The Late Cretaceous arc flare-up event from 90 to 70 in the Transverse Ranges of the Southern California Batholith was temporally and spatially associated with the development of a large contractional shear system that includes discontinuous segments of the Tumamait shear zone (Mt. Pinos), the Alamo Mountain-Piru Creek shear zone, the Black Belt shear zone (Cucamonga terrane), and the Eastern Peninsular Ranges shear zone. The age and kinematics of these shear zones inform the tectonic setting of the continental arc in Southern California during the beginning of the Laramide orogeny and during postulated large-magnitude dextral translations along the margin (the Baja-BC hypothesis).
The Mt. Pinos sector of the Southern California Batholith preserves the intra-arc, transpressional Tumamait shear zone and the ductile-to-brittle Sawmill thrust, both of which record Late Cretaceous deformation. The batholith and shear zone are hosted by Mesoproterozoic biotite gneisses and migmatites (1750-1760 Ma), Neoproterozoic biotite granites (660 Ma), Permo-Triassic granitic gneisses and amphibolite (260-250 Ma), and Late Jurassic granites and gneisses (160-140 Ma). Late Cretaceous rocks are variably deformed and include porphyritic granodiorite gneisses and peraluminous granites emplaced at 86 to 70 Ma. Mylonites of the Tumamait shear zone affect all rocks in the area and generally strike NW-SE and dip moderately to the NE and SW. Mineral stretching lineations plunge shallowly to the SE. Mylonitic fabrics are folded into a regional, SE-plunging synform that results in alternating bands of sinistral and dextral shear fabrics. Syn-kinematic titanites from 5 mylonitic samples give a 720-700°C temperature range, and lower-intercept 206Pb/238U dates of 77.0 Ma, 76.8 Ma, 75.1 Ma, 74.2 Ma, and 74.0 Ma. Subsequent folding of the mylonite is linked to N-directed motion on the Sawmill thrust. 40Ar-39Ar thermochronology ages of 67-66 Ma and onlapping Eocene shales indicate Latest Cretaceous activity on the thrust, prior to Eocene arc collapse. Based on the age of the Tumamait shear zone, we speculate that it is related to sinistral deformation observed in the nearby Alamo Mountain-Piru Creek and the Black Belt shear zones. We attribute the younger Sawmill thrust to collision of the Hess oceanic plateau with the Southern California Batholith after 70 Ma.
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This content will become publicly available on September 10, 2024
The Black Belt Shear Zone records the earthquake cycle at the brittle-ductile transition: implications for the SCEC Community Rheology Model
Abstract. The inception of the Laramide Orogeny in Southern California is marked by a Late
Cretaceous arc flare-up in the Southern California Batholith (SCB) that was temporally and
spatially associated with syn-plutonic development of a regionally extensive, transpressional
shear zone system. This ~200 km-long system is the best analog for the shear zones that
extend into the middle crust beneath the major lithotectonic block-bounding faults of the San
Andreas Fault system. We focus on the Black Belt Shear Zone, which preserves an ancient
brittle-ductile transition (BDT), and is exposed in the SE corner of the San Gabriel lithotectonic
block. The mid-crustal Black Belt Shear Zone forms a ~1.5-2 km thick zone of mylonites
developed within hornblende and biotite tonalites and diorites. Mylonitic fabrics strike SW and
dip moderately to the NW, and kinematic indicators from the Black Belt Shear Zone generally
give oblique top-to-SW, sinistral thrust-sense motion (present-day geometry). U-Pb zircon
ages of host rock to the Black Belt mylonites demonstrate crystallization at ~86 Ma and
metamorphism at ~79 Ma at temperatures ~753 ¡C. Syn-kinematic, metamorphic titanite
grains aligned with mylonitic foliation in the Black Belt Shear Zone give an age of ~83 Ma.
These data indicate syn-magmatic sinistral-reverse, transpressional deformation. The BDT
rocks in the Black Belt Shear Zone are characterized by a ~10 m-thick section of high strain
mylonites interlayered with co-planar cataclasite and pseudotachylyte (pst) seams.
Microstructural and electron backscatter diffraction (EBSD) analysis shows that the mylonites
and cataclasites are mutually overprinted, and pst seams are overprinted by mylonitic fabric
development. Pst survivor clasts show the same shear sense as the host mylonite, and this
kinematic compatibility demonstrates a continuum between brittle and ductile deformation that
is punctuated by high strain rate events resulting in the production of frictional melt. EBSD
analysis reveals a decreasing content of hydrous maÞc mineral phases in host mylonite with
increasing proximity to pst seams. This suggests that pst was generated by melting of
hornblende and/or biotite, implying that coeval development of mid-crustal mylonites and pst
does not require anhydrous melting conditions. Rather, the production of pst may liberate
water, implying that BDT rock rheology is affected by transient pulses of water inßux and
strain rate increases.
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- Award ID(s):
- 2138733
- NSF-PAR ID:
- 10495008
- Publisher / Repository:
- Poster Presentation at 2023 SCEC Annual Meeting
- Date Published:
- Subject(s) / Keyword(s):
- ["Type: Poster Presentation\nCategory: FARM\nSCEC Award: 21140\nSCEC Theme\/Topic: Community Models"]
- Format(s):
- Medium: X
- Location:
- Palm Springs, CA
- Sponsoring Org:
- National Science Foundation
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