Upper plate contraction north of the migrating Mendocino triple junction, northern California: Implications for partitioning of strain

Abstract

Geologic measurement of permanent contraction across the Cascadia subduction margin constrains one component of the tectonic deformation along the convergent plate boundary, the component critical for the seismic hazard assessment of crustal faults. A comprehensive survey of active faults in onshore subduction margin rocks at the southern end of the Cascadia subduction zone indicates that these thrust faults accommodate ∼10 mm/yr of convergence oriented 020°–045°. Seismotectonic models of subduction zones typically assign this upper plate strain to the estimate of aseismic slip on the megathrust. Geodetic models include this permanent crustal strain within estimates of elastic strain accumulation on the megathrust. Both types of models underestimate the seismic hazard associated with crustal faults. Subtracting the observed contraction from the plate convergence rate (40–50 mm/yr; directed 040°–055°) leaves 30–40 mm/yr of convergence to be partitioned between slip on the megathrust, contraction within the southern Juan de Fuca plate, and crustal contraction outside the subduction complex rocks. This simple estimate of slip partitioning neglects the discrepancy between the plate convergence and contraction directions in the vicinity of the Mendocino triple junction.The San Andreas and Cascadia limbs of the Mendocino triple junction are not collinear. The eastern edge of the broad San Andreas boundary is ∼85 km east of the Cascadia subduction boundary, and across this zone the Pacific plate converges directly with the North America plate. The skewed orientation of crustal structures just north of the leading edge of the Pacific plate suggests that they are deforming in a hybrid stress field resulting from both Juan de Fuca‐North America motion and Pacific‐North America motion. The composite convergence direction (50 mm/yr; directed 023°) is consistent with the compressive stress axis (020°) inferred from focal mechanisms of crustal earthquakes in the Humboldt region. Deformation in such a hybrid stress field implies that the crustal faults are being loaded from two major tectonic sources. The slip on crustal faults north of the Mendocino triple junction may consume 4–5 mm/yr of Pacific‐Humboldt convergence. The remaining 17–18 mm/yr of convergence may be consumed as distributed shortening expressed in the high rates of uplift in the Cape Mendocino region or as northward translation of the continental margin, north of the triple junction.