T51A-2840
Evidence of Failure on Low-Angle Normal Faults from Thermochronology and Paleomagnetism: A Case Study from South Mountains Metamorphic Core Complex, Arizona

Friday, 18 December 2015
Poster Hall (Moscone South)
Jack Ryan Hoehn1, Dana Marion Smith2, Laurel B Goodwin2, Joshua M Feinberg3, Matthew Thomas Heizler4, Bradley S Singer2 and Brian R Jicha5, (1)University of Wisconsin Madison, Geoscience, Madison, WI, United States, (2)Univ Wisconsin - Madison, Madison, WI, United States, (3)University of Minnesota, Minneapolis, MN, United States, (4)New Mexico Institute of Mining and Technology, Bureau of Geology and Mineral Resources, Socorro, NM, United States, (5)University of Wisconsin Madison, Madison, WI, United States
Abstract:
The South Mountains metamorphic core complex records progressive extension and

exhumation of Miocene granodiorite. Early mylonites are cut by younger brittle faults,

including locally abundant, shallowly dipping, pseudotachylyte-lined slip surfaces. These

frictional melt generation veins can be grouped based on mesoscopic character of both

pseudotachylyte and host granodiorite mylonite. All vein types are subparallel to biotite-lined,

host rock C-surfaces. Thin (<2mm), foliated type 1 veins define networks either confined to,

or located at margins of C-surface dominated ultramylonite. Type 2 veins are thicker (2-4mm)

with abundant host rock survivor clasts. Though largely parallel C-surfaces, these veins

possess S-surface-parallel segments up to several cms long, and cut host rock with a strong

S-C fabric. Type 3 veins lack mesoscopic foliation and cut host rock ranging from

protomylonite to ultramylonite. They are similar in thickness to type 2 veins, but include fewer

survivor clasts.

Previous 40Ar/39Ar dates on pseudotachylyte veins (16.24 ± 0.23 Ma and 17.44 ± 0.20

Ma) show pseudotachylyte-producing seismicity occurred over at least 1 million years. Multi-

diffusion-domain analysis of host rock K-feldspar demonstrates cooling below 150°C by 21.8

Ma. Assuming a geothermal gradient between 25°C and 50°C/km, pseudotachylyte veins were

generated at a maximum depth of 2.5-5 km. Fabric intensity of anisotropy of magnetic

susceptibility (AMS) corresponds to vein types. Paleomagnetic analyses yield 3 clusters of

characteristic remanent magnetizations (ChRMs) that correlate to AMS/vein types. ChRMs of

nonfoliated type 3 veins are close to the Miocene pole. By contrast, ChRMs for type 1 and 2

veins are deflected close to the foliation due to strong anisotropy of their magnetic mineral

assemblages.

Normal faults dipping <30° are poorly oriented for slip according to Andersonian fault

mechanics. Previous workers therefore have suggested they failed at steeper dips, then

rotated to current orientations. The uniformity of South Mountains generation vein orientations

records no rotation between formation of the oldest and youngest veins. The proximity of the

ChRMs of type 3 veins to the Miocene pole similarly indicates no rotation. Slip may have been

facilitated by low friction of biotite-lined C-surfaces.

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