[1] The Pamir plateau forms a prominent tectonic salient along the western end of the Tibet-Tarim margin. Despite its tectonic significance, relatively little is known about the timing of major Cenozoic tectonic events in the Pamir. Here we present new apatite and zircon (U/Th)-He ages, bulk rock geochemistry, and Al-in-hornblende barometry results from the Karakul graben, a prominent north-south oriented rift basin located ∼50 km south of the Main Pamir Thrust. Although cooling ages do not record the onset of extension, graben-bounding normal faults provide exposures of otherwise slowly eroding rocks which record two Cenozoic thermal events. Existing geochronology and new results suggest that granitic rocks in the Karakul region were shallowly emplaced, cooled very quickly through ∼300°C, and have experienced less than 10 km of exhumation since the late Triassic. A long period of relatively slow exhumation throughout much of the late Mesozoic and Cenozoic was punctuated by two periods of accelerated exhumation during the middle Eocene (∼50-40 Ma) and early Miocene (∼25-16 Ma). We interpret the first period of accelerated exhumation as a result of tectonic uplift and subsequent erosion due to the northward propagation of the India-Asia collision. We attribute the second period of rapid exhumation to a renewed phase of tectonism and plateau uplift in the Pamir, perhaps related to a break off event along the down-going Indian plate at ∼25 Ma or to the onset of slip along the nascent Karakoram fault.
The environmental impacts of shale-gas development on water resources, including methane migration to shallow groundwater, have been difficult to assess. Monitoring around gas wells is generally limited to domestic water-supply wells, which often are not situated along predominant groundwater flow paths. A new concept is tested here: combining stream hydrocarbon and noble-gas measurements with reach mass-balance modeling to estimate thermogenic methane concentrations and fluxes in groundwater discharging to streams and to constrain methane sources. In the Marcellus Formation shale-gas play of northern Pennsylvania (U.S.A.), we sampled methane in 15 streams as a reconnaissance tool to locate methane-laden groundwater discharge: concentrations up to 69 μg L(-1) were observed, with four streams ≥ 5 μg L(-1). Geochemical analyses of water from one stream with high methane (Sugar Run, Lycoming County) were consistent with Middle Devonian gases. After sampling was completed, we learned of a state regulator investigation of stray-gas migration from a nearby Marcellus Formation gas well. Modeling indicates a groundwater thermogenic methane flux of about 0.5 kg d(-1) discharging into Sugar Run, possibly from this fugitive gas source. Since flow paths often coalesce into gaining streams, stream methane monitoring provides the first watershed-scale method to assess groundwater contamination from shale-gas development.
Focused groundwater discharge in closed basins provides opportunities to investigate mechanisms for closing hydrologic and solute budgets in arid regions. The Salar de Atacama (SdA), adjacent to the Central Andean Plateau in the hyperarid Atacama Desert, provides an extreme example of halite (>1800 km3) and lithium brine (~5000 ppm) accumulation spanning late Miocene to present. Minimum long‐term water discharge needed to sustain halite accumulation over this timescale at SdA is 9–20 times greater than modern recharge (and double wet‐climate paleorecharge) within the topographic watershed. Closing this imbalance requires sourcing water from recharge on the orogenic plateau in an area over 4 times larger than the topographic watershed. Prolonged water discharge at SdA requires long residence times, deep water tables in recharge zones coupled with persistent near‐surface water tables in discharge areas, and large contributing areas characterized by strong gradients in landscape and climate resulting from plateau uplift.
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