T31E-01 INVITED
WAS THE PALAEO-TETHYS RESPONSIBLE FOR ENDING THE PALAEOZOIC?
When Pangaea was assembled during the course of the late Palaeozoic, a triangular oceanic space was left between Laurasia and Gondwana-Land. This space has been called the Palaeo-Tethys and long believed to have always been a giant gulf of the Panthalassa. However, stratigraphical/structural, palaeontological and palaeomagnetic data clearly show that the Palaeo-Tethys was separated by a land bridge, called the Cathaysian Bridge, from the Panthalassa. By later medial to late Permian time no deep-water connexion had remained between the two oceans and the Palaeo-Tethys acquired the character of an inland sea of Pangaea. This restriction, its location astride the Permian equator, the endorheic drainage of Pangaea towards it and the increasing aridity of the Permian world, led to the development of anoxic conditions in the Palaeo-Tethys. This anoxia occupied the abyssal areas by the Guadalupian, reached the lower shelves by the Wuchiapingian and occupied the entire Palaeo-Tethys by the end of the Changhsingian. This led to widespread decimation of the benthic organisms in these areas and in that order. Evidence of gas-release- caused submarine erosion surfaces, anomalous increase in chemical weathering onland and a 2000-km-wide halo of fungal spikes indicating the presence of abundant dead organic matter surrounding the Palaeo- Tethys suggest that it may have erupted toxic gases to kill the terrestrial organisms in its vicinity, including the airborn insects. This was the only time in the Phanerozoic that the insects experienced significant extinction. The Permian extinction was not global but was confined to the Palaeo-Tethys, its surroundings, and other marine areas it could pollute. Since these areas constituted some 90% by biodiversity of the niches in the Permian, decimation of their inhabitants created an impression of universal extinction. In boreal and austral areas (except around the Gulf of Malagasy penetrating deep into Gondwana-Land from the Palaeo-Tethys) life continued normally, where, for example, no chert gap developed. Most nectonic organisms (for example the fishes) escaped the great killing most likely by migrating to Panthalassa. The analysis of the Permian conditions indicate that regional geology must form the basis of the studies of the history of the biosphere and any hypothesis based on few stratigraphic sections or on few taxa is unlikely to yield a correct picture of life at any given time in the past of the earth.
T31E-02
The Mongol-Okhotsk Ocean: No Relationship to the Paleo-Tethys
The Mongol-Okhotsk Ocean (MOO) closed when the Amuria-North China Block (NCB) and the southern margin of Siberia collided in Late Jurassic to Early Cretaceous times. As such, the MOO could be considered as a branch of the Paleo-Tethys, given the position of the latter between NCB and Siberia in the reconstructions of the classical series of papers by Sengor et al. in 1984-1987. However, there are two enigmas that result from this idea. One is that the Mongol-Okhotsk suture must have terminated in Mongolia, because no evidence exist for any westward prolongation of the MOO near the Tarim block, which in the Mesozoic was already part of Eurasia. A connection between the MOO and the Paleo-Tethys is therefore unlikely. The other enigma is that the Mongol-Okhotsk suture runs east-west, whereas tomographic images of the subducted Mongol-Okhotsk slab in the deeper mantle appear to have a north-south strike. No sensible explanation can be constructed for a rotation of some 90 degrees of this slab. There is a solution, however, to these enigmas if we consider that the MOO did exist east of an initially north-south paleo-Pacific margin of continental fragments of Eurasia and was subducting westward during the Triassic-Early Jurassic. This would readily explain the tomographic slab orientation. As the Siberian and NCB approached each other and were overriding the MOO, they rotated towards each other in opposite senses (Siberia clockwise and NCB counterclockwise). The subduction-related volcanics above the subduction zone, following the outline around the core of the Tuva-Mongol Belt in the Altaids between NCB and Siberia, formed a tightening orocline. This large-scale oroclinal bending of the crust above a disappearing ocean is reminiscent of the similarly tightening orocline in Kazakhstan, which closed earlier, in the late Paleozoic, by subduction of the Junggar-Balkhash Ocean.
T31E-03 INVITED
Coupling of strain rate, orogen width and plate convergence speed in continental collisions
Most models that describe the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary. Yet a growing body of evidence in the Tibetan orogen suggests that deformation occurred early in the orogen's history at the far northern extent of the modern plateau and thus, our current mechanical understanding of orogenic plateau development is incomplete. Regardless of whether or not high topography was built simultaneously as a result of this deformation, early Cenozoic - present deformation in northern Tibet signifies that the modern limit of the orogen has been held fixed since continental collision began. Therefore strain has not significantly propagated further away from the plate boundary in time and the orogen width has narrowed as convergence continued since collision. Using a uniform average strain rate across the plateau derived from geodetic measurements, I predict changes in plate convergence rate through time as the orogen narrows. Observed decreases in plate convergence speed of India with respect to Eurasia through the Cenozoic (Molnar and Stock, in press) can be explained by the simple condition of a fixed orogen boundary and constant strain rate. This same relationship can also be derived for the Arabian-Eurasia collision however fewer geologic data and fewer plate velocity measurements are available to constrain such a model compared to Tibet.
T31E-04 INVITED
Closure of Neotethys and Mesotethys in Tibet
The modern phase of geological exploration in Tibet began with collaborative international expeditions in the 1980s. Plate tectonics was in its infancy at that time but the new concept was applied with alacrity to interpretation of rock associations across the plateau. Over the past decades our understanding of modern tectonic settings has advanced greatly. However in many examples, despite the existence of a wealth of new observations, geologists remain wedded to their earlier models. As was common at the time, occurrences of ophiolitic rocks were paradigmatically interpreted a fragments of ancient ocean floor and the areas where they are exposed were referred to as suture zones. We now know that many, if not most, preserved ophiolites were generated in supra-subduction zone settings and did not originate as classic mid-ocean ridge crust. Their preservation commonly involves collision with continental crust as it enters a subduction zone associated with an intra-oceanic island arc. Such collisions are typically ephemeral and difficult to recognize in an imperfect geological record. Not only is the genesis and preservation of ophiolites more complicated than previously envisaged, the nature of the entire subduction factory is complex. From DSDP, ODP and other oceanographic studies we know that ocean basins commonly have complex histories and are marked by mid-ocean ridges, oceanic plateaus and other features. Thus, the material entering a trench cannot be simply treated as a homogenous entity and the attempted subduction of various asperities is likely to greatly affect the development of convergent plate margins. Applying this knowledge to ancient systems such as the closure of Tethyan ocean basins across Tibet permits a more sophisticated analysis of their histories. Increased knowledge over the past decade allows us to recognize the probability that just like the modern-day Pacific Ocean basin, the extensive Neotethyan ocean basin was complex and likely contained intra-oceanic island arcs and other features. The expectation that such features once existed and collided at consuming plate boundaries where they were accreted or otherwise subducted leaving only a fragmentary record drives investigations of the subtleties of the geological record. The record of Neotethyan ocean closure along the Yarlung Tsangpo suture zone contrasts greatly with that for closure of Mesotethys along the Bangong-Nujiang suture zone indicating that the same model cannot be simply applied to both basins as the latter was a much more restricted ocean basin. However, we contend that by examining these two systems we can demonstrate that the development of the Mesotethyan ocean basin in this region may in fact be genetically related to partial consumption of the Neotethyan ocean basin.
T31E-05 INVITED
Equatorial Convergence of India and Early Cenozoic Climate Trends
India's northward flight and collision with Asia was a major driver of global tectonics in the Cenozoic and, we argue, of atmospheric CO2 concentration (pCO2) and thus global climate. Subduction of Tethyan oceanic crust with a carpet of carbonate-rich pelagic sediments deposited during transit beneath the high productivity equatorial belt resulted in a component flux of CO2 delivery to the atmosphere capable to maintain high pCO2 levels and warm climate conditions until the decarbonation factory shut down with the collision of Greater India with Asia at the Early Eocene climatic optimum at ~50 Ma. At about this time, the India continent and the highly weatherable Deccan Traps drifted into the equatorial humid belt where uptake of CO2 by efficient silicate weathering further perturbed the delicate equilibrium between CO2 input to and removal from the atmosphere towards progressively lower pCO2 levels thus marking the onset of a cooling trend over the Middle and Late Eocene that some suggest triggered the rapid expansion of Antarctic ice sheets at around the Eocene-Oligocene boundary.
T31E-06
Persistently shallow paleomagnetic inclinations in Asia: implications for the Indo-Asia collision
Defining the Cenozoic position of Asia is essential for the kinematics of the Indo-Asia collision. The latitude of
Asia, usually assumed stable during the collision, has been constrained using paleomagnetic datasets from
Europe rather than Asian datasets because the latter are affected by a recognized bias toward low
paleomagnetic inclination directly affecting paleolatitude estimates. This bias has been attributed mainly to
sedimentary processes producing 'inclination shallowing' during sedimentation and compaction. However, we
report here new paleomagnetic data from Cenozoic volcanic rocks of Mongolia, in principle devoid of
inclination shallowing, that are systematically ~5-15° lower than predicted from European poles.
This trend is confirmed from 50 to 20 Ma, when our results are combined with a review of existing Cenozoic
Asian paleomagnetic datasets including only volcanic rocks and sediments corrected for inclination
shallowing. In order to estimate continental shortening during the collision, this review also includes data from
the either side of the Indo-Asia suture zone. Two end-member hypothesis are investigated to resolve the low
Asian latitudes during that period: (1) a non dipolar field contribution or (2) lower paleolatitudes for Asia. The
former hypothesis would require a zonal octopular component between 4 and 12 percent. The latter
hypothesis would require the total Indo-Asia convergence to be lower than previously assumed and to have
occurred for the most part at high rates in the early stage of the collision (60-40 Ma) and, for a smaller part at
lower rates since ca. 40 Ma. According to estimates from seismic tomographic reconstructions, this change
would coincide with break-off of subducted Neotethyan lithosphere from the Indian plate.
http://www.geo.uu.nl/~forth/people/Guillaume
T31E-07
Tectonostratigraphy of the Lesser Himalaya of Bhutan: Deducing the Paleostratigraphy of the Northern Indian Margin
New mapping, U-Pb detrital zircon (DZ) dating and δ13C analyses, defines the stratigraphy of the Lesser Himalayan (LH) section in the eastern Bhutan Himalaya. The LH section contains 5 units with a combined thickness of up to 16 km. The Paleoproterozoic Daling-Shumar Group is divided into the lower Shumar Formation, which consists of 2-6 km of cliff-forming quartzite, and the upper Daling Formation, which consists of 3-4 km of schist and quartzite. The Daling-Shumar Group is intruded by ~1.9 Ga granitic gneiss bodies, and DZ samples yield youngest peaks centered at ~1.92 Ga, bracketing deposition age as Paleoproterozoic. The upper LH contains three Paleozoic units, the Baxa Group, Diuri Formation, and Gondwana sequence, which all contain Cambrian DZ peaks. The Baxa Group consists of 2-3 km of coarse- grained quartzite interbedded with phyllite and dolomite. The Baxa Group yields DZ peaks between ~1.7-.9 Ga, and a youngest peak of ~520 Ma, indicating a Cambrian maximum depositional age. Baxa dolomites yield δ13C values between +4 and +6‰, which make either a lower Cambrian or lower Mississippian deposition age the most likely. This suggests an unconformity representing between ~1.4-1.5 billion years between the Baxa and Daling-Shumar groups. The Baxa Group is overlain by the Diuri Formation, which consists of 2-3 km of diamictite. The Diuri Formation yields a prominent ~500 Ma DZ peak, and a youngest DZ peak at ~390 Ma, indicating a maximum age of Devonian. The youngest LH unit is the Gondwana sequence, which contains 1-2 km of sandstone, siltstone, shale, and coal, yields a ~500 Ma youngest DZ peak, and contains Permian fossils. The Permian Gondwana age brackets the upper depositional age of the Baxa Group and Diuri Formation as Permian. A maximum age of ~390 Ma and a minimum Permian age allows the Diuri Formation to be correlated with the Mississippian to Permian glaciation of Gondwana. The clear separation in age between the Paleoproterozoic and Paleozoic LH units is significant because 1) it reinforces suggestions that Paleoproterozoic LH stratigraphy is continuous along the entire east-west length of the orogen and 2) it highlights the along-strike variability of younger, regional- scale LH basins. Our data also indicate time-equivalent deposition of Paleozoic upper LH units in proximity to the Indian continent and the more distal Tethyan section, and show that deposition simultaneously overlapped both the GH and lower LH sections.
T31E-08 INVITED
Localization of Pliocene to present Arabia-Eurasia convergence in the Kura fold-thrust belt, Azerbaijan
In the central Arabia-Eurasia collision, closure of Neotethys has produced the WNW-trending Kura fold-thrust belt, which is separated from the Greater Caucasus to the N by the Alazani-Äyrichi intermontaine basin, and from the Lesser Caucasus to the S by the Kura basin. To test the hypothesis that over half of total Arabia-Eurasia convergence since ~5 Ma has been localized within the Kura fold-thrust belt, we are conducting 1:100,000-scale structural mapping and geochronologic and stratigraphic studies to establish both the magnitude and timing of deformation and their potential along-strike variation within the Kura fold- thrust belt. Although the Greater Caucasus range front is a prominent topographic break, our preliminary observations yielded no evidence of significant recent upper-crustal shortening across this boundary west of 48.25°E. In contrast, our 2008 structural mapping, together with prior work, attests to significant Pliocene to recent shortening i n the Kura fold-thrust belt, with the geometry and style of deformation varying systematically along strike. In the east (48.25°E), the belt is ~20 km wide across strike and comprises two main S-vergent anticlines, the Längäbiz in the north and Qaramäryäm in the south. The Qaramäryäm is a fault-propagation fold cored by middle Pleistocene(?) clastic sediments and is clearly active, as indicated by a folded geomorphic surface of probable late Pliestocene(?) age and both wind and water gaps. By contrast, in the west (47°E), the belt is ~50 km wide across strike and from N to S comprises at least four main structures: the Dashuz, Sarija, Khojashen, and Bozdagh anticlines. Although future work, including dating of interbedded volcanics found in 2008, is needed to constrain the ages of the folded sediments, prior biostratigraphic work indicates the folds deform lower Pliocene Productive series, upper Pliocene Akchagyl, lower Pleist ocene Apsheron, and younger Pleistocene(?) strata. Preliminary observations indicate consistent upward coarsening and thickening trends within sections exposed in each fold, with sections in the more northerly folds (Dashuz, Sarija) generally coarser-grained than those to the south (Khojashen, Bozdagh). All structures but Dasuz are asymmetric, south-vergent anticlines with moderately dipping (35-45°N) backlimbs in the north and steeply dipping (75°S) to locally overturned (75°N) forelimbs. Exposed structural depth decreases systematically from north to south. For example, in Dashuz, only the backlimb is preserved, juxtaposing Productive Series(?) in the hanging wall against Quaternary(?) units in the footwall. Likewise, in the core of the Sarija anticline, Akchagyl(?) deposits are thrust over Apsheron(?), but here the forelimb is locally preserved. In contrast, in the Khojashen and Bozdagh anticlines, the underlying faults are blind, with only folded Akchagyl(?) and Apsheron(?) strata exposed in the cores. The N to S decrease in exposed structural level at ~47°E suggests that the Kura fold-thrust belt has formed by systematic basinward propagation of the active deformation front. We envision that deformation is localized on a single, active fault-propagation fold at the thrust front, which grows rapidly until the basal decollment steps out into the foreland basin, terminating slip on the former deformation front and initiating a new active fold. The pattern of present-day deformation is consistent with this scenario, with active deformation apparently localized primarily along the southern margin of the Kura fold-thrust belt.