Granular Dynamics in Shallow Earthquake Slip Zones: Insights from Microstructural Analysis of Clast-Cortex Aggregate Grains

Abstract

Understanding the dynamics of shallow earthquake rupture and coseismic slip in carbonate-dominated fault zones is exceptionally important in many areas worldwide. Although constraints are available from rock mechanics experiments and paleoseismological investigations of surface breaks, little is currently known about the dynamic processes that occur in narrow fault slipping zones in the near-surface (<3 km) environment.

The Tre Monti Fault, central Italy (exhumed from depths <2 km) is an active carbonate-dominated fault zone that has probably hosted large (Mw >6) earthquakes. In previous studies of the principal slip zone (PSZ) of the Tre Monti Fault, peculiar aggregate- type grains were recognised and termed “Clast-Cortex Aggregates” (CCAs). Such grains contain a central clast (often angular) of host rock or reworked cataclasite surrounded by an outer, often laminated cortex containing much finergrained material. CCAs were only identified in the high-strain PSZ of the Tre Monti Fault (i.e. they are restricted to an ultracataclastic layer <2 cm thick) and as such they potentially record the effects of localized dynamic slip processes during the seismic cycle. However, because of their monomineralic composition, fine grain size and extremely low porosity, previous attempts to image CCAs using standard SEM-based techniques were not successful.

Using high-resolution Electron Backscatter Diffraction (EBSD) analysis of a single CCA, this thesis presents new observations regarding the microstructure and evolution of CCAs in carbonate-dominated fault zones. Results indicate that: 1) the diameter of individual grains comprising the CCA outer cortex and surrounding matrix of the PSZ is on the order of 1-5 μm; 2) mean grain size decreases progressively from the inner laminations towards the outer laminations; 3) in all locations around the CCA, and in the surrounding matrix of the PSZ, individual grains are markedly elongate (mean aspect ratio between 1.5 and 2.1) and are aligned at c. 70°-90° to the principal slip surface, and; 4) there is no significant crystallographic preferred ordinations of grains within the laminations of the outer cortex.

These new results support a model in which CCAs form by accretion of grains on to the outside of a central clast rotating clast within the cataclastic PSZ at shallow depths (<2 km). The PSZ is progressively crushed during slip events decreasing the grain size and resulting in grain size grading within the laminations of the CCAs. The formation of CCAs may be related to coseismic slip, however the data does not allow tight constraints on slip rates during formation. Following the formation of the CCAs, compaction within the fault zone, perhaps accommodated by pressure solution processes, results in individual grains within the CCA and surrounding matrix developing a systematic elongate shape. such distributed compaction may correspond to the shallow interseismic creep that has been observed at shallow depths following major recent earthquakes in carbonate-dominated fault zones.