The Tien Shan are the quintessential intracontinental range, situated more than 1000 km north of the suture between India and Asia. Their initiation and growth in the Cenozoic, however, remain poorly understood. In this study we present stratigraphic, detrital fission-track, and magnetostratigraphic results that provide a basis for reconstructing the Cenozoic tectonic evolution of the Kyrgyz Range and adjacent Chu basin in the northwestern Tien Shan. Detrital fission-track thermochronology indicates that the northwestern Tien Shan was tectonically quiescent for much of the Cenozoic. Prior to uplift and exhumation in the late Miocene, the Kyrgyz Range was buried by sediments shed from highlands to the south and/or east. Paired bedrock fission-track and [U-Th]/He ages from a sampling transect of 2.4 km relief demonstrate that rapid exhumation commenced at ca. 11 Ma. Initial thrusting in the hinterland was followed by evaporite accumulation (ϳ0.4 km/m.y.), which coincided with erosion of the pre-11 Ma strata that mantled the Kyrgyz Range. Between 10 and 3 Ma, bedrockexhumation rates decreased to Ͻ0.3 km/ m.y., while sedimentation rates decelerated initially to ϳ0.25 km/m.y. before accelerating to ϳ0.4 km/m.y. at 4-5 Ma. Detrital fission-track results indicate that by 4.5 Ma,

A B S T R A C TPaired apatite fission track and U-Th/He dates provide the first Late Cenozoic cooling ages for the northern Tien Shan. These data clearly argue for pulsed deformation since the Late Miocene, with early (10-11 Ma) and late (0-3 Ma) intervals of rapid exhumation separated by an extended interval of much slower rates. By integrating these bedrock cooling rates with shortening estimates derived from a balanced section, detrital cooling ages, and geomorphological estimates of conditions before deformation, we reconstruct a four-stage history of range growth and exhumation. Following ∼100 m.yr. of tectonic quiescence, abruptly accelerated rock uplift, exhumation, and cooling in the Kyrgyz Range commenced at ∼11 Ma with rates exceeding ∼1 km/m.yr. During the subsequent 7 m.yr., deformation and cooling rates decreased three-to sixfold before accelerating by comparable amounts during the past 3 m.yr. Since mid-Miocene times, the surface elevation of the Kyrgyz Range has increased ∼2 km, consistent with the reconstructed magnitude of crustal shortening (∼11 km) and thickening (∼12 km) across the range. The highly pulsed deformation rates indicate that the locus of deformation probably shifted repeatedly within the Tien Shan from the Miocene to present. Even at their most rapid, Cenozoic shortening rates in the Kyrgyz Range were equivalent to only 10%-20% of the modern geodetic convergence rate across the entire Tien Shan. This requires several ranges within the Tien Shan to have deformed simultaneously since the Middle Miocene, a situation analogous to the distributed shortening seen today.

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Single-crystal dating of detrital mineral grains confers a remarkable ability to reconstruct cooling histories of orogens and to place limits on the timing, magnitude, and spatial variations of erosion. Numerous grains from a detrital sample are typically dated, and the statistical variability between populations of ages in different samples provides keys to variations in cooling histories and exhumation rates within the hinterland. Given that detrital samples comprise minerals drawn from an entire catchment, they offer an integrated perspective that is almost always unattainable with bedrock samples. Moreover, because detrital ages are preserved within stratigraphic successions, the evolution of populations of cooling ages through time and across an orogen can be reconstructed from the sedimentary record. When combined with a known hinterland 'stratigraphy' of bedrock cooling ages, studies of detrital ages in modern river systems demonstrate the fidelity of the detrital signal, and reveal both the power and limitations of detrital single-crystal dating in sedimentary basins. Low-temperature thermochronometers can be sensitive to variations in hinterland erosion of as little as 1-2 km. Although recognized previously from a theoretical viewpoint, the impact exerted on modern detrital ages by the interplay between erosion rates and lithology within tributary catchments has only recently been documented and provides a basis for refining orogenic histories using detrital ages. Documentation of the downstream evolution of detrital ages emphasizes that the distribution of ages that reaches the mouth of a river may bear little resemblance to age distributions in the headwaters. Similarly, because lithological concentrations of minerals used for singlecrystal dating can vary by many fold within the hinterland, rapidly eroding tributary catchments do not necessarily dominate populations of detrital ages. An ability to exploit detrital ages to place limits on kinematic rates within collisional orogens as a function of cooling rates provides a potent new analytical tool. If uncertainties regarding kinematic geometries, related particle pathways through orogens and steady-state assumptions can be reduced, detrital ages in both modern rivers and the recent stratigraphical record can serve to reconstruct rates of deformation and erosion and to test the viability of proposed models of orogenic evolution.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 AbstractUsing a custom-built, implantable pulse generator, we studied the effects of small pulsed currents on the viability on rat aortic-derived cells (RAOC) in vitro. The pulsed currents (0.37 A/m 2 ) underwent apoptosis within 24 h as shown by the positive staining for cleaved caspase-3 and classically apoptotic morphology. Based on these findings, we examined the effects of nanocurrents in vivo. The pulse generator was implanted subcutaneously in the rat model. The electrode|tissue interface histology revealed no difference between the active platinum surface and the neighboring control surface, however we found a large difference between electrodes that were functional during the entire experiment and non-active electrodes. These non-active electrodes showed an increase in impedance at higher frequencies 21 days post-implantation, whereas working electrodes retained their impedance value for the entire experiment. These results indicate that applied currents can reduce the impedance of implanted electrodes.