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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[degrees]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. doi: 10.1029/2009TC002589.

The Rio Icacos watershed in the Luquillo Mountains (Puerto Rico) is unique due to its extremely rapid weathering rates. The watershed is incised into a quartz diorite that has developed a large knickzone defining the river profile. Regolith thickness within the watershed generally decreases from 20 to 30 m at the ridges to several meters in the quartz diorite‐dominated valley to tens of centimeters near the major river knickpoint, as determined from previous studies. Above the knickzone, we observe spheroidal corestones, but below this weathering is much less apparent. Measured erosion rates from previous studies are also high in the knickzone compared with upper elevations within the river profile. A suite of near‐surface geophysical methods (i.e. ground penetrating radar and terrain conductivity) capable of fast data acquisition in rugged landscapes, was deployed at kilometer scales to characterize critical zone structure. Concentrations of chaotic ground penetrating radar (GPR) reflections and diffraction hyperbolas with low electrical conductivity were observed in vertical zones that outcrop at the land surface as areas of intense fracturing and spheroidally weathered corestones. The width of these fractured and weathered zones showed an increase with proximity to the knickpoint, and was attributed to dilation of these sub‐vertical fractures near the knickpoint, as postulated theoretically by a stress model calculated for the topographic variability across the knickzone in the Rio Icacos, and that shows a release of compressive stress near the knickpoint. We hypothesize that erosion rates increase in the knickzone because of this inferred dilation of fractures. Specifically, opened fractures could enhance access of water and in turn promote spalling, erosion, and spheroidal weathering. This study shows that ground‐based hydrogeophysical methods used at the landscape‐scale (traditionally applied at smaller scales) can be used to explore critical zone architecture at the scales needed to explain the extreme variability in erosion rates across river profiles. © 2018 John Wiley & Sons, Ltd. A suite of near‐surface geophysical methods was deployed at kilometer scales to characterize fracturing zones across the Rio Icacos Watershed in the Luquillo Mountains (Puerto Rico). Results were combined with stress models calculated for the topographic variability across the knickzone and showed an increase in fracturing width with proximity to the knickpoint that is indicative of a release in compressive stress near the knickpoint. [ABSTRACT FROM AUTHOR]

A broad array of new provenance and stable isotope data are presented from two magnetostratigraphically dated sections in the south-eastern Issyk Kul basin of the Central Kyrgyz Tien Shan. The results presented here are discussed and interpreted for two plausible magnetostratigraphic age models. A combination of zircon U-Pb provenance, paleocurrent and conglomerate clast count analyses is used to determine sediment provenance. This analysis reveals that the first coarse-grained, syntectonic sediments (Dzhety Oguz formation) were sourced from the nearby Terskey Range, supporting previous thermochronology-based estimates of a ca. 25-20 Ma onset of deformation in the range. Climate variations are inferred using carbonate stable isotope (d18O and d13C) data from 53 samples collected in the two sections and are compared with the oxygen isotope compositions of modern water from 128 samples. Two key features are identified in the stable isotope data set derived from the sediments: (1) isotope values, in particular d13C, decrease between ca. 26.0 and 23.6 or 25.6 and 21.0 Ma, and (2) the scatter of d18O values increased significantly after ca. 22.6 or 16.9 Ma. The first feature is interpreted to reflect progressively wetter conditions. Because this feature slightly post-dates the onset of deformation in the Terskey Range, we suggest that it has been caused by orographically enhanced precipitation, implying that surface uplift accompanied late Cenozoic deformation and rock uplift in the Terskey Range. The increased scatter could reflect variable moisture source or availability caused by global climate change following the onset of Miocene glaciations at ca. 22.6 Ma, or enhanced evaporation during the Mid-Miocene climatic optimum at ca. 17-15 Ma. [ABSTRACT FROM AUTHOR]

The Payún Matru volcanic field encompasses more than 12,000 km2 in the west-central Argentine province of Mendoza. The northwestern part of the field, called Los Volcanes, includes multiple basaltic lava flows and numerous cinder cones and scoria fields. We sampled primary lava flow features (tumuli, pahoehoe and aa flow surfaces) from two different basaltic lava flows and one fluvial strath for cosmogenic 3He exposure age dating in the Los Volcanes area. Olivines from the sampled basalts have low concentrations of U and Th and a minimal radiogenic 4He correction. We used the recently established R correction factor to correct our measured 3He concentrations for magmatic 3He and 4He and radiogenic 4He. Exposure ages for primary lava flow features covering a large area just east of where Route 40 crosses the Río Grande range from 41 to 43 ka with external uncertainties on the order of 7%. A recently published K→Ar age of 26 ± 5 ka was sampled within a few hundred meters of three of our cosmogenic exposure age samples and appears to be from the same basalt flow. Given the large uncertainties associated with dating young lava flows, the K→Ar age and our cosmogenic ages are in reasonable agreement; however, they do not overlap within uncertainties and suggest possible systematic discrepancies between the two methods of up to 20% in this instance. A sample from a bedrock strath carved by the Río Grande into basalt yielded a surface exposure age of 17 ± 1 ka. The toe of a much younger-looking aa flow about 5 km west of the Rt. 40 Río Grande bridge yielded two ages of 0 and 2 ka, and is presumed to be historical. [ABSTRACT FROM AUTHOR]

Andesitic boulder deposits mantle straths cut in sedimentary bedrock high above the northern tributaries of the Escalante River in south-central Utah. The andesitic gravel deposits are derived from the southern escarpments of Boulder Mountain and Aquarius Plateau. The sedimentology and geomorphic expression of these deposits suggest they are from slurry-flow mass movements that have been reworked by fl uvial processes. The andesitic boulders are signifi cantly tougher than the local sedimentary bedrock and cause boulder armoring and topographic inversion. The andesitic boulders are also effective tools for fl uvial incision when transported across weaker bedrock. Cosmogenic 3He exposure-age dating of some of the largest boulders exposed on the treads of four different deposits range from 303 ± 48 to 1395 ± 241 ka. The tallest boulders exposed on the deposit surfaces tend to yield the oldest exposure ages, suggesting that boulder erosion and deposit erosion are controlling the exposure-age populations and indicating that even the oldest exposure ages from a given deposit are likely minimum age estimates. Using the oldest exposure age from each surface, we estimate maximum Escalante River northern tributary incision rates of 151-323 m Ma-1 for the period since 0.6-1.4 Ma. [ABSTRACT FROM AUTHOR]