SEDIMENTOLOGIC AND STRATIGRAPHIC EVIDENCE FOR EARLY MIOCENE PASSAGE OF THE MENDOCINO TRIPLE JUNCTION UNDER THE SOUTHERN SAN JOAQUIN VALLEY

Recent published plate reconstructions indicate that the Mendocino Triple Junction (MTJ) and associated slab window migrated northward under the southern San Joaquin Valley (SSJV) between Chron C7 (24.7Ma) and Chron C6b (23.0Ma). Along the rim of the SSJV, ≈200-700m of nearshore/non-marine coarse-clastic strata (Ttc) and interbedded volcanic flows and breccias (Tv) (21-24Ma) of the Tecuya formation have been uplifted, tilted to the N, and exposed semi-continuously for 10 km. We describe detailed sedimentologic and stratigraphic data from Ttc that underlies and is interbedded with Tv. The base of Ttc consists of 50-100m of exotic resedimented conglomerates (Ttce) with WNW flowing paleocurrents that unconformably overlies gneissic basement or Eocene sediments. Exotic composition, broken rounds, and clasts of resedimented conglomerates indicate erosion and resedimentation of pre-existing conglomerates from a cryptic source somewhere in the W Mojave. Down to the N movement on the WNW trending Tunis fault cut Ttce and caused erosion of 10-50m from the footwall and deposition in the hanging wall of nearshore and fluvial conglomerates that thicken towards the fault, show decreasing stratal dip up section, contain local rocks, and paleocurrents to the NE. An intraformational unconformity cuts all older rocks and is overlain by a megabrx containing 1-5m clasts of gneiss, schist, marble, and underlying sediments and a locally sourced gneiss boulder conglomerate. Channels cut into this conglomerate are filled with 21-24Ma lava flows. The sedimentary record shows the following sequence: uplift and erosional rejuvenation to the S; initiation of N-S extension on WNW trending, north-dipping normal faults; northward migration of uplift and erosion with associated change from W to N flowing paleocurrents, short local drainages, and exposure and erosion of local basement; development of a regional erosional unconformity as northward migrating uplift crossed the field area; eruption of lava flows on the erosional unconformity. We interpret this sequence as recording the northward movement of the MTJ fracture zone followed by volcanism generated by slab window processes. A similar signature of triple junction migration may be recognizable in the stratigraphic record of continental margins.