Edward R. Sobel

Thrusting and exhumation around the margins of the western Tarim basin during the India‐Asia collision

The Cenozoic collision between India and Asia has deformed a large part of central Asia. To the north of Tibet around the margins of the western Tarim basin, major basin-vergent thrusting has uplifted and exhumed thick Jurassic to Neogene sedimentary sections; this presumably reflects the propagation of collision-induced deformation into the basin. Apatite fission track data from five sections involved in this thrusting record strong late Oligocene to middle Miocene exhumation and cooling. On the northwest margin of the basin on the piedmont of the Tian Shan, a section exhumed by thrusting yields an exhumation age of 13.6±2.2 Ma (±1σ). Four Miocene sandstones from a second section 40 km to the east yield detrital source area cooling ages which decrease upsection from 25.0±3.9 to 13.1±2.2 Ma. Landsat imagery suggests that probable sediment source areas were dominated by Neogene thrusting, so these ages likely record progressive unroofing in Tian Shan thrust systems. Deformed Miocene to Pleistocene strata indicate that thrusting has continued and propagated basinward up until the present. Previously published apatite data from the Junggar basin on the northern flank of the Tian Shan yield a similar age of 24.7±3.9 Ma. On the southwest margin of Tarim on the piedmont of the western Kunlun Shan, three sections yield cooling ages of 19.8±0.9 Ma, 20.0±3.1 Ma, and roughly 20 Ma. Farther south at Kudi, previous work has yielded apatite cooling ages of 17±2 Ma and a zircon cooling age of 22±2 Ma. These similar cooling ages over a ≈250 km long belt in the western Kunlun Shan are associated with the transpressional Kumtag fault and the Main Pamir Thrust (MPT). Geologic relations within the western Kunlun Shan suggest that the MPT-Kumtag fault system offsets the originally linear trend of the Paleozoic-early Mesozoic Kunlun arc system by 200–300 km, accommodating much of the Neogene northward indentation of the Pamir block. We propose that the ≈20 Ma ages slightly postdate the initiation of this indentation and consequent crustal thickening. Taken together, the Tian Shan and Kunlun Shan results indicate that crustal thickening, in part accommodated by strike-slip faulting, became the dominant mode of deformation by ≈25–20 Ma in a large region extending from the Pamir and west Kunlun Shan north to the Tian Shan.

Uplift, exhumation, and deformation in the Chinese Tian Shan

The terranes composing the basement of the Tian Shan were originally sutured together during two collisions in Late Devonian–Early Carboniferous and Late Carboniferous–Early Permian time. Since then, the range has repeatedly been uplifted and structurally reactivated, apparently as a result of the collision of island arcs and continental blocks with the southern margin of Asia far to the south of the range. Evidence for these deformational episodes is recorded in the sedimentary histories of the Junggar and Tarim foreland basins to the north and south of the range and by the cooling and exhumation histories of rocks in the interior of the range. Reconnaissance apatite fission-track cooling ages from the Chinese part of the range cluster in three general time periods, latest Paleozoic, late Mesozoic, and late Cenozoic. Latest Paleozoic cooling is recorded at Aksu (east of Kalpin) on the southern flank of the range, at two areas in the central Tian Shan block along the Dushanzi-Kuqa Highway, and by detrital apatites at Kuqa that retain fission-track ages of their sediment source areas. Available Ar/Ar cooling ages from the range also cluster within this time interval, with very few younger ages. These cooling ages may record exhumation and deformation caused by the second basement suturing collision between the Tarim–central Tian Shan composite block and the north Tian Shan. Apatite data from three areas record late Mesozoic cooling, at Kuqa on the southern flank of the range and at two areas in the central Tian Shan block. Sedimentary sections in the Junggar and Tarim foreland basins contain major unconformities, thick intervals of alluvial conglomerate, and increased subsidence rates between about 140 and 100 Ma. These data may reflect deformation and uplift induced by collision of the Lhasa block with the southern margin of Asia in latest Jurassic–Early Cretaceous time. Large Jurassic intermontane basins are preserved within the interior of the Tian Shan and in conjunction with the fission-track data suggest that the late Mesozoic Tian Shan was subdivided into a complex of generally east-west–trending, structurally controlled subranges and basins. Apatite data from five areas record major late Cenozoic cooling, at sites in the basin-vergent thrust belts on the northern and southern margins of the range, and along the north Tian Shan fault system in the interior of the range. The thrust belts *Now at ExxonMobile Exploration Company, P.O. Box 4778, Houston, Texas 77060, USA Dumitru, T.A., et al., 2001, Uplift, exhumation, and deformation in the Chinese Tian Shan, in Hendrix, M.S., and Davis, G.A., eds., Paleozoic and Mesozoic tectonic evolution of central Asia: From continental assembly to intracontinental deformation: Boulder, Colorado, Geological Society of America Memoir 194, p. 71–99. 72 T.A. Dumitru et al.

A possible middle Paleozoic suture in the Altyn Tagh, NW China

Preliminary 40Ar/39Ar thermochronologic results from two transects across the Altyn Tagh (AT) range allow new interpretations for the genesis of the range and provide constraints for its subsequent displacement. Recent reports of Middle to Upper Ordovician pillow lavas in the NE portion of the range as well as the first reports of high-pressure metamorphic rocks within the range are interpreted as the remnants of a lower Paleozoic ocean subducted within the AT. Muscovite and biotite ages from schist, gneiss, and granodiorite document a significant metamorphic and intrusive event throughout the range at 435±20 Ma. This fabric is crosscut by undeformed, postorogenic microgranite plutons dated at 383±7 Ma by muscovite. These data suggest that an ocean basin closed after the Early Silurian and prior to the Middle Devonian, forming the herein named Lapeiquan suture. The lack of similary aged plutons to the north and the suggestion of a synchronous arc to the south implies that subduction was south dipping (present geography). If the Altyn Tagh contains a middle Paleozoic suture, lateral continuations should be identifiable. Possibilities to the west include the Kudi suture in the western Kunlun Shan. Comparisons of lithologies and radiometric data reveal significant similarities between the Kudi suture and the Altyn Tagh, suggesting that the AT originated as part of the same middle Paleozoic orogenic belt. Reconstruction of the Paleozoic structure of northern Tibet is complicated by Cenozoic deformation, particularly along the Altyn Tagh fault. Although this could be used as a measure of the finite post-Devonian deformation, a more thorough understanding of Paleozoic geodynamics is a prerequisite to unraveling subsequent displacements.

From sea level to high elevation in 15 million years: Uplift history of the northern Tibetan Plateau margin in the Altun Shan

Approximately 1300 m of Oligocene-Miocene clastic strata are exposed along the Miran River in the southeastern Tarim basin, where the adjacent Altun Shan form the topographic escarpment of the northern Tibetan Plateau. The sedimentary section is faulted against Proterozoic rocks of the Altun Shan in the footwall of the south-dipping, oblique reverse Northern Altyn Tagh fault. Oligocene-Lower Miocene strata consist of fine-grained rocks that record low-gradient depositional systems. Mid-Miocene and younger rocks consist of coarse conglomerate, derived from the Altun Shan and deposited by high-gradient depositional systems. The change to coarse, high-gradient depositional systems with detrital source areas coincident with the modern Miran River drainage is interpreted to mark the onset of uplift of the Altun Shan on the Northern Altyn Tagh fault and its erosional exhumation. The age of the change from pre-orogenic to synorogenic sedimentation is constrained by a foraminifera assemblage at the base of the conglomeratic section that includes Early-Middle Miocene planktonic foraminifera. This interpretation is also supported by apatite fission track and (U-Th)/He ages and thermal models that indicate rapid Miocene cooling, and hence, rapid exhumation of the Altun Shan. In addition to defining the age of the synorogenic section, the foraminifera assemblage contains planktonic taxa, indicating a connection to open marine waters, and benthic assemblages typical of brackish to near-sea level paleobathymetry. Thus, micropaleontologic evidence demonstrates that the Miran River locality, now at ∼1400 m elevation, was at sea level approximately 15 million years ago. Thus, in addition to constraining the age of surface uplift and exhumation of the Altun Shan, the principal mountain range of the Tibetan Plateau in this region, as ∼15 to 16 Ma, the foraminifera assemblage indicates that the SE Tarim basin, off the northern edge of the plateau, had an average surface uplift rate of nearly 100 m/m.y. for the past 15 million years. These results suggest that shortening in the Altun Shan and uplift of the range significantly post-dated the initiation of large-scale strike-slip on the Altyn Tagh fault, and that regional surface uplift mechanisms operated in the Tarim basin, beyond the margins of the Tibetan Plateau.

Basin analysis of the Jurassic–Lower Cretaceous southwest Tarim basin, northwest China

The Jurassic through Cretaceous southwest Tarim basin, northwest China, contains more than 6 km of fluvial and lacustrine strata deposited in a foreland setting during the successive collisions with Eurasia of the Changtang block during Late Triassic–Early Jurassic time and with the mega-Lhasa block during Late Jurassic–Early Cretaceous time. This tectonism is chronologically linked with the creation of a narrow lower Middle Jurassic transtensional basin with thick sedimentary infill, succeeded by a broader Upper Jurassic–Lower Cretaceous compressional(?) basin with thinner sedimentary infill. The older basin formed between a north-northwest–striking dextral fault on the eastern side of the southwest Tarim basin in the Tian Shan and a postulated strike-slip or normal fault on the western margin of the basin along the Kunlun Shan. The former fault is now the Talas-Ferghana fault; the latter may be a predecessor to the Main Pamir thrust. Subsidence analysis of the thickest sedimentary section suggests thermal subsidence, interpreted as the result of transtension between the two basin-bounding faults. The younger basin extends farther east and west and does not preserve evidence of activity along the Talas-Ferghana fault. The change in basin style between these two episodes of basin development likely reflects either a small counterclockwise rotation of basin-bounding structures during the first episode or a small clockwise rotation of the maximum compressive stress between the two episodes.

From tectonically to erosionally controlled development of the Himalayan orogen

Whether variations in the spatial distribution of erosion influ- ence the location, style, and magnitude of deformation within the Himalayan orogen is a matter of debate. We report new 40 Ar/ 39 Ar white mica and apatite fission-track (AFT) ages that measure the vertical component of exhumation rates along an ;120-km-wide NE-SW transect spanning the greater Sutlej region of northwest India. The 40 Ar/ 39 Ar data indicate that first the High Himalayan Crystalline units cooled below their closing temperature during the early to middle Miocene. Subsequently, Lesser Himalayan Crys- talline nappes cooled rapidly, indicating southward propagation of the orogen during late Miocene to Pliocene time. The AFT data, in contrast, imply synchronous exhumation of a NE-SW-oriented ;80 3 40 km region spanning both crystalline nappes during the Pliocene-Quaternary. The locus of pronounced exhumation de- fined by the AFT data correlates with a region of high precipita- tion, discharge, and sediment flux rates during the Holocene. This correlation suggests that although tectonic processes exerted the dominant control on the denudation pattern before and until the middle Miocene; erosion may have been the most important factor since the Pliocene.

Climatic forcing of asymmetric orogenic evolution in the Eastern Cordillera of Colombia

New apatite fission-track data, paleoelevation estimates from paleobotany, and recently acquired geological data from the Eastern Cordillera of Colombia document the onset of increased exhumation rates in the northeastern Andes at ca. 3 Ma. The Eastern Cordillera forms an efficient orographic barrier that intercepts moisture-laden winds sourced in the Amazon lowlands, leading to high rainfall and erosion gradients across the eastern flank of the range. In contrast, the drier leeward western flank is characterized by lower rates of deformation and exhumation. In light of the geological evolution of the Eastern Cordillera, the combination of these data sets suggests that the orographic barrier reached a critical elevation between ca. 6 and ca. 3 Ma, which ultimately led to protracted, yet more focused erosion along the eastern flank. Sequentially restored structural cross sections across the eastern flank of the Eastern Cordillera indicate that shortening rates also have increased during the past 3 Ma. From fission-track and structural cross-section balancing, we infer that accelerated exhumation led to increasing tectonic rates on the eastern flank, creating a pronounced topographic and structural asymmetry in the Eastern Cordillera. The tectonic and climatic evolution of this orogen thus makes it a prime example of the importance of climatic forcing on tectonic processes.

Oligocene range uplift and development of plateau morphology in the southern central Andes

[1] The Puna-Altiplano plateau in South America is a high-elevation, low internal relief landform that is characterized by internal drainage and hyperaridity. Thermochronologic and sedimentologic observations from the Sierra de Calalaste region in the southwestern Puna plateau, Argentina, place new constraints on early plateau evolution by resolving the timing of uplift of mountain ranges that bound present-day basins and the filling pattern of these basins during late Eocene-Miocene time. Paleocurrent indicators, sedimentary provenance analyses, and apatite fission track thermochronology indicate that the original paleodrainage setting was disrupted by exhumation and uplift of the Sierra de Calalaste range between 24 and 29 Ma. This event was responsible for basin reorganization and the disruption of the regional fluvial system that has ultimately led to the formation of internal drainage conditions, which, in the Salar de Antofalla, were established not later than late Miocene. Upper Eocene-Oligocene sedimentary rocks flanking the range contain features that suggest an arid environment existed prior to and during its uplift. Provenance data indicate a common similar source located to the west for both the southern Puna and the Altiplano of Bolivia during the late EoceneOligocene with sporadic local sources. This suggests the existence of an extensive, longitudinally oriented foreland basin along the central Andes during this time. Citation: Carrapa, B., D. Adelmann, G. E. Hilley, E. Mortimer, E. R. Sobel, and M. R. Strecker (2005), Oligocene range uplift and development of plateau morphology in the southern central Andes, Tectonics, 24, TC4011, doi:10.1029/ 2004TC001762.

Dome formation and extension in the Tethyan Himalaya, Leo Pargil, northwest India

Metamorphic dome complexes occur within the internal structures of the northern Himalaya and southern Tibet. Their origin, deformation, and fault displacement patterns are poorly constrained. We report new fi eld mapping, structural data, and cooling ages from the western fl ank of the Leo Pargil dome in the northwestern Himalaya in an attempt to characterize its post‐middle Miocene structural development. The western fl ank of the dome is characterized by shallow, west-dipping pervasive foliation and WNW-ESE mineral lineation. Shear-sense indicators demonstrate that it is affected by east-west normal faulting that facilitated exhumation of highgrade metamorphic rocks in a contractional setting. Sustained top-to-northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation. Garnet and kyanite indicate that the Leo Pargil dome was exhumed from the mid-crust. 40 Ar/ 39 Ar mica and apatite fi ssion track (AFT) ages constrain cooling and exhumation pathways from 350 to 60 °C and suggest that the dome cooled in three stages since the middle Miocene. 40 Ar/ 39 Ar white mica ages of 16‐14 Ma suggest a fi rst phase of rapid cooling and provide minimum estimates for the onset of dome exhumation. AFT ages between 10 and 8 Ma suggest that ductile fault displacement had ceased by then, and AFT track-length data from high-elevation samples indicate that the rate of cooling had decreased signifi cantly. We interpret this to indicate decreased fault displacement along the Leo Pargil shear zone and possibly a transition to the Kaurik-Chango normal fault system between 10 and 6 Ma. AFT ages from lower elevations indicate accelerated cooling since the Pliocene that cannot be related to pure fault displacement, and therefore may refl ect more pronounced regionally distributed and erosion-driven exhumation.

Cretaceous–Paleogene basaltic rocks of the Tuyon basin, NW China and the Kyrgyz Tian Shan: the trace of a small plume

Abstract The Tuyon basin, NW China, and the central portion of the Kyrgyz Tian Shan preserve a volumetrically small series of basaltic extrusive and intrusive units emplaced primarily in Mesozoic–Paleogene sedimentary rocks. Four units from the Tuyon basin yield 40 Ar / 39 Ar ages of 67, 59, 46 and between ∼110 and 120 Ma. Major and trace element analysis combined with Sr, Nd and Pb isotopic data suggest that these units were derived from apparently ocean island basalts (OIB)-like mantle melts, which have characteristics close to asthenospheric PREMA, mixed with an enriched component in the continental mantle lithosphere or emplaced with some crustal contamination. Two basalts from Kyrgyzstan were also dated by 40 Ar / 39 Ar as ∼54 Ma; these results agree with previous K–Ar analysis which show that most Kyrgyz basalts were emplaced during the Eocene. Major element data from the Kyrgyz basalts ( Knauf et al., 1980 ; Fortuna et al., 1994 ) show strong similarities with the Tuyon data, suggesting that the more extensive Tuyon data can be taken as representative of the entire basaltic suite. There is little geological data to support either Late Cretaceous–Paleogene rifting or the presence of a large plume beneath the region. Instead, these magmas point to the presence of a small thermally anomalous region in the mantle beneath the central Tian Shan which might best be described as a small plume which probably rooted at a shallow level in the asthenosphere.