T42A-01
Deformation of Taiwan from Continuous GPS Monitoring
As of 2006 there are more than 290 continuously recording GPS stations(1,2) now in and around Taiwan and its surrounding islands. We have determined horizontal and vertical velocity vectors, the areal strain rate, the shear strain rate and the volume strain rate. Because of the rapid convergence of Eurasian plate with the Philippine Sea plate (PSP) near Taiwan the trend in the GPS displacement time series is large enough to derive the above- mentioned quantities on a yearly basis and thus track their changes as a function of time. In the following discussions we shall use the Penghu island station (S01R) as reference when relevant. The overall patterns of the velocity vectors in Taiwan reflect the geometry of the plate convergence that created Taiwan. In the mature collision zone of central Taiwan, situated on the Eurasian plate, the velocities are largest on the eastern side and decreases toward the west to very small values in the Coastal Plain. In the south the western side of the Hengchun Peninsula is still moving westward or southwestward at about 50 mm/yr, apparently as a part of the PSP. In general, in central and northern Taiwan, the velocity vectors are oriented WNW but north of about 24ON the velocities become very slow because the PSP has subducted toward the north near that latitude so the collision takes place at depth and the deformation on the surface becomes small. The areal strain rate patterns derived from the horizontal velocities show that although much of Taiwan is under compression, two areas consistently show dilatation: one along the high Central Range and the other one in the northeast coastal area south of Ilan. In the Central Range the dilatation is probably related to the building of the mountain range and in the northeast the subduction of the PSP leaves the top portion a "free boundary". Although the noise level of the vertical time series is about 3 times higher than the corresponding horizontal measurements the continuous data show patterns of deformation. In the few years after the 1999 Chi-Chi earthquake subsidence occurred in many areas but by 2007 many areas had resumed rising. Rates higher than 10 mm/yr are found in a few areas. Several coastal areas show rapid subsidence as a result of water well pumping. Over the period of more than six years, time-dependent variations can be detected. For example, from 2002 through 2006 there appear to be a detectable increase in areal strain. We can expect to see significant changes in deformation of the Taiwan orogen through continuous GPS monitoring. (1) Operated by Central Weather Bureau, Central Geological Survey, Land Survey Bureau, Ministry of Interior, Institute of Earth Sciences, National Taiwan University, and others. (2)For details of the network please see GPSLAB web page of Institute of Earth Sciences, Academia Sinica: http://gps.earth.sinica.edu.tw http://gps.earth.sinica.edu.tw
T42A-02 INVITED
Numerical Models of Subduction to Collision in Taiwan.
The Island of Taiwan is formed by the collision of the Philippine Sea plate with the Eurasian plate. In the south, the Philippine Sea plate overlies a seismically active subduction zone. On the other hand the Central Range is underlain by only crustal seismicity. Does the Asian continent actively subduct into the upper mantle under the Central Range or does it underplate the Luzon arc? Do the resulting large-scale structures provide the dynamic forces to explain the pattern of deformation observed across Taiwan? We study the dynamic evolution of the Taiwanese orogeny using 2D elastic-plastic and viscoelastic numerical models of deformation of the lithosphere. We drive this modeling exercise from the assumption that the present day structure and motion depend on the long-term stress and strain history of the lithosphere. We start the models from the subduction of the Eurasian plate under the Philippine Sea plate and let it evolve to the collision of the Eurasian plate with the Luzon arc. We find that whether or not the Asian crust is dragged in the mantle is dependent on the crustal structure of the South China Sea margin. If the ocean continent transition there is mainly formed of thinned continental crust the accumulated buoyancy of the thickened crust during collision generates forces large enough to tear the subducting slab off the margin. If it consists of accreted oceanic crust the margin is dragged into the upper mantle and the slab stays attached to the Asian plate. Both scenarios lead to predictions on the type of structures and motions that should be observed by future seismic experiments in Taiwan.
T42A-03
Mega-thrust and Intra-slab Earthquakes beneath Tokyo Metropolitan Area around subduction and collision zones in JAPAN
In central Japan the Philippine Sea plate (PSP) subducts beneath the Tokyo Metropolitan area, the Kanto region, where it causes mega-thrust earthquakes, such as the 1703 Genroku earthquake (M8.0) and the 1923 Kanto earthquake (M7.9). The vertical proximity of this down going lithospheric plate is of concern because the greater Tokyo urban region has a population of 42 million and is the center of approximately 40 % of the nation's economic activities. A M7+ earthquake in this region at present has high potential to produce devastating loss of life and property with even greater global economic repercussions.The M7+ earthquake is evaluated to occur with a probability of 70 % in 30 years by the Earthquake Research Committee of Japan.We started the Special Project for Earthquake Disaster Mitigation in Tokyo metropolitan areas, a project to improve information needed for seismic hazards analyses of the largest urban centers. Under the project we will deploy a 400-sation dense seismic array in metropolitan Tokyo and Kanto, referred to as the Metropolitan Seismic Observation network (MeSO-net) in next 4 years. The target area of the present project is unique in tectonic setting because two oceanic plates, Philippine Sea plate (PSP) and Pacific plate (PAC), are subducting beneath the Kanto and also a volcanic arc, Izu-Bonin arc, is colliding with Honshu arc. The situation makes the tectonics complicated: there are both zones of smooth subduction and collision of the oceanic plate with the landward plate, either the Eurasian plate or the North American plate. Furthermore, the PSP encounters the PAC at shallow depth in the eastern Kanto region. The newly developing MeSO-net will contribute to understand the generation mechanism associated with the plate subduction and collision. Assessment in Kanto of the seismic hazard requires identification of all significant faults and possible earthquake scenarios and rupture behavior, regional characterizations of the PSP geometry and the overlying Honshu arc physical properties. Our study addresses (1) improved regional characterization of the PSP geometry based on new deep seismic reflection profiles (Sato etal.,2005), reprocessed off-shore profiles (Kimura et al.,2005), and a dense seismic array in the Boso peninsula (Hagiwara et al., 2006) and (2) identification of collision of internal heterogeneity (Wu et al., 2007). We compile these results and present a new model which will be verified by data from the planned MeSO-net. We present a relatively high resolution tomographic image from so far obtained data to show a low velocity zone which suggests a possible internal failure of the slab; a source region of the M7+ intra-slab earthquake. Our study contributes a new assessment of the seismic hazard in the Tokyo metropolitan area. http://www.eri.u- tokyo.ac.jp/shuto/EN/index.html
T42A-04
From Oceanic Lithosphere Subduction To Continental Collision: Influence Of The Plate Contact
We showed recently that the overall dynamics of oceanic subduction differ depending on whether the plate contact is a fault or a channel (De Franco & al., 2007. GJI, doi: 10.1111/j.1365-246X.2006.03498.x). Here we investigate how the plate contact affects the transition from oceanic lithosphere subduction to continental collision. We use a finite element method to solve the heat and the time dependent momentum equations for elastic, (power law) viscous and plastic rheologies. For the same rheological properties and driving forces , varying the nature of the plate contact leads to three types of responses: subduction of the entire continental lithosphere, shear delamination of the continental crust or slab break-off. We make the following observations from our numerical experiments. The presence of a subduction channel promotes coherent and, when the boundary conditions allow it, plate-like subduction of the continental margin. In models with a subduction fault, coherent subduction of the incoming continental lithosphere occurs when the colliding passive margin has a gentle ocean-continent transition. The approaching continental sliver starts to subduct and the subduction is characterized by a non-plate-like behavior, slower subduction velocity than in channel models and strong slab deformation. If the continental margin is steep and the strength of the incoming continental crust is high, fault models result in locking of the trench, eventually leading to slab break-off. If the crustal strength is relatively low, shear delamination of the upper crust is expected. In the channel model this type of delamination never occurs. The tectonic setting does not significantly affect the nature of the model response. We conclude that the plate contact type, together with the geometrical and rheological properties of the incoming continental fragment, is a crucial subduction characteristic controlling the response of continental collision during the transition from oceanic subduction to continental collision. During the early stage of continental collision, the plate contact plays a more relevant role than the magnitude of slab pull and the tectonic setting.
T42A-05 INVITED
Sedimentary Response to Arc-Continent Collision, Permian, Southern Mongolia
The Eurasian Tien Shan-Yin Shan suture is a ~3000 km-long boundary between Paleozoic arc and accretionary complexes (the Altaids) and Precambrian microcontinental blocks (Tarim and North China block). Stratigraphic data are presented from localities in southern Mongolia spanning more than 800 km along the northern margin of the suture. Facies descriptions, climatic indicators, sandstone provenance, and paleocurrent data help reconstruct Permian basin evolution during and following arc-continent collision, and results are integrated with previously published data to create a preliminary regional synthesis. Upper Permian strata of southern Mongolia comprise fluvial successions in the southwest, and marine turbidite deposits in the southeast. Floral assemblages show mixing of Siberian craton and North China block communities, indicating their close proximity to Mongolia by Permian time. There is a rapid transition from humid environments in the Late Permian to more arid conditions in the Early Triassic, which corresponds to the global Permian-Triassic boundary event, but may also reflect more local driving mechanisms such as rain shadow effects. Permian sandstones from Mongolia have undissected to dissected arc provenance, with little input from continental or recycled orogen sources. Timing of the nonmarine-marine facies transition and cessation of arc magmatism broadly supports earlier collision along the western part of the suture zone than the eastern part (e.g., Late Carboniferous-Late Permian). However when regional geologic constraints are integrated, a more complex model involving differential rotation of Tarim and the North China block is preferred. Late Paleozoic rocks of southern Mongolia have been subsequently dismembered along Mesozoic-Cenozoic strike-slip faults, and thus also represent the long-term record of intracontinental deformation within accreted, heterogeneous crust.
T42A-06 INVITED
Anatomy of Intra-Oceanic Arc Systems
Intra-oceanic arc systems (IOAS) are ultimately embedded in orogenic belts and added to the continental crust. Reconstructing fossil IOASs in collision zones requires understanding the salient features of a typical IOAS. IOASs have the relative dimensions of tagliatelle (flat) pasta: much wider (~250 km) than thick (10-30 km), much longer (1000's of km) than wide. IOASs begin to form when subduction begins, either spontaneously (SNSZ) or by forced convergence (INSZ). For SNSZ, IOASs start as broad zones of seafloor spreading associated with subsidence of the adjacent lithosphere, whereas INSZ IOASs are built on trapped crust. IOAS magmatism manifests the evolution of its subduction zone and indirectly the breadth of the subducted ocean. Two stages in SNSZ IOAS magmato-tectonic evolution exist: infancy and maturity. Infancy lasts 5-10 Ma and results in broad zones of seafloor spreading of tholeiite/boninite; this becomes forearc for the mature IOAS and is emplaced as ophiolite during collision (subduction zone failure). Arc maturity begins with true subduction, as the subducted slab reaches depths ~130 km, focusing magmatism to begin building the magmatic arc ~200km away from the trench and allowing the forearc to cool and hydrate. Mature magmatic arcs mostly yield low-K tholeiitic and medium-K calc-alkaline magmas. Magmatic focusing begins crustal thickening beneath the magmatic arc, at ~500m/Ma for the Izu-Bonin-Mariana IOAS. No systematic compositional evolution to more LIL-enriched primitive magmas occurs once IOAS maturity is reached, except when upper plate stress regime (BAB formation, strike- slip faulting) or the nature of subducted material (more/different sediments, young oceanic crust) changes. Thickening is accompanied by processing of crust beneath the magmatic arc, with progressive differentiation into upper volcanic, middle tonalitic, and lower mafic layers, producing an increasingly effective density filter for magma ascent. Crustal layer formation involves anatexis of amphibolite and mafic melt fractionation to form nests of felsic plutons, accompanied by drip-delamination of pyroxene-rich residues and cumulates back into the mantle. Active IOASs thus have mass transfer in both directions across the crust-mantle boundary beneath the magmatic arc, leading to small P-wave velocity differences between gabbroic lower crust and pyroxenitic upper mantle. Forearcs, in contrast, are underlain by serpentinized harzburgite. Intra-oceanic arc systems are rarely associated with accretionary prisms; because most are far-removed from continental sources of sediment, they subduct oceanic lithosphere with thin sediments and have naked forearcs subjected to tectonic erosion. These aspects of IOASs should be revealed in accreted ancient arcs: 1) Ancient IOAs should be large, both wide and thick; and 2) Ancient IOASs should be asymmetric. Scraps of IOASs could be smaller slivers of crust, brought into place by strike-slip faulting, but a true accreted arc should be as obvious to a geologist as a beached whale is to a beachcomber.
T42A-07
New geochemical and isotopic constraints on magmagenesis in an Ordovician arc-continent collision, Ireland: The Tyrone Igneous Complex
The Tyrone Igneous Complex (TIC) of County Tyrone, Northern Ireland, has previously been identified as an ophiolite dating to 471 Ma. Although this age suggested along-strike equivalence with igneous and volcaniclastic units of Connemara and the South Mayo Trough in western Ireland, no detailed petrologic or geochemical analyses were previously available from the TIC to substantiate this inference. We present new trace-element and Nd isotopic analyses from 37 spatially and lithologically comprehensive samples of the TIC, including gabbros, basaltic sheeted dikes, a tonalite pluton, and felsic lavas. The geochemical affinity of these samples compares favorably to western Irish plutons and volcanogenic sediments of similar age. Felsic TIC units, in particular, display rare-earth-element enrichment and other trace-element patterns (including relative high-field- strength-element depletion) characteristic of supra-subduction-zone magmatism. Together, the TIC and western Irish units correspond to pre-, syn-, and post-collisional phases of an Early Ordovician collision between an intra- oceanic island arc and the Laurentian continental margin during the closure of the Iapetus Ocean. The new data improve constraints on the magmatic evolution and timing of tectonic events (e.g., initial collision, subduction of continental sediment) within an evolving arc-continent collision zone, processes important for the production of continental crust.
T42A-08
Crustal Recycling in Accretionary Orogens: LA-ICP-MS Geochronology and Hf Isotope Evidence of Detrital Zircons in Late Paleozoic Turbidite Units of the Southern Central Andes
Active margin accretionary orogens are considered major sites of formation of juvenile continental crust. In the southern central Andes, Nd model ages of the Proterozoic metamorphic basement, the Neoproterozoic and Paleozoic sedimentary cover and the coeval magmatic rocks cluster between 1.8 and 1.4 Ga and vary between 2.2 and 1.2 Ga, respectively. This indicates a dominant involvement of Meso- and Paleoproterozoic crustal material in crustal cycling. We applied LA-ICP-MS to obtain U-Pb age spectra of detrital zircons and to analyze individual dated zircons for their Hf isotope systematics. We selected 7 samples of Late Devonian to Permian turbidite units of northern Chile because their detritus most likely reflects the provenance evolution of this margin prior to the onset of the Andean active margin cycle. The studied rocks are parts of an accretionary wedge at Gondwana's late Paleozoic active margin in the Central Andes. The units overstep the boundary between the enigmatic Chilenia terrane of alleged Laurentian origin in the south and coeval autochthonous regions of Gondwana to the north. Zircon populations of overstepping units deposited on and derived from Chilenia should ideally demonstrate the Laurentian heritage of the terrane by an absence of geochronologic events typical of Gondwana and absent from Laurentia, i.e. the Brasiliano orogenic cycle. Of each sample we analysed 140 to 145 grains for their U-Pb systematics. We only considered grains falling within 10% of concordia, i.e. between 99 and 118 grains per samples. Subsequently c. 20 dated grains per sample were analyzed for Hf isotopes. The U-Pb data of all samples reflect the same distribution of main events, the Rhondonian orogeny of the southwestern Amazon craton between 1.5 and 1.2 Ga, the "Grenvillian" orogenic cycle between 1.2 and 0.9 Ga, the Brasiliano cycle between 0.8 and 0.55 Ga, and the early Paleozoic Famatiniano cycle between 0.55 and 0.42 Ga. The younger, Late Carboniferous-Permian turbidites also register a cluster of ages between 0.33 and 0.25 Ga marking the re-initiation of subduction magmatism after a period of magmatic, tectonic and metamorphic quiescence in the Late Silurian and Devonian. Evidence of the Gondwanan Brasiliano cycle in all samples casts doubt on a Laurentian origin of the Chilenia terrane. The Hf data demonstrate that all analyzed sedimentary rocks contain a significant number of juvenile zircons with positive epsilon Hf values. These grains, however, mostly fall in the 1.5 to 0.9 Ga age bracket and indicate formation of their protoliths during the Rhondonian and "Grenvillian" orogenic cycles. Grains with negative epsilon Hf values can also be traced back to the same juvenile Mesoproterozoic crustal domains. In conclusion, the available data indicate that formation of juvenile crust mainly took place between 1.5 and 0.9 Ga. During all subsequent orogenic cycles affecting the region of the southern central Andes between 0.9 and 0.25 Ga, the crustal evolution of this accretionary orogen seems to have been characterized by repeated recycling of the continental crust originally formed in the Mesoproterozoic.