San Luis Obispo Transform Fault and Middle Miocene rotation of the Western Transverse Ranges, California


  • Clarence A. Hall Jr.


A fault-disrupted, linear belt of lower middle Miocene pyroclastic and volcanic rocks was erupted 15–17 m.y. B.P. along a trend 140 to 200 km long in the southern Coast Ranges and along the northern flank of the western Transverse Ranges of California. Offset segments of the belt are coincident with the Oceanic-West Huasna, Santa Maria River-Little Pine, and Lompoc-Solvang fault zones. The volcanic rocks thin eastward and also vary in thickness along strike, the latter reflecting the undulation of ‘porpoising’ highs and lows of basins. To account for the linear distribution of over 3000 km2 of bimodal volcanic rock types, it is proposed that they were intruded, extruded, and ejected into a continental submarine margin environment along a feature here named the San Luis Obispo transform (SLOT). The fault was near the mid-Miocene continental margin and evidently joined the early mid-Miocene Mendocino triple junction with the crest of the East Pacific Rise. Subduction of the Farallon Plate had stopped along the adjacent continental margin before Miocene magmatism in this region. The inferred position of the East Pacific Rise crest was too far to the south, and the inferred position of the Mendocino triple junction was too far to the north between 15 and 20 m.y. B.P. to have been directly related to the Miocene volcanic event near 35°N latitude at that time. A part of the western Transverse Ranges was transported northwestward immediately following Miocene volcanism. Earlier movement is possible, but evidence is inconclusive. Microplate transport was concurrent with the development of pull-apart structures and the ≃12- to ≃16-m.y.-old period of volcanism along the southern margin of the microplate within the Los Angeles basin. Variable paleomagnetic directions that display clockwise deflections in the Oligocene (e.g., Morro Rock-Islay Hill Complex) and Miocene igneous rocks (e.g., western Transverse Ranges) in the vicinity of SLOT can be accounted for by extension and rotational tectonics during pull-apart development, rotation of blocks between faults during the northwestward transport of the western Transverse Ranges along SLOT, and by rotation and possible northward translation of a part of the western Transverse Ranges.