Douglas W. Burbank

Late Cenozoic tectonic evolution of the northwestern Tien Shan: New age estimates for the initiation of mountain building

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 (;0.4 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 ,0.3 km/ m.y., while sedimentation rates decelerated initially to ;0.25 km/m.y. before accelerating to ;0.4 km/m.y. at 4‐5 Ma. Detrital fission-track results indicate that by 4.5 Ma,

Middle-late Miocene (>10 Ma) formation of the Main Boundary thrust in the western Himalaya

Three independent data sets from northwestern India and Pakistan suggest initial displacement along >1000 km of the Main Boundary thrust prior to 10 Ma, at least 5 m.y. earlier than previously reported. Regionally extensive changes in the depositional characteristics and rates of the foreland-basinfill between 11 and 9.5 Ma are interpreted to reflect new hinterland loading due to the formation of the Main Boundary thrust. Sediment-accumulation rates, sandstone-siltstone ratios, and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 Ma in well-dated stratigraphic sections from Pakistan to Nepal across the Indo-Gangetic foreland basin. In the Himachal Pradesh reentrant of northwestern India, a newly discovered 8.7 Ma conglomerate derived from the hanging wall of the Main Boundary thrust indicates that source-area uplift and denudation must have occurred prior to 9 Ma and probably prior to 10 Ma, assuming a gravelprogradationrateof3cm/yr.Threeapatitefission-trackages fromstructuresattheleadingedgeoftheMainBoundarythrustin the Kohat region of northwest Pakistan indicate that rapid cooling below ;105 C1‐2 m.y. earlier. These data indicate that the Main Boundary thrust in the western Himalaya formed synchronously along strike in the middle-late Miocene, has a displacement rate of ;10 mm/yr, and has a displacement history that is coeval with late displacement on the Main Central thrust.

A study of the 1999 monsoon rainfall in a mountainous region in central Nepal using TRMM products and rain gauge observations

Raingauge data from the 1999 monsoon were compared with precipitation derived from the precipitation radar (PR) and the microwave imager instruments on board the Tropical Rainfall Measuring Mission (TRMM) satellite. The raingauges are part of a new hydrometeorological network installed in the Marsyandi river basin, which extends from the edge of the Tibetan Plateau to the Gangetic basin. TRMM-derived precipitation showed better detection of rain at low altitude stations as compared with high elevation stations, with good scores for the PR product for rain rates >0.5 mm/hr. The 3D PR rain rates suggest strong interaction between mesoscale convective systems and steep terrain at elevations of 1–2 km, which is consistent with the very high rainfall measured at those locations. Analysis of the raingauge data shows that even at altitudes as high as 4,000 m the cumulative monsoon rainfall is comparable to the highest amount recorded in the Indian subcontinent.

Building the Northern Tien Shan: Integrated Thermal, Structural, and Topographic Constraints

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 (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.

A 900 k.y. record of strath terrace formation during glacial- interglacial transitions in northwest China

The timing of the development of strath terraces with respect to climatic variability remains equivocal. Previous studies attribute strath-terrace formation to glacial or interglacial climates or to variations in sediment and water fluxes that cause lateral erosion followed by vertical incision. A chronology of strath-terrace formation spanning similar to900 k.y. has been generated on the basis of loess-paleosol couplets and paleomagnetic, thermoluminescence, and radiocarbon dating of strath terraces in the Qilian Shan of northeastern Tibet. Repetitive stratigraphic and geomorphic patterns on each terrace indicate that they formed during glacial-interglacial transitions. Long-term bedrock incision rates and inferred rock uplift rates appear steady and unrelated to strath formation over the past 900 k.y.

Rates of erosion and their implications for exhumation

Abstract At time-scales of 102 to 105 years, erosion by rivers, landslides and glaciers can exceed 5 mm/y. Sustained denudation at these rates is sufficient to account for many of the rapid rates of unloading or cooling that are revealed by geobarometric or thermochronologic studies. Because feedbacks exist among many surface processes, determinations of erosion rates on a few geomorphic processes can be adequate to estimate mean rates across an entire landscape. Rates of fluvial and glacial incision exert a dominant control on landscape lowering, because these rates set the local base level and modulate the flux from adjacent hillslopes. When rates of deformation are sufficiently rapid and sustained, a collisional orogen approaches a dynamic equilibrium or topographic steady state. Due to variations in erosion rates as a function of Late Cenozoic climate changes, such a steady state should be defined at time-scales longer than one climate cycle. During dynamic equilibrium, the geomorphic system displays a predictable configuration of interacting rivers, hillslopes and glaciated process zones. A dynamic equilibrium appears to prevail near Nanga Parbat, Pakistan, and the Southern Alps of New Zealand, where spatial variations in geomorphic erosion rates mimic variations in bedrock cooling rates at time-scales of 106 y.

Magnetochronology of the Upper Cenozoic strata in the Southwestern Chinese Tian Shan: rates of Pleistocene folding and thrusting

Abstract The southwestern Chinese Tian Shan of Central Asia is an actively deforming part of the Indian–Asian collision system. Paleomagnetic investigations of two Plio–Pleistocene terrestrial successions provide the first detailed magnetostratigraphy for the upper Cenozoic strata in this region. Paleomagnetic samples were collected from 358 sites within the Atushi Formation and Xiyu Formation across the Atushi–Talanghe anticline near Atushi. Thermal demagnetization behavior, reversal and fold tests of paleomagnetic stability indicate that the characteristic remanence directions were acquired before tilting and folding of the strata. A composite magnetostratigraphic section for this sequence correlates with the Mammoth subchron to the Jaramillo subchron, between 3.3 and 1.07 Myr, of the geomagnetic polarity timescale of Cande and Kent [J. Geophys. Res. 100 (1995) 6093–6095]. The mean declination (349±3°) in the Boguzihe section indicates a counterclockwise vertical-axis rotation (−11±2°) during the past 1.4 Myr. The conglomeratic Xiyu Formation is time-transgressive along its progradational contact with the underlying Atushi Formation across a map distance of ∼6 km; the basal contact ranges from less than 1.0 Myr in the Ganhangou section to 1.9 Myr in the Boguzihe section to the southwest, and to ∼2.8 Myr on the northwest limb of the anticline. Northeastward lateral propagation and growth of the Atushi–Talanghe anticline initiated at ∼1.4 Myr in the Boguzihe, and ∼1.2 Myr in the Ganhangou. The cross section in the Boguzihe provides a conservative (maximum) estimate of shortening rate of ∼3.3 (4.4) mm/yr and uplift rate of ∼3 mm/yr. This rate represents about half of the geodetically determined shortening rate between the northern Tarim Basin and the Kyrgyz Tian Shan [Wang et al., Acta Seismol. Sin. 22 (2000) 263–270; Reiger et al., Earth Planet. Sci. Lett. 191 (2001) 157–165]. During fold growth over the past 1.4 Myr, the crest of the Atushi–Talanghe anticline eroded at an average rate of 2.6–2.7 mm/yr.

Signatures of mountain building: Detrital zircon U/Pb ages from northeastern Tibet

Although detrital zircon has proven to be a powerful tool for determining provenance, past work has focused primarily on delimiting regional source terranes. Here we explore the limits of spatial resolution and stratigraphic sensitivity of detrital zircon in ascertaining provenance, and we demonstrate its ability to detect source changes for terranes separated by only a few tens of kilometers. For such an analysis to succeed for a given mountain, discrete intrarange source terranes must have unique U/Pb zircon age signatures and sediments eroded from the range must have well-defi ned depositional ages. Here we use ~1400 single-grain U/Pb zircon ages from northeastern Tibet to identify and analyze an area that satisfi es these conditions. This analysis shows that the edges of intermontane basins are stratigraphically sensitive to discrete, punctuated changes in local source terranes. By tracking eroding rock units chronologically through the stratigraphic record, this sensitivity permits the detection of the differential rock uplift and progressive erosion that began ca. 8 Ma in the Laji Shan, a 10‐25-km-wide range in northeastern Tibet with a unique U/Pb age signature.

Impulsive alluviation during early Holocene strengthened monsoons, central Nepal Himalaya

The steep-walled bedrock gorges of the Greater Himalayan rivers currently lack significant stored sediment, suggesting that fluvial erosion and transport capacity outpace the supply of sediment from adjacent hillsides. Despite this appearance of sustained downcutting, such rivers can become choked with sediments and aggrade during intervals of higher precipitation. Cosmogenic dating ( 10 Be and 26 Al) of fluvially carved bedrock surfaces indicates that sediment at least 80 m thick filled the Marsyandi River valley in central Nepal during a time of strengthened early Holocene monsoons. Despite threefold differences in height (43‐124 m) above the modern river, these fluvial surfaces display strikingly similar cosmogenic exposure ages clustering around 7 6 1 ka. We speculate that enhanced monsoonal precipitation increased pore pressure and the frequency of landsliding, thereby generating a pulse of hillslope-derived sediment that temporarily overwhelmed this alpine fluvial system’s transport capacity. After the easily liberated material was exhausted ca. 7 ka, the hillslope flux dropped, and the river incised through the aggraded alluvium. It concurrently eroded adjacent rock walls, thereby removing previously accumulated 10 Be and 26 Al and resetting the cosmogenic clock in the bedrock. Unlike previous studies, these exposure ages cannot be used to derive river-incision rates; instead they record a coupled fluvial-hillslope response to climate change.

Middle Miocene reorganization of deformation along the northeastern Tibetan Plateau

Temporal variations in the orientation of Cenozoic range growth in northeastern Tibet define two modes by which India-Asia convergence was accommodated. Thermochronological age-elevation transects from the hanging walls of two major thrust-fault systems reveal diachronous Miocene exhumation of the Laji-Jishi Shan in northeastern Tibet. Whereas accelerated growth of the WNW-trending eastern Laji Shan began ca. 22 Ma, rapid growth of the adjacent, north-trending Jishi Shan did not commence until ca. 13 Ma. This change in thrust-fault orientation refl ects a Middle Miocene change in the kinematic style of plateau growth, from long-standing NNE-SSW contraction that mimicked the plate convergence direction to the inclusion of new structures accommodating east-west motion. This kinematic shift in northeastern Tibet coincides with expansion of the plateau margin in southeastern Tibet, the onset of normal faulting in central Tibet, and accelerated shortening in northern Tibet. Together these phenomena suggest a plateau-wide reorganization of deformation.