Preliminary Investigation of the Footwall Transition of the Borrego Spring Shear Zone

Abstract:

The Borrego Springs Shear Zone in the Peninsular Range Batholith, California, is an east
over west thrusting regime that is part of the Eastern Peninsular Ranges Mylonite Zone. To
the west, undeformed tonalite is dominant. Moving east, tonalite becomes foliated and
mylonitized in places. Further east, the full transition to the footwall is unclear as it is covered
by Quaternary deposits. While roughly continuous on a regional level, there is no clear cut
band of mylonite that can be followed locally for greater than several hundred meters. The
mylonitized tonalite shows lower amphibolite to upper greenschist facies metamorphism,
which may indicate why there is no local uninterrupted band of mylonitization, as it
metamorphosed at the brittle/ductile transition. Dated at ~92MA, it is surmised that
deformation ended no later than 65-70MA due to ⁴⁰Ar/⁴⁰K dates from biotite in the
mylonitized tonalite. Felsic pegmatite dikes also cut through the area, showing evidence of
ductile shear in some places, but again, not consistently throughout the region.

Detailed mapping of the area shows a synformal structure of tonalitic mylonite between
undeformed tonalite. Foliations are roughly perpendicular to the hinge line of the synform.
This evidence, along with intermittent mylonitization, indicates non-uniform deformation. No
evidence of overprinting was seen in the synform, suggesting that the fold formed after the
majority of ductile deformation.