Open Session - SWISS GEOSCIENCE MEETINGs

1.2
Numerical modeling of poly-phase formation and exhumation of
HP-UHP rocks in continental subduction zone
Li Zhonghai* & Gerya Taras*
* Geophysical Fluid Dynamics Group, Institute of Geophysics, Department of Geosciences, ETH-Zurich, CH-8093 Zurich (lzhhai@gmail.com)
High-pressure-ultrahigh-pressure (HP-UHP) metamorphic rocks commonly form and exhume during the early continental
collision, with the protoliths mainly derived from subducted upper and middle continental crust. While geodynamic significance
of HP-UHP complexes is widely recognized and their appearance in the Neoproterozoic is considered as a “hallmark”
for establishing modern plate tectonic styles many questions related to their origin still remain unresolved. Of particular
importance is polymetamorphic origin of many HP-UHP terranes composed of tectonic units having strongly variable ages,
peak metamorphic conditions and P-T paths.
In order to address this issue we conducted 2D high-resolution thermomechanical numerical modeling of the continental
subduction associated with formation and exhumation of the HP-UHP rocks, with testing different geometrical configurations
and varied width of subducting continental margins, convergence velocity and sedimentation and erosion rates.
Most of our experiments confirm poly-phase origin of HP-UHP terranes and predict existence of several consequent episodes
of (U)HP rocks exhumation related to the inherently cyclic origin of continental crust subduction-detachment-exhumation
process. Periodicity of formation of rheologically weak zones (thrusting faults) controlling HP-UHP rocks exhumation processes
depends on the competing effects of downward directed subduction drag and upward directed crustal buoyancy
forces. The buoyancy forces and related deviatoric stresses accumulate in the subduction channel due to subduction of lowdensity
crustal rocks and are then reset back during rapid exhumation episodes.
Our reference model predicts three distinct phases of detachment/exhumation processes: 1) first detachment and exhumation
of HP rocks; 2) second detachment and exhumation of HP rocks; 3) exhumation of the UHP-HT rocks. Residence time of
UHP rocks in the overriding plate channel is on the order of several million years which allows strong rising of peak metamorphic
temperatures toward 800-900°C due to conductive heat exchange with underlying asthenospheric mantle. The
model results are applicable to the Sulu UHP terrane in eastern China with similar characteristics in terms of 1) spatial distribution
of metamorphic units (HP-UHP-Suture zone); 2) peak P-T conditions of the HP-UHP metamorphism; 3) temporal
constraints for polyphase processes (the formation and exhumation of HP-I slices are about 10 Ma earlier than the UHP slices);
and 4) provenance (mixed various crustal and mantle rock types with protoliths mainly from the subducted continental
margin).
The presence of the incoming continental margin with relatively thin crust is important for the onset of continental deep
subduction and exhumation processes. The smaller margin width doesn’t produce efficient exhumation before real collision
of the thick subducting continental interior and the overriding plate. Convergence velocity is another major parameter
controlling subduction/exhumation processes by affecting them into three different scenarios: (1) “first-step” exhumation
processes of the subducted rocks without intruding into the sub-lithospheric channel, under low convergence velocity; (2)
“polyphase” exhumation processes with three phases of the formation and exhumation of HP-UHP rocks, under intermediate
convergence velocity; (3) “last-step” exhumation processes without effective exhumation until the subducting continental
interior collides with overriding plate, under high convergence velocity. The surface processes (sedimentation and erosion
rates) affect the size and exhumation time of HP-UHP units but don’t change the polyphase characteristics of subductiondetachment-exhumation
processes.
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Symposium 1: Structural Geology, Tectonics and Geodynamics