Open Session - SWISS GEOSCIENCE MEETINGs

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Symposium 1: Structural Geology, Tectonics and Geodynamics
1.
Transitions in tectonic mode based on calculations of self-consistent
plate tectonics in a 3D spherical shell
van Heck Hein* & Tackley Paul*
*Institute of Geophysics, Schafmattstrasse 30, CH-8093 Zuerich (hvanheck@erdw.ethz.ch)
In the past decade, several studies have documented the effectiveness of plastic yielding in causing a basic approximation of
plate tectonic behavior in mantle convection models with strongly temperature dependent viscosity, strong enough to form
a rigid lid in the absence of yielding. The vast majority of such research to date has been in either two-dimensional, or threedimensional
cartesian geometry. Also, scalings for mixed internally and bottom heated convection are not well established.
In the present study, mantle convection calculations are done to investigate the planforms of self consistent tectonic plates
in three-dimensional spherical geometry. Several diagnostics are used to analyze how successful each model is in producing
tectonic plates. We study the proposed transitions in tectonic mode (e.g. changes in plate size, rigid lid convection to tectonic
plates, smoothly evolving plates to more episodic, time dependent, tectonics) as a function of yield stress envelope,
Rayleigh number and heating mode.
Cases with zero bottom heat flux are compared to cases which have both internal heating and bottom heating. This enables
us to investigate which tectonic mode prevails as function of lithospheric yield stress, Ra and heating mode. The results are
compared to analytical scalings for boundary regimes as well as scalings for heat flux. This allows us to scale to different
planets of different sizes and can be applied to the evolution of Earth, Mars and Venus as well as terrestrial extra-solar planets.
1. 8
Late Quaternary slip rates of active faults adjacent lake Qinghai,
northeastern margin of the Tibetan Plateau
Yuan Dao-Yang*, Champagnac Jean-Daniel**, Ge Wei-Peng*, Molnar Peter***, Zhang Pei-Zhen****, Zheng Wen-Jun****, Zhang
Hui-Ping****, Liang Ming-Jian*
* Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou 730000, China
** Institute of Mineralogy, University of Hannover, Callinstrasse 3, Hannover D-30167, Germany
*** Department of Geological Sciences, and CIRES, University of Colorado, Boulder, Colorado, 80309, USA
**** State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China
Combining the terrace riser offsets with terrace ages dated by 14 C, OSL and 10 Be techniques, we determine average strike slip
rates of Elashan and Riyueshan faults, two north-northwest-trending strike-slip faults along the western and eastern sides of
the lake Qinghai, northeastern margin of the Tibetan plateau, to be about 1.0 ± 0.2 mm/yr and 1.2 ± 0.2 mm/yr, respectively.
Between them, the Qinghainanshan fault consists of three secondary thrust faults, whose total vertical slip rate and shortening
rate are ≈0.4-1.0 mm/yr and ≈0.2-1.2 mm/yr, respectively.
The relatively low slip rates in this region reflect distributed deformation. The total right-lateral offsets of the geological
contacts, which were interpreted from 1:200,000-scale Qinghai regional geological maps of the region, are about 8.8-11.9 km
along the Elashan fault and 10.9-11.6 km for the northern segment of Riyueshan fault. If long–term slip rates were constant
during late Cenozoic time, initiation of dextral movement would be 10.3 ± 3.6 Ma and 9.4±2.3 Ma for the two strike-slip faults,
consistent with records of tectonic deformation in Cenozoic basins nearby.
Our study highlights a stage of tectonic deformation in the northeastern margin the Tibetan plateau beginning near ≈10 Ma,
long after the collision between India and Eurasia began.