BUILDING THE NORTHERN TIEN SHAN: INTEGRATED THERMAL, STRUCTURAL, AND TOPOGRAPHIC CONSTRAINTS

Abstract

Paired 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 (11–10 Ma) and late (3–0 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 geomor-phological estimates of conditions before deformation, we reconstruct a four-stage history of range growth and ex-humation. 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.