TY - JOUR
T1 - High temperatures and inverted metamorphism in the schist of Sierra de Salinas, California
AU - Kidder, Steven
AU - Ducea, Mihai N.
N1 - Funding Information:
The manuscript benefited from extended interactions with M. Carlos, A. Martin, E. Nadin, J. Saleeby, A. Walser and P. Wetmore, as well as brief but enlightening exchanges with M. Brandon, B. Dickinson, R. Jamieson, S. Paterson, L. Rosenberg and C. Teyssier—though they may not agree with all the interpretations expressed here. Field and laboratory work was possible only by the generosity of K. Domanik, the Dorrances, Violinis, P. Kephart at the Rana Creek Ranch and M. Stromberg at the University of California Hastings Reserve. Thoughtful reviews by J. Platt and S. Peacock were appreciated. Supported by NSF EAR 0229470 (Ducea).
PY - 2006/1/31
Y1 - 2006/1/31
N2 - New field and thermobarometric work in the Californian Salinian block clarifies current and pre-Tertiary relationships between the schist of Sierra de Salinas and Cretaceous arc-related granitic rocks. The contact is variably preserved as a brittle fault and high-temperature mylonite zone, the Salinas shear zone, which represents the contact between North America and sediments accreted above the Farallon slab between ∼76 Ma and ∼70 Ma. Near granulite facies, prograde replacement of hornblende with clinopyroxene is associated with deformation of plutonic rocks at the base of the upper plate. In the lower plate, the schist of Sierra de Salinas, garnet-biotite thermometry indicates decreasing temperatures down-section from at least 714 °C to∼575 °C over an exposed thickness of ∼2.5 km, consistent with petrologic evidence of an inverted metamorphic gradient. The measured temperatures are significantly higher than observed at shallow levels above subducting slabs or predicted by 2D computational models assuming low shear stresses. Previous workers have called upon shear heating to explain similar observations in the correlative Pelona schist, an unlikely scenario given the results of recent rock deformation experiments which predict that feldspar-quartz-mica aggregates are far too weak to withstand stresses of ∼70 MPa required by the shear heating hypothesis. As an alternative, we propose that high temperatures resulted from conductive heating while the leading edge of the schist traveled ∼150 km beneath the recently active Salinian continental arc during the initiation of shallow subduction. Weakening of the schist due to high temperatures helped facilitate the collapse of the Salinian arc as the schist was emplaced. Schist emplacement coincided with loss of lower, mafic portions of the arc, and therefore evolution of the Southern California crust towards a more felsic composition.
AB - New field and thermobarometric work in the Californian Salinian block clarifies current and pre-Tertiary relationships between the schist of Sierra de Salinas and Cretaceous arc-related granitic rocks. The contact is variably preserved as a brittle fault and high-temperature mylonite zone, the Salinas shear zone, which represents the contact between North America and sediments accreted above the Farallon slab between ∼76 Ma and ∼70 Ma. Near granulite facies, prograde replacement of hornblende with clinopyroxene is associated with deformation of plutonic rocks at the base of the upper plate. In the lower plate, the schist of Sierra de Salinas, garnet-biotite thermometry indicates decreasing temperatures down-section from at least 714 °C to∼575 °C over an exposed thickness of ∼2.5 km, consistent with petrologic evidence of an inverted metamorphic gradient. The measured temperatures are significantly higher than observed at shallow levels above subducting slabs or predicted by 2D computational models assuming low shear stresses. Previous workers have called upon shear heating to explain similar observations in the correlative Pelona schist, an unlikely scenario given the results of recent rock deformation experiments which predict that feldspar-quartz-mica aggregates are far too weak to withstand stresses of ∼70 MPa required by the shear heating hypothesis. As an alternative, we propose that high temperatures resulted from conductive heating while the leading edge of the schist traveled ∼150 km beneath the recently active Salinian continental arc during the initiation of shallow subduction. Weakening of the schist due to high temperatures helped facilitate the collapse of the Salinian arc as the schist was emplaced. Schist emplacement coincided with loss of lower, mafic portions of the arc, and therefore evolution of the Southern California crust towards a more felsic composition.
KW - Accretion
KW - POR schists
KW - Pelona schist
KW - Salinia
KW - Schist of Sierra de Salinas
KW - Shear heating
KW - Subduction
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U2 - 10.1016/j.epsl.2005.11.037
DO - 10.1016/j.epsl.2005.11.037
M3 - Article
AN - SCOPUS:30744464897
SN - 0012-821X
VL - 241
SP - 422
EP - 437
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 3-4
ER -