Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

Abstract

Abstract. The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

Constraints on the Emplacement of Martian Nakhlite Igneous Rocks and Their Source Volcano From Advanced Micro‐Petrofabric Analysis

“…In other words, simple twins impact the M-Index calculations by underestimating true CPO strengths. The values that are observed sit in a similar range to reported ultramafic rocks on Earth (Hidas et al, 2019;Van Der Werf et al, 2017), although clinopyroxene is not the dominant phase in these studies. For the analyzed sections, the CPO strength from M-Index calculations is at odds with the estimates from the CPO plots but does agree with the observed SPO (Figure 4).…”

Migmatite and leucogranite in a continental-scale exhumed strike-slip shear zone: Implications for tectonic evolution and initiation of shearing

“…Crustal anatexis generally occurs at different geological settings including environments of crustal thickening or post-collisional collapse (e.g., Brown, 2001;Cao et al, 2019;Shakerardakani et al, 2020). Melt segregation and extraction are evidenced by the presence of leucosomes within migmatites, which record the melt flow network through the crust (e.g., Brown, 2001;Searle et al, 2010;Vanderhaeghe, 2009;Yu et al, 2019). Highly evolved peraluminous leucogranites are widespread throughout orogenic belts, especially the leucogranites associated with continental collision formed by partial melting of aluminum-rich metasedimentary rocks buried at lower crustal depth (e.g., Patiño-Douce and Johnston, 1991;Scaillet et al, 1995;Brown, 2007Brown, , 2013Shakerardakani et al, 2020).…”

“…Exhumation of migmatites and leucogranites within the crust is generally favored by post-collisional collapse or exhumation of metamorphic core complexes along crustal-scale strike-slip shear zones (e.g., Hutton, 1988;Searle, 2006;Searle et al, 2010;Cao et al, 2011a;Liu et al, 2015bLiu et al, , 2020. The presence of melt weakens rock strength and promotes faster deformation, ultimately influencing how orogenic belts form (e.g., Rosenberg and Handy, 2005;Vanderhaeghe, 2009;Alina et al, 2019). Therefore, accurate dating of the timing of melting, determining the protoliths, and characterizing the structure and pressure-temperature (P-T) conditions of partial melting are important for understanding the evolutionary processes and dynamics of the late stages of orogenesis (e.g., Labrousse et al, 2011).…”

Can the Magmatic Conditions of the Martian Nakhlites be Discerned via Investigation of Clinopyroxene and Olivine Intracrystalline Misorientations?

“…Thus, the activation criteria for olivine's crystallographic slip-systems over a variety of parameters are fairly well constrained (Bernard et al, 2019;Karato et al, 2008;. Augite, on the other hand, is only starting to be studied in the same level of detail (Tedonkenfack et al, 2021;Van der Werf et al, 2017). Previous work exploring slip-systems in clinopyroxene has predominantly focused on diopside (monoclinic with a similar crystal lattice structure to augite), but mostly in laboratory settings (Bascou et al, 2002;Bystricky & Mackwell, 2001;Ingrin et al, 1991;Mauler et al, 2000;Skrotzki, 1994), where many of these crystallographic slip-systems have yet to be observed within naturally occurring rocks (Keppler, 2018;Skrotzki, 1994;Tedonkenfack et al, 2021;Van der Werf et al, 2017).…”

“…Augite, on the other hand, is only starting to be studied in the same level of detail (Tedonkenfack et al, 2021;Van der Werf et al, 2017). Previous work exploring slip-systems in clinopyroxene has predominantly focused on diopside (monoclinic with a similar crystal lattice structure to augite), but mostly in laboratory settings (Bascou et al, 2002;Bystricky & Mackwell, 2001;Ingrin et al, 1991;Mauler et al, 2000;Skrotzki, 1994), where many of these crystallographic slip-systems have yet to be observed within naturally occurring rocks (Keppler, 2018;Skrotzki, 1994;Tedonkenfack et al, 2021;Van der Werf et al, 2017). Current knowledge of clinopyroxene crystallographic slip-system activation (often based off numerical simulations for example, Ulrich and Mainprice (2005)), shows strong dependence of crystal orientation relative to the principal stress axes.…”