The Mendocino triple junction: Active faults, episodic coastal emergence, and rapid uplift

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Abstract
A complex zone of rapid Holocene surface uplift and deformation occurs at the Mendocino triple junction, the juncture of three plate‐bounding faults: the Cascadia subduction zone, San Andreas fault, and Mendocino fault. Within this mountainous structural knot, up to 1.4 m of coastal emergence occurred during the 1992 Cape Mendocino MS 7.1 earthquake. Surveying and radiometric dating of ancient marine strandlines (7.5) along the Cascadia subduction zone megathrust. However, another plausible interpretation of the data is that multiple earthquakes resulting in smaller amounts of net surface uplift per event (similar to the 1992 earthquake) occurred closely spaced in time, giving the appearance in the geologic record of less frequent and larger events. Regardless of the number and timing of paleoearthquakes, the number of platforms is a minimum of the number of events that resulted in sudden, rapid uplift, because platform preservation is also a function of rising Holocene sea level. At present, rates and patterns of net surface uplift are better constrained than the timing and magnitude of paleoearthquakes. Periods of rapid Holocene emergence also are identified as far south of the area of 1992 uplift and the Mendocino fault as ∼30 km, along the unlocated San Andreas fault. These also might be associated with coseismic uplift. Based on new mapping of active faults in the region, it is proposed here that this uplift is the result of multiple discontinuous thrust and strike‐slip fault segments which distribute plate boundary deformation along a restraining bend at the termination of the San Andreas fault. This interpretation is consistent with the fact that a single, prominent, strike‐slip fault has not been identified in the Mendocino triple junction region, while several small reverse faults crossing Holocene deposits and geomorphic surfaces have been mapped and dated.