245 Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part I
We welcome contributions inspired by the diverse research of John Platt in structural geology and tectonics showing how the rock record can be used to illuminate the fundamental processes of crustal deformation and mountain building.
Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217D
Poorly-constrained Paleosubduction Interface Slip Directions from Subduction Complexes: An Example from the Franciscan Complex of California.
Paper number: 245-5
Program: Technical Sessions
Time: 9:10 AM – 9:25 AM
Authors
Abstract
Obliquity of subduction slip on exhumed subduction interfaces is crucial for determining partitioning of oblique convergence in various parts of orogenic belts, but determination of the interface slip direction is difficult, as shown by studies in the Franciscan subduction complex of California. About 13000 km of oceanic plate subducted during the ca. 176 to 12 Ma history of sporadic accretion recorded in the Franciscan. However, studies of subduction direction relative to subduction zone strike in the Franciscan are rare, representing a small fraction of spatial-temporal subduction interfaces. Marin Headlands (ca. 95 Ma subduction age), El Cerrito Quarry (ca. 95 Ma) of the San Francisco Bay area and the greater Yolla Bolly area of the northern Coast Ranges (ca.100 to 121 Ma) show strike-perpendicular thrusting, whereas the Skaggs Springs schist (ca. 120 Ma?) of the southwestern northern Coast Ranges shows oblique sinistral slip. The latter requires an evaluation of rotation of stretching lineation trends by later folding. On the other hand, the 70-km-long schist belt affords a test of whether the long axis of the current exposure belt is parallel to the original subduction zone strike. If the belt long axis represents the subduction zone strike (equal depth along the interface) then the belt should record the same metamorphic conditions along strike. Quantitative studies of metamorphic pressure-temperature conditions for Skaggs Springs schist metamorphism have not been conducted, but petrography shows the same mineral assemblages along the length of the belt. In contrast, petrographic observations of units in the Yolla Bolly and adjacent areas indicate a northward decrease metamorphism over a distance of about 400 km, suggesting that the northern exposures record more updip positions than those in the south. Most accreted Franciscan units lack sufficient exposed strike length to evaluate along-strike metamorphic differences and test parallelism with subduction zone strike. Most Franciscan subduction was associated non-accretion. Structures above and below non-accretionary horizons reflect accretionary slip of the units accreting before and after the non-accretionary event. Determining subduction slip directions on non-accretionary horizons is daunting (no attempts yet) but important.
Geological Society of America Abstracts with Programs. Vol. 57, No. 6
doi: 10.1130/abs/2025AM-412836
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