Calcite twinning strains from syn‑faulting calcite gouge: small‑offset strike‑slip, normal and thrust faults

We have evaluated the stress–strain behavior of calcite precipitated and mechanically twinned in small-offset strike-slip, normal and thrust faults of a variety of ages and from a variety of tectonic settings (n = 3001 twin measurements, 63 strain analyses from 18 field sites). Five strike-slip faults with syn-faulting, horizontally striated calcite (rake = 0°) were studied and we report the orientations of the contemporaneous stress–strain field associated with each fault: intrusion of the Marathon Large Igneous Province mafic dikes (~ 2.1 Ga in Archean crust, Minnesota, USA); post-Keweenaw rift (1.1 Ga) faulting (Island Lake fault, central Ontario, Canada); subduction associated with metamorphic core complex formation (Cretaceous, China); subduction (Cretaceous to Miocene, Italy), and continental extension (recently active Furnace Creek fault, Death Valley, California, USA). Seven normal faults with synfaulting, dip-slip striated calcite were studied and are from the following tectonic settings: a normal fault slip surface in an Ordovician Piedmont fold, Appalachian’s; paleo-subduction associated with Cretaceous metamorphic core complex formation (China, 3 sites); the paleo-extensional Atlantic margin (~ 55 Ma, Ireland, 1 site with a U–Pb calcite age); continental extension (1 active site, Mojave desert); a transcurrent margin (Jamaica, 1 active fault site), and subduction [2 active faults along the Eur-African margin in Italy (with calcite U–Th disequilibria ages) and Crete, respectively]. Six thrust fault examples are all from convergent orogenic settings: the basal thrust of the Penokean (1850 Ma) fold-and-thrust belt; the Penokean orogen foreland in Mesabi Range banded iron formation folds; an offset breccia body in the Permian Gondwanide belt, Ellsworth Mountains, Antarctica; the frontal thrust of the Gondwanide Cape belt, South Africa; the Paleocene frontal Prospect thrust, Sevier belt, Wyoming, and an Alpine foreland back-thrust, Lulworth Cove, U.K. For each strike-slip fault system the twinning shortening strain is horizontal and at an angle of 0°–60° to the respective fault plane (dextral or sinistral) although in the majority of cases the shortening axis is parallel to fault strike (13 of 23 results). In each normal fault example, dip-slip kinematic striations dominate the faulted surface yet the orientation of the maximum principal compressive stress (σ1) and shortening strain axis (ε1) are not 45° to the fault plane as predicted but are sub-horizontal and either strike-parallel (25 of 35 results) or strike-normal (10 of 35 results). Thrust faults preserve shortening strain axes parallel to the dip-slip kinematic direction, within the fault plane (plane strain) and not at 45° to the principal plane (5 of 5 results). None of the fault stress–strain field results reported here support the Andersonian or Mohr–Coulomb criteria for stress–strain relations predicted along faults.