Affiliation: UW-Madison

Title: A Record of Earthquakes and Fault Mechanics Written in Stone

Abstract: An exhumed fault zone can preserve a record of earthquakes that extends over hundreds of thousands to millions of years. Recent studies indicate that such archives of earthquake information can be accessed effectively through integrated structural and isotopic analyses of appropriate fault rocks. In this talk, I will describe and explore the implications of two such records for better understanding the mechanics of extensional fault systems. U-Th dating of thirteen coseismic calcite veins in the Loma Blanca high-angle normal fault of the Rio Grande rift, NM, demonstrates remarkably periodic earthquakes with a recurrence interval of ca. 40ka over more than 400,000 years, consistent with recurrence intervals determined for shorter time intervals throughout the Basin and Range province. Interestingly, it also records a brief period in which the recurrence interval declines to 5-11 ka. Stable isotope analyses of the latter veins record fluid influx from a new source that fundamentally changed the seismic cycle. In contrast, 40Ar/39Ar dates on four low-angle normal, pseudotachylyte generation veins from the South Mountains metamorphic core complex, AZ, record a mean recurrence interval of 557 ka. The latter dataset, though much more limited in size, suggests that earthquakes produced by low-angle normal faults are far less likely to be sampled by the comparatively short-term instrumental and paleoseismic records.

High- and low-angle normal faults are commonly linked in networks, with numerous high-angle faults floored in a single low-angle detachment. I consider the implications of recurrence intervals that differ by an order of magnitude in light of this geometry. One possibility is that seismic slip at 40 ka intervals on multiple high-angle faults, which are optimally oriented for failure in extensional tectonic regimes, progressively loads low-angle normal faults, which are not, resulting in much longer recurrence intervals for earthquakes on the shallowly dipping structures. A second possibility is that this particular distribution of normal faults and slip rates minimizes the energy required to accommodate extension. These options are not mutually exclusive.