Subduction, accretion, and exhumation of coherent Franciscan blueschist-facies rocks, northern Coast Ranges, California

Abstract

We present structural data and cross sections from four transects that together cover much of the Eastern Belt of the Franciscan accretionary complex. The westernmost, Middle Eel transect includes jadeite-lawsonite facies rocks (the Taliaferro Metamorphic Complex, TMC) intercalated with lawsonite-albite facies metagreywacke. The TMC shows subduction-related imbricate thrusting, refolded by upright folds with amplitudes of 100-1500 m, and it is cut by abundant normal faults that contributed to exhumation. East of the Coast Ranges divide, three linked transects along Thomes Creek cover the transition from lawsonite-albite facies metagreywackes to the blueschist facies South Fork Mountain Schist. The western section shows thick-bedded metagreywacke intercalated with broken formation, deformed by NWvergent folds and associated thrusts and pressure-solution cleavage, and intensively dissected by abundant low-angle normal faults. In the central section, thin-bedded metagreywacke, broken formation, and conglomerate show an early foliation and folds overprinted by E-vergent folds and crenulation cleavage. The South Fork Mountain Schist forms the easternmost section and records the most intense deformation. The dominant foliation is a differentiated crenulation cleavage that has been refolded by NW vergent folds with amplitudes of millimeters to hundreds of meters. Structural relationships in the South Fork Mountain Schist exposed in Cottonwood Creek farther north are similar to those in Thomes Creek, indicating that our observations have regional significance. All the contractional structures and ductile deformational fabrics in these transects formed under high-P low-T metamorphic conditions during subduction and accretion, and the dominant deformation mechanism was pressure solution. Exhumation was achieved primarily by intensive normal faulting on the outcrop scale, and normal sense motion on the Coast Range fault. This paper provides the first documentation of syn-subduction normal faulting within the Franciscan Complex.

Rheology and stress in subduction zones around the aseismic/seismic transition

“…Limited detrital zircon ages suggest that the youngest sedimentary components are~137 million years old, and Ar-Ar ages on metamorphic mica indicate crystallization or cooling at~121 Ma (Dumitru et al 2010), which suggests that the SFMS is the oldest largescale coherent body of metamorphic rock within the Franciscan Complex. This and its structural position immediately beneath the Coast Range ophiolite, which represents the upper plate of the subduction zone, suggest that it formed along the subduction zone interface in Early Cretaceous time (Schmidt and Platt 2018). The unit is~3.5 km thick and consists largely of pelitic schist, with subordinate amounts of metabasaltic rocks and metachert.…”

“…This places it at a position equivalent to the transition from seismic to aseismic creep on present-day active margins such as the Nankai trough (Peacock 2009) or the Cascadia margin (Peacock et al 2011), and in a temperature regime similar to that calculated for tremor regions beneath the Kii Peninsula (Peacock 2009). It is intensely deformed, with a strong differentiated fabric composed of alternating millimeter-scale laminae of quartz and sheet silicates, formed by solutionredeposition processes (Schmidt and Platt 2018). This fabric has been redeformed by a predominantly Wvergent set of folds, ranging in scale from millimeterscale crenulations to folds several hundred meters in wavelength.…”

“…10b). Plastic deformation alternated with hydraulic fracture, as younger veins show lesser degrees of plastic deformation than older ones (Schmidt and Platt 2018). The size of the dynamically recrystallized grains (~40 μm) indicates shear stress of the order of 10 MPa, but the evidence for alternating phases of plastic deformation and hydraulic fracture suggests that the shear stress and fluid pressure may have fluctuated repeatedly.…”

“…8). The source of the silica is likely to be the limbs of the crenulations, as these commonly show evidence for pressure solution, such as depletion in quartz and concentration of mica and insoluble materials such as graphite (Schmidt and Platt 2018). A train of these crenulations, such as that illustrated in Fig.…”

“…One of these is the Pelona schist in the San Gabriel Mountains of southern California; this is a body of volcanic and sedimentary rocks derived from the underthrust oceanic plate that was metamorphosed at a temperature of~500°C and a depth of~39 km during the latest Cretaceous Laramide flat-slab subduction event (Xia and Platt 2017). The other is the South Fork Mountain Schist in the Coast Ranges of northern California, which was metamorphosed at~350°C and a depth of~30 km along the subduction zone interface during early Cretaceous subduction (Broecker and Day 1995;Cooper et al 2011;Schmidt and Platt 2018). The Pelona Schist appears to represent the products of aseismic creep; the conditions under which the South Fork Mountain Schist formed correspond well to the transition zone, hence with a possible source region of slow earthquakes.…”