Timing of extension in the Pioneer metamorphic core complex with implications for the spatial‐temporal pattern of Cenozoic extension and exhumation in the northern U.S. Cordillera

Abstract

[1] The Pioneer core complex (PCC) in central Idaho lies along a transition between Early Eocene and ca. ≤40 Ma core complexes to the north and south, respectively. Thus, the age of extensional development of the PCC is important in understanding the spatial-temporal patterns of core-complex development in the North American Cordillera. New results, including structural observations and U-Pb zircon (SHRIMP and ICPMS) geochronology, constrain the early extensional history of the footwall for the first time. High-temperature strain with a top-WNW shear-sense is pervasive throughout metamorphic rocks of the northwestern footwall. An isoclinally folded dike yields a crystallization age of $48-47 Ma, whereas a crosscutting dike yielded an age of 46 Ma. Metamorphic rocks are also intruded by the $50-48 Ma Pioneer intrusive suite (PIS), a W-dipping granodiorite sheet displaying a magmatic fabric. Northwest-trending lineations are locally visible and also defined by anisotropy of magnetic susceptibility, indicating that during emplacement, the PIS was undergoing similarly oriented extensional strain as the enclosing metamorphic rocks. Therefore, WNW-directed extension spanning this structural section occurred between $50 and 46 Ma. Following emplacement of crosscutting 46 Ma dikes, deformation was partitioned into the WNW-directed Wildhorse detachment. Motion on the detachment occurred between $38 and 33 Ma, as documented by previous 40 Ar/ 39 Ar thermochronology. It is not clear, however, whether extension was continuous through the interval between these two time periods. Although Early Eocene extension in the PCC was synchronous with extension in core complexes to the north, rates of footwall exhumation in central Idaho were much lower. This southward slowing is compatible with N-S differences in inferred subduction zone geometry/kinematics and in the internal character of the orogenic wedge.

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

“…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 ., ].…”

Evidence for a long‐lived accommodation/transfer zone beneath the Snake River Plain: A possible influence on Neogene magmatism?

“…Geochronology, thermochronology, and crosscutting relationships in lower plate rocks of metamorphic core complexes provide evidence for crustal flow and rapid rise of deeper crustal rocks during Eocene magmatism (e.g., deformed 55–48 Ma intrusive rocks) [ Foster and Raza , ; Vogl et al ., ] (Figure ). Cooling of lower plate rocks of metamorphic core complexes to near‐surface conditions was complete by ~30 Ma as evidenced by the 45–30 Ma cooling ages from low‐temperature thermochronology studies [e.g., Foster and Raza , ; Vogl et al ., ] (Figure ). These once deep‐seated rocks were exposed and eroded, and this detritus was delivered westward by the Princeton and Tyee Rivers and deposited in Eocene strata of the Franciscan Complex and Tyee forearc basin [ Dumitru et al ., ].…”

“…Where directly dated using crosscutting relationships or high‐temperature thermochronology, ductile deformation in the core complexes related to extension is younger than ~42 Ma. For example, in the Albion Mountains, both metamorphism and top‐to‐the‐NW extensional ductile deformation are synchronous with the emplacement of 32–25 Ma plutons [ Strickland et al ., ; Konstantinou et al ., ] which is ~15 Ma younger than the end of ductile deformation in the Pioneer core complex just north of the SRP (based on a ~46 Ma age of crosscutting undeformed intrusions in the lower plate) [ Vogl et al ., ] (Figure ).…”

“…[] to be <10 km in the E‐W direction, which results in less than 10% stretching along the entire region of the Lost River to Beaverhead Mountains, a distance of ~120 km. These field‐based studies and crosscutting relationships are consistent with low‐temperature thermochronology in the core complexes north of the SRP (Figure ) which indicate little subsequent cooling of lower plate rocks after ~20 Ma, with the exception of some late slow cooling in the ranges fringing the Pioneer core complex [ Vogl et al ., ].…”