MECHANICAL CONSTRAINTS ON LOW ANGLE NORMAL FAULT STRENGTH: LONG-TERM AND SECULAR NUMERICAL MODELING OF CORE COMPLEXES (Invited Presentation)

Although our understanding of the mechanics and evolution of low-angle normal faults and core complexes has advanced greatly, whether low-angle normal faults creep aseismically or slip in earthquakes according to ~10-1000-year seismic cycles remains unresolved. Moreover, the diversity in style of core complexes and low angle normal faults imaged at rifted margins also suggests that other factors such as semi-brittle behavior and buoyancy forces influence the mechanics and structural evolution of low-angle normal faults. Here we show that localization of deformation at the brittle ductile transition in mixed brittle-ductile (semi-brittle) material may play a determinant role in the mechanical evolution of low-angle normal faults. We present numerical models of both the short-term and long-term evolution of core complexes. We show that the diverse styles of low-angle normal faults and their capacity to slip at low angles are likely controlled by the strength of the lower crust and the presence of sub-horizontal weak ductile shear zone near the brittle ductile transition. Using models of the secular evolution of low-angle normal faults including a rate-and-state frictional formulation we show that the frictional and rheological contrast between the brittle and ductile rocks in the shear zone controls the temporal pattern and seismic style of 1-10 kyr-timescale normal fault strain accommodation. These models show that over thousands of years normal faults can either creep (steadily or unsteadily), experience creep transients punctuated by periods of earthquake clustering, or slip seismically in a regular seismic cycle. We compare results of the secular models to observed slip histories and earthquake recurrence intervals on normal faults.