Francis T. Wu

Imaging the Indian subcontinent beneath the Himalaya

The rocks of the Indian subcontinent are last seen south of the Ganges before they plunge beneath the Himalaya and the Tibetan plateau. They are next glimpsed in seismic reflection profiles deep beneath southern Tibet, yet the surface seen there has been modified by processes within the Himalaya that have consumed parts of the upper Indian crust and converted them into Himalayan rocks. The geometry of the partly dismantled Indian plate as it passes through the Himalayan process zone has hitherto eluded imaging. Here we report seismic images both of the decollement at the base of the Himalaya and of the Moho (the boundary between crust and mantle) at the base of the Indian crust. A significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths. North of the Himalaya, the lower Indian crust is characterized by a high-velocity region consistent with the formation of eclogite, a high-density material whose presence affects the dynamics of the Tibetan plateau.

TAIWAN OROGENY : THIN-SKINNED OR LITHOSPHERIC COLLISION ?

Abstract The Taiwan orogeny is young and presently very active. It provides an excellent environment for studying ongoing orogenic processes, especially since the region is monitored intensively with dense seismological and geodetic networks, and new studies aiming at deciphering shallow and deep structures in and around Taiwan have been recently conducted or are being planned. The available data can be used continually to test critically hypotheses of the Taiwan orogeny. Hypotheses dealing with the mechanics of mountain building are basic to the understanding of Taiwan orogeny and are particularly amenable to testing. The widely cited ‘thin-skinned tectonics’ hypothesis was formulated to explain mainly the geologic and relatively shallow (

Fault plane solutions of shallow earthquakes and contemporary tectonics in Asia

Abstract The tectonics of Asia are interpreted as a result of convergence of the Indian and Eurasian plates. The Indian shield bends down and underthrusts the Himalayas to the northeast along a shallow dipping fault plane while the Eurasian plate underthrusts the Pamir mountains, and therefore presumably the Indian Plate, to the south. The convergence of the Indian and Eurasian plates appears to cause relatively high stress to be transmitted across a broad area, north and east of the Himalayas, and this stress in turn causes earthquakes and renewed tectonic activity in some of the ancient Paleozoic and Mesozoic fold belts that separate more stable, aseismic blocks in Asia.

Tomographic imaging of lithospheric structures under Taiwan

Abstract Tomographic images of the crustal and mantle velocity structures under Taiwan are obtained by simultaneous inversion of local earthquake P-wave arrival times for hypocenters and P-wave velocity structures. In northern Taiwan, a high-velocity zone, coinciding with the Wadati-Benioff zone, can readily be identified as the subducted Philippine Sea plate. The imaged zone dips toward the north at an angle of 40° from a depth range of 20–55 to 100–130 km. An upper-mantle low-velocity wedge, ranging from 40 to 80 km in depth, exists above the subducted slab. Above this wedge is the Ilan Plain of northern Taiwan which lies at the west end of the Okinawa Trough, a well recognized back-arc basin; the crustal velocities under the Plain are also relatively low. The well-defined high-velocity zone and the low-velocity wedge provide some constraints on the thermal structures of the subduction system under northern Taiwan. In tomographic images across the central section of Taiwan, thickening of the crust and up-arching of the lower-crustal materials under the Central Range are commonly observed; the crust under the Western Foothills region is clearly thinner and the near-surface low-velocity layers are well developed. The structures under the Central Range show that although the Taiwan orogeny is quite young, a root, deeper in the north and shallowing to the south, has formed. The results of our tomography show that a significant portion of the lithosphere is involved in the Taiwan orogeny.

Preliminary results from a geophysical study across a modern continent-continent collisional plate boundary - the Southern Alps, New Zealand

Abstract The Southern Alps of South Island, New Zealand, is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement. A joint US-NZ geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three-dimensional model of the deformation. The measurements undertaken include active source and passive seismology, magnetotelluric and electrical studies, and petrophysics. Preliminary models for the active source seismic measurements across South Island confirm, in general terms, a thickened crust under the Southern Alps, a high-velocity lower crustal layer, and a major crustal discontinuity associated with the Alpine fault. The anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements, shear wave splitting and resistivity. The mid-crust under the Southern Alps coincides with a major electrical conductivity high, which possibly corresponds to fluid in the crust. The top lies at about 15 km, close to the base of shallow seismicity east of the Alpine fault. Offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to South Island. These data are showing complex structure, particularly off western and southeastern South Island. The complexity in structure, high-quality data and consistency in results from several techniques indicates that the South Island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular, and the continental lithosphere in general.

Clay gouges in the San Andreas Fault System and their possible implications

SummaryIn the northern and central sections of the San Andreas Fault Zone, and along Calaveras and Hayward faults, clay gouges have been found to occur on the surface and at shallow depths.It is consistent with the available geochemical data that such gouges can exist at depths down to 10 km. If extensive gouge materials exist in a fault zone then their properties will determine, to a large extent, the behavior of the fault. From known properties of clays in the presence of water we can infer that, in such cases, the tectonic stress and the stress drops for earthquakes will be low and substantial creep will take place before earthquakes.

Upper mantle anisotropy in the New Zealand Region

Shear-wave splitting parameters of fast polarization direction (Φ) and delay time (δt) are determined using data from the Southern Alps Passive Seismic Experiment (SAPSE), on the South Island of New Zealand and in the surrounding region. Our results clearly show that Φ are subparallel to trends of the Alpine and Marlborough Faults, and to the Pacific-Australian plate boundary. The δt values range from 0.6–2.2 s with an average value of 1.6 s; the largest values are from the central South Island. The main source of the observed shear-wave splitting is an anisotropic region between 40–400 km. The width of the zone is approximately 200 km. We attribute the coincidence of surface structural trends with the measured Φ, and the large δt values, to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle.

Inversion of a hyper-extended rifted margin in the southern Central Range of Taiwan

Seismic reflection and wide-angle data acquired across, south, and west of Taiwan show that extended to hyper-extended continental crust of the Chinese continental margin is present more than 200 km south of the shelf and is subducting at the Manila Trench. Furthermore, crustal-scale tomographic velocity models show that this crust is underthrusted to ∼15 km depth below the accretionary prism, where it then is structurally underplated to the base of the prism. We document an increasing volume of accreted crust from south to north, and in our northern transect high-velocity material of the accretionary prism can be directly linked to outcrops of Central Range basement rocks. In map view the Central Range of Taiwan is clearly contiguous with the Hengchun Peninsula and Hengchun submarine ridge to the south. Accordingly, we propose a new model in which the Central Range forms directly from the accretionary prism, including the basement core, which originates from subducted, and then accreted, extended to hyper-extended continental crust.

Seismic image of the Tarim basin and its collision with Tibet

A broadband seismic deployment in 1998–1999 in southwestern Tarim provided data for imaging the crust and upper mantle across the contact between the Tarim block and the Tibetan Plateau. A profile composed of migrated teleseismic receiver functions clearly shows lateral structural changes. The crust under the Tarim basin is relatively simple. The Moho discontinuity is mapped at a depth of 42 km near the northern end of the array and dips gently toward the south to ∼50 km under the Kunlun foreland. The Tarim basin appears to be rigid, with little shortening. Farther to the south, the imaging reveals a complex of reflectors in the lower crust and the upper mantle. There are both north- and south-dipping upper mantle structures under the Kunlun foreland and Kunlun Shan region. We found the observations to be more consistent with a model of lithospheric collision in which the crust and the upper mantle on both sides interpenetrate and deform.

Source parameters for stick-slip and for earthquakes.

Source parameters of stick-slip friction events measured in the laboratory show particle and rupture propagation velocities which are similar to those observed for earthquakes and inferred from seismic source theory. This dynamic similarity strongly supports the idea that stick-slip is the mechanism for shallow earthquakes.