Guillaume Dupont-Nivet

Tibetan plateau aridification linked to global cooling at the Eocene-Oligocene transition

Continental aridification and the intensification of the monsoons in Asia are generally attributed to uplift of the Tibetan plateau and to the land–sea redistributions associated with the continental collision of India and Asia, whereas some studies suggest that past changes in Asian environments are mainly governed by global climate. The most dramatic climate event since the onset of the collision of India and Asia is the Eocene–Oligocene transition, an abrupt cooling step associated with the onset of glaciation in Antarctica 34 million years ago. However, the influence of this global event on Asian environments is poorly understood. Here we use magnetostratigraphy and cyclostratigraphy to show that aridification, which is indicated by the disappearance of playa lake deposits in the northeastern Tibetan plateau, occurred precisely at the time of the Eocene–Oligocene transition. Our findings suggest that this global transition is linked to significant aridification and cooling in continental Asia recorded by palaeontological and palaeoenvironmental changes, and thus support the idea that global cooling is associated with the Eocene–Oligocene transition. We show that, with sufficient age control on the sedimentary records, global climate can be distinguished from tectonism and recognized as a major contributor to continental Asian environments.

Greater India Basin hypothesis and a two-stage Cenozoic collision between India and Asia

Cenozoic convergence between the Indian and Asian plates produced the archetypical continental collision zone comprising the Himalaya mountain belt and the Tibetan Plateau. How and where India–Asia convergence was accommodated after collision at or before 52 Ma remains a long-standing controversy. Since 52 Ma, the two plates have converged up to 3,600 ± 35 km, yet the upper crustal shortening documented from the geological record of Asia and the Himalaya is up to approximately 2,350-km less. Here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic Indian lithosphere within the Himalaya between approximately 50 and 25 Ma. Paleomagnetic data show that this extended continental and oceanic “Greater India” promontory resulted from 2,675 ± 700 km of North–South extension between 120 and 70 Ma, accommodated between the Tibetan Himalaya and cratonic India. We suggest that the approximately 50 Ma “India”–Asia collision was a collision of a Tibetan-Himalayan microcontinent with Asia, followed by subduction of the largely oceanic Greater India Basin along a subduction zone at the location of the Greater Himalaya. The “hard” India–Asia collision with thicker and contiguous Indian continental lithosphere occurred around 25–20 Ma. This hard collision is coincident with far-field deformation in central Asia and rapid exhumation of Greater Himalaya crystalline rocks, and may be linked to intensification of the Asian monsoon system. This two-stage collision between India and Asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography.

Restoration of Cenozoic deformation in Asia and the size of Greater India

A long‐standing problem in the geological evolution of the India‐Asia collision zone is how and where convergence between India and Asia was accommodated since collision. Proposed collision ages vary from 65 to 35 Ma, although most data sets are consistent with collision being underway by 50 Ma. Plate reconstructions show that since 50 Ma ∼2400-3200 km (west to east) of India‐Asia convergence occurred, much more than the 450-900 km of documented Himalayan shortening. Current models therefore suggest that most post‐50 Ma convergence was accommodated north of the Indus‐Yarlung suture zone. We review kinematic data and construct an updated restoration of Cenozoic Asian deformation to test this assumption. We show that geologic studies have documented 600-750 km of N‐S Cenozoic shortening across, and north of, the Tibetan Plateau. The Pamir‐Hindu Kush region accommodated ∼1050 km of N‐S convergence. Geological evidence from Tibet is inconsistent with models that propose 750-1250 km of eastward extrusion of Indochina. Approximately 250 km of Indochinese extrusion from 30 to 20 Ma of Indochina suggested by SE Asia reconstructions can be reconciled by dextral transpression in eastern Tibet. We use our reconstruction to calculate the required size of Greater India as a function of collision age. Even with a 35 Ma collision age, the size of Greater India is 2-3 times larger than Himalayan shortening. For a 50 Ma collision, the size of Greater India from west to east is ∼1350-2600 km, consistent with robust paleomagnetic data from upper Cretaceous‐Paleocene Tethyan Himalayan strata. These estimates for the size of Greater India far exceed documented shortening in the Himalaya. We conclude that most of Greater India was consumed by subduction or underthrusting, without leaving a geological record that has been recognized at the surface.

A Female Homo erectus Pelvis from Gona, Ethiopia

Analyses of the KNM-WT 15000 Homo erectus juvenile male partial skeleton from Kenya concluded that this species had a tall thin body shape due to specialized locomotor and climatic adaptations. Moreover, it was concluded that H. erectus pelves were obstetrically restricted to birthing a small-brained altricial neonate. Here we describe a nearly complete early Pleistocene adult female H. erectus pelvis from the Busidima Formation of Gona, Afar, Ethiopia. This obstetrically capacious pelvis demonstrates that pelvic shape in H. erectus was evolving in response to increasing fetal brain size. This pelvis indicates that neither adaptations to tropical environments nor endurance running were primary selective factors in determining pelvis morphology in H. erectus during the early Pleistocene.

Tibetan uplift prior to the Eocene-Oligocene climate transition: Evidence from pollen analysis of the Xining Basin

Uplift of the Tibetan Plateau and the Himalayas since the onset of the Indo-Asia collision is held responsible for Asian aridifi cation and monsoon intensifi cation, but may also have gradually cooled global climate, leading to the 34 Ma Eocene-Oligocene transition. To unravel the interplay between Tibetan uplift and global climate, proxy records of Asian paleoenvironments constrained by accurate age models are needed for the Paleogene Period. Here we report the 38 Ma appearance of high-altitude vegetation recovered from palynological assemblages in precisely dated lacustrine sediments from the Xining Basin of the northeastern Tibetan Plateau region. This result confi rms previous evidence for important regional uplift in the central and northern Tibetan Plateau regions during the early stage of the Indo-Asia collision. This is consistent with the idea that the associated increase in rock weathering and erosion contributed to lowering of atmospheric CO 2 , leading to the Eocene-Oligocene transition.

Paleogene clockwise tectonic rotation of the Xining-Lanzhou region, northeastern Tibetan Plateau

[1] To help understand the deformational history of the northeastern Tibetan Plateau, paleomagnetic samples were collected from 177 sites and two magnetostratigraphic sections at 16 localities distributed among Upper Jurassic-Lower Cretaceous to Pliocene sedimentary and subordinate volcanic rocks within the Xining-Lanzhou region (34–37� N, 101–105� E). A total of 127 sites at 12 localities yielded primary magnetizations confirmed by fold, reversal, and conglomerate tests. Age control on sedimentary rocks is provided by regional synthesis of chronostratigraphic data and our own biostratigraphic and magnetostratigraphic analysis presented in the companion paper by Horton et al. [2004]. Analysis of paleomagnetic declination combined with results from previous studies yield a remarkably consistent trend of vertical axis tectonic rotations across the studied region. Whereas 19.0 ± 7.2� to 37.8 ± 10.6� clockwise rotations are recorded consistently in all paleomagnetic localities in Lower Cretaceous to Eocene rocks, all paleomagnetic localities in Oligocene to Pliocene rocks have recorded minor to insignificant rotations, indicating that the Xining-Lanzhou region has undergone a wholesale regional clockwise rotation during late Paleogene time. Consistent with regional chronostratigraphic and thermochronologic results, this late Paleogene tectonic rotation confirms that deformation reached regions of the northern Tibetan Plateau shortly after the initial collision of India with Asia. When compared to rotational paleomagnetic results from adjacent regions, several mechanisms can be proposed to explain the clockwise rotation. On the basis of consistency with geologic data we prefer a model involving clockwise rotation of the Xining-Lanzhou region through right-lateral shear, and associated shortening, related to northward indentation of the Qaidam basin. INDEX TERMS: 1525 Geomagnetism and Paleomagnetism: Paleomagnetism applied to tectonics (regional, global); 8102 Tectonophysics: Continental contractional orogenic belts; 9320 Information Related to Geographic Region: Asia; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: tectonics, paleomagnetism, Tibetan Plateau

Mesozoic‐Cenozoic evolution of the Xining‐Minhe and Dangchang basins, northeastern Tibetan Plateau: Magnetostratigraphic and biostratigraphic results

[1] Accurate stratigraphic ages are crucial to understanding the deformation history of the Tibetan Plateau prior to and during the Indo-Asian collision. Efforts to quantify MesozoicCenozoic ages are hindered by limited fossils and a paucity of volcanic horizons and regionally correlative strata. Magnetostratigraphic and biostratigraphic results from the Xining-Minhe-Longzhong basin complex and Dangchang basin provide an improved chronology of nonmarine basin development over a large region of the northeastern Tibetan Plateau (34–37� N, 101–105� E). Analyses of 171 magnetostratigraphic levels and 24 palynological assemblages (>120 species) indicate Late Jurassic-Early Cretaceous to mid-Tertiary deposition. Although magnetic polarity zonation is incomplete, independent palynological age control partially restricts possible correlations to the Geomagnetic Polarity Timescale. The sediment accumulation record, basin provenance, structural geology, and published thermochronological data support a history of Jurassic exhumation, Late Jurassic-Early Cretaceous fault-related basin initiation, and Cretaceous-Paleogene reduced accumulation. These patterns, which are compatible with Late Jurassic-Early Cretaceous extension and Cretaceous-Paleogene postrift thermal subsidence, were disrupted at about 40–30 Ma, when shortening related to the Indo-Asian collision induced localized range uplift, vertical axis rotation, and amplified subsidence. INDEXTERMS: 1520 Geomagnetism and Paleomagnetism: Magnetostratigraphy; 8102 Tectonophysics: Continental contractional orogenic belts; 9320 Information Related to Geographic Region: Asia; 9604 Information Related to Geologic Time: Cenozoic; 9609 Information Related to Geologic Time: Mesozoic; KEYWORDS: tectonics, magnetostratigraphy, Tibetan Plateau, Cenozoic, Mesozoic, sedimentary basins

Paleomagnetism indicates no Neogene rotation of the Qaidam Basin in northern Tibet during Indo-Asian collision

Paleomagnetic data were obtained from Tertiary red sedimentary rocks at two locations separated by several hundred kilometers within the Qaidam Basin. In the east-central part of the basin, 30 sites from the lower Pliocene Youshashan Formation yielded characteristic remanent magnetization (ChRM) directions with intermediate unblocking temperatures (100‐600 8C); ChRM with high unblocking temperatures (to 680 8C) was isolated from 14 sites. In the same area, ChRM directions were obtained from six sites within the Oligocene Lower Gancaigou Formation. Characteristic magnetization was also determined from 16 sites within the Lower Gancaigou Formation exposed in the E Bo Liang range of the north-central Qaidam Basin. When compared with equivalentage expected directions for Eurasia, the mean paleomagnetic directions indicate no Neogene vertical-axis rotation of the Qaidam Basin or the Altyn Tagh fault. The Qaidam Basin may act as an indentor translating without rotation toward the North China block and driving clockwise vertical-axis rotations by differential shortening within the Nan Shan fold-and-thrust belt.

Discordant paleomagnetic direction in Miocene rocks from the central Tarim Basin: evidence for local deformation and inclination shallowing

From exposures at the southeastern end of the Maza Tagh range in the central Tarim Basin (latitude: 38.5‡N; longitude: 80.5‡E), 55 paleomagnetic sites were collected from red mudstones and sandstones of the Miocene Wuqia Formation. Thermal demagnetization revealed a high unblocking temperature characteristic remanent magnetization (ChRM). Five sites collected across a kink fold yield a positive fold test at 99% confidence level. The mean directions computed from normal and reversed polarity sites are antipodal suggesting a primary origin for the ChRM. In stratigraphic coordinates, the final set of 30 site-mean ChRM directions yields a section-mean direction: inclination (I) = 29.4‡; declination (D) = 24.7‡; K95 = 6.2‡. When compared to the Miocene expected direction (at 20 Ma), the observed direction indicates 30.8 < 5.5‡ flattening of inclination and 15.3 < 6.7‡ clockwise vertical-axis rotation. Anisotropy of magnetic susceptibility measurements on 155 samples show a strong foliation of 1.092 with a subvertical minimum susceptibility axis. These observations indicate a rock-magnetic (depositional or compaction shallowed) origin for the inclination flattening. The clockwise deflection of the observed declination can be interpreted as either: (1) 15.3 < 6.7‡ clockwise rotation of the entire Tarim Basin since the Miocene; or (2) a local km-scale structural deformation. It is not a simple matter to discard the interpretation of 15.3 < 6.7‡ clockwise rotation of the Tarim Basin because the fastest rates of rotation determined from global positioning system and slip-rate studies of Quaternary faults could produce such a rotation if extrapolated to 20 Ma. Nevertheless, we argue that local deformation is the preferred interpretation because the map pattern of local structures shows V20‡ clockwise deflection toward the southeastern end of the Maza Tagh range where the paleomagnetic samples were collected. ? 2002 Elsevier Science B.V. All rights reserved.

Asian aridification linked to the first step of the Eocene-Oligocene climate Transition (EOT) in obliquity-dominated terrestrial records (Xining Basin, China)

Asian terrestrial records of the Eocene-Oligocene Transition (EOT) are rare and, when available, often poorly constrained in time, even though they are crucial in understanding the atmospheric impact of this major step in Cenozoic climate deterioration. Here, we present a detailed cyclostratigraphic study of the continuous continental EOT succession deposited between ∼35 to 33 Ma in the Xining Basin at the northeastern edge of Tibetan Plateau. Lithology supplemented with high-resolution magnetic susceptibility (MS), median grain size (MGS) and color reflectance (a) records reveal a prominent ∼3.4 m thick basic cyclicity of alternating playa gypsum and dry mudflat red mudstones of latest Eocene age. The magnetostratigraphic age model indicates that this cyclicity was most likely forced by the 41kyr obliquity cycle driving oscillations of drier and wetter conditions in Asian interior climate from at least 1 million year before the EOT. In addition, our results suggest a duration of∼0.9 Myr for magnetochron C13r that is in accordance with radiometric dates from continental successions in Wyoming, USA, albeit somewhat shorter than in current time scales. Detailed comparison of the EOT interval in the Tashan section with marine records suggest that the most pronounced lithofacies change in the Xining Basin corresponds to the first of two widely recognized steps in oxygen isotopes across the EOT. This first step precedes the major and second step (i.e. the base of Oi-1) and has recently been reported to be mainly related to atmospheric cooling rather than ice volCorrespondence to: H. A. Abels (abels@geo.uu.nl) ume growth. Coincidence with lithofacies changes in our Chinese record would suggest that the atmospheric impact of the first step was of global significance, while the major ice volume increase of the second step did not significantly affect Asian interior climate.