Orocopia Schist Detatchment Fault metamorphic rock - Laramide Orogeny of Orocopia Mountains. 50-80  million years ago

 

The Orocopia Schist is an ancient, deep-ocean sedimentary rock that was subducted and metamorphosed during the Laramide orogeny in the Late Cretaceous and Early Tertiary periods, primarily beneath what is now southern California and southwest Arizona. This rock, found in locations like the Orocopia Mountains and parts of Arizona, was likely underplated as a wide, thin layer at the base of the continental crust, though some models suggest it ascended as diapirs. Recent discoveries indicate a broader distribution than initially thought, suggesting it may be a more common feature of convergent plate margins. 

Formation and Location

• Origin:

  Orocopia Schist began as marine sediments, primarily metagraywacke (oceanic in origin) from a trench, deposited off the California coast. 

• Subduction:

  These sediments were carried down into the Earth's mantle as part of the Farallon slab during the Late Cretaceous subduction event. 

• Metamorphism:

  The intense pressure and heat from subduction transformed the sediments into schist, a metamorphic rock characterized by its layered structure of flat, sheet-like minerals, such as quartz and mica. 

Geological Significance

• Subduction Complex:

  The Orocopia Schist is part of the larger Pelona-Orocopia-Rand Schist (PORS) complex, a major geological feature that provides a unique record of shallow subduction processes. 

• Relamination:

  Some researchers propose that the Orocopia Schist was not merely subducted and underplated, but later "relaminated" and brought back to the middle to lower crust as a "sediment diapir" or crustal plume. 

• Exhumation:

  Following its deep burial and metamorphism, the schist was brought to the surface through a process of structural ascent and exhumation, likely driven by extension and core complex formation in the Paleogene. 

Key Locations

• California:

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  The Orocopia Mountains in southeastern California contain the principal exposure of the schist. 

• Arizona:

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  Further inland exposures in southwestern Arizona, such as at Cemetery Ridge and in the Plomosa Mountains, demonstrate the wide distribution of these subducted sediments. 

• The structure, petrology, and chemistry of the Orocopia Schist ...

  The Orocopia Schist and its lateral equivalents are primarily a quartzofeldspathic Late Cretaceous – early Tertiary metagraywacke ...

  SDSU Digital Collections

• Zircon ages from Orocopia Schist at Cemetery Ridge, southwestern ...

  Aug 4, 2017 — Orocopia Schist has recently been discovered at Cemetery Ridge, Arizona, even farther inland from the continental margi...

  GeoScienceWorld

• Laramide subduction and metamorphism of the Orocopia Schist, ...

  Sep 7, 2018 — Thin (<10 μm) zircon overgrowths on detrital cores, resolved using laser ablation–inductively coupled plasma–mass spect...

  GeoScienceWorld

• Show all

• The age of the Orocopia Schist itself (when the sediments were deposited) is Late Cretaceous to ~80 Ma, but the metamorphic event that formed the schist occurred later, during the Laramide orogeny and into the Paleogene, around 73 to 47 Ma. The schist was subsequently exhumed to the surface during the Miocene, and the metamorphic rocks continued to cool at this time. 

• Deposition and Metamorphism

• Deposition:

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• The sedimentary rocks that would become the Orocopia Schist were deposited in the Late Cretaceous, after 70-80 Ma. 

• Metamorphism:

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• The schist was metamorphosed under high-pressure, high-temperature conditions during the Laramide orogeny, which began around 73 Ma. The peak metamorphism and recrystallization occurred at depths of 20-35 km. 

• Exhumation and Cooling

• Paleogene Exhumation:

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• A significant event of exhumation brought the Orocopia Schist to upper-crustal depths. 

• Miocene Exhumation:

• .

• The schist was further exhumed to the surface during the Miocene, with cooling occurring in pulses, such as the one that started around 52-50 Ma and again from 24-22 Ma. 

• Key Takeaway

• The Orocopia Schist's age refers to both the age of its original sedimentary protolith (Late Cretaceous) and the time of its metamorphic transformation (Laramide-Paleogene). 

Orocopia Schist Detatchment Fault

Key indicator of the 300km split of pacific plate and continental plate

​Yes, the Orocopia Schist is a metamorphic rock that formed through burial and prograde metamorphism in a subduction complex and later experienced exhumation in a metamorphic core complex. It exhibits complex structural and metamorphic features due to multiple episodes of faulting and deformation, with the metamorphism predating its rise to the surface.

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