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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Himalayan-like Crustal Melting and Differentiation in the Southern North American Cordilleran Anatectic Belt during the Laramide Orogeny: Coyote Mountains, Arizona
The southern US and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (c. 80–40 Ma). The metamorphic sources of melt are not exposed at the surface; however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62 to 42 Ma. Ti-in-zircon temperatures (TTi-zr) correlate with changes in zircon rare earth elements (REE) concentrations, and indicate prograde heating, mineral breakdown, and melt generation took place from 62 to 53 Ma. TTi-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53 to 51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51 to 42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in TTi-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution–reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, large ion lithophile elements, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High-silica, muscovite± garnet leucogranite dikes that crosscut two-mica granite represent more evolved residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of (1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, (2) heat and fluid transfer related to the Laramide continental arc, and (3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny.
more » « less- NSF-PAR ID:
- 10472656
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Journal of Petrology
- Volume:
- 64
- Issue:
- 10
- ISSN:
- 0022-3530
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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A microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening, and regional metamorphism, including the Late Cretaceous-early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite-biotite-garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top-to-the-north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of non-coaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 ×10-11 s-1 to 5.0 × 10-13 s-1 at depth of ~11–14 km. Detailed 40Ar/39Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation post-dates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest.more » « less
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Granitic rocks, interpreted to be related to crustal melting, were emplaced into regions of thickened crust in southern Arizona during the Laramide orogeny (80–40 Ma). Laramide-age anatectic rocks are exposed as plutons, sills, and dike networks that are commonly found in the exhumed footwalls of metamorphic core complexes. This study investigates newly discovered exposures of granodioritic–leucogranitic rocks from three intrusive phases in the footwall of the Pinaleño–Jackson Mountain metamorphic core complex of southeastern Arizona, called the Relleno suite. Zircon U–Pb geochronology indicates that the suite was emplaced from 58 to 52 Ma. Zircon Lu/Hf isotope geochemistry, whole-rock Sr and Nd isotope geochemistry, and mineral O isotope geochemistry were used to investigate the source of these rocks and evaluate whether they are related to crustal anatexis. Average zircon εHf(t) values of the suite range from −4.7 to −7.9, whole-rock εNd(i) and 87Sr/86Sr(i) values range from −9.4 to −11.8 and 0.7064 to 0.7094 respectively, and quartz δ18OVSMOW values range from 6.8 to 9.4 ‰. Isotopic and geochemical data of these rocks are consistent with derivation from and assimilation of intermediate–mafic (meta)igneous rocks, at deep crustal levels, and are supported by thermodynamic melt models of Proterozoic igneous rocks equivalent to those exposed in the Pinaleño Mountains. In comparison with other Laramide-age anatectic granites in SE Arizona, those exposed in the Pinaleño Mountains are temporally similar but present compositional and isotopic differences that reflect melting and assimilation of different lithologies, producing distinct mineralogical and isotopic characteristics. The results suggest that crustal melting during this interval was not limited to metasedimentary protoliths and may have affected large portions of the deep crust. The early Paleogene Relleno suite in the Pinaleño Mountains strengthens the relationship between crustal melting and regions of thickened crust associated with the Sevier and Laramide orogenies.more » « less
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Abstract Dating ultra‐high–pressure (
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