Transient thermal regimes in the Sierra Nevada and Baja California extinct outer arcs following the cessation of Farallon subduction

Abstract

[1] We examine the thermal relaxation of the Sierra Nevada and Baja California extinct outer arc blocks following the progressive cessation of Farallon subduction under western North America beginning at $30 Ma. Being parts of the same outer arc until the inland jump of the San Andreas transform fault at $5 Ma, these two regions show many similarities in their geology, geomorphology, rigid body behavior, and their relatively low seismicity. In the thermal model, we combine results of different geophysical and geophysical studies to constrain the thermal state and geometry of the outer arcs before the cessation of subduction and then model the postsubduction temperature responses in these regions using the results of the tectonic reconstructions. A well-constrained regional thermal model of these blocks using the results of many earlier studies in these regions confirms that the present low heat flow values in these regions are the remnants of the very cold outer arc thermal regime of the subduction zone even as long as 30 Ma after cessation of subduction. Thus the entire Pacific boundary of the North American plate is still in a transient thermal state. The calculated low lithospheric temperatures in the Sierra Nevada and Peninsular blocks correlate very well with their rigid body behavior obtained from geodetic studies, and seismogenic layer thicknesses obtained from seismological studies. This is in contrast with the fact that both regions are surrounded by intense deformation associated with the western North America intraplate and extraplate motions. These low-temperature islands play important roles in the present interaction of the North American and Pacific plates and contribute to the broad deformation of the transform boundary. The thermal relaxation of the extinct outer arcs includes both vertical heating from the underlying asthenosphere and the lateral heating from the extinct back arc (Basin and Range), which has remained as a high heat flow region after the cessation of the subduction. We suggest that the significant lateral heat transfer from the Basin and Range in the Sierra Nevada (and from the Gulf of California spreading center in the Peninsular block since $5 Ma) may be the main driving mechanism of the postsubduction volcanism/magmatism along the extinct volcanic arc and the recent tilted uplift of the Sierra Nevada block. The low lithospheric temperatures in Sierra Nevada region may also explain the observation of the high seismic velocities in the mantle beneath the southern Sierra Nevada where the downwelling of the mantle lithosphere proposed.

Seismological estimates of means of isostatic support of the Sierra Nevada

“…4C, 4D). The regions in which colder crustal geotherms are employed match almost exactly with areas of low reduced heat fl ow (Saltus and Lachenbruch, 1991) and low surface heat fl ow (Erkan and Blackwell, 2009), <25 mW/m 2 and 40 mW/m 2 , respectively (Fig. 4C).…”

Fault geometry and cumulative offsets in the central Coast Ranges, California: Evidence for northward increasing slip along the San Gregorio-San Simeon-Hosgri fault

“…Maximum depth of the magnetic rocks is limited by the Curie isotherm for magnetite. Modeling of heat fl ow across the Coast Ranges suggests that temperatures are lower than 580 °C to depths of 20-25 km (Erkan and Blackwell, 2009)-essentially to the base of the crust. We did not try to fi t every short-wavelength magnetic anomaly, but instead concentrated on matching the shape and amplitude of the magnetic anomalies that pertain to fault geometry.…”

Late Cretaceous Construction of the Mantle Lithosphere Beneath the Central California Coast Revealed by Crystal Knob Xenoliths

“…Measured surface heat flows of 80-90 mW/m 2 for the central California coast (Erkan & Blackwell, 2009) suggest depths of ∼45-55 km for the Crystal Knob sample set. Geotherms corresponding to all reasonable surface heat flows suggest entrainment over a depth range of 5-10 km within the mantle lithosphere, with the hotter samples more deeply sourced.…”

“…Blackwell and Richards (2004) Nevada. Erkan and Blackwell (2009) attributed high surface heat flows in the Coast Ranges to nonconductive processes such as shear heating, rapid surface uplift, and fluid circulation along faults. Erkan and Blackwell (2009) attributed high surface heat flows in the Coast Ranges to nonconductive processes such as shear heating, rapid surface uplift, and fluid circulation along faults.…”

“…However, sustained rock uplift and erosion, such as that documented since 2 Ma in the Santa Lucia Range , can raise the crustal geothermal gradient through advection of material. In contrast, the intact batholithic domains underlying the Sierra Nevada, Central Valley, and Peninsular Ranges have relatively low heat flows (Erkan & Blackwell, 2009). The crust of the Coast Ranges and the Mojave Plateau underwent extensive Late Cretaceous crustal detachment faulting (Wood & Saleeby, 1997;Chapman et al, 2012), and the Coast Ranges underwent additional Neogene transform faulting.…”