RADIOGENIC ISOTOPE ANALYSES REFINE CONSTRAINTS ON THE SEISMIC CYCLE OF AN INTRAPLATE NORMAL FAULT

We present the results of Sr isotope analyses of syntectonic calcite veins in the Loma Blanca normal fault, Rio Grande rift, NM, USA. Previously determined U-Th dates on these veins – which formed in coseismic extension fractures – constrain earthquake recurrence intervals from 150-565 ka. Stable isotope data were used to distinguish samples with overlapping U-Th ages and define 13 distinct earthquake events. The combined U-Th and stable isotope data show that for more than 400 ka the Loma Blanca fault produced periodic earthquakes with a mean recurrence interval of 40 ± 7 ka, although a pronounced cluster of events at ~430 ka shows much shorter recurrence intervals of 5-11 ka. Breccia veins formed during this episode exhibit carbon isotope signatures consistent with having formed in association with degassing of a CO₂ charged brine during post-failure, fault-localized fluid migration.

Strontium isotope analyses were conducted to: 1) refine the Loma Blanca earthquake chronology by providing a more sensitive means of recognizing distinct fluid-flow events among samples with overlapping U-Th ages, and 2) provide additional constraints on the sources of post-seismic fluid flow throughout the Loma Blanca earthquake record. Strontium isotope results indicate that post-seismic fluids moving through the Loma Blanca fault fall into two broad groups of 0.70980 – 0.71008 and 0.71073 – 0.71097. The Sr isotope data show that events 6 and 12 of our original earthquake chronology, which are recorded by overlapping ages from multiple vein samples, each involved two distinct fluids and therefore likely represent separate earthquakes. Accordingly, we revise our original earthquake chronology of 13 events to 15 events, and calculate a new recurrence interval of 34 ± 8 ka. Higher ⁸⁷Sr/⁸⁶Sr values are interspersed throughout the record and define multiple temporal “peaks” relative to a baseline of low ⁸⁷Sr/⁸⁶Sr values. These trends indicate that the source (and likely depth) of fluid transport through the Loma Blanca varied over the timescale of 100’s of ka. Our data also demonstrate that radiogenic isotope analyses of syntectonic veins may be used to refine the timing of earthquake ruptures and recognize the presence of distinct fluids in paleoseismic records.