ALBERT

Description: Many low-angle normal faults (dip ≤30°) accommodate tens of kilometers of crustal extension, but their mechanics remain contentious. Most models for low-angle normal fault slip assume vertical maximum principal stress σ1, leading many authors to conclude that low-angle normal faults are poorly oriented in the stress field (≥60° from σ1) and weak (low friction). In contrast, models for low-angle normal fault formation in isotropic rocks typically assume Coulomb failure and require inclined σ1 (no misorientation). Here, a data-based, mechanical-tectonic model is presented for formation of the Whipple detachment fault, southeastern California. The model honors local and regional geologic and tectonic history and laboratory friction measurements. The Whipple detachment fault formed progressively in the brittle-plastic transition by linking of “minidetachments,” which are small-scale analogs (meters to kilometers in length) in the upper footwall. Minidetachments followed mylonitic anisotropy along planes of maximum shear stress (45° from the maximum principal stress), not Coulomb fractures. They evolved from mylonitic flow to cataclasis and frictional slip at 300–400 °C and ∼9.5 km depth, while fluid pressure fell from lithostatic to hydrostatic levels. Minidetachment friction was presumably high (0.6–0.85), based upon formation of quartzofeldspathic cataclasite and pseudotachylyte. Similar mechanics are inferred for both the minidetachments and the Whipple detachment fault, driven by high differential stress (∼150–160 MPa). A Mohr construction is presented with the fault dip as the main free parameter. Using “Byerlee friction” (0.6–0.85) on the minidetachments and the Whipple detachment fault, and internal friction (1.0–1.7) on newly formed Reidel shears, the initial fault dips are calculated at 16°–26°, with σ1 plunging ∼61°–71° northeast. Linked minidetachments probably were not well aligned, and slip on the evolving Whipple detachment fault probably contributed to fault smoothing, by off-fault fracturing and cataclasis, and to formation of the fault core and fractured damage zone. Stress rotation may have occurred only within the mylonitic shear zone, but asymmetric tectonic forces applied to the brittle crust probably caused gradual rotation of σ1 above it as a result of: (1) the upward force applied to the base of marginal North America by buoyant asthenosphere upwelling into an opening slab-free window and/or (2) basal, top-to-the-NE shear traction due to midcrustal mylonitic flow during tectonic exhumation of the Orocopia Schist. The mechanical-tectonic model probably applies directly to low-angle normal faults of the lower Colorado River extensional corridor, and aspects of the model (e.g., significance of anisotropy, stress rotation) likely apply to formation of other strong low-angle normal faults.

Description: Multiple climate proxies indicate episodic changes in moisture levels within an ∼1 Ma duration (early–mid Pliocene) interval. Limestones within the Opache Formation, Calama Basin, Atacama Desert region, Chile, contain evidence for wetter and drier periods on short time scales. Proxies include carbonate lithological changes, paleontology (stromatolites, oncolites, gastropods, ostracods and diatoms), O and C stable isotopes, geochemistry, and mineralogical changes (aragonite, calcite, Mg-calcite, dolomite and gypsum) throughout a 30 m stratigraphic section. Stromatolite fossil cyanobacteria dark and light laminations and mesohaline to hypersaline diatom species suggest Pliocene annual seasonality. Short-term changes between wetter and drier conditions indicate that at least this part of the Atacama region was experiencing relatively rapid early–mid Pliocene climate instability. The predominance of limestone in the Opache Formation, in contrast to the 1500 m of Oligocene-Miocene siliciclastic conglomerates and sandstones, interpreted as arid climate alluvium, that underlie it, indicates a shift from arid or hyperarid climate to a semi-arid climate. Semi-arid conditions promoted limestone deposition in a shallow lacustrine-palustrine environment. In this setting, events such as storms with associated surface water flow, erosion, siliciclastic sand, gravel, and intraclast deposition, coupled with significant biological activity, represent sedimentation during more humid periods in a shallow lacustrine depositional environment. In contrast, limestone characterized by mudcracks, Navicula diatoms, and vadose syndepositional cementation, reflect periods of enhanced evaporation, water shallowing, and episodic desiccation, characteristic of a palustrine depositional system. These facies shifts, in conjunction with geochemical and isotopic proxy evidence, yield a sedimentary record of wetter and drier climate shifts.

Description: Late Paleozoic large-scale transcurrent tectonics and synkinematic intrusions are prominent features in the Eastern Tianshan segment of the southwestern Central Asian Orogenic Belt. However, the spatial and temporal relationship between synkinematic intrusions and crustal-scale shear zones remains unclear. Here we report petrology, geochemistry, and geochronology of the Qiziltag pluton associated with the Kanggur-Huangshan Shear Zone (KHSZ) with a view to characterize the spatial and temporal relationship between synkinematic intrusions and large-scale transcurrent shearing. Field relations and zircon U-Pb ages indicate that the Qiziltag pluton was formed through two stages of magmatism, with earlier stage granitoids (gneissic biotite granite: 288.9 ± 1.9 Ma, biotite monzogranite: 291.5 ± 1.7 Ma, K-feldspar granite: 287.9 ± 3.1 Ma), and later stage bimodal intrusions (biotite quartz monzonite: 278.5 ± 1.8 Ma, gabbro: 278.1 ± 2.3 Ma). The earlier stage granitoids are high-K calc-alkaline, enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, Th, and U), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). Combined with their depleted isotopic compositions (εNd(t) = +6.29 to +7.48) and juvenile model ages (TDM2 = 450–610 Ma), we infer that the granitoids were derived from juvenile lower crust in a post-collisional tectonic transition (from compression to extension). The structural and temporal features indicate that the earlier stage (ca. 290 Ma) granitoids formed prior to the regional large-scale dextral strike slip. The later stage bimodal intrusions are dominated by biotite quartz monzonite as the felsic member and gabbro as the mafic component. The biotite quartz monzonite is high-K calc-alkaline with enriched LREEs and LILEs (e.g., Rb, Th, and U), and depleted HFSEs (e.g., Nb, Ta, and Ti), whereas the gabbro is subalkalic with depleted LREEs and HFSEs (e.g., Nb and Ta), resembling normal mid-ocean ridge basalt features. The bimodal intrusions show similar isotopic compositions (εNd(t) = +6.41 to +6.72 and εHf(t) = +9.55 to + 13.85 for biotite quartz monzonite; εNd(t) = +9.13 to +9.69 and εHf(t) = +4.80 to +14.07 for gabbro). These features suggest that the later stage (ca. 280 Ma) bimodal intrusions were derived from partial melting of depleted mantle and anatectic melting of lower crust materials induced by synchronous underplating of basaltic magma in a post-collisional extension. The structural features of the bimodal intrusions indicate that the later stage (ca. 280 Ma) magmatism was coeval with the development of the KHSZ. In conjunction with spatial and temporal evolution of magmatism and sedimentary records of Eastern Tianshan, we infer that transition between the northward closure of the North Tianshan Ocean and subsequent collision between the Central Tianshan Massif and the Qoltag Arc belt occurred at ca. 300 Ma.

Description: Paleosalinity is an important environmental feature but it is difficult to evaluate. In the present study, paleosalinity was assessed during the deposition of three sets of source rocks in the western Pearl River Mouth Basin, South China Sea, utilizing four different kinds of methods, i.e., saturated hydrocarbon biomarkers, strontium abundance, non-pollen microalgae assemblies, and carbon-sulfur relationships. Results show that the second member of the Eocene Wenchang Formation (E2w2) was deposited in a freshwater environment and the Oligocene Zhuhai Formation (E3z) was deposited in a shallow marine environment. The Oligocene Enping Formation (E3e), which was believed to be deposited in a freshwater environment, was actually deposited in a brackish water environment. Mechanisms of salinity increase during the non-marine E3e deposition were mainly deep hydrothermal fluid input through the south boundary fault and episodic marine transgressions, not evaporation. The effect of salinity on organic matter content and type was investigated. Results show that salinity has no significant influence on total organic carbon (TOC) and hydrogen index (HI) of the E2w2, which was caused by the balance between freshwater algae and euryhaline algae. TOC and HI decrease with increasing salinity for samples from the E3z, which is contrary to the conventional hypothesis that marine transgressions promote source rock deposition. The decrease of TOC with carbon/sulfur ratios for samples from the E3e actually reflect the influence of thermal maturity but thermal maturity only plays the second role in HI. The effect of salinity on HI during the E3e deposition can be divided into two stages. During the first stage, the increase of salinity was mainly caused by deep hydrothermal fluid input without an oxygen level increase. HI values, therefore, remained relatively stable. During the secondary stage, the increase of salinity was mainly caused by marine transgressions which increased the oxygen level and as a consequence, HI decreased sharply with increasing salinity. This study provides a long-term salinity evolution of the western Pearl River Mouth Basin and suggests that salinity is an important factor controlling source rock deposition. In addition, this study presents an example that goes against conventional wisdom that marine transgressions promote source rock deposition in a shallow marine environment. This study also suggested that marine transgressions had already begun at the end of the early Oligocene.

Description: The organic matter-rich shales in Wufeng-Longmaxi Formation, Jiaoshiba area, Southeast China, are showing a notable petrographic heterogeneity characteristic within the isochronous stratigraphic framework, which lead to vast differences in the mineral composition and organic matter abundance in the adjacent sections of the shale reservoir. The studied shale has been divided into three systems tracts: a transgressive systems tract (TST), an early highstand systems tract (EHST), and a late highstand systems tract (LHST). Multiple-scale petrographic observation and detailed mineralogical and geochemical analyses were combined to investigate the manifestation, origin, and the ways by which the shale heterogeneity is affected. The results indicate that polytropic depositional environments lead to different components in sediment. Subsequently, these differences among shale sections become more apparent through different diagenetic pathways. During the deposition of the section TST, the Hirnantian glaciation and regional volcanism played a crucial role, contributing to the abundant accumulation of fine-grained intrabasinal silica and organic matter. In diagenesis stage, authigenic quartz aggregates derived from siliceous organisms are formed. They filled in primary interparticle pores, forming a rigid particle-bracing structure that provide effective resistivity against the compaction and spaces for organic matter migration and occlusion. Finally, the migrated organic matter left plenty of newly created pore spaces that constituted a great portion of the total porosity of shale reservoir. The depositional process of section EHST is strongly influenced by contour current, which brings about more extrabasinal influx and impoverishes organic matter. In diagenesis stage, the rigid particle-bracing structure could only be preserved in limited areas, since insufficient siliceous supply could not produce enough authigenic quartz. Primary interparticle pores are significantly reduced owing to compaction, leaving less space for later organic matter migration and occlusion. As a result, the total porosity of shale reservoir declines in this section. In a rapid tectonic-uplifting background, the deposition of section LHST is associated with a rapid increase in terrigenous clay minerals, which further dilutes organic matter. Ductile clay experienced strong compaction and then occupies most of the primary interparticle space. Rigid particles are wrapped by a large number of clays, which has destroyed the particle-bracing structure. As a result, the nanoporous system in the shale could not be well preserved.

Description: The Wufeng and Longmaxi organic-rich shales host the largest shale gas plays in China. This study examined the petrography, rare earth element (REE) and other trace-element geochemistry, Sm-Nd geochronology, and isotope geochemistry (87Sr/86Sr, δ18O, δ13C) of fracture-cementing minerals within core samples of the Wufeng and Longmaxi Formations from the Jiaoshiba shale gas field in order to (1) characterize the mineral phases occurring in the veins (mineralized fractures); (2) determine the ages of the calcite by the Sm-Nd isochron dating method; (3) understand the sources of calcite-precipitating fluids; and (4) explore the possible mechanisms responsible for calcite vein formation in shale gas systems. The fractures hosted in the Longmaxi Formation are mineralized with quartz as the predominant fracture cement, and calcite as an intracementation phase postdating the earlier quartz cement. In contrast, the fractures hosted in the Wufeng Formation are dominantly mineralized by calcite, which occurs either as the only cement present or as a cement phase predating later quartz cement. Calcite veins within the Longmaxi Formation have a Sm-Nd isochron age of 160 ± 13 Ma and δ13C values of –4.71‰ to –3.11‰, δ18O values of 17.1‰–17.4‰, and 87Sr/86Sr values of 0.72437–0.72869. Calcite veins within the Wufeng Formation yielded a Sm-Nd isochron age of 133 ± 15 Ma and are characterized by δ13C values of –2.29‰ to –1.03‰, δ18O values of 17.3‰–17.7‰, and 87Sr/86Sr values of 0.72202–0.72648. The similarity between 87Sr/86Sr values of the calcite and those of their respective surrounding host rocks (0.72670–0.72875 of the Longmaxi shales; 0.72030–0.72648 of the Wufeng shales), combined with relatively depleted δ13C and uniform fluid δ18O isotopic features, indicates that the calcite-precipitating fluids within the Wufeng and Longmaxi Formations were derived largely from their respective surrounding host-rock sources. REE data equally indicate that the distinguishable Eu anomalies (6.20–19.35; 4.45–11.91), Y anomalies (1.03–1.50; 1.44–1.70), and Y/Ho ratios (28.80–39.16; 38.86–45.18) of calcite veins within the Longmaxi and Wufeng Formations were controlled by their respective surrounding host rocks. The Sm-Nd isochron ages and fluid inclusion data of fracture cements suggest that fracture opening and calcite precipitation in composite veins within the Wufeng and Longmaxi Formations were triggered by gas generation overpressurization.

Description: New geochemical and geochronological data are used to characterize the geodynamic setting of metasediments, felsic orthogneisses, and eclogite and amphibolite lenses forming the Beishan complex, NW China, at the southern part of the Central Asian Orogenic Belt. The metasediments correspond compositionally to immature greywackes receiving detritus from a heterogeneous source involving a magmatic arc and a Precambrian continental crust. Metagranitoids, represented by felsic orthogneisses, show both composition of greywacke-derived granitic melt with incompatible trace element patterns similar to the host metasediments. The eclogite lenses are characterized by high Nb contents (5.34–27.3 ppm), high (Nb/La)N (〉1), and low Zr/Nb ratios (

Description: The nature of coarse sediment in rivers emerging from mountain ranges determines rates of downstream fining, the position of the gravel-sand transition, sediment entrainment thresholds, and channel morphologies. Additionally, in the stratigraphic record, clast size distributions and lithologies are used to reconstruct paleo-hydraulic conditions and source area provenance. Using Himalayan rivers, we demonstrate that the signal of first-generation clasts derived from the hinterland of a mountain range can be significantly altered by recycling older, structurally exhumed foreland deposits. The Siwalik foothills of the Himalaya comprise Neogene fluvial sandstones and quartzite-rich conglomerates with well-rounded clasts that were deposited in the Indo-Gangetic foreland basin and later exhumed by erosion, following uplift along the Himalayan mountain front. Mass balance calculations reveal that the Upper Siwalik conglomerate may contribute a significant proportion of the total gravel flux exported from the main Himalayan catchments (up to 100%) despite forming

Description: Oxygen isotope analyses of diagenetic cements can provide detailed evidence of sedimentary burial processes and conditions, as the δ18O values of precipitating minerals reflect contemporaneous local δ18Owater and temperature conditions. Uncertainties in the timing and rates of pore water δ18O evolution in sedimentary basins can complicate interpretation of these records. Fracture-bridging (0.5–1 mm) quartz cements observed in sandstones of the Cretaceous Travis Peak Formation in the East Texas basin show clear growth-zoning by cathodoluminescence and contain detailed fluid inclusion records of temperature that make them excellent candidates for interrogating prolonged histories of basin temperature and the evolution of δ18O in basin pore water. New secondary ion mass spectrometer (SIMS) δ18Oquartz isotopic data from fluid inclusion-rich quartz bridges in Travis Peak sandstones record a steady increase of pore water δ18O values from ∼5 to 7‰ (VSMOW; Vienna Standard Mean Ocean Water) as the sandstone warms from ∼130 to 150 °C. To help evaluate whether this trend could be generated solely from local water-rock interactions in response to burial compaction, a one-dimensional closed system isotopic burial model was created to simulate how δ18Owater values change in a quartz-dominated sandstone during diagenesis. Using both directly measured and inferred rates of Travis Peak compaction, the magnitude of change in δ18Owater that we calculate from quartz bridge geochemistry cannot be reasonably modeled solely by local quartz mechanical compaction, pressure solution, and cementation processes, necessitating significant fluxes of silica and high-δ18O water from outside of the sandstones prior to maximum burial. This indicates that even units which appear surrounded by significant barriers to fluid flow (i.e., mudrock-bounded channel sandstones) may have been infiltrated and diagenetically modified by large fluxes of fluid on geologic time scales.

Description: Climate changes related to orographic barrier uplift have been in the research spotlight over recent years. Several works have focused on the interactions between climatic and tectonic processes in order to understand the development of a rain shadow. Patagonia is an ideal region in which to analyze such interactions, since a Miocene climate change, from wetter to drier conditions, has mainly been associated with Andean uplift. In this work, we analyzed a succession of stacked paleosols recorded in a Miocene North Patagonian foreland basin in order to understand how the paleosol moisture regime related to the atmospheric humidity changes caused by the uplift of the Patagonian Andes. Based on macromorphological, micromorphological, and geochemical studies, and supported by a high-resolution chronostratigraphic scheme based on U-Pb geochronology and magnetostratigraphy, the paleosols were characterized with corresponding mean annual paleoprecipitation (MAP) and mean annual temperature values. Alfisol-like paleosols were identified at the base of the foreland infill (15–14.6 Ma) with a MAP of 1229 ± 108 mm/yr. The Andisol-like paleosols recognized in the middle section of the sequence (14.6–12.75 Ma) exhibited a MAP of 1053 ± 108 mm/yr, whereas the Aridisol-like paleosols occurring in the upper section of the infill (12.75–11.5 Ma) presented a MAP of 677 ± 108 mm/yr. The determined Miocene mean annual temperatures (∼11 ± 2.1 °C) were similar to the present-day values (11 °C). Based on the complete tectonic record of the Patagonian Andes, the observed decrease in MAP was assigned to the rain shadow effect created by the uplift of the North Patagonian Andes. Results indicate that although the process started around 19 Ma, the rain shadow effect was not effectively recorded before ca. 14.6 Ma.