From the Tropic of Cancer to Southeastern Alaska: Perspectives from Studying the Himalayan-Tibetan Orogen, the Indian Shield and the Taiwan Collision Zone

Wang-Ping Chen¹, Andre Pugin², Tai-Lin Tseng¹, Zui-Hong Zou¹ and Honn Kao³ (¹Dept. of Geology, Univ. of Illinois and ²IL State Geological Survey, Urbana, IL, ³Geological Survey of Canada, Sidney)

Based on on-going projects to study the Himalayan-Tibetan and Taiwan orogens, we proposed the following three types of experiments to provide important constraints on the interaction between tectonics and climate changes in southeastern Alaska.

1) Crustal thickness on regional scales. Isostatic compensation is a principal, long-term response of the crust to denudation. Thus deviations from Airy isostasy provide important indications for either continual trends of rapid denudation, or other mechanisms of compensation (e.g., Pratt isostasy through thermal anomalies in the mantle lithosphere). To detect deviations from Airy isostasy, a combination of gravity anomalies and precise measurements of crustal thickness is required. The latter can be readily determined from earthquake data using techniques such as joint inversion of crustal receiver-functions and surface waves, or pseudo-reflection profiles from interferometry of transmitted wavefields. We will present examples from the Taiwan orogen and the Indian shield.

2) Configuration and kinematics of crustal structures. Geodetic measurements of how the free surface moves, such as those from GPS and InSAR, are important boundary conditions for understanding continental dynamics. Correct interpretation of such data, however, relies on knowing the geometry and kinematics of crustal structures such as active faults and shear zones. As such, a combination of earthquake source studies (precise location of seismicity in 3-D, fault plane solutions, and seismic moment) and high-resolution seismic imaging (e.g., inversion of forward- and back-scattering, travel-time tomography) is indispensable. The design of seismic arrays for Hi-CLIMB, an international, multi-year (2001-2006) project in Tibet and Nepal (supported by NSF Continental Dynamics Program) can serve as a blueprint to achieve objectives 1) and 2) in southeastern Alaska.

3) High-resolution imaging of lacustrine deposits. A debate is brewing on how to tie marine sediments in the Indian Ocean with the Himalayan-Tibetan orogen, because sedimentary records in the Arabian Sea (Indus fan) and in Bengal fan seem to differ. Contrary to the popular notion of accelerated exhumation, sediments in the Bengal fan since about 7.4 Ma ago (mainly from the High Himalayan Crystalline series) seem to show increased intensity of weathering and residence time in the foreland that appear to be related to decreased sediment delivery. If true, the active deformation front and the development of out-of-sequence thrusts have a close connection with the marine sedimentary records. In anticipation of a similar issue in southeastern Alaska, we propose a judicious study of lacustrine deposits -- sediments en route to the marine record. A particularly economic and efficient method is to conduct high-resolution seismic reflection profiles using a light boat and simple acoustic sources. Pilot studies conducted in Canada and Europe show strong signals in the kHz range, probably sufficient to reveal fluctuations in sedimentation rates over 10 to 100 kyr, comparable to time-scales for crustal isostasy.

(JOI/NSF Continental Dynamics Workshop: Interplay of Collisional Tectonics and Late Cenozoic Glacial Climate in Alaska and the Northeastern Pacific Ocean, Austin, TX, May 4-5, 2003)