T51A-0293
Active Forearc Response to CO-NZ-CA Triple Junction Migration, Southern Central America
Southeast migration of the CO-NZ-CA triple junction at a rate of ~55 mm/yr results in an abrupt increase in convergence rate, slab thickness and subduction direction within the upper plate of the Central American convergent margin. At the triple junction, an active transform fault (the dextral Panama Fracture Zone) subducts beneath the Caribbean plate at the Middle America Trench, and juxtaposes the thick, orthogonal and shallow subduction of the Cocos plate against the thin, oblique and steeper subduction of the Nazca plate. New bedrock geology, Quaternary mapping and Ar/Ar dates of fluvial and volcanic deposits inboard of the triple junction provide evidence that both the outer and inner forearc of this system is actively responding to the dynamic changes presented by triple junction migration. Our results confirm that the Fila Costeña, a thin-skinned inner forearc thrust belt, is active and likely propagating in concert with triple junction migration. Mapping within the area overriding the Panama Fracture Zone indicates that thrusting develops only in those areas experiencing Cocos subduction; the thrust belt dies out coincident with the on-shore projection of the Panama Fracture Zone, and balanced cross-sections indicate a lateral gradient in the amount of shortening near the termination of the thrust belt. Along-strike variations in drainage basin morphometry suggest that drainage divides of the Fila Costeña are propagating to the southeast with the triple junction, resulting in hook-shaped drainage patterns and asymmetric basin shapes. A survey of a flight of 3-4 fluvial terraces along the Río Chiriquí Viejo indicates recent thrusting along a prominent thrust fault of the Fila Costeña. These terraces are also inset into multiple lahar flows with an upper surface tentatively constrained at ~507 ka based on an Ar/Ar hornblende plateau age. Recent work indicates that this thrust fault displaces surficial lahar deposits, suggesting that it must have become emergent less than 500,000 years ago. At least 4 marine terraces occur along the southern tip of the Burica Peninsula in the outer forearc that straddles the subducting transform approximately 20 km inboard of the triple junction. The oldest (late Pleistocene) and highest (60-70 m) terrace surface appears tilted to the southeast, possibly as a consequence of the change in slab thickness at the Cocos-Nazca plate boundary that migrates to the southeast with the triple junction. These observations indicate that the changing plate tectonic framework associated with triple junction migration has a large impact on forearc structure and landscape evolution through time.
T51A-0294
Proto-orogeny: stratigraphy, tectonics and neotectonics at the northern Australia collisional margin
The ongoing arc-continent collision at the northern Australian margin causes a variety of crustal responses, both at the collisional front and in the hinterland. From fieldwork in East Timor (collisional front) and extensive seismic structural interpretation across the North West Shelf (hinterland) we propose that continental collision initiated with the arrival of an offshore plateau of the Australian margin at the subduction zone at 10.8 Ma, and that remnants of this collided plateau occur throughout Timor. Detailed stratigraphic analyses and biostratigraphic age determinations have been crucial to re-evaluating the stratigraphy of East Timor, and thus re-defining kinematic and geodynamic models for deformation in this very young orogenic belt. Using these tools in addition to structural mapping and geochemical analyses, we have significantly altered age determinations and tectono-stratigraphic affinities for key units (e,g the type section of the Miocene is a thrust stack of Triassic-Cretaceous material), recognised new stratigraphic associations (e.g. thick Australian-derived pelagites in association with thrust slices of exotic material), identified a previously unrecognised slices of ocean floor material, and documented new structural relationships between Australian and exotic units (shear zones, identification of a crush breccia terrane boundary along late, high-angle faults). In addition we have recognised various basaltic compositions indicating both mid-ocean ridge and ocean-island basalts within thrust sequences, at unique structural levels, and proposed new, well-constrained models, for tectonic events. Significant shortening at the collisional front protected much of the hinterland from collisional strain, to the extent that shortening behind the collision is limited to approximately 1per cent. We attribute hinterland deformation to elastic flexure and structural re-amplification of pre-existing topography. Further west, the collisional segment of the margin transitions into a passive margin, where deformation is dominated by far-field stresses generated elsewhere along the Australian plate margin, generating anomalous intraplate seismicity.
T51A-0295
Multi-Stage origin of the ophiolites along the Bangong-Nujiang suture zone: Implications for the evolution of central Tibet
The history of the BNS ophiolite is important for models of the tectonic evolution of Central Tibet during the Mesozoic. Current petrologic, geochemical, and stratigraphic data all favor the interpretation of the ophiolite as having formed in a transitional MORB-IAT setting above a supra-subduction zone. Detailed studies of field relationships at several localities indicate that two distinct ophiolitic suites are present, especially in the western and middle sectors of the suture zone. One suite includes all elements of an entire ophiolite sequence albeit dismembered or disrupted through faulting. These rocks are mainly scattered along the northern part of the BNS, e.g. at Bangong Tso, Dong Tso and Dongqiao-Amdo. Radiometric ages for this ophiolitic suites are limited to the Early to Middle Jurassic. The other zone of ophiolitic rocks is located in the southern part of the BNS, such as Shiquanhe, Guchang, Lagkor Tso, Baila-Jueweng and Xainza, and is associated with a remnant basin allowed the deposition of Upper Jurassic to Lower Cretaceous cherts along the south part of the suture, and shallow marine deposition continued on the structural highs elsewhere along the suture.
T51A-0296
The youngest blueschist belt in SW Japan
We report discovery of the youngest blueschist belt in SW Japan and describe exhumation process of the high- P/T metamorphic belt. The Cretaceous Sanbagawa belt is distributed in SW Japan and is one of the typical high- P/T metamorphic belts in the world. In the Oboke area, central Shikoku, schists are widely distributed and were considered as a part of the Sanbagawa belt. However, recent zircon chronological study has revealed the older limit of the depositional age of this area, which indicates that this area belongs not to the Sanbagawa belt but to the Northern Shimanto belt (Aoki et al., 2007). The Northern Shimanto belt is known for a Cretaceous-Tertiary accretionary complex in Japan. The redefinition of the attribution of the Oboke schists to the Northern Shimanto belt raised an importance of studies from view points of metamorphic petrology and geochronology in the area, those are essential for understanding the tectonic evolution of the of the Northern Shimanto belt. In this study, tectonic evolution of the Northern Shimanto belt was examined based on petrological and geochronological studies in the Oboke area. Some basic schists in this area contain Na-amphibole (magnesioriebeckite close to crossite and winchite). The peak metamorphic P-T condition with Na-amphibole-bearing basic schist and Na- amphibole-free basic schists are estimated to P = 4-4.5 kbar and T = 240-270° (12-15 km depth), which corresponds to the transition among blueschist, greenschist and pumpellyite-actinolite facies. K-Ar ages of phengite from schists showed 64.8 +/- 1.4 Ma and 64.4 +/- 1.4 Ma for the basic schists, and 65.0 +/- 1.4 Ma, 61.4 +/- 1.3 Ma and 63.6 +/- 1.4 Ma for the pelitic schists. Metamorphic temperature in the Oboke area is below the closure temperature of the K-Ar phengite system. Hence, the K-Ar ages (66-61 Ma) in this area represent the peak metamorphic age of the Northern Shimanto belt. In the broad sense of definition of blueschist facies, the highest- grade part of the Northern Shimanto metamorphic rocks belongs to the blueschist facies metamorphism, according to Miyashiro (1973). These results suggest that the Northern Shimanto belt is the youngest blueschist belt in Japan. Integrating the results of this study with previous studies, the tectonic evolution of the Northern Shimanto belt and the Sanbagawa belt are as mentioned below. The Northern Shimanto metamorphic rocks were formed as an accretionary complex by a subduction of the oceanic plate between 100-65 Ma (e.g., Taira et al., 1982). At that time, the Sanbagawa metamorphic rocks had already begun exhumation, and juxtaposed onto the Northern Shimanto metamorphic rocks at 12-15 km depth, associated with an extensive fluid infiltration from the underplated Northern Shimanto belt between 88 and 72 Ma. After that time, both the Sanbagawa and the Northern Shimanto metamorphic rocks were exhumed to the surface by domal up-lift until 52-42 Ma (Isozaki and Itaya, 1990), and at which time the Sanbagawa schists were eroded to be transported to fore arc-basins.
T51A-0297
JUNE 03, 2007 NATURAL DISASTER IN THE VALLEY OF GEYSERS IN KAMCHATKA
The famous Valley of Geysers along with active volcanoes appears to be a beautiful visiting card of Kamchatka. It is well known in Russia and other countries as the most popular tourist place. Annually it is visited by thousands of Russian and foreign tourists. The Valley of Geysers is the most potentially hazardous area in Kamchatka because of intense development of landslides, avalanches and frequent mudflows occurring within its boundaries. June 03, 2007 landslide, followed by a mudflow, resulted in a north - west faced horse-shoe amphitheater consisting of two adjacent circuses. The height of north-eastern sub-vertical wall is 150 m with a length 800 m; the length of a flatly inclined bottom 400 600 m. Initially estimated volume of collapse and avalanche made up 8-15 millions cubic meters. Avalanching and formation of a dam at the Geysernaya River caused completion of some geysers and open thermal water discharge at sites blocked off by the avalanche and a dammed lake. However beyond the boundaries of the avalanche and the lake, the geysers are still operating. It is likely that some geysers could be brought back if water level in the lake decreases. Possibly new geysers could appear. Based on results of routine survey we estimated specific areas that nowadays pose a hazard as well as a possibility of new avalanches and landslides that may occur in the future. Estimation and forecast of new avalanches and landslides require continuous observations to be performed in the Valley of Geysers to monitor deformation and seismic processes.
T51A-0298
The morphological changes after hillslope erosion and the stability of hillslope in the Dokdo
The goals of studies are finding the evidence of landform change, classifying landform by geographical features, and finally analyzing stability of hillslope of the Dokdo volcano. In these studies, the 3-D topographic maps of the Dokdo were made using bathymetry around the Dokdo and DEM data of the subaerial portion of the Dokdo. The stability of hillslope of the Dokdo was estimated by analyses of morphological elements such as altitude, slope and slope aspect. In submerged part of the Dokdo, the erosion landforms formed by mass-movements and channels were confirmed in slopes of 14~ 40°. The debris flows formed a irregular salient at slopes or were piled up with 5~ 15° gradients at bottom of the Dokdo. About 70% of subaerial slopes of the Dokdo are steep with high gradient of more than 40°. There are the debris collapsed from the slopes from the costal area to -30~ -40 m in water depth. The north and east slopes of Seo-do and the west slopes of Dong-do in the Dokdo, except wave cut platform and low gradient slope, are unstable with a possibility of slump or mass-movement. The areas of weak structure in the Dokdo have the possibilities of corrosions and collapses by wave cuts, typhoons, tidal waves.
T51A-0299
Neo-Tethyan Ocean evolution of southwest Tibet inferred from a melange structurally beneath the Kiogar ophiolite
Pre-collisional tectonic evolution of Neo-Tethyan Ocean is best recorded in the Yarlung Tsangpo suture zone (YTSZ) ophiolite complexes and associated melanges, one of which occurs in the Kiogar area of SW Tibet. Structural, sedimentological, palaeontological and geochemical work on a subophiolitic melange has been carried out in order to better constrain the tectonic evolution of this part of the Neo-Tethyan Ocean. The melange includes a series of south-verging dismembered thrust sheets and blocks, separated from the overlying ophiolite by a north-dipping thrust fault. The rocks include radiolarian cherts, pelagic limestones, reefal limetones, turbiditic sandstones and basaltic blocks. Biostratigraphic age data show that radiolarian cherts interbedded with basalts of a coherent succession were deposited in mid-Cretaceous time. Similar-age radiolarian cherts are depositionally associated with ophiolitic rocks elsewhere along the YTSZ in the Xigaze area of southeast Tibet. Extrusive rocks consists mainly of vesicular to highly vesicular, plagioclase-phyric and porphyritic-ankaramitic lavas. Geochemical evidence shows that the extrusive rocks, of inferred Cretaceous age, range from E-MORB to OIB, interpreted to be related off-axis seamount volcanism. MORB-type basaltic blocks, the ages of which remain unconstrained, were also found elsewhere. The melange was emplaced when the passive margin north of India collapsed, creating a foredeep ahead of advancing thrust sheets of ophiolitic rocks, prior to the closure of the Neo-Tethys.
T51A-0300
Arc arc collisional tectonics within the Central Mobile Belt of the Newfoundland Appalachians
The Central Mobile Belt of Newfoundland Appalachians records the Ordovician arc arc collision between the peri-Laurentian Red Indian Lake Arc of the Annieopsquotch accretionary tract (c. 480460 Ma), and the peri- Gondwanan Victoria Popelogan Arc (c. 473453 Ma), which marks the closure of the Cambro-Ordovician Iapetus Ocean. Although the arc systems are in part coeval, they are distinguishable by the preservation of distinct structural histories and stratigraphies, unique basement characteristics as demonstrated by lead isotopic values of volcanic massive sulphide deposits and faunal differences. A modern analogue of such an arc - arc collision is observed in the Molucca and Solomon seas of the southwest Pacific. From such modern analogues it is evident that the Victoria Popelogan Arc occupied a lower-plate setting during collision. This tectonic setting is demonstrated by subsidence of the Victoria Popelogan Arc similar to the collision induced subsidence that is developed on the Australian active margin and Halmahera arcs of the Southwest Pacific. The timing of Victoria Popelogan Arc subsidence is constrained by three age dates that form the last vestiges of arc volcanism (457 ± 2; 456.8 ± 3.1; 457 ± 3.6 Ma). These volcanic rocks are immediately overlain by Caradocian black shale of the Point Leamington Formation that marks the base of the Badger Group and the initiation of a successor basin. Caradocian black shale is noticeably absent from the top of the Red Indian Lake Arc with this time interval instead represented by a sub-Silurian unconformity, formed in response to collisional uplift. Emergence of the peri- Laurentian margin is demonstrated by detritus from it preserved in the Badger Group, which as it stratigraphically overlies the peri-Gondwanan Victoria Popelogan Arc, requires that Iapetus was closed by this time. Following this collision, subduction stepped back into the outboard Tetagouche Exploits back-arc basin. Whereas correlative tracts representing Ordovician arc arc collision occur elsewhere within the Northern Appalachians and British Caledonides, none of them have as well preserved evidence as in Newfoundland.
T51A-0301
Dynamic implications of drowned reefs and raised coastlines along the Bismarck volcanic arc
Dynamic implications of drowned reefs and raised coastlines along the Bismarck volcanic arc Raised and drowned reefs reveal the distribution of uplift and subsidence along the north coast of New Guinea and the island volcanoes of the Bismarck volcanic arc. Extinct volcanic islands in the Bismarck volcanic arc are fringed by well developed coral reefs. Drowned reefs offshore from these islands provide evidence for subsidence in the central section of the arc, north of the Finisterre Range, which has been colliding with the Australian continent since the Pliocene. Bathymetric and backscatter data collected from R/V Kilo Moana in 2004 reveal regularly spaced drowned reef terraces and drowned atolls at up to 1100 meters below sea level. The interval between terraces varies from about 200 meters around Bagabag Island to the west, to about 100 meters around Umboi Island to the east. Across-arc variations are poorly constrained because of the narrow width of the arc. However, the adjacent mainland coast has well documented raised reefs indicating long-term uplift. Farther from the Finisterre Range, islands show evidence for either constant sea level or slight uplift. Supposing that each terrace represents drowning of a lowstand reef during postglacial sea level rise, then roughly uniform subsidence rates of 1 to 2 mm per year can be inferred, decreasing from Bagabag to Umboi. Subsidence may be due to cessation of magmatic activity and cooling, flexural loading by the uplifting Finisterre Range, and/or sediment loading on the seafloor north of the Finisterre Range.
T51A-0302
Preservation of Oxygen Isotope Stage 3 Marine Terrace Deposits along the Southwest Coast of the Osa Peninsula, Corcovado National Park, Costa Rica
Subduction of the aseismic Cocos Ridge at the Middle American Trench outboard of the Osa Peninsula results in rapid late Quaternary surface uplift. The distribution of surface uplift corresponds with the imaged bathymetric relief. On the Osa Peninsula, inboard of the northwest flank of the Cocos Ridge, exposures of the Late Pleistocene Puerto Armuelles Formation are recognized. The Puerto Armuelles Fm consists of poorly consolidated sands, silts, and muds from shallow marine, estuarine, and mangrove systems. Along the southwest coast, Puerto Armuelles Fm sediment infills paleo-topographic depressions. AMS radiocarbon dates obtained on 4 marine macrofossil samples yield ages ranging from 38.51 ka B.P. to 42.35 ka B.P. Dates obtained on multiple samples from individual sections are internally consistent, recording younger ages at higher stratigraphic levels within two fining upward deposits. The sections are displaced relative to one another across a northeast striking fault. The two measured stratigraphic sections are used to quantify a minimum Late Pleistocene to recent separation rate of 0.54 m ka-1. A suite of 15 radiocarbon dates on exposures of the Puerto Armuelles Fm from the eastern portion of the Osa Peninsula (Gardner et al., 1992) and 14 radiocarbon dates obtained from equivalent strata along the northwestern portions of the Osa Peninsula (Sak et al., 2004) clearly and consistently indicate a Late Pleistocene age of deposition during Oxygen Isotope Stage 3.
T51A-0303
Remote sensing investigation into the correlation between landslides caused by the 2002 November 3rd, 7.9M Denali Fault earthquake and a surge of the SE fork of McGinnis Peak Glacier.
The 2002 November 3rd, 7.9 M Denali Fault caused a large landslide that deposited 11.4 x 106m3 feet of rock and ice onto the SE fork of McGinnis Peak Glacier. The landslide left a large mass on the upper ice reservoir of the glacier and blocked the terminus outlet of the glacier with debris. Satellite images and historic photos were used to examine the terminus history of the glacier. Further landslides/mass wasting events during the spring of 2003, documented via Land Sat 7 images, covered the same upper ice reservoir as the 2002 landslides. Between 2004 and 2006 the SE fork Glacier of McGinnis Peak experienced a dramatic surge. There is no known history of such dramatic surge occurrences on the glacier of interest. Though causation is difficult to prove preliminary evidence points towards the implication that the landslides generated by the 2002 earthquake most likely either caused or contributed to the documented glacial surge. Examination of more satellite images is planned to further examine McGinnis Peak's glacier history and to refine the timing of the possible correlation between the earthquake generated landslides and the dramatic surge of the SE glacial fork of McGinnis Peak
T51A-0304
A Detailed 3D Seismic Velocity Structure of the Subducting Pacific Slab Beneath Hokkaido, Tohoku and Kanto, Japan, by Double-Difference Tomography
Three-dimensional heterogeneous structure beneath northeastern (NE) Japan has been investigated by previous studies and an inclined seismic low-velocity zone is imaged in the mantle wedge sub-parallel to the down-dip direction of the subducting slab (Zhao et al., 1992, Nakajima et al., 2001). However, the heterogeneous structure within the slab has not been well studied even though it is very important to understand the whole process of water transportation from the slab to the surface. Here we show a detailed 3D seismic velocity structure within the subducted Pacific slab around Japan and propose a water-transportation path from the slab to the mantle wedge. In this study, we estimated 3D velocity structure within the Pacific slab by the double-difference tomography (Zhang and Thurber, 2003). We divided the study area, from Hokkaido to Kanto, into 6 areas due to the limitation of memory and computation time. In each area, arrival-time data of 7,500-17,000 events recorded at 70-170 stations were used in the analysis. The total number of absolute travel-time data was about 140,000-312,000 for P wave and 123,000-268,000 for S wave, and differential data were about 736,000-1,920,000 for P wave and 644,000-1,488,000 for S wave. Horizontal and vertical grid separations are 10-25 km and 6.5 km, respectively. RMS residuals of travel times for P wave decreased from 0.23s to 0.09s and for S wave from 0.35s to 0.13s. The obtained results are as follows: (1) a remarkable low-Vs zone exists in the uppermost part of the subducting slab, (2) it extends down to a depth of about 80 km, (3) the termination of this low-Vs zone almost corresponds to the "seismic belt" recently detected in the upper plane of the double seismic zone (Kita et al.,2006; Hasegawa et al., 2007), (4) at depths deeper than 80 km, a low-Vs and high-Vp/Vs zone is apparently distributed in the mantle wedge, immediately above the slab crust. We consider that these features reflect water-transportation processes from the slab to the mantle wedge. A low- Vs zone in the uppermost part of the subducting slab corresponds to the hydrous oceanic crust since its absolute velocity is about 4.0 km/s, comparable to that expected for the oceanic crust (Hacker et al., 2003). Dehydration reactions occur in the oceanic crust as temperature and pressure increase, and a relatively large amount of water is released at depths of about 80-100 km. The water generated by dehydration reactions could migrate upward and react peridotite at the base of the mantle wedge, forming a thin-serpentine layer there. Then, the layer is dragged by the subducting slab to deeper depths (e.g. Iwamori, 1998). Such water-transportation processes from the slab to the mantle wedge are partly constrained by a recent receiver function analysis (Kawakatsu and Watada, 2007). We further found an along-arc variation of the termination depth of the low-velocity oceanic crust, suggesting the along-arc variation in the amount of fluids released from the slab.
T51A-0305
Subduction in Central Kermadec: Crustal Structures from the Incoming Plate and the Arc- Backarc Region From Wide-Angle Seismics
The central part of the 2500-km long Tonga-Kermadec Trench is characterized by the subduction of the Louisville Ridge and unusually large seismicity approximately 200-300 km to the south of this ridge subduction. From this region we show preliminary results which have been derived from the recently acquired interpretation of seismic wide-angle reflection/refraction data. The data were collected along an almost 500-km long transect carried out in April 2007 using the R/V Sonne in order to determine the upper lithospheric structures of the incoming Pacific Plate and the overriding Australian Plate across the Colville and Kermadec Ridges. This transect lies immediately north of Raoul Island, the largest of the Kermadec Islands and which is presently a highly active volcano. This study is part of the MANGO project (Marine Geoscientific Inverstigations on the Input and Output of the Kermadec Subduction Zone) which comprises a 1000-km long working area north of New Zealand's North Island. It covers the transition from subduction of the Hikurangi Plateau in the south to erosive subduction of normal Pacific oceanic crust in the centre and thence accretionary subduction further north. Overall the subduction is accompanied by northward increasing seismicity. The aim of this project is to understand the transition throughout the different regimes, the variation of the structures to explain the accompanying seismicty, and the role and evolution of the stratovolcanoes. This will be achieved by analysing the structures of the sediment, crust and upper mantle and also material transfers from its input and output through subduction zone processes.
T51A-0306
Spatiotemporal correlation between seismicity and strain rate change in Japan
Large spatial change of strain rate is considered to be a manifestation of the abrupt spatial change of underground structure such as a subsurface fault with high gravity anomaly, where seismicity is higher than the surrounding area in many cases. Meanwhile, large temporal change of strain rate is considered to be a reflection of the abrupt temporal change of stress accumulation rate, for example, due to an episodic geophysical event. Possible physical causality between seismicity and these spatial and temporal underground condition changes motivated us to examine spatiotemporal correlation between seismicity and strain rate change in Japan. Inquiring into spatiotemporal correlations between various geophysical indices is very important for understanding the subsurface conditions for earthquake occurrence. For this research, we developed a tool for constructing spatially continuous strain rate distribution using daily position coordinates of GPS stations operated by Geographical Survey Institute (GSI), Japan with MATLAB script. Strain rate distribution for a time period, which is composed of gridded data covering the Japanese islands, was calculated based on the position changes per year for GPS stations densely deployed over the Japanese islands. We also made some gridded indices for seismicity using hypocentral data provided by Japan Meteorological Agency. We introduce the preliminary result on spatiotemporal correlation between each index for seismicity and strain rate change.
T51A-0307
New Developments in the Study of Seismicity and Crustal Structure in the Western Hellenic arc
The western part of the Hellenic Arc between Pyrgos and Pylos, western Peloponnese, is one of the most seismically active areas in the whole Mediterranean. This area has been repeatedly affected by large magnitude earthquakes that have caused severe destruction and human loss (i.e. 1886 Philiatra M7.3, 1893 Zante-Keri M6.5, 1899 Kiparissia M6.5, 1947 Pylos M7.0, and 1997 Gargaliani M6.6). Some of the largest regional tsunamis in the Mediterranean Sea have also been observed in association with large earthquakes (i.e. 1630 and 1866, south-western Hellenic Arc), affecting near field as well as remote coastal segments in western Peloponnese, Crete, and as far as Alexandria (Egypt), Adriatic Sea and east Sicily. This situation requires urgent solutions for an effective risk management and mitigation plan. For this, it is essential to study the local seismic activity and define the active fault zones on/ and offshore with high accuracy and resolution required for a reliable seismic and tsunami hazard assessment. In the present study, we built a combined on/offshore seismic array consisting of 17 4C Ocean bottom Seismographs (OBS) and 15 land-stations with three 4.5-Hertz geophones in each locality, and recorded the seismic activity for a period of 2 months. We located more than 3,500 earthquakes by using minimum of 6 stations, and applied a local velocity model that was obtained from active seismic experiments. The microseismicity map is associated with the active fault zones in the area. The shallow activity (0-15 Km depth) shows the deformation of the sediments and the upper crust. It is mainly concentrated around Zakynthos island and the offshore zone of Pylos. Seismicity associated with the deeper part of the crust (16 - 30 Km) coincides again with the tectonically uplifted block of Pylos, Messinia and also with the deformation below the island of Zakynthos. Deeper seismicity follows the subduction zone below western Peloponnese with constantly increasing depth towards the western and eastern parts of it. In a specific zone associated with the transtensional basin between Zakynthos and Pylos, the seismicity extends to depths more than 80 Km. This deep activity is associated with a major fault zone that is displacing vertically two different crustal units of the western Hellenic collision zone. Focal mechanisms associated with this fault zone demonstrate mainly dextral strike slip movements. The active seismic experiments that mapped the geometry of the sediments and crust clearly show that the internal Hellenic zones are thrusted over the external ones and the uplifted units exposed on the island of Zakynthos and that of Messinia are highly elevated blocks of the thrust belts. The present paper is a contribution to the EC, FP6 FrameWork, SEAHELLARC project no. 037004.
T51A-0308
Arc-continent collision in the Southern Urals
The transition from intraoceanic subduction to arc-continent collision in the Southern Urals of Russia is arguably one of the best-preserved examples in any Paleozoic orogen. The arc-continent collision history recorded in the rocks of the Southern Urals began in the Early Devonian with the onset of intra-oceanic subduction and the formation of the Magnitogorsk Arc and ended with its collision with the margin of Laurrusia during the Late Devonian. Arc-continent collision led to the development of an accretionary complex that includes shallowly and deeply subducted continental margin rocks, ophiolite fragments, and sediments that were deposited in a foreland-basin setting. Data from high pressure rocks preserved in the accretionary complex indicates that at the same time as new arc crust was being added to the continent its margin was being subducted beneath the accreting arc to a depth of at least 60 km and perhaps as much as 120 km or more. Geophysical datasets indicate that the Eurasian continental crust currently reaches a maximum thickness of 53 km beneath the Uralides, with normal mantle P-wave and S-wave velocities below a well defined refraction Moho. This suggests that any of the Laurrusia margin that was subducted to current sub-Moho depths (53 km) has been assimilated into the mantle and its identity as continental crust destroyed.
T51A-0309
Exhumation history of the Yushan area- the highest peak of the Taiwan Orogenic belt in Central Taiwan
Taiwan Orogeny to be the result of an oblique arc-continental collision between the Philippine Sea Plate and Eurasia Plate. This collision results in that there are more than 100 peaks of mountain are high than 3000 m. This Yushan is located on Central part of Taiwan Orogenic belt and its elevation is 3952 m which is the highest mountain around the Taiwan area. Although there are some fission track data to study the erosion rate or exhumation rate of mountain area, but there is no any data around the Yushan area. In order to identify the exhumation history of the Yushan area we use the zircon and apatite fission track dating method. The total reset zone of zircon fission track is limit in western side of Dili fault and the highest peak of Yushan and surrounding area all show total reset. The zircon fission track age of the Yushan is 5.6 Ma and the other lower topographic area shows from 3.7 to 5.3 Ma. Assuming the geothermal gradient is 30’J and the annealing temperature of zircon fission track is 235’J, the average exhumation rate is near 1.28 mm/yr from 5.6 Ma to now. This average exhumation rate is rather lower comparing with other area. We supposed that the later stage exhumation rate will accelerate and we will identify by following apatite fission track dating.
T51A-0310
Exhumation history of the Puli basin and surrounding area in central Taiwan
Taiwan Orogeny to be the result of an oblique arc-continental collision between the Philippine Sea Plate and Eurasia Plate. The Taiwan orogenic belt is major composed of Hshishan and Central Range located in western and eastern side separately. The Central Range is major comprised of pre-Miocene metamorphic rock and the Hsihshan Range is comprised of Eocene to Miocene passive margin sediment rocks. On the contrast to the Central Range that has high denudation and uplift rate, the Hsihshan Range has lower denudation rate that can reveal the early stage denudation history and variation mechanism of mountain building. Our new zircon fission track data shows that the oldest of total reset age is ca 5-6 Ma that along the western flank of the Hsihshan Range that identify that the initial stage of mountain building started from 6 Ma. We also found the variation of denudation rate along the strike direction especially around the Puli Basin. Puli basin shows lower topography along the Hsihahan Range. The total reset zircon fission track ages are ca. 6Ma around the Puli basin that indicates a lower denudation rate contrast to the surrounding area. We supposed that this lower exhumation rate could be controlled by subsurface structure or strength of the crust.
T51A-0311
Dimensionality Analysis and Geo-Electric Structure of Long-period Magnetotelluric Data, Southern Taiwan, TAIGER project
The Taiwan orogen has formed as a result of the arc-continent collision between the Eurasian continental margin and the Luzon island arc over the last 3 million years. It is the type example of an arc-continent collision. In 2004, the Taiwan Integrated Geodynamical Research (TAIGER) project was formed and began a systematic investigation of the crustal and upper mantle structure beneath Taiwan. This included new magnetotelluric (MT) data collection to study the geo-electrical structure beneath Taiwan. High quality long period MT data has been collected through collaboration between National Central University, Taiwan, and the University of Alberta, Canada. In total, 82 long-period MT stations were deployed on 4 cross- island profiles in Taiwan with a remote reference station located on Penghu Island in the Taiwan Strait. The remote reference is ~50km from the main island of Taiwan and is used to reduce cultural noise effects in these data from the populated mainland. Dimensionality analysis from tensor decomposition has been performed on these data using the McNeice-Jones algorithm. The results of this analysis indicate that the electrical structures are two-dimensional with dominant strike directions N45° E, N37° E and N29 ° E in northern, central and southern Taiwan, respectively. As expected, these strike directions are essentially parallel to the regional geology. The decomposition parameters of shear, twist and anisotropy for these profiles are small, indicating the 2-D strike directions are well constrained. The dimensionality analysis presented implies that the generation of 2- D inversion models will be appropriate for these data. The results of 2-D inversion show that the collision boundary between the Eurasian and Philippine Sea Plates is beneath the central range in the southern profile. A low resistivity zone is located beneath the western foothills. At mid-crustal depth, a boundary is imaged between conductive western sedimentary rocks and the resistive metamorphic rocks to the east which form the main orogenic belts of the central ranges. This margin occurs near the trace of the Cauchow fault where there is evidence of a conductor rising to the surface. This conductor may be related to interconnected fluids and/or thermal effects in the mid crust. In this paper, the analysis of these data will be examined in detail and the tectonic implications discussed.
T51A-0312
Seismic and Petrofabric studies from Red Mountain Ophiolites, South Island, New Zealand
We study deformation mechanisms within the crust and upper mantle by comparing seismic and structural measurements of peridotite fabric at Red Mountain. We measure fabric from microscopic through km scales by comparing crystallographic orientations of peridotites with field measurements of seismic anisotropy along lines of 100-200 m length, and with shear-wave splitting of local and teleseismic earthquakes. SKS splitting from stations on and near Red Mountain are similar to previous measurements on South Island. Polarizations of the first-arriving waves (φ) are in the range of 15 to 60 deg with 1.9 3.1 s delay times (dt), consistent with previous interpretations of crystal preferred orientation of olivine due to shearing parallel to the Alpine Fault at mantle depths. Preliminary average shear wave splitting measurements for local earthquakes with depth >100 km have φ of 55 deg, consistent with the SKS results, but dt is much smaller, at 0.03 ± 0.01 s (1 σ). Shallower events yield φ of 145 deg with dt=0.32 ± 0.003 s. Similar to shallow S-wave splitting, shallow seismic P-wave speeds for waves traveling in the upper few meters are faster (1.8 ± 0.1 km/s) along a profile oriented at 150 degrees than along the orthogonal profile (1.4 ± 0.15 km/s), yielding anisotropy of 23 ± 9.5 %. The S-wave velocity for the average top 30 m calculated from surface wave analysis on the radial components yielded 14% anisotropy, with a fast direction of 150 degrees. Shear wave splitting measurements from hammer shots yield delay times of 0.01± 0.005 s with the fast orientations in the 120-150 deg range. On Red Mountain, macroscopic foliation attitudes defined by compositional layering and orthopyroxene (opx) shape fabrics strike ~70 deg., and dip ~ 80 NW. Stretching lineations defined by opx prism alignment have a mean pitch of ~50 SW. Near-surface cracks have strikes of ~120 degrees, almost perpendicular to the foliation and lineation. Electron Back-Scattered Diffraction studies of crystallographic orientation of olivine were carried out on three peridotite samples taken from near the seismic lines. Olivine [100] axes are oriented in the lineation direction, [010] axes are normal to the foliation, and [001] is within the foliation plane and perpendicular to lineation. This suggests dislocation creep within a high-temperature [100] (010) slip system. Average S wave anisotropy of these samples based on the single-crystal elastic constant tensors is a maximum of 7.3 % for a wave traveling in the foliation plane normal to the flow direction, and φ is sub-parallel to the olivine [100] axes representing the flow direction marked by lineation. Shallow anisotropy measured from near surface studies and shallow local earthquake shear-wave splitting is parallel to the near-surface cracking, consistent with anisotropy caused by cracks rather than by mineral alignment. This suggests that the NE/SW anisotropy measured from teleseismic phases is not being contaminated by near surface crustal mineral alignment, but instead may be caused by shear-induced olivine alignment at mantle depths.
T51A-0313
Preliminary results of surface deformation in suburban central Taiwan observed by PSInSAR
Precise leveling revealed up to 10 cm/yr of land subsidence occurred in the suburban central Taiwan during the past decades. Although the general trend of subsidence is known, the spatial pattern of subsidence and the exact nature of the subsidence is still far from clear. In order to quantify different types of deformation and further mitigate the hazards caused by both tectonic and anthropogenic activities, many geodetic measurements were deployed in Taiwan. However, the spatial density of these geodetic measurements were generally very low due to the high cost of the instrument or the time required for the campaign. An alternative method was using SAR interferometry, however, the vegetations and farmland in central Taiwan prevent the radar signal from having high enough coherence to form meaningful interferogram. Persistent scatterer InSAR was therefore deployed in our study area to acquire the deformation signal. Radar images were processed using Diapason software to form interferograms and the phase signals were processed using Matlab. The result showed that a significant amount of area in central Taiwan was subjected to a very rapid subsidence rate of up to 10 cm/yr. Precise leveling in this area also provided evidence for such a rapid subsidence in this area. Further works are needed in order to precisely map out the area affected and find out the causes of the phenomenon.
T51A-0314
Seismotectonics of Northernmost Manila Subduction ‘V in Lights of Two OBS Arrays SW offshore Taiwan
The Manila trench is well-known as the convergent boundary where the South China Sea plate subducting easternward beneath the Philippine archipelago. As the oblique subduction/collision of plates taken place around the Taiwan area, the topographic feature of the Manila trench lose its topographic identification when approaching northernward to Taiwan. Systematic study for the seismic source parameters along the Manila trench is an important clue to resolve the issue of the complicated plate interaction in the vicinity of northern Manila trench. However, seismological monitoring is largely limited by land territory, where we can readily install established seismometers. In order to compensating the scarcity of earthquake observation at the northern Manila trench, we used Ocean Bottom Seismometer (abbr. OBS) to detect local earthquakes at the relevant area. This study is constructed by two short-period OBS arrays deployed at the offshore SW Taiwan in the years of 2005 and 2006, respectively using 7 and 9 OBS. Local seismic signals have been well logged by the used OBS machines. Thousands earthquakes including micro-events are successfully compiled from these seismograms, and focal mechanisms can be determined by means of near-field waveform simulations or P-wave first-motion polarization for some of the large earthquakes. The preliminary study with these seismic data shows a clear seismicity inclined plane dipping to the northeast at SW Taiwan island. The near-field focal mechanisms reveal apparent convergent forces acting in this area in NW-SE direction. The seismic velocity structure of the relevant region is refined with the OBS records in this study. A clustering algorithm will be applied to relocate earthquakes and consequently to obtain tighter earthquake clouds and thus better-defined seismogenic zone at the northernmost Manila trench.
T51A-0315
Data Processing and Structure Investigation From Dili Test Shot Experiment Across Central Taiwan
A 500 Kg explosive test shot experiment detonated in the town of DiLi with receiver arrays deployed across central Taiwan was carried out on 1:05 am, 6 October, 2006. The scientific objectives are for preliminary structure studies, wave propagation effects, efficiency of the energy propagation and attenuation through western sedimentary basin as well as the data processing strategy for the TAIGER-CD project. More importantly, such a unique field survey activity provide an important opportunity to verify the strong lateral change of Moho depth derived from receiver function analysis base on the results of H and Vp/Vs ratio (£e) study across SSLB and YULB broadband stations reported by Wang et al., (2006) and the proposed in-active subducting slab in central Taiwan by Chen et al., (2004). Overall speaking, the generated energy seems able to reach both end of survey deployment and island-wide as first arrival travel-time picks are apparent. However, detail data analysis and travel-time inversion studies all show potential difficulties may encountered in the coming TAIGER active source survey. The identification of latter phases is highly contaminated by the scattered energy generated from both source signature and near-surface heterogeneity effects which makes conventional data processing insurmountable. The strong oscillating source signature makes the conventional spiking and predictive decon ineffective to suppress long tails generated at shot hole. The nonlinearity of source signal, highly scattered energy, multiples and 3D effects may further mask later arrivals and obfuscate phase identification. The observed non-impulsive and ringing source signature my attribute to large borehole diameter, compaction of the explosive and un-evenly distribute booster, PVC borehole casing and poorly selected site condition. Moho reflections can not be identified from DiLi test shot experiment. We further implemented the HHT technique of Huang (1998) for signal processing to identify nonlinear and non- stationary properties of recorded data. The key part is to decompose data into a finite numbers of intrinsic mode function (IMF) components with time-variable amplitudes and frequencies by using the empirical mode decomposition (EMD) combined with the Hilbert spectrum analysis (HSA) and power spectral density analysis. First P- and S- wave arrivals can be identified from decomposed IMFs. The decomposed IMF3 show satisfactory minimal value of orthogonal index but the significance test indicates that the signals are mainly compose of noises. Signal-to-noise ratio is low for the recorded Dili test shot data. Study of CMP profile indicates that the explosive energy decayed within six second with a noticeable but relatively weak signal occurred around 10 sec. Therefore, the identification of reflected Moho and detachment phases with the characteristic of relative high frequency content is suspicious. Travel-time refraction and reflection inversion studies show sedimentary basin across western Taiwan. CMP profile revealed shallow velocity structure variations but hard to recognize deep reflectors. The maximum depth of investigation for velocity structure profile obtained is around 25 to 30 km. Careful cooperative plan involves using one ton explosive on land, safety concerns, field deployment operations, more shot points, geophone group stacking, timing and detail data processing strategy and preparatory efforts must be carry out immediately.
T51A-0316
Evidence of a slab of subducted lithosphere beneath central Taiwan from teleseismic S waves
Knowing whether there is a slab of subducted lithosphere beneath central Taiwan is important for our understanding of the regional tectonic evolution. Seismic waves propagating through a slab exhibit earlier arrivals, smaller amplitudes, and broader waveforms relative to those undisturbed. While P waves from Tonga- Kermadec deep and intermediate-depth earthquakes, relative to Kinmen station, exhibit reduced amplitudes and travel times for central Taiwan stations (Chen et al., 2004), we incorporate observations of S wave in this study to have better constraint. The Generic Array Processing (GAP) software package (Koper, 2005) is applied to process data recorded by Broadband Array in Taiwan Seismology (BATS). Genetic algorithm (GA) is used to simultaneously determine relative arrival times of BAT's stations with the cost function based on sum of cross- correlation over each pair stations. Preliminary results show that S waves of Tonga-Kermadec events exhibit stronger travel time reductions than those of P waves for central Taiwan stations, consistent with the effects of an eastern dipping aseismic slab. The possibility of crustal effects can be eliminated by having contrary observations for Hindu Kush events. Amplitude information will be included in further study.
T51A-0317
Crustal Q Tomography in Taiwan
While 3-D crustal velocity structures in an active orogen can be derived from travel time tomography, other rock properties are needed for a more complete characterization of the orogen. The anelastic quality factor (Q) provides constraints on the thermal condition or perhaps fluid activity within the rocks. With the recent deployment of a PASSCAL broadband array in Taiwan, enhancing the existing BATS and Central Weather Bureau networks, the station density has increased significantly and a rich archive of broadband local earthquake data has become available. The advantages of using broadband data, rather than the previously available short period data, include the increase of spectral width, better instrumental calibration and stronger S wave signals. As a first step we derive an approximate structure using dt* calculated from the ratio of P and S wave spectra, after Roth et al. (1999), for paths between earthquakes and stations. By using their "best" Qp/Qs value of =1.75 we perform a tomographic inversion to derive a regional measurements of Qs in three dimensions. These preliminary Qs results from broadband data indicate higher values of Qs in Eastern Taiwan when compared to the western plain, both at the surface and at a depth of 30 km. These results are similar in distribution to the Qs tomography results of Chen (1998). We will use the methodology of Roth et al. (1999) to develop more robust 3-D Qs as well as Qp distributions for Taiwan using the broadband data now available.
T51A-0318
Surface strain field in transition from arc-continent collision to subduction: GPS measurements in northern Taiwan, 1995-2005
We present Global Positioning System (GPS) measurements for the period 1995-2005 at 126 sites in northern Taiwan. Based on surface strain field from the results, we describe the transitional tectonics from arc-continent (Luzon-Chinese) collision to the converging Ryukyu arc-trench subduction. Station velocities relative to the stable Chinese continental margin station, S01R, were derived from coordinate time-series of each station by solving weighted least-squares problems. Horizontal velocity field shows vectors of 0.3-7.3 mm/yr toward the NW in the south-western part of the network (Hsinchu area); velocities of 1.0-7.8 mm/yr from south to north rotating from 8° to 143° in the northern area of the network (Taipei area); and a clockwise rotation from west to east with vectors of 9.3-41.2 mm/yr from 53° to 146° in southeastern part of the network (Ilan area). Patterns of strain rate field show, from west to east, dominant NNW-SSE shortening in the Hsinchu area; N-S shortening and E-W lengthening in the Taipei area; and predominant NE-SW compression and NW-SE extension in the Ilan area. Furthermore, prominent clockwise block-rotation is indicated, respectively, in the northernmost Foothills-Hsuehshan Range and the northernmost Eastern Central Range. The northernmost Foothills- Hsueshan Range, bounded by a strike-slip fault to the north, is characterized by coherent motion (low internal strain rate of < 0.20 μstrain/yr) with the best fitting Euler pole located near the northeastern tip of the Ilan plain. For the northernmost Eastern Central Range, although showing block-like rotation motion, a large uncertainty was found for the best fitting Euler vector. Trench roll-back and back-arc opening probably superpose onto the arc-continent collision-induced rotation in the northernmost Eastern Central Range.
T51A-0319
S-Splitting Measurements and Taiwan Orogeny
Shear wave splitting is the most widely used method of quantifying anisotropy in the upper mantle and crust. We use broadband teleseismic shear waves (SKS and SKKS) recorded at the newly deployed TAIGER stations as well as BATS and CWB networks in Taiwan to obtain the splitting parameters: the fast wave polarization direction (φ) and delay time (δt). Altogether 50 good quality measurements were added to the database. By adding our measurements to those made previously by Rau et al. (2000) and Huang et al. (2006) we see an interesting pattern that appears in many compressive orogens, i.e., the fast direction following the local structural trend. This is especially true for the new measurements in the area around the Peikang basement high: they wrap around the high. In northernmost Taiwan the structural trend turns to nearly EW and the measured fast directions, both new and from the previous results, follow it. In terms of delay times at a station significant variations are sometimes observed; they appear to be a function of event back-azimuth and/or phase used (ScS, SKS or SKKS). The maximum delay time (2.6 sec) is at ENLB near the northern end of the Coastal Range and the minimum delay time (0.4 sec) is at KNMB just offshore of southeastern China. The connection between the crustal imprint and fast direction measurements, the effect of lateral variation of anisotropy as well as comparing mantle and crustal fabrics are to be elucidated.
T51A-0320
Inversion Accompanying Arc Collision in the Taiwan Orogen
A kinematic model for the Western Foothills fold and thrust belt (WFFTB) of southern Taiwan based on serial balanced sections shows that inversion of pre-existing normal fault basins is common. The basal fault is interpreted to follow both stratigraphic horizons and pre-existing normal faults. The estimated aggregate shortening of the easternmost fault of the WFFTB is about 40 km. Restoration of the WFFTB structures places the Eurasian margin at one time at least as far east as the present position of the Coast Range. A regional structure, the sub-Yuching anticline, explains the uplift of the Yuching and Tignpinglin synclines above their regional level. This structure is on-line with the interpreted continuation of the Manila trench beneath Taiwan. There is not a strict forward-breaking thrust sequence in southern Taiwan. The Tachieshan, Schuangtung and Pinghchi faults first moved before deposition of the Cholan Fm between 6 and 3.3 Ma. These faults were reactivated at 1.5 Ma continuing until today. The most western thrust sheets contain structures closely associated with the Peikang high, implying that the structures are either formed by inversion of faults of the Peikang high or new thrust faults whose ramps are located on pre-existing normal faults. While inversion has not been described in either the on- land or offshore accretionary prism portion of the Taiwan orogen, we propose that the collision portion should be best viewed as a series of inverted structures built upon pre-existing Neogene basins. The province of inversion follows the Eurasian margin and may not yet have propagated offshore.
T51A-0321 INVITED
Collision Tectonics of Taiwan and TAIGER Experiments
By any measure of tectonic activity Taiwan can be described as vigorous. The results from abundant permanent GPS station network on the island show that the convergence of the Philippine Sea and Eurasian plate near Taiwan is about 80 mm/yr and much of the island is sustaining a compressional strain rate in excess of 2 ppm per year in and a dilatational strain of more than 1 ppm per year along the northeastern shore and in a significant portion of the Central Range. Judging from ample thermochronometric data now available on the island it is clear that the rapid uplifting has been ongoing for the last million years. The high seismicity in Taiwan defines parts of the plate tectonic environment and the1999 M7.6 thrust faulting event in central Taiwan and the 2006 M7.1 normal faulting event offshore of Hengchun Peninsula show the variations from subduction to collision from south to north. For longer time scale the Geology of Taiwan shows clearly the accretion of the arc onto the continental shelf. Several tectonic models of Taiwan have been proposed on the basis of available data and tectonic concepts. The implications of these models on the collisional tectonics do not always agree. To resolve them and to build a more comprehensive model of Taiwan detailed information regarding the crust and upper mantle is particularly needed. The TAIGER (TAiwan Integrated GEodynamic Research) program mounts several major field experiments and aims to fill the gap of information. Through active and passive seismic experiments we wish to obtain multi-scale crustal and upper mantle seismic images not only under the island but also around Taiwan for the purpose of defining plate geometry and the configuration of crust and mantle boundaries. The imaging will also target key components in proposed models. For example, the subduction of the Eurasian plate is often included; can we image it as a high velocity anomaly even though it is not seismically active? What is its geometry? At the same time the broadband instruments on land and at ocean floor, mainly to the east and south of Taiwan, will record earthquakes. The passive seismological data will provide a refined map of where the earthquakes, whether interplate or intraplate, are occurring and the plate kinematics. To assess the material properties and the ambient temperature field conductivity profiles from magnetotellurics and laboratory petrophysics have been largely completed. Land broadband seismic stations have been in operation now for six months to more than a year and analysis has begun. A test explosion in Central Taiwan was carried out in October 2006. By early November 2007 broadband OBS deployment should be complete. Three land-based active source profiles and a CDP line across the main boundary between the Foothills and Central Range are scheduled for February/March of 2008. In our preparation to digest this large amount of information and to construct more comprehensive tectonic models we have begun to conduct complete numerical geodynamic experiments, incorporating geometry, ambient conditions, rheological properties of rocks, plate velocities, etc. These experiments, initially only two- dimensional, will allow us to explore the range of initial models that can arrive at an end state that resembles the observations.
T51A-0322
Buoyancy control on continent subduction and implications for the dynamics of India-Asia convergence
At some point during their evolution, most trenches encounter continental lithosphere. Because average continental lithosphere is positively buoyant, collision is often the result. However, there is evidence that some continental crust and lithosphere does get subducted. For example, Indian plate kinematic reconstructions show that Meso-Cenozoic subduction consumed a highly heterogeneous lithosphere including small oceanic basins and substantial portions of the flanking continental lithosphere, which had undergone various degrees of extension. Here we study how continental lithosphere can affect the subduction process in general, and to what extent it can be responsible for the variations in plate motions during Greater Indian continent subduction and subsequent collision onto Asian plate. We model a visco-elastic slab, which subducts without external forcing, i.e. driven by slab pull and ridge push only, into a passive viscous mantle. We find that lithospheric buoyancy is the primary control on continental subductability. Only if a significant amount of upper crust is stripped off, and the mantle lithosphere is not strongly melt depleted, does continental lithosphere become negatively buoyant. If in addition, the lower crust is eclogitized, continental lithosphere can even be as negatively buoyant as old oceanic lithosphere. Oceanic slab pull and ridge push modulate to which depth continental material is subducted (usually between 100 and 400 km) before subduction stalls, but can not change whether sustained continental subduction occurs or not. However, the combination of continental buoyancy, slab pull and ridge push strongly affects the partitioning of subduction velocities, i.e. plate advance, subduction velocity and trench motions. We find that the combination of a decreased slab pull, due to continent subduction, and continued strong ridge push is required to explain the peculiar overturned morphology of the Indian upper mantle slab, and match the variations in relative trench advance and subduction rates, including the strong trench advance which resulted in indentation.