STRIKE-SUBPARALLEL BLUESCHIST FACIES STRETCHING LINEATION, SKAGGS SPRINGS SCHIST, FRANCISCAN SUBDUCTION COMPLEX, CALIFORNIA: EARLY-CRETACEOUS STRIKE-SUBPARALLEL SUBDUCTION INTERFACE MOVEMENT DURING SUBDUCTION OR EXHUMATION

The obliquity of movement on paleo-subduction interfaces is difficult to access owing to later modification of the interface geometry, particularly by folding. Many (most?) studies assume pure thrust movement on the interface with the strike-parallel convergence component accommodated by strike-slip faulting inboard of the subduction complex. In contrast, modern subduction zones show a range in strain partitioning of oblique convergence with pure thrusting on the interface as one end member. The Skaggs Springs Schist (SSS) is the oldest large coherent metamorphic unit in the Franciscan subduction complex of western California and is the only Franciscan unit with a pervasive stretching lineation. The main SSS outcrop belt in Sonoma and southern Mendocino County extends ~ 70 km NW-SE with a width of ~ 3 km. This outcrop belt preserves the axial part of a regional west-vergent overturned synform, with foliation striking parallel to the belt axis and dipping dominantly moderately to steeply NE. Structurally lower units flank the belt on both sides. The dominant lithology of the SSS is a quartz-rich glaucophane ± lawsonite ± phengite ± titanite ± garnet ± jadeite ± relict epidote schist. Long axes of glaucophane and lawsonite are oriented subparallel to the lineation in a syntectonic relationship. Some lawsonite grew later with more random orientations. The lineation trends subparallel to foliation strike with a sub-horizontal plunge. There is no systematic relationship between the obliquity of lineation trend to foliation strike and foliation dip, so the lineation-foliation relationship orientation is not an artifact of post-lineation steepening of the foliation. There is no change in mineral assemblages or metamorphic grain size along the 70-km-long exposure belt, so the foliation strike reflects the paleo-subduction zone strike, albeit with the possibility of belt-wide late vertical axis rotation. It is unclear whether the lineation formed during subduction or deep exhumation along the interface, but the geometry suggests highly oblique subduction at the time of formation. Sinistral shear sense is associated with the lineation. The SSS has yielded young U-Pb detrital zircon ages of ca. 144 – 148 Ma and ⁴⁰Ar – ³⁹Ar phengite ages of ca. 132 Ma, so the stretching lineation and associated fabric reflects early Cretaceous sinistral oblique movement on the subduction interface.