T33C-2061
The Late Eocene climatic transition from greenhouse to icehouse conditions in the Neo- Tethys
The middle Eocene to early Oligocene period represents the most important transition in Earth's climate: from greenhouse conditions to the icehouse conditions of the present day. This global transition was preceded by long-term cooling with superposed short-term variations in various marine proxies, which indicate instability in the paleoceanographic state prior to the key climatic transition. I integrate previous multidisciplinary studies with recent data and summarize interpretations from Massignano section (Eocene/Oligocene boundary Global Stratotype Section and Point , Umbria-Marche basin), the Contessa Highway section (Gubbio, proposed Lutetian/Bartonian boundary), and ODP 738B. Based on the many data sets from this sections, with an emphasis on the rock magnetic data, I propose that the fluctuations and final cessation of a westward sub-tropical Eocene Neo-Tethys (STENT) current was a driver in the climatic transition from greenhouse to icehouse conditions. I propose the STENT current hypothesis, which aim to explain that global variations in climatic conditions were synchronous with large variations in circulation in the Neo-Tethys Ocean. Tectonic closure of the gateway between the Arabian and Eurasian plates represents a threshold that caused late Eocene paleoceanographic variations in the Neo-Tethys and in global ocean circulation. The hypothesis is based on new paleomagnetic and environmental studies of Eocene sequences from the Tethys sector and Indian Ocean drill core, which provide important insights about the timing and nature of paleoclimatic events and sedimentary processes that affected the Eocene oceans.
T33C-2062 INVITED
On the Opening and Closure History of the Palaeo- and Neotethys
Gondwana was by far the largest tectonic entity in the Lower Palaeozoic, stretching from the South Pole to north of the Equator. South China was located close to Gondwana whilst e.g. North China, Tarim and Annamia were not attached to core Gondwana in the Lower Palaeozoic. Most of the area from the Taurides (Turkey) to at least East of India represented a passive margin for the whole of the Lower Palaeozoic, and Ordovician palaeomagnetic data from northern India now confidently place Tethyan Himalaya at the northern margin of cratonic India at this time. The Palaeotethys opened no earlier than the late Silurian when the Armorican Terrane Assemblage separated from Gondwana and by the early Carboniferous (c. 350 Ma), Palaeotethys had grown to more than 3000 km between NW Africa and southern France. This part of the Palaeotethys was subsequently closed at c. 320 Ma during the most important growth phase of Pangea when Laurussia, Gondwana and intervening terranes collided. Although some continental elements were still adjusting their positions along the Pangea perimeter. The Neotethys probably began opening at c. 265 Ma while Palaeotethyan oceanic crust was being subducted beneath Eurasia. North and South China and Annamia were not part of Pangea and located to tropical- subtropical latitudes in the eastern part of the Palaeotethys, separated by a wide Mongol-Okhotsk Ocean from the central and northern Asian parts of Pangea. Using a new reconstruction method we can now reconstruct the 258 Ma Emeishan large igneous province and hence South China in longitude. That positioning also determines the previously unknown width of the Palaeotethys Ocean between South China and Pangea at that time, which was as much as 7000 km. Palaeotethys had essentially vanished by the Late Triassic as a result of the collisions of many peri- Gondwana terranes (Cimmeria) with Eurasia. The Late Triassic also coincided with an important reorganization in the North Atlantic and a phase of crustal shortening in NW Siberia. The Early Jurassic witnessed the assembly of the Asian part of Pangea but simultaneously saw the break-up of Pangea in the Central Atlantic. At this time, India was still part of Pangea and the Neotethys was more than 9000 km wide across the Indian-Asian sector. India experienced a tormented post-Early Jurassic journey, first drifting away from East Africa, then East Antarctica/Australia, Madagascar and finally the Seychelles before its dramatic collision with Asia and destruction of the Neotethys in the process. The northern margin of India and the Tethyan Himalaya, however, must have remained very stable during most of the Phanerozoic as no significant rotation or deformation is recognised between Early Ordovician and Eocene times.
T33C-2063
Neotethys Opening and the Pangea Transformation During the Permian
We studied the stratigraphy, composition, and paleomagnetism of lateritic weathering profiles of Permian age from north Iran and western Karakoram, Pakistan. A limited set of samples yielded stable low inclination paleomagnetic components carried essentially by hematite of chemical origin isolated in massive, fine- grained, and homogeneous ferricrete facies. These laterites demonstrably originated at equatorial paleolatitudes characterized by intense weathering processes under warm and humid climatic conditions. Paleomagnetic estimates of paleolatitude from Iran, Karakoram, and north Tibet from this study and the literature, albeit sparse, provide testable constraints on the motion of the Cimmerian terranes as the result of the opening of the Neotethys Ocean along the eastern margin of Gondwana during the Permian. We confirm and help refine previous suggestions that the Cimmerian terranes migrated from southern Gondwanan paleolatitudes in the Early Permian to subequatorial paleolatitudes by the Middle Permian to Early Triassic. As a novel conclusion, we find that the timing, rates, and geometry of Cimmerian windshield wiper tectonics are broadly compatible with Neotethyan opening taking place during the internal transformation of Pangea essentially in the Permian.
T33C-2064
The End of Tethys: Opening and Closing of Oceans between Australia, India and SE Asia
SE Asia has grown by closure of Tethyan oceans south of Asia, principally by addition of fragments rifted from the Gondwana margins, resulting in a mosaic of continental crust and arc/ophiolite sutures. A new reconstruction identifies the blocks rifted from West and NW Australia in the Late Jurassic. They are now in Borneo, Java and Sulawesi, not West Burma as often assumed. Rifting in the Banda and Argo regions began at about 160 Ma, possibly due to south-directed subduction at the north Gondwana margin. Greater India is proposed to have extended north to the northern edge of the Exmouth Plateau and began to separate from Australia at about 140 Ma. The Banda and Argo blocks collided with the SE Asian margin between 110 and 90 Ma. At 90 Ma the Woyla intra-oceanic arc also collided with the Sumatra margin. This terminated subduction beneath Sundaland. The Indian and Australian plates were separated by a leaky transform from about 90 to 75 Ma which became a slightly convergent transform from about 75 to 55 Ma. This transform boundary is considered the eastern end of Tethys from the mid Cretaceous. There was a completely different history of subduction north of India compared to that north of Australia. The subduction history is recorded in the deep mantle by distinctive velocity anomalies which change from east to west abruptly at about 110°E. Between 90 and 45 Ma, India moved rapidly north with north-directed subduction within Tethys and at the Asian margin. It collided with an intra-oceanic arc at about 57 Ma, west of Sumatra, but continued to move north. The first contact of India with Asia was probably about 45 Ma, an estimate dependent on the shape of Greater India and the Asian margin; final ocean closure was later. North of Australia, between 90 and 45 Ma, there was no subduction beneath Sumatra and Java. During this interval south Sundaland was a mainly passive margin with some strike-slip deformation and extension. At 45 Ma Australia began to move north and subduction resumed beneath Indonesia. This was a time of major changes in lengths of subduction boundaries which may be of global importance. Subduction has continued to the present. The structure of the now-subducted ocean floor south of Indonesia, and the rifted NW Australian margin, subsequently influenced the Cenozoic development of SE Asia.
T33C-2065
Constraints to the timing of Neo-Tethys closure determined from the Indus Group molasse, Ladakh Himalaya, NW India
The Indus Group is a Tertiary aged sequence composed of marine and terrestrial sedimentary rocks which were deposited in an evolving late-forearc to intermontane basin setting during the closure of Neo-Tethys and onset of India-Asia collision (Brookfield and Andrews-Speed 1984, Van Haver 1984, Searle 1990, Sinclair and Jaffey 2001, Clift et al. 2002). Clift et al. (2002) have constrained the age of collision by determining the lowermost stratigraphic point in the Indus Group that contains detritus from both Indian and Asian plates, and also by identifying where the Asian plate derived Indus Group unconformably overlies Indian plate margin sediments. The Chogdo Formation, dated by an overlying limestone at older than 50.5 Ma (Green et al. 2008) is identified by Clift et al. (2001), to be the oldest unit of the Indus Group to contain detritus from both the Indian and Asian plates, and to stratigraphically overly Lamayuru Group Indian slope turbidites and Jurutze forearc basin rocks, thereby constraining the timing of ocean closure at prior to 50.5 Ma. However, despite its importance, these previous evaluations of the Indus Group have been hampered by poor stratigraphic knowledge and uncertain lateral correlations, largely due to the relatively complex deformation of the rocks and poor biostratigraphic control. We use a combination of geological mapping, biostratigraphy, facies analysis, petrography, bulk rock geochemistry, and isotopic characterisation of single detrital grains to 1) create an accurate and more widely representative stratigraphy for the Indus Group, 2) determine the nature of the contacts which separate the overlying Indus Group from underlying Indian and Asian plate formations and 3) determine the provenance of the Group, in particular the stratigraphic level within the Indus Group at which both Indian and Asian plate detrital minerals occur together, in order to constrain the time of collision and discover which geological terranes where exhumed and actively eroded during the early stages of the Himalayan orogeny. Our Initial analyses indicate that: 1) the Chogdo Formation may not be as widely occurring as previously interpreted, partly due to obscured tectonic contacts and problems with lateral correlations along strike; 2) there is no apparent location where the Chogdo overlies Indian Plate sediments; 3) there is no confirmed evidence to suggest that the Chogdo Formation contains Indian Plate detritus. Reassessment of constraints to the timing of Neo-Tethys closure as determined from the Chogdo Formation is required.
T33C-2066
The Paro Formation Provenance and its Tectonometamorphic History, Bhutan Himalaya
In western Bhutan, a unique package of rocks that comprises garnetiferous mica schist, quartzites, marbles, calc-silicates, and slivers of ortho-gneiss locally known as Paro Formation has posed an intriguing question on its lithostratigraphic correlation. The lithostratigraphic correlation of Paro Formation either to Greater Himalaya Sequence, Lesser Himalayan Sequence, or Tethyan sediments is important to define its contact with the surrounding rocks. Recent mapping in conjunction with U-Pb ages of detrital zircons, whole rock Nd isotopes, and petrologic study re-defines its stratigraphy and allows for provenance interpretation, lithostratigraphic correlation, and metamorphic history. U-Pb ages of detrital zircons show a strong peak at ~1.8 Ga while whole rock εNd isotopes are less negative and range from -9 to -12.5. The presence of much older (>1.6 Ga) detrital zircons in PF strongly suggests that the PF is LHS. However, an average εNd value of -10.8 requires the PF to contain young detritus. A 440 Ma crystallization age of ortho-gneiss within the PF requires PF to be older than Silurian. The mineral assemblages show that PF has attained upper green-schist to amphibolite facies metamorphism. The occurrence of sillimanite as kyanite pseudomorph suggests that rocks of PF have undergone metamorphism at temperature/pressure conditions in the sillimanite field but below the second sillimanite isograd. The metamorphic grade and thickness of the PF is significantly greater than what is documented immediately below the Main Central Thrust (MCT) in eastern Bhutan (~500 m of upper green-schist facies rocks). Metamorphism in the PF is as high as that identified in portions of the MCT Zone in Nepal and India but is significantly thicker. The combined provenance and metamorphic data may suggest that PF has a LH provenance and has been subjected to pressures and temperatures typical of GH rocks. Also, a preliminary balanced cross-section and the sequential restoration puts Paro Formation in between the lower LHS and the GHS but at the same stratigraphic level. This balanced cross-section estimates minimum shortening at ca. 497 km. This is similar to shortening in eastern Bhutan (ca. 420 km).
T33C-2067
Trace Element Geochemistry of Mafic Clasts in the Indus Molasse Conglomerates: Source Region and Implications for Timing of Initiation of Indo-Eurasian Collision
It has been suggested recently that the source terrane for mafic clasts in the Indus Molasse conglomerates is the Shyok Suture Zone, which lies to the north of the Ladakh Batholith, whereas previous workers have described a southern source area such as the Nidar and/or Spontang Ophiolites. If the source is to the south, this implies that initiation of Indo-Eurasian collision had begun by the time Indus Molasse deposition began, thereby substantiating the need to constrain the timing of Molasse deposition as a proxy for timing of collision. If the source is to the north, which is currently counter to conventional wisdom, then the depositional age of the Molasse becomes less important unless collision initiation can be proved in some other way. This problem can be examined in detail using the trace element geochemistry of mafic clasts in the conglomeratic horizons in the Indus Molasse. It has been demonstrated that the trace element geochemistry of mafic igneous rocks of the Shyok Suture Zone rocks is enriched relative to that of mafic igneous rocks in the Indus Suture Zone (Nidar, Spontang Ophiolites, etc.) We have collected mafic clasts from four conglomerates from the Zanskar Gorge transect through the Indus Group, as well as from conglomerates along strike from both Eastern and Western Ladakh. We will demonstrate the source region for these mafic clasts, thereby pinpointing the stratigraphic horizon within the Indus Group that can constrain the timing of initiation of collision, if the depositional age can be determined.
T33C-2068
Identifying the Himalayan Hinterland-Foreland Transition in Central Nepal
Recumbent isoclinal folds and orogen transport direction-parallel stretching lineations indicative of extending flow commonly characterize orogenic hinterlands. Strain preserved in orogenic forelands, in contrast, is typified by thrust faulting and related folding characteristic of compressing flow. Exhumed mid-crustal rocks exposed in central Nepal, which are mapped as part of the Greater Himalayan sequence (GHS), record a progressive, multi-stage metamorphic and deformation history. The GHS comprises material in the hanging wall of the Main Central thrust (MCT), here mapped at the base of Tertiary pervasive deformation and metamorphism. In the study area the GHS is separated into two distinct tectonometamorphic domains. Metamorphism and deformation of the migmatitic upper domain is interpreted to have occurred synchronously at ca. 20 Ma. Metamorphic depth estimates define an apparent field gradient of 62 MPa/km, twice that expected for pelitic gneiss density. The distorted pressure field gradient of the upper domain is interpreted to reflect post-metamorphic vertical thinning. Assuming plane strain conditions, as indicated by petrofabric data, vertical thinning of the GHS would have been paired with horizontal stretching. In contrast, metamorphism and strain in the lower domain is diachronous, younging away from the migmatitic core toward the MCT. The lower domain is interpreted to comprise thrust slices added to the hanging wall of the MCT while the fault migrated downward structurally beneath the base of the migmatitic rocks after ca. 20 Ma. The net effect of the migration of the MCT was to vertically thicken and horizontally shorten the GHS. The vertical thinning and horizontal stretching of the upper domain of the GHS indicates hinterland-style extending flow. In contrast, the vertical thickening and horizontal shortening of the lower domain of the GHS is more typical of compressing flow commonly observed in foreland regions. The transition between the upper and lower domains, therefore, represents the change from hinterland-style deformation to foreland-style deformation as the mid-crustal material was extruded laterally from the back of the Himalayan orogenic wedge. The identification of the Himalayan hinterland-foreland transition may serve to reconcile some of the current contrasting interpretations of the GHS.
T33C-2069
Cenozoic Deep Ocean Sediments in the Himalaya Mountains
After over 150 years research history, it is generally accepted that formation of the Himalaya orogen is due to the subduction of the Tethyan oceanic crust underneath the Euroasia plate, resulting in the closure of the Neo-Tethys ocean and the collision between the Indian plate and the Euroasia plate. However, the geometry, kinematics and geodynamic evolution of the Himalayan orogen remain poorly understood. The latest marine deposits of the Neo-Tethyan ocean not only constrain a minimum age to collision, but also provides information about the transition from marine to continent. However, the distribution, age, and their stratigraphic and sedimentologic characters of the latest marine deposits are still poorly constrained, not much more than that of Hayden's investigation. Fortunately, due to 1:250,000 scale geological mapping by the Chinese geologists, many stratigraphic outcrops and sections were newly found and investigated closely southern to the Yarlung Zangbo Sure Zone, such as the Jiachala Formation near Gyangze within the northern zone of the Tethyan Himalaya, the Sangdanlin section near Saga within the northern zone, and the Qumiba section near Tingri within the southern zone. Here we report the distribution of the latest marine deposits found state-of-art in the Himalayan area, especially the Eocene radiolaria cherts and turbidite deposits in the areas of the Gyangze-Saga. Our detrital zircon U-Pb dating from those turbiditic sandstones also confirm an age of Paleogene, very close to the age suggested by radiolaria. Our detailed stratigraphic and sedimentologic investigation indicates that due to tectonic control, some of them belong to shallow marine deposits, whiles many of them deposited in submarine pelagic environments with gravity currents (turbidic) development. Based on the spatial distribution of the latest marine deposits, we propose that most area of the Himalayas, now becoming the highest mountain in the world, are still remain marine environments during the Paleogene, and some of them remain submarine environment. Therefore, the previously geological models of the Himalayan orogen and of the Neo-Tethyan need to be re-accessed.
T33C-2070
Paleomagnetic constraints on the timing of initial collision between India and Asia
The onset of India-Asia collision was widely regarded as during early Paleogene of about 55-50 Ma, however, it is controversial about the initial contact between the two continents from very early collision of about 70-65 Ma to very late collision around the Eocene/Oligocene boundary of about 34 Ma. The neotectonic ongoing impingement of India into Asia has undoubtedly triggered a dramatic crustal shortening in both continental margins of India and Asia, direct comparison between the master APW paths of India and Eurasia plates is then impossible to constrain the initial contact between the Greater Indian and Asian continents. In this study, we report paleomagnetic results from Linzizong volcanic rocks from the Linzhou Basin of the Lhasa Block, which has recently been dated to be formed during 64-43 Ma comprising three sub-units namely Dianzhong, Nianbo, and Pana formations in ascending order. Detailed rock magnetic and paleomagnetic studies on the samples collected yield characteristic remanent magnetization with positive fold and reversal tests. The results imply a low paleolatitude of about 10 degrees in the Northern Hemisphere for the Lhasa Block during the eruption times of the Linzizong volcanic rocks. Together with previous results from the Himalaya Block and the master APWP for India, our results permit at least an initial India-Asia collision not later than about 50 Ma. However, further late Cretaceous and Paleogene paleomagnetic results obtained directly from the northern edge of the Himalaya Block are clearly desirable to provide more details about the initiation age of the India-Asia collision. In addition, paleomagnetic results from Paleogene Linzizong volcanic rocks yield also some constraints to the overall neotectonic crustal shortening in areas to the north of the Yalu-Zangbu collision zone.
T33C-2071
A new Meso-Cenozoic Apparent Polar Wandering Path for East Eurasia and Tectonic Implications.
Based on the analysis of more than 490 Asian Mesozoic and Cenozoic paleomagnetic paleopoles compiled from the Global Paleomagnetic Database 2004 (GPMDB04) and bibliographic review of 2004-2008 publications, we present a synthesis of Asian paleomagnetism, focusing on the Tertiary Inclination Anomaly in Central Asia. We show that (1) this anomaly is present not only in mobile lithospheric segments of Central Asia, but also in more stable blocks (Siberia, Amuria, and South and North China blocks), and (2) that it affects paleomagnetism of igneous rocks as well as sedimentary formations. From these observations and in the frame of a dipolar magnetic field hypothesis, we propose a new Meso-Cenozoic (0-130 Ma) Apparent Polar Wandering Path (APWP) for East Eurasia, computed on 225 paleopoles from South China, North China, Amuria, Siberia, Kazakhstan and Middle-East Asia areas. It appears to be significantly discordant from Europe APWP in the 10-80 Ma time period. We discuss implications of this APWP on some critical problems such as inclination flattening in sediments, amount of relative movements of lithospheric blocks in Asia under the effect of ongoing India penetration into Asia, Tethyan margin reconstruction and age of India collision. We finally address the possible impact of persistant non-dipolar components of the magnetic field during the Cenozoic on this analysis.
T33C-2072
Paleomagnetic data from Upper Cretaceous Red Beds, Northwest Vietnam (Song Da Terrane), and Their Bearing on the Extrusion History of Indochina and Deformation Along its Margins
Northwest Vietnam mainly consists of the Song Da terrane, which is bounded to the east by the NW-oriented Ailao Shan/Red River (ASRR) fault system, interpreted to be the southwest margin of the South China Block, and the NW-oriented Song Ma fault. The northern termination of the Song Da terrane is considered to be where the NE-oriented, right lateral Dien Bien Phu fault intersects the ASRR. Whether the Song Da terrane is part of the extruded Indochina Block, paleomagnetic data from which suggest some 10°+ southward latitudinal displacement, can be evaluated with paleomagnetic data from rocks of the appropriate age. Our paleomagnetic sampling concentrated on the Upper Cretaceous Yen Chau Formation, which unconformably overlies Paleozoic and Triassic sedimentary rocks. The Yen Chau Formation is locally up to about 1300 m thick, and is characterized by medium to thick bedded, coarse to fine-grained sandstones and siltstones, all of which are partially cemented by hematite. Samples were collected from 10 localities using a portable drill, with 6 to 19 sites collected per locality, and 7 to 15 samples collected from each site. This approach allows evaluation of the integrity of the remanence at the locality level, where, presumably, considerable time is recorded in each section. Each locality is a homoclinal road cut exposure, with bedding dips varying from sub-horizontal to moderately overturned. NRM intensities range from about 0.7 mA/m to about 25 mA/m; values which are relatively low in comparison to many red beds. A varied response to alternating field (AF) demagnetization indicates that magnetite carries a considerable (over 50 percent) of the remanence; the finest grained samples of relatively high NRM intensity reveal little response to AF treatment, indicating a dominance by hematite, as also supported by three-component IRM thermal demagnetization. Samples with the highest NRM intensities and the least contribution by magnetite respond favorably to thermal demagnetization, with full remanence unblocking by about 685°C, and yield characteristic magnetization directions of north-northeast to northeast declination and moderate positive inclination (about 30 to 35°). Our preliminary results are comparable to those of Takemoto et al. (2005, EPSL, 229, 273- 285) and we tentatively conclude that there has been no significant latitudinal translation of the Song Da terrane, since the Early Cretaceous, with respect to the South China Block. We continue to explore the possibility of local scale, vertical axis rotation of parts of the Song Da terrane. Extrusion of the Indochina Block, in association with its own style of internal deformation, appears to have been facilitated by displacement along structures west of the Song Ma fault.
T33C-2073
Origin of a Voluminous Pulse of Eocene Arc Magmatism in Iran
The Late Triassic to Miocene closure of Neotethys via subduction beneath central Iran was characterized by slow (~2-3 cm/yr) and relatively constant convergence between Arabia and Eurasia. Despite this protracted history of subduction, the record of shallow marine arc volcanism in Iran is dominated by an Eocene pulse that is not readily explainable by changes in the rate or style of plate interactions between south Asian and Neotethyan lithosphere. New U-Pb and 40Ar/39Ar geochronology of volcanic arcs in central and northern Iran constrains the duration of this pulse to <22 My. Eocene volcanic rocks are enriched in large ion lithophile elements (LILE) and depleted in high-field strength elements (HFSE), a pattern typical of arc magmatism. In contrast, Oligocene basalts from the Urumieh-Dokhtar arc and the Alborz Mtns. are enriched in both LILE and HFSE. Together with the recent recognition of Eocene metamorphic core complexes in central and east-central Iran and stratigraphic evidence for Eocene subsidence, these geochemical and geochronological data suggest that the magmatic pulse was generated by extension-related decompression melting of lithosphere hydrated by slab-derived fluids, followed by Oligocene upwelling and melting of enriched mantle that was less extensively modified by hydrous fluids. Based on the inboard position of Cretaceous arc magmas relative to Eocene volcanism, we suggest that extension was driven by an episode of slab retreat or rollback, analogous to the western US. In contrast to the western US, slow subduction rate and restricted Mesozoic magmatism in Iran resulted in a long (~150 My) period of "preconditioning" the arc lithosphere, resulting in a much more voluminous magmatic episode during extension than in the western US.
T33C-2074
Zagros Geodynamics, From Subduction to Collision: The Fate of the Neotethys
The Zagros orogen preserves the record of three main periods/regimes in the convergence history of the Neotethys: (1) Long-lasting subduction processes and arc magmatism (>150-35 Ma). Trace and rare-earth element systematics on the upper plate Sanandaj-Sirjan and Urumieh-Dokhtar magmatic arcs show that they originated from similar, subduction-related mantle sources. The inward shift of arc magmatism (~300 km) from the former to the latter at the end of the Tertiary likely resulted from a change in kinematics and was predated by the formation of the Paleocene-Eocene intraoceanic arc of Kermanshah (earlier thought to represent a remnant of the Peri-Arabic obduction). Oceanic subduction proper ended by 35-30 Ma and was followed by collision. (2) A distinctive period of pertubation of subduction processes (115-85 Ma). A sharp rise of convergence velocities across the Neotethys at ~115 Ma was followed by two regional- scale (i.e., > 3000 km along strike), coeval (~100-80 Ma), short-lived major tectonic phenomena: - the transient exhumation of oceanic blueschists all along the Neotethyan subduction zone, which testifies to a change in plate-slab coupling, - the development of intra-oceanic subduction ultimately leading to the obduction of oceanic crust onto Arabia. (3) Collision and slab tear at depths (~25-0 Ma). Collision started before ~25-23 Ma (in Lorestan) and resulted in 70 km of shortening in the internal zones alone over the last 20-15 My. Calc-alkaline magmatism resumed in the Urumieh-Dokhtar magmatic arc after collision, mainly from the Mio-Pliocene onwards. In Central Zagros this syn-collision magmatism shows a distinctive adakitic trend attributable to the melting at depths of mafic material in response to localized slab breakoff (i.e. 200-300 km along strike), as further supported by tomography. The timing of this event is thus broadly coeval with slab-breakoff below southern Turkey and supports the view that slab tearing propagated in the Neotethyan slab during the period c. 10-5 Ma. This period also significantly coincides with the intensification of collision in Zagros, as witnessed by deformation and progressive unconformities in the Arabian foreland.
T33C-2075
Constraints on the Rates and Timing of Exhumation of the Greater Caucasus from Low- Temperature Thermochronology
Constraining the timing of onset and rates of deformation within the Greater Caucasus mountains is key to understanding the role of this orogen in accommodating total deformation across the Arabia-Eurasia plate boundary, as well as for comparison with modern observations of geodetic shortening and seismic strain release. Bedrock maps of the Greater Caucasus display two geologically distinct domains, separated at the longitude of Mt. Kazbek, possibly by the transverse Borjomi-Kazbek fault. West of Kazbek, Paleozoic amphibolite facies metamorphic rocks are thrust southward over highly deformed Mesozoic shelf and continental slope sediments. East of Kazbek, no rocks older than Jurassic are exposed, although Mesozoic and early Cenozoic strata are also deformed in a primarily south vergent sense. We present new low-temperature thermochronologic (apatite (U-Th)/He and fission track (AFT)) data from the western Greater Caucasus in Russia, and from the eastern Greater Caucasus in Azerbaijan. Cooling ages derived from thermochronology were combined with geologic constraints and fission-track length modeling to develop thermal histories for these regions. Samples from the western Greater Caucasus were collected from Permo-Triassic granitoids. Along the northern flank of the range, a sample from the Bezengi Valley yields AFT and (U-Th)/He ages of 22 and 11 Ma, respectively. Thermal modeling of fission-track length distributions from this sample suggest slow cooling, at rates of ~3°C/Myr, from 30--5 Ma. Cooling rates increase significantly at 5 Ma, to ~11°C/Myr. Samples collected closer to the southern range front, around Baksan Valley, yield AFT ages of ~5 Ma, with cooling rates of ~20°C/Myr. In contrast, (U-Th)/He ages on detrital apatite grains from Jurassic sandstones along the northern edge of the eastern Greater Caucasus are consistent with geologic relations indicating exhumation no greater than ~2 km since Early Cretaceous time. A sample from Early Cretaceous volcaniclastic sandstones of the Vandam zone on the southern flank of the range yields AFT and (U-Th)/He age of 88 and 2.8 Ma, respectively. Fission-track length modeling indicates that this sample remained at ~90°C from 20--5 Ma, when it was exhumed to the surface at ~15°C/Myr. These data bear on several issues regarding the tectonic evolution of the Greater Caucasus. First, modern rates of deformation can account for the observed deformation across the range if extrapolated back 5--7 Ma. Our thermochronologic data suggest that exhumation of the eastern Greater Caucasus has occurred within this time, but that the western Greater Caucasus was subject to an earlier exhumational history. This is consistent with geologic observations indicating Oligocene onset of exhumation in the western Greater Caucasus. Our data question lateral propagation of the Greater Caucasus, suggesting instead that the range may have had two distinct stages of growth, an Oligocene to Late Miocene stage in the western Greater Caucasus, and a post-Miocene stage across the modern range as a whole. Interestingly, the dividing line between these two zones roughly corresponds with a change in geodetic shortening and seismic moment release rates. The western Greater Caucasus are historically aseismic, with modern GPS shortening rates near zero, while significant seismicity and geodetic shortening rates of ~12 mm/yr are observed across the eastern Greater Caucasus. Whether this indicates a recent cessation of tectonic activity in the western Greater Caucasus remains an area of ongoing research.
T33C-2076
Title: Structural styles of active shortening within the Kura fold-thrust belt: Insights from the Qaramaryam fold, Azerbaijan
Closure of Neotethys during the Arabia-Eurasia collision has produced a wide belt of deformation in the southern foreland of the Greater Caucasus. Since 5 Ma, the main locus of shortening has shifted from within the Greater Caucasus southward to the Kura fold-thrust belt in Georgia and Azerbaijan. This belt is characterized by an eastward decrease in structural complexity and depth of exposure, which we interpret as evidence for diachronous initiation and eastward propagation of the belt along strike. The eastern termination of Kura fold-thrust belt(48.25°E) is defined by the S80°E- trending Qaramäryäm anticline, 20 km south of the Greater Caucasus rangefront. Our preliminary (2008) neotectonic and structural mapping focused on the eastern 15 km of this 45 km long fold, where the Girdiman river crosses the fold nose in seven strike-perpendicular water gaps. Exposures within these gaps indicate that Qaramäryäm is a south-verging, east-plunging, fault-propagation fold with a cross-strike width of 5 km that deforms an upward-coarsening package of middle Pleistocene(?) coarse sand and channelized gravel. The fold geometry varies systematically along strike. In the west, a single fold axis separates a 30°N-dipping backlimb from a subvertical/overturned forelimb. To the east, the 10°N- dipping backlimb is separated from the 30°S-dipping forelimb by a sub-horizontal crest, with the across- strike width of the crest increasing eastwards. We interpret these variations to result from an eastward increase in the depth of the tip of an inferred blind thrust. On the ridges between the water gaps, deformed relict geomorphic surfaces of probable late Pleistocene age are preserved on the backlimb of the fold, with the backlimb, crest, and forelimb preserved along one ridge. If dated, this surface will provide an important marker for determining the rate of shortening at Qaramäryäm. Because the structural geometry and sedimentology of Qaramäryäm closely resembles older folds seen to the west within the Kura fold- thrust belt, we hypothesize that it can serve as an actualistic model for both the structural evolution and southward migration of the active deformation front those parts of the Kura fold-thrust belt that are no longer active.
T33C-2077
Mediterranean-style closure of the Paleo-Tethys ocean
The Alpine-Himalayan belt expresses lateral variations in the architecture of continent-continent collisions and includes two end-member examples. One end-member, the Mediterranean, formed from a complex interaction between the Eurasian and Africa-Apulian plates which initiated rollback of Alpine-Tethys lithosphere and opened new ocean basins coeval with plate convergence. As the other end-member, the India-Asia collision is generally considered to have initiated following the closure of a single large ocean basin through the subduction of Neo-Tethys oceanic lithosphere beneath Eurasia. We propose that Mediterranean-style tectonics may have dominated during the closure of the Paleo-Tethys in central Asia based on new and previous observations. These include: (1) coeval Mid-Triassic HP-UHP metamorphism within the Qinling-Dabie orogen, Qiangtang metamorphic belt, and Indochina block; (2) coeval Late Triassic exhumation of the QinlingDabie orogen and Qiangtang metamorphic belt and accumulation of vast volumes of Upper Triassic flysch within the Paleo-Tethys ocean; (3) the north-south orientation of the Longman Shan thrust belt, the triangular map pattern of the Hoh-Xil-Songpan-Ganzi flysch complex, the oroclinal map patterns of Paleo-Tethys arc terranes, and the abrupt eastern termination of the Kunlun; (4) wide distribution of 224-210 Ma adakitic rocks; and (5) evidence for Late Triassic extension within terranes adjacent to the Paleo-Tethys. We propose near synchronous Mid-Triassic continent-continent collisions between the Qiangtang terrane of western Cimmerian-Gondwanan affinity and a Paleo-Tethys arc terrane or the Kunlun terrane, eastern Cimmeria-Indochina and South China block, and the South China and North China blocks. Resistance to continental subduction and oceanic slab breakoff led to a decrease in the northward drift of Gondwana-derived terranes and promoted rollback of subducting remnant Paleo-Tethys oceanic lithosphere. In particular, we suggest that the eastern Kunlun arc rifted from Eurasia and was swept southward above the retreating Paleo-Tethys slab, leaving a hot and strongly attenuated lithosphere in its wake which in turn provided the ideal setting for the accumulation of immense volumes of flysch. A broader implication is that HP-UHP metamorphism may mark the beginning, rather than the end, of the suturing process.
T33C-2078
The 150-100 Ma Apparent Polar Wander of Adria and Africa Using Data From the Northern Apennines (Italy)
We reconstructed a 150 to 100 Ma high-resolution Apparent Polar Wander Path (APWP) for Adria from 6 magnetostratigraphic sections in the Northern Apennines (Italy). To solve the problem of tectonic rotations induced during the orogenesis, we computed relative rotations between sections by considering paleomagnetic directions from common time overlaps and realigned them into a common declination reference frame. We computed a 25° counterclockwise rotation between our segment and the equivalent age 10 Myr sliding window APWP from Besse and Courtillot (Besse, J., Courtillot, V., 2002, Apparent and true polar wander and the geometry of the geomagnetic field over the last 200 Myr. J. Geophys. Res. 107, doi:10.1029/2000JB000050), by minimizing the sum of angular distance between poles. The two segments are in very good agreement with inclinations and declinations showing the same temporal evolution and being most often statistically indistinct after the rotation correction. The good agreement found strengthens the fact that Adria was part of Africa at least during this period, whereas it has been affected by thrust sheet rotations most probably during the Apennines orogenesis. The high quality of the obtained segment demonstrates that is possible to reconstruct APWP using data from orogenic belts, when it is possible to solve tectonic rotations linked to the orogenesis. The segment computed from the Northern Apennines sections documents two APWPs loops. The first one is around 140 Ma and could be linked either to a large-scale collision (closure of the Mongol-Okhotsk Ocean) or to a true polar wander event. A smaller event is detected at 100-110 Ma, with apex at ca. 105 Ma, and could evidence a possible southward motion of Adria, undetected from classical African APWPs.
T33C-2079
Low Eurasian Latitudes in the Late Jurassic: Evidence for True Polar Wander
Recently, data from Adria suggested a more southern position of Africa in late Jurassic times than expected from the apparent polar wander paths (APWP). Those southern latitudes were attributed to large-scale soutward plate movements of Eurasia and Africa in middle Jurassic lower Cretaceous times, followed by a return to more northern latitudes (Muttoni et al., 2005). To test this scenario for Eurasia, we collected paleomagnetic samples from middle Jurassic to lower Cretaceous rocks, both in Crimea (Ukraine) and in the Pontides (northern Turkey), to determine their paleolatitudinal position. Large datasets were sampled to enable correction for the inclination shallowing in sediments using the elongation/inclination (E/I) method (of Tauxe and Kent, 2004). The results clearly show low paleolatitudes that position the southern margin of Eurasia ~1200 km further to the south than expected from the APWPs, in agreement with the data from Adria. However, we argue that large-scale plate movements during this time span are unlikely, because of the considerable amount of subduction zones surrounding the African and Eurasian continents in late Jurassic times, making large movements of the associated slabs in the mantle implausible. Therefore, we suggest that the low latitudes of our new Eurasian sites result from true polar wander (TPW) which is in agreement with the TPW scenario recently proposed by Steinberger and Torsvik (2008).
T33C-2080
Structure of the Southeast Anatolian Suture Zone, Turkey
The Southeast Anatolian belt of the Alpine-Himalaya chain is situated in the region of collision between Anatolian and Arabian micro plates. Contractional tectonic due to the closure of the Neo-Tethys Ocean has led to folding, thrusting and large-scale strike-slip faulting of Phanerozoic units in the belt. The study area is a part of this belt including the Puturge and Malatya metamorphites, the Maden complex and magmatites. The Puturge metamorphites are composed mainly of penetratively-deformed pelites, psamites, metabasites and metacarbonates, reaching up high-grade metamorphic conditions, and involve at least three foliations dipping moderately and steeply to mainly the northwest and northeast, although southwest and southeast dipping record. One of these foliations is characterized by mylonitic foliation. Preliminary petrologic observations (e.g. recrystallized quartz, syn-deformational growth of the chlorite and muscovite) in mylonites indicate mainly greenschist grade syn-deformational conditions. The Malatya metamorphites are dominated by Paleozoic-Mesozoic metacarbonates with low-grade metamorphism reveal open and tight folds and thrust faults. The Maden complex is composed of Eocene volcanic and sedimentary rocks. Magmatites, comprising small mafic and granitic bodies, was intruded before onset of the Miocene faulting. Today morphotectonic architecture of the region is result of (1) that the Malatya metamorphites emplaced over the Puturge metamorphites, the Maden complex and magmatites, called Turkdag thrust (TT), during Early Miocene, and (2) that major left-lateral East Anatolian Fault zone (EAFZ). The Turkdag thrust, low- angle south directed fault, is characterized by zone including variably products of the brittle deformation (e.g., cataclasites, fractures) and is possibly Early Miocene in age. The TT is truncated and or folded by faults of the EAFZ. The zone display displacement at about 10 km in the region, striking N30°E and N55°E.
T33C-2081
U-Pb Detrital Zircon Ages From the Internal Hellenides Using LA-SF-ICP-MS
Zircon is a common accessory phase in clastic sedimentary rocks as well as one of the most stable minerals in sedimentary environments. Investigating the age spectra of detrital zircons is important for sedimentary provenance analysis to glean information regarding ancient source areas and major magmatic events, crucial for plate-tectonic and palaeogeographic reconstructions. Furthermore, in the absence of fossil and other stratigraphic data, the youngest zircon grain in a sedimentary rock can indicate a maximum limit for the age of deposition. Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) is a rapid, comparably inexpensive and sufficiently precise technique for in-situ Pb-Pb and U-Pb age determinations of igneous, metamorphic and detrital zircons. In this study, we applied a simple method for in-situ U-Pb age determinations of zircons using a ThermoFinnigan Element2 magnetic sectorfield ICP-MS coupled to a frequency quintupled Nd-YAG (213 nm) laser ablation system. This method takes advantage of the very high sensitivity of the magnetic sectorfied ICP-MS and allows U-Pb age dating of zircons with significantly better spatial resolution (i.e. spot diameters of 30 microns and below and a depth resolution of 15 to 20 ìm) compared to quadrupole ICP-MS based methods. For this study, samples were collected from the Serbo- Macedonian Massif and the Vardar Zone of northern Greece. Both areas belong to the Internal Hellenides. There, the relationships between different pre-Alpine crustal fragments are now masked by younger (Mesozoic to Cenozoic) complex structural and metamorphic events. This, together with the scarcity of biostratigraphic, geochronological and palaeomagnetic data, has given rise to equivocal palaeotectonic models and interpretations. Detrital zircon geochronology is used here to constrain terrane accretion processes and the provenance of crustal sources for sediments during Palaeozoic and Mesozoic times. The age and origin of pre-Alpine basement units in the Internal Hellenides has important implications in our understanding the evolution of North Gondwana-derived terranes in more detail and consequently in alternative plate-tectonic reconstruction for the Palaeozoic and Mesozoic.
T33C-2082
Plio-Quaternary Shortening on the Algerian Margin: Evidence From Multibeam Bathymetry and Seismic Reflection Survey off Boumerdes
The northern limit of Algeria is one of the most seismically active regions of the western Mediterranean, with potential magnitudes estimated at up to 7,5. Instrumental seismicity is detected mainly onshore and expresses a NW-SE dominant shortening. However, since the May 2003 Boumerdès earthquake, offshore deformation attracts scientists' attention. The aim of this note is to describe a system of Plio-Quaternary folds and blind thrusts at the foot of the continental slope offshore Boumerdès based on data acquired in 2003 and 2005 (Maradja 1 and Maradja 2/ Samra cruises). On a S-N oriented transect offshore Boumerdès, three uplifted basins are observed from the mid-continental slope down to 30-40 km within the Balearic abyssal plain. These basins are limited by scarps corresponding to the north-western flanks of Plio-Quaternary anticlines. The geometry of the sedimentary units allows to distinguish Messinian salt features (developed early) from other tectonic (s.s.) compressional structures that formed later as a series of diachronous folds. The folding of the Miocene layers is clearly tectonically (s.s.) controlled. It initiated during the Plio-Quaternary and progressively migrated from the slope toward the abyssal plain. The pattern of perched basins and the folding distribution strongly suggest the occurrence of a system of flat and blind thrust ramps. As no thrusts are observed in the Miocene layers, flats and thrust ramps have to be deeper, probably rooted in the basement, as evidenced during the 2003 Boumerdes Mw 6.9 event. The position of basement highs below the Miocene deposits compared to the active fronts indicates that the shape of the Plio-Quaternary fold and thrust belt is controlled by these previous basement highs. Uplifted basins are less developed in size and depth on the slope than in the abyssal plain, suggesting that the flat length increases from the slope to the abyssal plain. We interpret this increase as being directly related to crustal heterogeneity located at the boundary between continental and oceanic crust. We propose that this boundary exert a major influence in the present-day deformation pattern. On the basis of shortening estimates and time constraints, we document that a significant part of the finite Africa-Europe convergence is accommodated here.
T33C-2083
Was Korean Peninsula shown a same tectonic motion with Eurasian Plate since Mesozoic?
A comparison of apparent polar wander paths (APWP) for the different two terrains has been provided a relative tectonic history of each terrain for the geological times. In the present study, we refined the Korean APWP since Cretaceous by compilation of internationally peer-reviewed and age-controlled paleomagnetic data. In order to trace a tectonic movement of the Korean Peninsula, compiled Korean APWP was compared with the Eurasian APWP. Most importantly, the Korean APWP showed a hair-pin curvature in the position of Early Tertiary, after convergence of Eurasian APWP and Korean APWP at around 80 Ma. As anticipated, the Eurasian APWP also displayed a copycat hair-pin near 60-40 Ma, suggesting that the Korean Peninsula belonged to Eurasian plate as a part of single tectonic unit since Late Cretaceous. However, slightly different paleomagnetic poles in the Tertiary Korean APWP as well as a significant divergence of Cretaceous poles from the Eurasian APWP imply that Korean Peninsula recorded little tectonic modifications in Tertiary.
T33C-2084
Paleomagnetism, Rock Magnetism, and Magnetic Mineralogy of the Eocene Disang Group, Changlang Area, Arunachal Pradesh, Northeast India
As part of an integrated provenance, structural, and paleomagnetic study to determine the location of the plate boundary, or western extent of a diffuse plate boundary, between the northeast corner of India (between Assam and Bangladesh) and Indochina that existed during early Cenozoic time, we have conducted preliminary paleomagnetic studies on a three km thick Eocene marine sandstone sequence of the Disang Group. These strata have been interpreted to have an orogenic source from the Indo-Burman ranges and/or easternmost Himalayas, as part of Indochina. The paleomagnetism of rocks of Eocene age in this area, assuming a primary magnetization is preserved, should differ considerably from that of comparable age rocks deposited on or adjacent to the Indian Craton at this time. A total of 12 oriented samples were collected from two separate localities along the Changlang road section (about 27.141N/95.740E and 27.153N/95.744E), northeast of Dibrugarh, Assam. At these localities, strata dip to the northeast at about 60 degrees. All of the samples are light to medium-gray, well indurated fine-grained, quartzolithic sandstones. Each oriented block sample was prepared into multiple (10-20+) specimens for measurement, to assess the internal consistency of the remanence and rock magnetic properties, as well as to, hopefully, test for the possibility of substantial inclination shallowing due to compaction. Preliminary study of these samples shows that the remanence is principally carried by magnetite, with the randomization of a magnetization that is consistent at the specimen and locality level between peak fields of about 10 and 60 mT. Overall, the in situ direction of magnetization identified in the preliminary assessment of these samples does not resemble a modern field direction, or a time-averaged Plio-Pleistocene field direction; further work will strive to assess the reliability of this remanence as a primary, Eocene-age magnetization representative of this understudied area.
T33C-2085
A Preliminary Study of Magnetic Proxies for the Late Quaternary Paleoclimate Using Gravity Cores From Lake Hovsgol, Northern Mongolia
Lake Hovsgol (50°30'~51°35'N, 100°15'~100°40'E) in northern Mongolia is the second largest and deepwater basin within the Baikal rift system, which is affected dominantly by East Asian Winter Monsoon. Lake Hovsgol is 135 km long, average 20 km wide, and has a maximum depth of 262 m. Lake Hovsgol sediments are potentially excellent paleoclimate recorders due to their altitude of 1645 m above sea level (about 1100 m higher than Lake Baikal) and minimal human influence. Recent studies reported magnetic susceptibility data along with detailed sedimentologic, absolute age-dating, and paleontologic data from Lake Hovsgol (Prokopenko et al., 2005; Hovsgol Drilling Project Group, 2007). Although magnetic susceptibility is one of the useful parameters for analyzing climatic change recorded by lake sediment, loess and paleosol around the world, other enviromagnetic parameters (e.g., frequency- dependent susceptibility, isothermal remanent magnetization, anhysteretic remanent magnetization, and bivariate ratios) can be more sensitive to climatic change in some cases. The aims of this study are to find the optimal magnetoclimate proxies for Lake Hovsgol sediments and to provide high resolution paleoclimate change model in the study area. We have measured various magnetic parameters representing magnetic composition, concentration, and granulometry of seven gravity cores from Lake Hovsgol. Preliminary correlations of magnetic properties with other paleoclimate proxies (e.g., lithology, AMS radiocarbon ages, sedimentary structure, and diatom analysis) yield three paleoenvironmental units that span about 20000 years: Last Glacial Maximum, Deglacial, and Holocene. Ongoing analytical work will further provide high resolution records for abrupt paleoclimate change within the units.