INTEGRATING GEOCHRONOLOGY AND STRUCTURAL ANALYSIS AT MULTIPLE SCALES TO BETTER UNDERSTAND FAULT MECHANICS

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 geochronologic analyses of appropriate fault rocks. In this contribution, we explore the implications of two such records for better understanding the mechanics of extensional fault systems. U-Th dating of 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. 40 ka over more than 400,000 years, consistent with recurrence intervals determined for shorter time intervals throughout the Basin and Range province. In contrast, ⁴⁰Ar/³⁹Ar dates on four low-angle normal, pseudotachylyte generation veins from the South Mountains metamorphic core complex, AZ, record a much longer recurrence interval of ca. 1 Ma. The latter dataset, though 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. We consider the implications of recurrence intervals that differ by two orders 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.