Long-term viscous creep versus short-term brittle/plastic deformation in the seismogenic zone - the microstructural record of cherts from Mount Diablo, California

Exhumed metamorphic rocks from fossil subduction zones represent a unique source of information on the microscale deformation mechanisms and stress history within the seismogenic domain in subduction zones. Microstructural analysis of these rocks yields insight into processes operating on length and time scales generally inaccessible for active systems due to limitations in surface-based geophysical and geodetic experiments.

We studied high-pressure – low-temperature blueschist facies metamorphic cherts, representative of the upper oceanic crust, exhumed from the Franciscan subduction complex and exposed at Mt. Diablo, California. These rocks underwent intense deformation at about 30 km depth. Their microstructural record reflects repeated superposition of different deformation stages, including long-term ductile deformation by viscous creep, short-term brittle failure followed by vein formation, and transient crystal plastic deformation. As such, the samples are taken to be a representative rock volume reflecting processes active in the seismogenic zone and are used as a gauge, recording the history of stress and fluid pressure near the plate interface at depth.

The microstructural record of episodic changes in the far field stress is to be isolated from small-scale heterogeneities in the stress field due to contrasting material properties. For instance, pure quartz veins record pronounced crystal plastic deformation at high stress, while embedded in a fine-grained polyphase matrix that undergoes viscous deformation by dissolution precipitation creep. In this case, the weak matrix causes stress concentration in the stiff veins.

Despite the limits given by the sample size and heterogeneity, the microstructural characteristics indicate distinct deformation stages at variable stress levels, repeatedly superimposed on each other. Here, the formation of tensile cracks is attributed to sudden stress changes at sufficient pore fluid pressure, widening and sealing of these cracks to transient deformation during stress relaxation, and stages of crystal plastic deformation, particularly distinctive in vein quartz, to high peak stresses attained by rapid loading. Based on these systematic observations, we present a conceptual generic model for the recorded episodic changes in the mode of deformation and the underlying cyclic stress history. We then discuss how stress changes as reflected by the microstructural record can be ascribed to the seismic cycle, with the respective seismic events having occurred at some time, and somewhere in the vicinity of the sample, along the plate interface. 

Interestingly, the number of recorded stress and deformation cycles is limited, generally not exceeding two or three cycles. When comparing the expected residence time of a HP - LT metamorphic rock in the given depth range with the present-day frequency of seismic events along the plate interface in a subduction zone, this observation indicates that only a small portion of the expected large number of seismic events has left a marked imprint, whereas the effects of the vast majority remain beneath the limits of detection. We suspect that the noticeable high-stress events are related to nearby fault propagation resulting in a vertical shift of the plate interface, presumably being prerequisite for the transfer of a rock from the subducted lower plate to the hanging wall, in order to become exhumed.