Stress, microstructure, and deformation mechanisms during subduction underplating at the depth of tremor and slow slip, Franciscan Complex, northern California

Subduction zone shear stress is thought to influence the topography of accretionary wedges as well as earthquake magnitude and frequency, and it has traditionally been calculated indirectly using methods that include relating surface heat flow and wedge topography to shear stress on the megathrust. The Eastern Belt of the Franciscan Complex in northern California is part of an exhumed accretionary complex deformed and metamorphosed under high P/T ratio conditions. Its lack of a retrogressive overprint makes it an ideal location to examine the mechanics of accretion within a fossil subduction zone. Blueschist-facies metasediments were deformed by a combination of dislocation and pressure-solution creep. EBSD analysis of dynamically recrystallized quartz across both the upper and lower boundaries of an accreted packet of metasediments indicates shear stresses of 32.8-46.6 MPa along the bounding faults and similar to 26 MPa within the main body of the accreted packet. A novel method of determining minimum strain due to pressure solution yields finite stretches of similar to 1.6-1.9 and corresponds to strain rates generally on the order of 10-14 s(-1) over the 2-6 m.y. period of deformation. A maximum strain rate on the order of 10-13 s(-1) was calculated based on plate convergence rate and shear zone width. These independently determined strain rates are shown to be best predicted by a thin film pressure solution flow law. Deformation mechanism maps show that pressure solution and dislocation creep operated with similar strain rates, and that progressive differentiation then increased the favorability and domination of dislocation creep. Dilational microcracking associated with pressure solution produced opening increments in crenulation arcs of similar to 1 mm. These events have the potential to produce microseismic events with an effective seismic moment comparable to that of low-frequency earthquakes. Concatenation of these events to form shear zones or folds on scales up to hundreds of meters could explain tremor bursts and slow slip events.

Automated summary

This study examines the mechanics of accretion within a fossil subduction zone in the Eastern Belt of the Franciscan Complex, revealing the role of pressure solution and dislocation creep in deformation, shear stress distribution, and strain rates during the deformation process. The results indicate that pressure solution and dislocation creep operate with similar strain rates, progressively differentiating and favoring dislocation creep, with dilational microcracking associated with pressure solution potentially producing microseismic events. The concatenation of these events could explain phenomena such as tremor bursts and slow slip events on a larger scale.