Paleomagnetic analyses of pseudotachylyte veins constrain fault mechanics
Abstract
Low-angle normal faults (LANFs) are poorly oriented for slip according to Andersonian fault mechanics. The geologic record generally supports slip at current shallow (<30°) dips; however, the seismic record generally suggests such faults do not produce earthquakes >M5.5. One explanation for the discrepancy between these data sets is that LANFs were re-oriented by isostatic rebound as they were exhumed, allowing them to slip at steeper dips before rotating into their final, shallow orientations. We tested this hypothesis through a systematic investigation of both the magnetic characteristics of, and remanent magnetization recorded by, pseudotachylyte veins formed via frictional melting at seismic slip rates in the South Mountains metamorphic core complex of Arizona. The pseudotachylyte veins studied line faults that dip 11°-14°, and cut Miocene granodiorite mylonite in the footwall of the core complex. UltraViolet Laser Ablation MicroProbe (UVLAMP) 40 Ar/ 39 Ar dating of four individual veins yielded five distinct weighted mean apparent ages between 17.21±0.13 and 18.88±0.18 Ma - a period in which the geomagnetic time scale shows multiple changes in polarity. In addition, they demonstrate reactivation of one vein, which records two separate ages in distinct tabular domains.
Our approach to integrating paleomagnetic with structural and geochronologic analyses of fault rocks, though challenging to implement, is a particularly effective way to interrogate the rock record of fault mechanics. The 40 Ar/ 39 Ar ages allow us to calculate an expected geomagnetic field direction for the study site using previously published apparent polar wander paths for North America, as well as an error ellipse corresponding to the secular variation expected for the site latitude. Paleomagnetic directional analysis provides evidence for pseudotachylyte formation - and therefore earthquakes—in present, low-angle, orientations. Our remanence results provide examples of pseudotachylyte veins within a few 10s of meters of each other in outcrop recording opposite polarity magnetizations; this includes reactivated pseudotachylyte veins, which record both normal and reverse polarity events. These results indicate that this low-angle normal fault zone was active in its present orientation for a prolonged period of time.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T11G0230L
- Keywords:
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- 8010 Fractures and faults;
- STRUCTURAL GEOLOGYDE: 8045 Role of fluids;
- STRUCTURAL GEOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS