Mississippian zircons with highly evolved eHf( t ) signatures. Comparison of these age-Hf patterns with data from nearby assemblages suggests the following: (1) Results from YTTs are similar to (or compatible with) available data from rocks of the Yukon-Tanana terrane in eastern Alaska, Yukon, and northern British Columbia (YTTn) and pericratonic strata in east-central Alaska (NAa). (2) YTTs contains abundant Late Ordovician–Early Devonian magmatism that is not recorded in YTTn and NAa. (3) The eHf( t ) values from YTTs display two excursions from juvenile to evolved eHf( t ) values, which are interpreted to record cycles of crustal thinning and then thickening within a convergent margin system. (4) Available data from both YTTs and YTTn support Neoproterozoic(?)–early Paleozoic positions along the northern Cordilleran margin. (5) The Late Ordovician–Early Devonian magmatic record of the southern Alexander terrane is very similar to that of YTTs, which raises the possibility that these assemblages evolved in the same convergent margin system along the northern (Alexander) and northwestern (YTT) margins of Laurentia. These results support a tectonic model in which: (1) YTTs formed outboard of (or northward along strike of) YTTn and NAa along the northern Cordilleran margin during Neoproterozoic(?)–early Paleozoic time; (2) initial subduction-related magmatism during Late Ordovician to Early Devonian time records a progression from crustal thinning to crustal thickening, and is preserved only in YTTs; (3) a second phase of magmatism records Middle–Late Devonian crustal thinning followed by early Mississippian crustal thickening; (4) YTTs and YTTn evolved as an intra-oceanic arc outboard of the Slide Mountain ocean basin during Carboniferous–Permian time and were accreted to the continental margin during Triassic time; and (5) YTTs is interpreted to have been displaced ∼1000 km southward, from an original position outboard of YTTn/NAa to its present position outboard of the Stikine terrane, along a sinistral fault system of Late Jurassic–Early Cretaceous age.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Roberts Mountains allochthon: New insights into the early Paleozoic tectonics of western North America

Abstract: Detrital zircon U-Pb geochronology and Hf isotope geochemistry provide new insights into the provenance, sedimentary transport, and tectonic evolution of the Roberts Mountains allochthon strata of north-central Nevada. Using laser-ablation inductively coupled plasma mass spectrometry, a total of 1151 zircon grains from six Ordovician to Devonian arenite samples were analyzed for U-Pb ages; of these, 228 grains were further analyzed for Hf isotope ratios. Five of the units sampled have similar U-Pb age peaks and Hf isotope ratios, while the ages and ratios of the Ordovician lower Vinini Formation are significantly different. Comparison of our data with that of igneous basement rocks and other sedimentary units supports our interpretation that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units sampled, as well as Ordovician passive margin sandstones of the western Laurentian margin, had a common source in the Peace River Arch region of western Canada. We propose that the Roberts Mountains allochthon strata were deposited near the Peace River Arch region, and subsequently tectonically transported south along the Laurentian margin, from where they were emplaced onto the craton during the Antler orogeny.

Sources of volcanic detritus in the basal Chinle Formation, southwestern Laurentia, and implications for the Early Mesozoic magmatic arc

Abstract: The Upper Triassic Chinle Formation in southwestern Laurentia is the oldest distinctive record of Early Mesozoic Cordilleran arc magmatism, in the form of detrital zircons and volcanic clasts. Initial deposition of the basal Shinarump and Mesa Redondo members, herein collectively called the Shinarump conglomerate, began in Late Triassic time, yet the earliest known arc magmatism is older by as much as 40 m.y. Analysis of detrital zircons from eight sites in southeastern Nevada, southern Utah, and northeastern Arizona and volcanic-clast zircons from four of these sites provides a basis for understanding the evolution of the Early Mesozoic arc. Most Permian and Triassic detrital zircons from the Shinarump conglomerate have ages from ca. 260 to 220 Ma with rare grains as old as 280 Ma. These ages are compatible with derivation from sources in the magmatic arc to the west and southwest, including plutons of corresponding age in the Mojave Desert. Volcanic clasts are uniformly in the range 232–224 Ma; their age and zircon geochemistry argue against a source in currently exposed Mojave Desert Triassic plutons. As a further test, we compared Th/U ratios of clast and detrital zircons with those of possible sources to the west. Th/U values of many detrital grains support their derivation from Triassic Mojave Desert plutons. Some detrital grains and those from the clasts, however, have Th/U values that are uniformly higher than those in Permo-Triassic Mojave Desert plutons and therefore argue for a different, unexposed source. We propose that the early arc lay offshore of western Laurentia. Over time, plutons were emplaced across a range of continental crustal thicknesses that likely increased toward the east. At approximately 235–230 Ma, a land connection between the arc and retro-arc areas was established and fluvial sedimentation began. The observation that the youngest grain ages in our detrital samples are variable suggests that this land connection was tenuous for perhaps 10 m.y. until well into Chinle Formation sedimentation.

An evaluation of Mesozoic rift-related magmatism on the margins of the Labrador Sea: Implications for rifting and passive margin asymmetry

Abstract: The Labrador Sea is a small (∼900 km wide) ocean basin separating southwest Greenland from Labrador, Canada. It opened following a series of rifting events that began as early as the Late Triassic or Jurassic, culminating in a brief period of seafloor spreading commencing by polarity chron 27 (C27; Danian) and ending by C13 (Eocene-Oligocene boundary). Rift-related magmatism has been documented on both conjugate margins of the Labrador Sea. In southwest Greenland this magmatism formed a major coast-parallel dike swarm as well as other smaller dikes and intrusions. Evidence for rift-related magmatism on the conjugate Labrador margin is limited to igneous lithologies found in deep offshore exploration wells, mostly belonging to the Alexis Formation, along with a postulated Early Cretaceous nephelinite dike swarm (ca. 142 Ma) that crops out onshore, near Makkovik, Labrador. Our field observations of this Early Cretaceous nephelinite suite lead us to conclude that the early rift-related magmatism exposed around Makkovik is volumetrically and spatially limited compared to the contemporaneous magmatism on the conjugate southwest Greenland margin. This asymmetry in the spatial extent of the exposed onshore magmatism is consistent with other observations of asymmetry between the conjugate margins of the Labrador Sea, including the total sediment thickness in offshore basins, the crustal structure, and the bathymetric profile of the shelf width. We propose that the magmatic and structural asymmetry observed between these two conjugate margins is consistent with an early rifting phase dominated by simple shear rather than pure shear deformation. In such a setting Labrador would be the lower plate margin to the southwest Greenland upper plate.

Extraction of three-dimensional fracture trace maps from calibrated image sequences

Abstract: The routine application of digital survey technologies such as terrestrial lidar and photogrammetry to the characterization of fault and fractures in outcrop over the past decade has resulted in major advances in terms of the efficiency of discontinuity data acquisition. However, the reliance upon mesh- and point-cloud–based analysis approaches means that data sets obtained from these sources commonly offer heavily abstracted views of the measured fracture network due to the limited resolution of the input model. Here, we pre­sent an alternative approach that combines conventional two-dimensional (2D) image analysis with ray-tracing techniques to extract three-dimensional (3D) fracture trace maps from photogrammetrically calibrated image sequences. These 3D trace objects may be interrogated to obtain fracture network properties (trace length, intensity, and connectivity), with probabilistic methods used to estimate fracture orientation for high collinearity traces. Our approach possesses a number of advantages over existing digital surface reconstruction-based methods, with the use of a 2D pixel-based approach allowing established image-processing routines (e.g., edge detection/connected components analysis) to be applied to the characterization of fracture and fault properties. Moreover, the innately high resolution of the input images results in practically lossless 3D fracture trace representation, limiting truncation effects. As a result, the method is capable of resolving local varia­bility in higher-order fracture properties such as fracture intensity, which are difficult to derive using existing approaches. We demonstrate the approach on pervasively faulted Permian age exposures of the Vale of Eden Basin, UK.

A calibration-free approach for measuring fracture aperture distributions using X-ray computed tomography

Abstract: Various methods have been proposed to measure fracture aperture distributions, including X-ray computed tomography (CT) imaging, which has the advantage that it can be combined with dynamic flow experiments. In this paper, we present a calibration-free missing CT attenuation (CFMA) imaging method for measuring fracture apertures that avoids time-consuming calibration. In addition, this model does not assume a homogeneous matrix and thus provides a good estimate of fracture apertures even when rock properties are heterogeneous. The validity of the CFMA model is established by four approaches: comparing apertures calculated with the conventional calibration-based method; evaluating model predictability at different scanner voxel sizes; comparing with calibration coefficients in the literature from a number of experiments with different rocks and X-ray scanners; and comparing aperture measurements for dry and wet scans. We analyze the systematic error and the random error introduced by rock heterogeneities and CT scanning and show that by averaging 5 replicate scans, we reduce the aperture measurement error to ∼22 µm.

Stratigraphic and geochemical expression of Barremian-Aptian global climate change in Arctic Svalbard

Abstract: Significant changes in global climate and carbon cycling occurred during the Early Cretaceous. This study examines the expression of such climatic events in high-latitude Svalbard together with the stratigraphic utility of carbon-isotope stratigraphies. Isotopic analysis of fossil wood fragments (from the Rurikfjellet, Helvetiafjellet, and Carolinefjellet formations, Festningen, Spitsbergen) record a distinctive pattern including a negative isotope excursion preceding a positive event, correlatable with the global early Aptian isotope event. Our carbon-isotope profile improves the stratigraphic correlation and relative dating of the succession. We show that the upper part of the Helvetiafjellet Formation was deposited during the early Aptian, and not the late Barremian, as previously thought. Furthermore, we estimate an age for the abrupt contact of the Rurikfjellet Formation with the overlying Helvetiafjellet Formation (associated with a pulse of igneous activity) to be ca. 129 Ma or ca. 124 Ma, depending on which age model for the Early Cretaceous is used. The well-known dinosaur footprints of the Helvetiafjellet Formation at Festningen are constrained to the middle Barremian and, coupled with floral data, support a warm late Barremian prior to the Aptian carbon-isotope event. The appearance of glendonites at 655 m in the Carolinefjellet Formation is consistent with global cooling in the late Aptian–early Albian.

Miocene relative sea level on the New Jersey shallow continental shelf and coastal plain derived from one-dimensional backstripping: A case for both eustasy and epeirogeny

Abstract: Onshore drilling by Ocean Drilling Program (ODP) Legs 150X and 174AX and offshore drilling by Integrated Ocean Drilling Program (IODP) Expedition 313 provides continuous cores and logs of seismically imaged Lower to ­Middle Miocene sequences. We input ages and paleodepths of these sequences into one-dimensional backstripping equations, progressively accounting for the effects of compaction, Airy loading, and thermal subsidence. The resulting difference between observed subsidence and theoretical thermal subsidence provide relative sea-level curves that reflect both global average sea level and non-thermal subsidence. In contrast with expectations, backstripping suggests that the relative sea-level maxima in proximal onshore sites were lower than correlative maxima on the shelf. This requires that the onshore New Jersey coastal plain has subsided relative to the shelf, which is consistent with models of relative epeirogeny due to subduction of the Farallon plate. These models predict subsidence of the coastal plain relative to the shelf. Although onshore and offshore sea-level estimates are offset by epeirogeny, the ampli­tude of million-year–scale Early to Middle Miocene sea-level changes seen at the New Jersey margin is generally 5–20 m and occasionally as great as 50 m. These events are interpreted to represent eustatic variations, because they occur on a shorter time frame than epeirogenic influences. Correction for epeiro­genic effects largely reconciles differences between onshore and offshore rela­tive sea-level estimates and suggests that backstripping provides a testable eustatic model for the Early to Middle Miocene.