Material and Stress Rotations: The Key to Reconciling Crustal Faulting Complexity with Rock Mechanic...

Prof. AMOS NUR

Geophysics Department, Stanford University, Stanford, California

20180524-15:00:00

Room 2829, No. 2 Science Building
 

A perennial problem in fault mechanics is that the fault geometries in situ—especially of strike- slip faults—often contradict theoretical predictions. According to experimental and theoretical rock mechanics as captured by Coulomb’s law, fault directions and motions should correspond simply to stresses in the crust. However, the complex geometrical distribution and regional trends of observable faults in the crust often seem at odds with the regional state of stress. Fortunately, these discrepancies can be neatly reconciled with Coulomb’s law if we recognize that many faults did not form in their current orientations, but have rotated over time, and/or the stress field has rotated as well.

I describe a comprehensive tectonic model for the strike-slip fault geometry, seismicity, material rotation, and stress rotation, in which new, optimally oriented faults can form when older ones have rotated about a vertical axis out of favorable orientations. The model was successfully tested in the Mojave region using stress rotation and three independent data sets: the alignment of epicenters and fault plane solutions from the six largest central Mojave earthquakes since 1947, material rotations inferred from paleomagnetic declination anomalies, and rotated dike strands of the Independence dike swarm.

The success of the rotation model in the Mojave has applications well beyond this special region alone. The implication for crustal deformation in general is that rotations—of material (faults and the blocks between them) and of stress—provide the key link between the geology of faults and the mechanical theory of faulting. Excluding rotations from the kinematical and mechanical analysis of crustal deformation makes it impossible to explain the complexity of what geologists see in faults, or what seismicity shows us about active faults. However, when we allow for rotation of material and stress, Coulomb’s law becomes consistent with the complexity of faults and faulting observed in situ.