Geoscience undergraduate student Ellen Lamont (’10) has been working with Dr. Jon Lewis on an NSF-funded project to understand earthquake-related deformation in the rocks of central Taiwan. Dr. Lewis is presenting this research at the national meeting of the American Geophysical Union (AGU) in San Francisco on December 18, 2009.
Lamont’s inverse models of earthquake data reveal that the southern termination of the Hseuhshan Range is experiencing active horizontal stretching parallel to the mountain belt. This stretching appears to be accommodated by faults that allow the mountain belt to collapse in response to gravity and to shear horizontally in response to tectonic forces. Lamont’s findings shed light on what happens when the lower plate in a tectonic collision has an irregular shape inherited from when the oceanic rocks adjacent to the Chinese coast were initially formed.
Lewis will present the results at the fall meeting of the American Geophysical Union in San Francisco on December 18, 2009. His talk will be part of a special session on subduction and tectonic collision.
Crustal extrusion of the hangingwall at the southwestern termination of the Taiwan orogen: Constraints from seismogenic strain inversions J. C. Lewis1; E. A. Lamont1; T. B. Byrne2; J. M. Crespi2; R. Rau3 1. Geoscience, Indiana Univ. of Pennsylvania, Indiana, PA, United States.2. Center for Integrative Geosciences, Univ. of Connecticut, Storrs, CT, United States.3. Earth Sciences, National Cheng Kung Univ., Tainan City, Taiwan.
Partial strain tensors inverted from earthquake focal mechanisms in the Luliao region of the Taiwan orogen reveal relatively fine spatial-scale partitioning of contemporary, non-recoverable strain (i.e., seismogenic strain) at the southwestern termination of the Hseuhshan range. We show that the region is dominated by widespread orogen-parallel stretching, and subordinate subvertical stretching localized on western slope of the Hseuhshan range.
The modeled seismic events define an elongate NW-SE swarm with three easily identified spatial clusters; a diffuse northwestern subset, a tight central subset, and a diffuse southeastern subset. Each subset of focal mechanisms is inverted for a best-fitting partial strain tensor using an adaptation of the micropolar continuum model. Prior work shows that homogeneous strain yields a single, best-fitting model tensor, whereas heterogeneous strain is reflected by two or more preferred orientations for model principal strain axes. The events in the central subset yield a single stable solution. In contrast, the northwestern and southeastern subsets yield multiple viable geometries for the model principal strain axes. These data are therefore subdivided on the basis of kinematics, to separate events with one steeply plunging kinematic axis (P or T) from events with two shallowly plunging kinematic axes.
The central subset and each of the kinematically-defined subsets for the northeastern and southwestern areas are inverted for best-fitting strain tensors. The two populations defined for the southeastern subset yield strain tensors that are consistent with normal faulting and strike-slip faulting, both of which are characterized by generally NE-SW maximum stretching. The central subset yields a strain tensor consistent with normal faulting in a regime of NE-SW maximum stretching. The three populations defined for the northwestern subset each yield stable model tensors. Two of these are characterized by NE-SW maximum stretching, and as with the southeastern subset, one is consistent with normal faulting and one is consistent with strike-slip faulting. The third population is characterized by sub-vertical maximum stretching and is consistent with thrust faulting in a regime of WNW-directed shortening.
These results are interpreted to reflect crustal extrusion related to the southwestward propagation of arc-continent collision over continental crust footwall with an inherited irregular geometry. Magnetic anomalies suggest that the trend of the boundary between transitional and full-thickness continental crust in the footwall is oriented ~240° whereas the orogen is oriented ~200°. As a consequence, the southwestern termination of the Hseuhshan range is situated immediately outboard of this boundary, and the upper plate to the south is situated above increasingly attenuated footwall crust. This tectonic architecture appears to play a role in lateral extrusion of the upper plate along widely distributed normal and strike-slip faults within the orogen.