The Imaging of Subsurface Structures Under Taiwan, New
Zealand and Himalaya and the Mechanisms of Mountain
Building
Francis T. Wu
Department of Geological Sciences
State University of New York
Binghamton, New York 13902-6000
USA
Although mountain building has been one of the centers of attention in geology
for a number of years, how mountains are built mechanically and how deep does an
orogen extend are still being debated. Surface observations provided the basis of
much of the initial understanding of orogeny and to complete the picture subsurface
information becomes important. Recently the improvement of the resolution of the
geophysical methods made it possible to test the proposed concepts of mountain
building.
In this paper we take advantage of the modern geophysical studies in
three active ranges to explore several questions regarding mountain building. By
studying several major ranges of different ages and scales we hope to arrive at some
generalization as to the common mechanisms in operation. The geophysical imaging
of these mountain ranges were not of the same quality. For example, the seismic
networks deployed in each case differ in density and coverage and thus resolution.
Marine geophysical imaging mapped clearly the crust under Central New Zealand and
in Hengchun area of southern Taiwan. Seismological data in the high Himalaya only
became available the last few years. Nevertheless, by looking at these ranges at the
same time we can address common key questions concerning them as well as their
unique aspects.
Active orogeny lend particularly well to such studies because the local
earthquakes generated by the orogenic processes provide clues to the stress and strain
in the orogen and can be used as sources for illuminating the internal structures of the
orogen. In active orogens geophysical imaging will provide a glimpse of the
processes in progress. Geophysical data provides little or no time constraints to the
evolution of a mountain range, yet the geometry of the orogen is certainly the
integration over time of all the deformation that has taken place. Here we discuss
three of the world's major mountain ranges and the models proposed as a result of
recent studies.
Situated between the Ryukyu and the Manila subduction zones, the Taiwan
orogen was created by the collision of the Philippine Sea plate and the Eurasian plates
beginning about four to five million years ago.
Available images and seismicity
affirm that (1) the crust under the western Foothills is about 35 km thick, (2) the
crust is much thickened between 23-24.3ON, but thins toward the north and the south,
(3) under the Hengchun Peninsula the presence of the subduction system is still clear,
(4) although the Central Range is undergoing rapid uplift it is generally a region of
low seismicity, but under both the Foothills to the west and the Coastal Range to the
east seismicity extend down the lower crust, (5) high-angle west-dipping reverse
faulting, in the depth range of 20-35 km, on the western side of the Central Range
indicates a mode under which the root could have been formed, (6) crustal thickens on
the Philippine Sea plate side under the Coastal Range to more than 30 km, and (7) the
splitting of the S waves imply that upper mantle under the island is highly anisotropic
with the fast direction essentially parallel to that of the geologic structures on the
surface. These and other evidence point to the complex rheology in the Taiwan
orogen and extensive and pervasive deformation occur in the orogen down to at least
50 km depth. A pure shear deformation in the orogen had led to the rising of the
mid-crustal material to shallow depth and the downthrust of lower crustal material to
form the root. The shearing along the plate boundary may extend down to upper
mantle depths to generate an isotropic upper mantle.
The orogeny of South Island (SI), New Zealand, is also quite young, and similar
to Taiwan, the southern Alps orogen is located between two subduction zones with
opposite polarities.. Noticeable transcurrent tectonics began in the area more than 10
millions years ago but changes in plate motion about 6 million years ago led to more
compression and the beginning of the South Alpine orogeny as we know today.
The crust under the Southern Alps had thickened to about 35 km probably from a
“Chatham Plateau-like” crust of ~20 km. No significant thickening of the Tasman
crust on the western side of the Alpine fault is seen. The seismicity under the high
South Alpine is concentrated in the upper 10 km while that under the eastern foothills
earthquakes as deep as 20 km are common; no deep seismicity occurs under it. The
S wave splitting in SI is very pronounced; the largest delay between fast and slow S is
about 2.1 second (vs 1.2 second in Taiwan and 2.7 second in Tibet), with the fast
direction parallel to the geologic structures at the surface. Both teleseismic travel
time delays and a teleseismic tomography image show the presence of a high
velocity anomaly in the upper mantle ((about 60-180 km) directly under the Southern
Alps. These data can be interpreted in terms of a vertically coherent orogen from the
surface to at least 200 km; while the thickening of crust may have induced the
formation of the high velocity anomaly, the coincidence of the fast splitting direction
and the structural trend again may mean the shearing in the zone extend from the
surface a few hundred kilometers into the upper mantle.
The Himalayan orogen is intimately tied to the formation of the Tibetan plateau;
its remoteness and great elevation render it one of the last frontier of geophysical
studies. One INDEPTH reflection line extend into the high ranges of southern Tibet
and identified a reflector under the Himalaya and was associated to the main frontal
thrust. The 2001-2002 HIMNT experiment deployed a seismic network on both
sides of Mount Jolmolungma in an area of about 300 x 280 km2. Based on receiver
function results the transition from the 45 km crust on the Indian Plate side to the 70
km thick crust on the Eurasian plate side occurs smoothly under the high Himalayas.
Relocated hypocenters show that seismicity occurs in distinct zones during the
16-month recording period. One of the remarkable zones lies directly under the high
Himalaya, reaching 140 km in places. Even under the Gangetic Plain subcrustal
events (>50 km) exist. Focal mechanisms within and near the network show
relatively high angle normal faulting and some reverse and strike-slip faulting. If the
Southern Tibet Detachment exists it is seismically quiescent. The upper mantle
seismicity may be related to subduction or to upper mantle deformation related to the
formation of the high Himalaya. So far the S-splitting anomaly is not very noted.
That mountain building involves processes in the upper mantle appears to be
clear in all three cases. Both in Taiwan and in New Zealand crustal root formation is
fairly rapid, at the rate of about 2.5 mm/yr in New Zealand and 5-6 mm/yr in Taiwan.
Interpreting S-splitting as resulting from shearing in the upper mantle due to oblique
convergence, the degree of anisotropy is proportional to the rate of shearing on SI and
Taiwan. As revealed by the location of the crustal seismicity, the rheology of
materials in an orogen could be quite complex. Overall the orogenies may involve
more pure shear deformation than simple shear deformation.