T41D-01
3-D Imaging of Marlborough, New Zealand, Subducted Plate and Strike-Slip Fault Systems
We present a 3-D seismic velocity model of the northern South Island, New Zealand, using several passive and active data sets. This area is the transition from Hikurangi subduction in the North Island to oblique continental collision along the Alpine fault and the Southern Alps, and geodetically observed deformation is consistent with plate motion primarily taken up on a series of rotating strike-slip faults. We carry out simultaneous inversion for Vp and Vp/Vs and hypocenters using earthquake data from three temporary seismic arrays and the permanent seismic network, and active data recorded along offshore profiles and at onshore stations. We image the slab as a continuous unbroken feature extending from the active subduction zone to the north. Across the Marlborough region, the 3-D hypocenters define the slab as a smooth curved feature to over 250-km depth. Slab seismicity below 100-km depth abruptly ends in the section where the Awatere fault transfers to the Alpine fault. The subducting Pacific plate has a thick crust, including greywacke upper crust, which slows subduction. The downwarping interaction with the translating overlying plate forms a 50-km long zone of apparent crustal thickening with Vp < 7.0km/s to 40-km depth. The Australian plate has a high-velocity block to 65-km depth, aligned with the Separation Point pluton and trending oblique to the subducted slab. This zone of thicker Australian lithosphere may also hinder subduction. The Marlborough faults trend about 25 degrees to the strike of the underlying slab, so that their relation to the slab varies along the fault, and the overlying plate ranges from 20 to 50-km thick along the faults. Thus the fault interaction with the slab varies from translating material directly above the subducting plate to bottoming into a thick ductile region. The lower crust has high Vp/Vs under the Marlborough faults apart from the region of Haast schist, which may indicate excess fluid released by the subducted plate.
T41D-02
Electrical resistivity structure of a transitional ocean-continent subduction zone: the Marlborough strike-slip system, northern South Island, New Zealand
The Marlborough strike slip fault system of the northern South Island, New Zealand, lies in the transition of plate convergence from near decoupling between upper and lower plates in true subduction (northern North Island), to essentially complete coupling between the plates with near-continuous lithospheric thickening and no subduction (central South Island). Here, four major dextral fault zones at a low angle to the present Australian- Pacific plate convergence direction accommodate most of the component of motion parallel to the plate boundary, while Pacific plate underthrusting and modest uplift take up most of the boundary-normal plate motion. Study of the Marlborough system may clarify controls on internal physical state of strike slip fault zones because of the differing degrees and rates of fault slip in a crust of unusually uniform composition. It should also fill a gap in understanding of transpressional orogens between uncoupled and fully coupled plate situations such as regards geometry and anisotropy of lower crustal conductors, and regions of production of lower crustal and slab related fluids. Preliminary results from a transect of 63 broad-band MT soundings across the northern South Island suggest enhanced deep crustal/uppermost mantle conductivity near the center of the island overlying a steep increase in plate dip and possible fluid release. Internal resistivity structure of the strike-slip faults is subtle and still being tested. A moderately southeast dipping mid-crustal conductor in the Westland province projects to the surface near the active Buller-Murchison thrust trend, which we tentatively correlate with damage and fluidized porosity along the deeper fault zone. This resistivity structural picture contrasts with previous study of the central South Island, where apparent continuum compression and crustal thickening creates an anisotropic conductive root zone from prograde metamorphism, fluid release and shearing.
T41D-03 INVITED
THE SOUTHERN ALPS OROGEN, SOUTH ISLAND, NEW ZEALAND
The central part of the South Island of New Zealand is a product of the transpressive continental collision of the Pacific and Australian plates during the past 5 million years, prior to which the plate boundary was largely transcurrent for over 10 My. Subduction occurs at the north (west dipping) and south (east dipping) of South Island. The deformation is largely accommodated by the ramping up of the Pacific plate over the Australian plate and near-symmetric mantle shortening. The initial asymmetric crustal deformation may be the result of an initial difference in lithospheric strength or an inherited suture resulting from earlier plate motions. Delamination of the Pacific plate occurs resulting in the uplift and exposure of mid-crustal rocks at the plate boundary fault (Alpine fault) to form a foreland mountain chain. In addition, an asymmetric crustal root (additional 8 - 17 km) is formed, with an underlying mantle downwarp. The crustal root, which thickens southwards, comprises the delaminated lower crust and a thickened overlying middle crust. Lower crust is variable in thickness along the orogen, which may arise from convergence in, and lower lithosphere extrusion along, the orogen. Low velocity zones in the crust occur adjacent to the plate boundary (Alpine fault) in the Australian and Pacific plates, where they are attributed to fracturing of the upper crust as a result of flexural bending for the Australian plate and to high pressure fluids in the crust derived from prograde metamorphism of the crustal rocks for the Pacific plate.
T41D-04 INVITED
From Convergence to Subduction – Plate Boundary Formation through New Zealand
In contrast to the normal ‘Wilson cycle' sequence of subduction leading to continental collision and associated mountain building, the evolution of the New Zealand plate boundary in the Neogene reflects the converse – initially a period of continental convergence that is followed by the emplacement of subduction. Plate reconstructions allow us to place limits on the location and timing of the continental convergence and subduction zones and the migration of the transition between the two plate boundary regimes. Relative plate motions and reconstructions since the Early to Mid-Miocene require significant continental convergence in advance of the emplacement of the southward migrating Hikurangi subduction – a sequence of tectonism seen in the present plate boundary geography of Hikurangi subduction beneath North Island and convergence in the Southern Alps along the Alpine Fault. In contrast to the transition from subduction to continental convergence where the leading edge of the upper plate is relatively thin and deformable, the transition from a continental convergent regime, with its associated crustal and lithospheric thickening, to subduction of oceanic lithosphere requires substantial thinning (removal) of upper plate continental lithosphere to make room for the slab. The simple structure of the Wadati-Benioff zone seen in the present day geometry of the subducting Pacific plate beneath North Island indicates that this lithospheric adjustment occurs quickly. Associated with this rapid lithospheric thinning is the development of a series of ephemeral basins, younging to the south, that straddle the migrating slab edge. Based on this association between localized vertical tectonics and slab emplacement, we argue that the tectonic history of these basins record the effects of lithospheric delamination driven by the southward migrating leading edge of the subducting Pacific slab. Although the New Zealand plate boundary is often described as simply two subduction zones linked by the transpressive Alpine Fault, in actuality the present is merely a snapshot view of an ongoing and complex evolution from convergence to subduction.
T41D-05
Mountain Building Mechanisms in the Southern Central Range of the Taiwan Orogenic Belt - from Accretionary Wedge Deformation to Arc-Continental Collision
Most researches consider the Taiwan Orogeny to be the result of an oblique arc-continental collision between the Philippine Sea Plate and Eurasia Plate. According to kinematic modeling, the mountains started to build from the north and progressively propagated southward at a rate of 60-90 km/my. Because of the oblique nature of the collision, the influence of the collision on mountain building resulted in the southern Central Range experiencing orogenic processes more recently than in the north. In order to test this model, we studied a critical area using zircon and apatite fission-track data to reveal the early exhumation history of the southern Central Range. We find that exhumation started about 6 Ma, which is earlier than the previously predicted timing of mountain building. We also find that the exhumation history can be separated into two stages: an initial stage starting at ca. 6 Ma and continuing to ca. 1 Ma with a slow uplift rate of <1mm/yr; and a second stage starting at ca. 1 Ma until the present with a high uplift rate of 4-10 mm/yr. The initial stage of mountain building is considered to be related to accretionary wedge deformation as the South China Sea Plate subducted beneath the Philippine Sea Plate whereas the second stage mountain building resulted from the arc-continental collision. Combining the ages of isotopic dating and fission-track dating in the northern Central Range, we find that the northern Central Range also could start exhumation at ca. 6 Ma and that its exhumation history can also be separated into two stages with similar exhumation patterns and mechanisms to that of the southern Central Range. The most notable difference between the exhumation history of the northern and southern areas of the range is the more extensive degree of exhumation in the north; this could be attributed to the northern Central Range having experienced a longer collision history.
T41D-06 INVITED
Evolution of Subductions Indicated by Mélanges in Taiwan
Two mélange zones occur in Taiwan, the Lichi Mélange in the Coastal Range of eastern Taiwan and the Kenting Mélange in the Hengchun Peninsula of southern Taiwan. Because of the the southward propagation of the Taiwan orogen, these two mélanges now crop out at the western front of the Coastal Range (Lichi Mélange) and the west side of the southern Central Range (Kenting Mélange). These two mélanges are similar in appearance and controversial in origin, being interpreted either as subduction complexes, or as olistostromes. Remnants of neighbouring stratigraphic units, present as slices or clasts in the mélange zones, shed light on the nature and origin of these two mélanges and help deciphering the tectonic evolution of Taiwan orogen and its leading subductions. Based on multiple lines of evidence, including fossil age, clay mineral composition and nature of exotic ophiolitic blocks, we conclude that the Kenting Mélange zone, which was probably active over a longer period of Late Cenozoic time, was a direct result of subduction process, as a relict of subduction fault zone at the front of the Taiwan accretionary prism. The Lichi Mélange, for which the fossil age indicates a shorter Pliocene time span, was part of the Northern Luzon Trough forearc, is regarded as a collision mélange formed during forearc closure in probable relation with arcward backthrusting, when the Luzon arc started to collide with the Eurasian continent margin following subduction of the oceanic crust of the forearc area. After comparing the geological situation of these two mélanges, we obtain a new perspective for the evolution of a trench-prism system, from subduction to arc-continent collision.
T41D-07
Structural Variations of an Accretionary Wedge in the Transition Zone from Subduction to Collision Offshore Southern Taiwan
The area offshore southern Taiwan is the place where the Luzon subduction complex transforms into an incipient arc-continent collision complex near Taiwan. Morphologically, the subduction complex shows a classic trench (Manila Trench) - accretionary wedge (Hengchun Ridge) - forearc basin (North Luzon Trough) - volcanic arc (Luzon Arc) system north of the Luzon Island. The accretionary wedge expanded toward north, the forearc basin was closed north of 21.5N offshore SE Taiwan while the frontal portion of the accretionary wedge encroached on the passive China continental margin off SW Taiwan. A prominent out-of-sequence thrust was developed that divides the accretionary wedge into an upper slope domain and a lower slope domain. Seismic reflection profile data collected in the region provide insights on differentiating morphological and structural characters of the accretionary wedge in this transition zone. The closure of the forearc basin near Taiwan has been attributed to the development of a back thrust system along the rare side of the accretionary wedge. However, strike-slip faulting and block rotation due to oblique arc-continent collision also played significant roles in creating the present morphology off SE Taiwan. At the frontal portion of the accretionary wedge, folds and thrusts are the prominent structural features in the lower slope domain. The distances between the fault-bend-folds increase north of 21.5N, the sediment thickness also increases northward, from about 1000 m thick at 20.5N to about 4000 m thick at 21.5N. The upper slope domain consists of highly deformed accretionary wedge material that was uplifted probably due to under-plating processes. Seismic data also suggest that the activation of the out-of-sequence thrusts migrates westward near SW Taiwan. In the near shore area off SW Taiwan, due to the rapid deposition on top of a thick mud layer, mud diapirs developed. Many of the mud diapirs could be connected to the similar features on land SW Taiwan. The present seismic profiles reveal mostly the upper crustal structures. It is hoped that the deep seismic profiling survey to be conducted in 2008 during the TAIGER active source experiment could provide critical information in constraining the structural evolution from subduction to collision off southern Taiwan.
T41D-08
Magnetotelluric Imaging of an Arc-Continent Collision Beneath Central Taiwan
Arc-continent collisions are a fundamental part of the plate tectonic cycle and play an important role in mountain building and the growth of continents. Studying this process in ancient orogens is often hindered by tectonic overprinting and poorly resolved synorogenic plate kinematics and tectonic settings. The Taiwan arc-continent collision between the Luzon arc and the Eurasian continental margin is one of the most active arc-continent collisions in the world. The current plate motions and surface geology are well understood, making Taiwan an ideal location for geophysical investigations of this important tectonic process. The Taiwan Integrated Geodynamical Research (TAIGER) project was initiated in 2004 to image the lithospheric structure beneath Taiwan. The project includes the acquisition of both active and passive seismic data plus detailed magnetotellurics (MT) surveys. MT is a passive geophysical technique which records time variations of natural electromagnetic fields at the surface of the Earth and can determine the subsurface resistivity. This parameter can be used to constrain lithospheric composition and strength. Over the fall of 2006 and spring of 2007, 82 long-period MT soundings were acquired in Taiwan. The fieldwork was accomplished by collaboration between the University of Alberta and National Central University of Taiwan. In fall 2007, collection of additional broadband MT will complement these data. The long-period data were processed using robust techniques and remotely referenced with a station on the PengHu islands (~100km from the nearest site) to reduce the influence of cultural noise. The conductive Taiwan strait separating PengHu from the mainland acts as an effective attenuator of cultural EM fields. Two closely spaced parallel transects across central Taiwan are formed by 46 of these stations with a nominal spacing of 5km. Dimensionality analysis of these transects using the McNeice-Jones tensor decomposition algorithm reveal local 3-D effects in some data. However, an overall regional 2-D strike direction is defined that is parallel to the coastline of Taiwan. Regularized 2-D inversions of these transects image conductive sedimentary rocks in the western foreland basin and an east dipping mid-crustal conductor beneath the fold-and-thrust belt. A sharp boundary is observed between the western sedimentary rocks and the more resistive slates and metamorphic rocks to the east in the Central Range. This boundary coincides with a conductor rising from the mid-crust that may be related to interconnected fluids.