Autori: Kidder, S., and Ducea, M.N

Editorial: Earth and Planetary Science Letters, 241, p.422-437, 2006.

Rezumat:

Abstract: 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 hornblende quartz diorite 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 in other forearcs and 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 extremely 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. As lower, mafic portions of the
arc were delaminated, schist emplacement coincided with evolution of the Southern California crust towards
a more felsic composition. Dewatering associated with deformation in the Salinas shear zone may represent
a geologic counterpart to long-period seismic tremors associated with „slow earthquakes” along modern-day
subduction megathrusts.

Cuvinte cheie: shallow subduction, inverted metamorphic gradient