Earth and Planetary Sciences

Aerial thermal imaging is used to study grain-size distributions and induration on a wide variety of alluvial fans in the desert southwest of the United States. High-resolution aerial thermal images reveal evidence of sedimentary processes that rework and build alluvial fans, as preserved in the grain-size distributions and surface induration those processes leave behind. A catalog of constituent sedimentary features that can be identified using aerial thermal and visible imaging is provided. These features include clast-rich and clast-poor debris flows, incised channel deposits, headward-eroding gullies, sheetflood, lag surfaces, active/inactive lobes, distal sand-skirts and basin-related salt pans. Ground-based field observations of surface grain-size distributions, as well as morphologic, cross-cutting and topographic relationships were used to confirm the identifications of these feature types in remotely acquired thermal and visible images. Thermal images can also reveal trends in grain sizes between neighboring alluvial fans on a regional scale. Although inferences can be made using thermal images alone, the results from this study demonstrate that a more thorough geological interpretation of sedimentary features on an alluvial fan can be made using a combination of thermal and visible images. The results of this study have potential applications for Mars, where orbital thermal imaging might be used as a tool for evaluating constituent sedimentary processes on proposed alluvial fans.

Abstract—We address the problem of adaptive informationoptimal data collection in time series. Here a remote sensor or explorer agent throttles its sampling rate in order to track anomalous events while obeying constraints on time and power. This problem is challenging because the agent has limited visibility—all collected datapoints lie in the past, but its resource allocation decisions require predicting far into the future.

The Dynamic Albedo of Neutrons (DAN) instrument, to be included on-board the Mars Science Laboratory (MSL) rover, will quantify the hydrogen abundance and stratigraphy as it traverses the surface of Mars [1]. After a pulse of high energy (14 MeV) neutrons is fired into the surface, the DAN instrument will detect those neutrons that have interacted with materials from within the subsurface (< ~0.5 m deep) and scattered back toward the detectors. When binned by time, the flux of neutrons arriving at the detector will be a function of the amount of hydrogen and the stratigraphy of hydrogen-rich materials within the subsurface. Early studies [2], and recent experimental and modeling results [1, 3, 4] have confirmed that neutron die-away curves acquired from DAN will be sensitive to hydrogen-content of the nearsubsurface, the depth of a buried hydrogen-rich layer and possibly buried phyllosilicates. These results reveal the potential of the Dynamic Albedo of Neutrons (DAN) instrument on-board the Mars Science Laboratory (MSL) rover to identify local regions within the subsurface with increased amounts of hydrogen as it moves across the Martian surface.

Structural transects through the South Tibetan Detachment system (STDS) in the Dzakaa Chu valley, Tibet reveal a ∼1000-m thick, low-angle (<35°) zone of distributed ductile shear that displaces Paleozoic sediments over amphibolite facies gneisses, calc-mylonites and leucogranites of the Greater Himalayan Series (GHS). Within the shear zone, grain-size reduction with dynamic recrystallisation of quartz and growth of secondary phyllosilicates accommodated ductile deformation at elevated temperatures. Small-scale brittle normal faults and extensional shear veins overprint ductile features recording deformation at lower temperatures. Our structural data indicate that the Dzakaa Chu STDS records a progression from ductile- to brittle-deformation without development of a discrete detachment fault(s) that is common to many STDS sections. U(–Th)–Pb dating of post-kinematic leucogranites suggest that, in the lower part of the shear zone, mylonitic fabric development occurred prior to ∼20 Ma. By integrating structural and geochronological evidences we propose that the Dzakaa Chu STDS represents a deeper structural position than elsewhere in the Himalaya and provides important insight into the early ductile exhumation of the GHS that was dominated by movement along a 1-km wide shear zone without discrete brittle detachments. These findings are an important step towards understanding the development of low-angle detachment fault systems active during continental collision.

The use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The most common methods, broadly grouped into those that use tailed and tailless porphyroclasts, have been applied to many different tectonic settings; however, attempts have not been made to unify the various methods. Here, we propose the Rigid Grain Net (RGN) as an alternative graphical method for estimating Wm. The RGN contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution (PHD) method. We use the RGN to unify the most commonly used Wm plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix. Test samples from the South Tibetan detachment, Tibet yield indistinguishable results when the RGN is compared with existing methods. Because of its ease of use, ability for comparing natural data sets to theoretical curves, potential to standardize future investigations and ability to limit ambiguity in estimating Wm, the RGN makes an important new contribution that advances the current methods for quantifying flow in shear zones.

The Ama Drime Massif is a north–south trending antiformal structure located on the southern margin of the Tibetan Plateau that is bound by the Ama Drime and Nyönno Ri detachments on the western and eastern sides, respectively. Detailed kinematic and vorticity analyses were combined with deformation temperature estimates on rocks from the Ama Drime detachment to document spatial and temporal patterns of deformation. Deformation temperatures estimated from quartz and feldspar microstructures, quartz [c] axis fabrics, and two-feldspar geothermometry of asymmetric strain-induced myrmekite range between ∼400 and 650 °C. Micro- and macro-kinematic indicators suggest west-directed displacement dominated over this temperature range. Mean kinematic vorticity estimates record early pure shear dominated flow (49–66% pure shear) overprinted by later simple shear (1–57% pure shear), high-strain (36–50% shortening and 57–99% down-dip extension) dominated flow during the later increments of ductile deformation. Exhumation of the massif was accommodated by at least ∼21–42 km of displacement on the Ama Drime detachment. Samples from the Nyönno Ri detachment were exhumed from similar depths. We propose that exhumation on the Nyönno Ri detachment during initiation of orogen-parallel extension (11–13 Ma) resulted in a west-dipping structural weakness in the footwall that reactivated as the Ama Drime detachment.

Springs issuing from different faults and shear zones along the crest of the Himalayas tap three different levels of crust beneath the Tibetan Plateau. From structurally highest to lowest these are the Tingri Graben, the South Tibetan Detachment System (STDS), and the Ama Drime massif (ADM). The aqueous chemistry reflects water-rock interactions along faults and is consistent with mapped rock types. Major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the N-S trending faults that bound the Tingri Graben and Ama Drime detachment (ADD) compared to the STDS, suggesting that these structures penetrate to greater depths. Springs have excess CO2, N2, He, and CH4 compared to meteoric water values, implying addition from crustal sources. The 3He/4He ratios range from 0.018 to 0.063 RA and are consistent with a crustal source for He. The δ13C values of dissolved inorganic carbon (DIC) and CO2 gas range from −5.5 to +3.8‰ and −13.1 to −0.3‰ versus Peedee belemnite, respectively. Sources of carbon are evaluated by calculating isotopic trajectories associated with near-surface effervescence of CO2. Positive δ13C values of the Tingri graben and STDS springs are consistent with decarbonation of marine carbonates as the source of CO2. Negative values for the ADD springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon. The δ15N values of N2 range from −2.2 to +2.1‰ (versus AIR) and are explained by mixtures of air-derived nitrogen, metamorphic devolatilization of sedimentary nitrogen, and nitrogen from near-surface biogenic processes. CO2 flux is estimated by scaling from individual springs (∼105 mol a−1 per spring) to extensional structures across the southern limit of the Tibetan Plateau and likely contributes between 108 and 1011 mol a−1 (up to 10%) to the global carbon budget.

U(–Th)–Pb geochronology, geothermobarometric estimates and macro- and micro-structural analysis, quantify the pressure–temperature–time–deformation (P–T–t–D) history of Everest Series schist and calcsilicate preserved in the highest structural levels of the Everest region. Pristine staurolite schist from the Everest Series contains garnet with prograde compositional zoning and yields a P–T estimate of 649 ± 21 °C, 6.2 ± 0.7 kbar. Other samples of the Everest Series contain garnet with prograde zoning and staurolite with cordierite overgrowths that yield a P–T estimate of 607 ± 25 °C, 2.9 ± 0.6 kbar. The Lhotse detachment (LD) marks the base of the Everest Series. Structurally beneath the LD, within the Greater Himalayan Sequence (GHS), garnet zoning is homogenized, contains resorption rinds and yields peak temperature estimates of ∼650 ± 50 °C. P–T estimates record a decrease in pressure from ∼6 to 3 kbar and equivalent temperatures from structurally higher positions in the overlying Everest Series, through the LD and into GHS. This transition is interpreted to result from the juxtaposition of the Everest Series in the hangingwall with the GHS footwall rocks during southward extrusion and decompression along the LD system. An age constraint for movement on the LD is provided by the crystallization age of the Nuptse granite (23.6 ± 0.7 Ma), a body that was emplaced syn- to post-solid-state fabric development. Microstructural evidence suggests that deformation in the LD progressed from a distributed ductile shear zone into the structurally higher Qomolangma detachment during the final stages of exhumation. When combined with existing geochronological, thermobarometric and structural data from the GHS and Main Central thrust zone, these results form the basis for a more complete model for the P–T–t–D evolution of rocks exposed in the Mount Everest region.

The Leo Pargil dome, northwest India, is a 30 km-wide, northeast-trending structure that is cored by gneiss and mantled by amphibolite facies metamorphic rocks that are intruded by a leucogranite injection complex. Oppositely dipping, normal-sense shear zones that accommodated orogen-parallel extension within a convergent orogen bound the dome. The broadly distributed Leo Pargil shear zone defines the southwest flank of the dome and separates the dome from the metasedimentary and sedimentary rocks in the hanging wall to the west and south. Thermobarometry and in-situ U-Th-Pb monazite geochronology were conducted on metamorphic rocks from within the dome and in the hanging wall. These data were combined with U-Th-Pb monazite geochronology of leucogranites from the injection complex to evaluate the relationship between metamorphism, crustal melting, and the onset of exhumation. Rocks within the dome and in the hanging wall contain garnet, kyanite, and staurolite porphyroblasts that record prograde Barrovian metamorphism during crustal thickening that reached 530-630°C and 7-8 kbar, ending by c. 30 Ma. Cordierite and sillimanite overgrowths on Barrovian assemblages within the dome record dominantly top-down-to-the-west shearing during near-isothermal decompression of the footwall rocks to 4 kbar by 23 Ma during an exhumation rate of 1.3 mm year )1 . Monazite growth accompanied Barrovian metamorphism and decompression. The leucogranite injection complex within the dome initiated at 23 Ma and continued to 18 Ma. These data show that orogen-parallel extension in this part of the Himalaya occurred earlier than previously documented (>16 Ma). Contemporaneous onset of near-isothermal decompression, top-down-to-the-west shearing, and injection of the decompression-driven leucogranite complex suggests that early crustal melting may have created a weakened crust that was proceeded by localization of strain and shear zone development. Exhumation along the shear zone accommodated decompression by 23 Ma in a kinematic setting that favoured orogen-parallel extension.

The Neogene elevation history of the Mount Everest region is key for understanding the tectonic history of the world's highest mountain range, the evolution of the Tibetan Plateau, and climate patterns in East and Central Asia. In the absence of fossil surface deposits such as paleosols, volcanic ashes, or lake sediments, we conducted stable isotope paleoaltimetry based on the hydrogen isotope ratios (δD) of hydrous minerals that were deformed in the South Tibetan detachment shear zone during the late Early Miocene. These minerals exchanged isotopically at high temperature with meteoric water (δD water = −156‰ ± 5‰) that originated as high-elevation precipitation and infi ltrated the crustal hydrologic system at the time of detachment activity. When compared to age-equivalent near-sea-level foreland oxygen isotope (δ 18 O) paleosol records (δ 18 O water = −5.8‰ ± 1.0‰), the difference in δ 18 O water is consistent with mean elevations of ≥5000 m for the Mount Everest area. Mean elevations similar to modern suggest that an early Himalayan rain shadow may have infl uenced the late Early Miocene climatic and rainfall history to the north of the Himalayan chain.