Departmental Seminar

3 downloads 286 Views 289KB Size Report
Mar 13, 2014 - Active faults accommodate motion between crustal blocks through a range of behaviors. At one end ... comp
Departmental Seminar Smart Clays: Fault Lubrication and Creep on the San Andreas Fault Ben van der Pluijm University of Michigan Active faults accommodate motion between crustal blocks through a range of behaviors. At one end, earthquakes represent sudden and violent rupture; at the other end lies a steady, creeping motion that does not generate significant seismic activity. The properties that govern these behaviors are central to fault rupture processes and associated seismic hazards, and have been variably attributed to (i) low values of normal stress, (ii) elevated pore-fluid pressure, and (iii) low frictional strength. The San Andreas Fault in California exhibits different types of slip behavior, including large damaging earthquakes and segments that are creeping aseismically today. However, faults are not simple planar surfaces marking the contact between moving blocks. Instead, they are complex and evolving structures, and are often filled with fault rock. Fault rocks collected from SAFOD drilling are abundantly coated by polished thin-films of hydrated clay with occasional striations. Microscopy and X-ray studies of these surface coatings reveal the occurrence of both illite-smectite (I-S) and chlorite-smectite (C-S) phases. Ar dating of illitic clay coatings shows that they are neocrystallized phases that grew in the modern fault zone. It is proposed that fault creep is controlled by networks of thin (~ 100 nm thick) nanocoatings on fracture surfaces that are sufficiently interconnected to allow slip with minimal breakage of stronger matrix clasts. Displacements are accommodated by frictional slip along particle surfaces coated with smectitic phases. Indeed, recent experimental work on I-S fault rocks shows very low frictional strength and significant weaknesses compared to neighboring wall rocks. The stability of smectite in chloritic phases extends the role of clays to depths up to ~10 km, so we conclude that the localization of smectitic clay minerals promotes fault creep behavior without requiring scenarios involving talc/ serpentine phases or localized fluid pressure as explanations for mechanically-weak upper crustal faults.

Thursday, March 13, 2014 3:30 p.m. Room 1252, HAMP Bldg.

Refreshments at 3:00pm Room 2201/HAMP