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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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The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland
Abstract Below the seismogenic zone, faults are expressed as zones of distributed ductile strain in which minerals deform chiefly by crystal plastic and diffusional processes. We present a case study from the Caledonian frontal thrust system in northwest Scotland to better constrain the geometry, internal structure, and rheology of a major zone of reverse-sense shear below the brittle-to-ductile transition (BDT). Rocks now exposed at the surface preserve a range of shear zone conditions reflecting progressive exhumation of the shear zone during deformation. Field-based measurements of structural distance normal to the Moine Thrust Zone, which marks the approximate base of the shear zone, together with microstructural observations of active slip systems and the mechanisms of deformation and recrystallization in quartz, are paired with quantitative estimates of differential stress, deformation temperature, and pressure. These are used to reconstruct the internal structure and geometry of the Scandian shear zone from ~10 to 20 km depth. We document a shear zone that localizes upwards from a thickness of >2.5 km to <200 m with temperature ranging from ~450–350°C and differential stress from 15–225 MPa. We use estimates of deformation conditions in conjunction with independently calculated strain rates to compare between experimentally derived constitutive relationships and conditions observed in naturally-deformed rocks. Lastly, pressure and converted shear stress are used to construct a crustal strength profile through this contractional orogen. We calculate a peak shear stress of ~130 MPa in the shallowest rocks which were deformed at the BDT, decreasing to <10 MPa at depths of ~20 km. Our results are broadly consistent with previous studies which find that the BDT is the strongest region of the crust.
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- Award ID(s):
- 1650173
- PAR ID:
- 10208652
- Publisher / Repository:
- DOI PREFIX: 10.2113
- Date Published:
- Journal Name:
- Lithosphere
- Volume:
- 2020
- Issue:
- 1
- ISSN:
- 1941-8264
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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We investigate the deformation conditions of coeval mylonites and pseudotachylytes (pst) exposed in the brittle-ductile transition (BDT) in the Black Belt Shear Zone (BBSZ) in the Southern California Batholith using SEM (Scanning Electron Microscope) imaging, and Electron Backscatter Diffraction (EBSD) analysis. We selected four representative samples along a strain gradient of the BBSZ. The BBSZ is a transpressional shear zone developed within hornblende and biotite tonalites and diorites. The shear zone is discontinuous over a ~ 1.5 - 2 km wide zone, and kinematic indicators show oblique top-to-SW, sinistral-reverse to thrust-sense motion. Metamorphic titanite grains aligned within the mylonitic fabric date the deformation to ~ 83 Ma. SEM and EBSD data show mm-thick seams of pst contained within and parallel to mylonitic foliation, and mutually overprinting relationships between brittle and plastic deformation. We observe a brittle overprint of mylonitic fabric in sample 46 and fractured porphyroclasts reworked into mylonitic fabric in samples 45 and 47. EBSD maps from sample 45 and 47 show decreasing modal percentages of hydrous mafic minerals (biotite and hornblende) in the mylonites with proximity to pst seams, suggesting these melted to form pst. In pst seams, there are embayed and rounded/elliptical plagioclase survivor clasts and acicular and aligned biotite microlites parallel to mylonitic fabric (45 & 47). EBSD maps show pst survivor clasts with the same shear sense as the mylonitic fabric, suggesting co-development. Pole figures show weak CPO in hornblende and plagioclase of sample 46. Samples 45 and 47 have no CPO present in plagioclase, however samples 45, 46, and 47 show strong CPO patterns for quartz that are consistent with prism slip. We interpret dislocation creep as the deformation mechanism accommodating plastic deformation in host mylonites. Quartz CPO patterns provide evidence of mylonitic deformation at temperatures ~ 600o C, and the presence of plagioclase survivor clasts as evidence of pst temperatures of ~1100oC. The kinematically consistent sense of shear between pst and host mylonitic fabrics suggests coeval development that indicate shifts from brittle to ductile deformation. Our results suggest periodic pst-generating events involving melting of hydrous mafic minerals aided the development of coeval mylonites and pst in the BDT.more » « less
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