Oligo‐Miocene extensional tectonics and fluid flow across the Northern Snake Range detachment system, Nevada

Abstract

[1] The Northern Snake Range (Nevada) represents a spectacular example of a metamorphic core complex and exposes a complete section from the mylonitic footwall into the hanging wall of a fossil detachment system. Paired geochronological and stable isotopic data of mylonitic quartzite within the detachment footwall reveal that ductile deformation and infiltration of meteoric fluids occurred between 27 and 23 Ma. 40 Ar/ 39 Ar ages display complex recrystallization-cooling relationships but decrease systematically from 26.9 ± 0.2 Ma at the top to 21.3 ± 0.2 Ma at the bottom of footwall mylonite. Hydrogen isotope (dD) values in white mica are very low (−150 to −145 ‰) within the top 80-90 m of detachment footwall, in contrast to values obtained from the deeper part of the section where values range from −77 to −64 ‰, suggesting that time-integrated interaction between rock and meteoric fluid was restricted to the uppermost part of the mylonitic footwall. Pervasive mica-water hydrogen isotope exchange is difficult to reconcile with models of 40 Ar loss during mylonitization solely by volume diffusion. Rather, we interpret the 40 Ar/ 39 Ar ages of white mica with low-dD values to date syn-mylonitic hydrogen and argon isotope exchange, and we conclude that the hydrothermal system of the Northern Snake Range was active during late Oligocene (27-23 Ma) and has been exhumed by the combined effects of ductile strain, extensional detachment faulting, and erosion.

Fluid–Rock Interaction and Strain Localization in the Picacho Mountains Detachment Shear Zone, Arizona, USA

“…For the three minerals analyzed, oxygen isotope values fall within a narrow 2‰ range, with 10.2‰ < δ 18 O Qtz < 11.9‰, 4.1‰ < δ 18 O Bt < 5.7‰, and 5.3‰ < δ 18 O Hbl < 7.0‰, similar to values reported by Kerrich and Rehrig (). Such values are somewhat depleted compared to typical metamorphic rock compositions (Taylor & Sheppard, ), and are similar to δ 18 O values reported for other MCCs (Holk & Taylor, ; Holk et al, ; Mulch et al, , ; Gébelin et al, , ; Gottardi et al, , ; Quilichini et al, , ). Using the calibrations by Bottinga and Javoy () and the temperatures estimated by geothermometry (Table ), we estimate that the δ 18 O value of the fluid present during fluid–rock exchange ranges from 7.3 to 9.4‰.…”

Eocene and Miocene extension, meteoric fluid infiltration, and core complex formation in the Great Basin (Raft River Mountains, Utah)

“…Interpretation of 40 Ar/ 39 Ar ages in recrystallized rocks that were deformed within the closure temperature interval for 40 Ar diffusion is challenging [e.g., Mulch and Cosca , ; Harrison et al ., ; Cosca et al ., ], yet combined isotopic and elemental tracer data can provide additional insight into the 40 Ar retention behavior especially for white mica in ductile shear zones [e.g., Mulch et al ., , ; Gébelin et al ., ; Cosca et al ., ]. The fact that the individual sections across the RAG‐MCC preserve distinct “clusters” of very low δ 2 H ms values (Figure ) strongly supports the idea that both δ 2 H ms and 40 Ar isotope systematics remained undisturbed after deformation.…”

“…Undoubtedly, Miocene Basin and Range extension played a major role in establishing the present‐day architecture of crust and lithospheric mantle underlying the Great Basin. However, there is increasing evidence that deformation structures preserved in the exhumed mylonitic footwall of MCCs may preserve geological information that dates back to earlier episodes of their tectonic history [e.g., Wells et al ., , ; Foster et al ., , ; Gébelin et al ., , ; Vogl et al ., ; Wong et al ., ]. This also holds true for the RAG‐MCC where middle to late Miocene exhumation along the RRDSZ was associated with unroofing and doming of the MCC postdating Oligocene intrusion of plutons [e.g., Wells et al ., ; Konstantinou et al ., ].…”

Titanium concentration in quartz as a record of multiple deformation mechanisms in an extensional shear zone

“…These quartzites form a markedly thick unit in a sequence of platform sediments that accumulated along the continental shelf of the ancestral North American margin and that today compose the dominant lithologies preserved in the detachment zone of the Shuswap MCC [ Brown et al ., ; Price , ]. Metamorphosed sandstone units have been identified in a number of extensional shear zones bounding metamorphic core complexes in the North American Cordillera (Kettle [ Mulch et al ., ]; Pioneer [ Vogl et al ., ]; Raft River [ Gottardi et al ., ; Wells et al ., ]; northern Snake Range [ Gébelin et al ., ; Miller et al ., ]). These quartzites may have accommodated decoupling between the underlying crystalline basement and overlying brittle crust and could therefore define a significant rheological interface in the lithosphere [ Whitney et al ., ].…”