129-3 Delineation of a Mylonitized and Metamorphosed Pseudotachylyte System in the Homestake Shear Zone, Colorado

Abstract:

The nature of earthquakes in the mid-crust has been subject to extensive debate. We use mylonitic and metamorphosed pseudotachylyte (mpt) to characterize the nature of mid-crustal earthquakes along an 80-m-long outcrop in the Homestake shear zone (HSZ) as part of an NSF REU study. Mpt’s are notably difficult to identify and trace in the field; we use injection veins, branching veins, and a distinctive internal foliation and light-gray color to identify mpt. To aid in identification on the outcrop, we utilize handheld XRF and note a distinctive mpt chemistry reflected by the ratio (FeO+MgO+CaO+TiO₂)/SiO₂ which averages 0.27 ± 0.01 for mpt, versus 0.16 ± 0.01 for host rock and 0.15 ± 0.08 for nearby pristine pseudotachylyte (pst).

Mpt zones are traceable for tens of meters, striking 060° parallel to the host-rock composite foliation. Mpt is composed of finely foliated biotite with s-c fabrics and elongated quartz porphyroclasts. Decimeter-scale sinistral offsets with an oblique reverse component are observed along 060° mpt zones. A Riedel-like fracture system striking 015° and 290° crosscuts the 060° mpt. The 015˚ set was reactivated and offsets the 060˚ mpt with a synthetic, sinistral shear sense by as much as 105 cm. Some 015˚ sets are lined with a second generation of mpt.

Field observations suggest an early and previously unrecognized, midcrustal polydeformation sequence in the HSZ pst system. Pristine pst was generated along the 060˚ foliation, and subsequently mylonitized and metamorphosed, followed by development of 015˚ and 290˚ brittle tensile fractures. The 015˚ fracture set was subsequently reactivated and locally generated mpt with a 3-5-m-length scale. Contrasting 015˚ and 060˚ orientations accommodating sinistral shear suggest the stress field rotated from ~N-S to ~NW-SE. The limited length scale of 015˚ mpt suggests ruptures originated in the mid-crust as opposed to downdip propagation from the upper crust. 

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