Only relatively rare fault rocks record paleoseismicity (e.g., pseudotachylyte), though most fault slip probably accumulates during earthquakes. We describe two fault-rock assemblages, made of common fault-rock types, and argue that fault-rock assemblages can provide criteria for inferring seismogenic slip on exhumed faults that are more common than individual fault-rock types. Such assemblages allow direct study of fault-slip products formed under various crustal conditions that are not accessible on active faults. One assemblage, of layered cataclasites, is preserved several meters below the detachment. It is rare in the rock record and is best interpreted as recording parts of multiple seismic cycles. A second assemblage is the two-part detachment fault core: inner fault-core ultracataclasite layers separated by principal slip surfaces and outer fault-core cataclasites. We hypothesize that this common assemblage forms mainly during seismogenic slip. We interpret a third assemblage of clay gouge and breccia as recording slip and alteration above the seismogenic zone.
Detailed structural analysis of the layered cataclasites shows that they formed and were deformed episodically, most consistent with multiple seismic cycles that probably were defined by West Salton detachment fault (WSDF) main shocks. Cataclasite layers formed sequentially at the expense of overlying quartz diorite, while older, deeper layers were abandoned, folded, and faulted. Upward growth of the stack of layers probably reflects both strain weakening of the overlying rocks and strain hardening of the layered cataclasites. Each layer formed in two stages: (1) Lithologically heterogeneous and/or cataclastically foliated, fault-bounded precursor slivers above the top layer record grain-size reduction by cataclasis and cataclastic foliation development and probably record inter-seismic shear. (2) One or more slip event(s), interpreted as seismogenic, transformed precursor slivers into layers of homogeneous, random-fabric cataclasite. Many layers are overprinted by weakly developed foliation, probably of postseismic origin.
The detachment fault core formed at ≥2.3–4 km depth, in the upper seismogenic zone. Pseudotachylyte along the detachment nearby also records multiple seismogenic WSDF slip events. Similarities between the cataclastic layers and detachment fault-core rocks support the interpretation of both as products of seismogenic slip. In contrast, clay-rich gouge and upper-plate breccias formed at depth ≤2–3 km. The footwall fault core is clay poor and thus apparently passed through the uppermost crust with little overprint.
The layered cataclasites and detachment fault core display only minor chemical alteration, and the layers record limited shear strain relative to fault-core rocks. Thus, the layered cataclasites may be good natural analogs for experimentally formed fault rocks. Both assemblages formed from intermediate plutonic protoliths, similar to “average” mid-crustal rocks, and from the same lithologies as cut by the active San Jacinto and Elsinore faults. Thus, these two assemblages provide accessible, paleoseismically formed analogs to fault rocks presumably present at seismogenic depth along active faults.