3D ANALYSIS OF A MULTIPLE DIKE INTRUSION AND LATER DEFORMATION, INDEPENDENCE DIKE SWARM, CALIFORNIA

Most past studies of the Late Jurassic Independence Dike Swarm (IDS) exposed in the Sierra Nevada (California) have focused on glacially polished outcrops where vertical sections are difficult to observe. Because of this, stress analysis was based mainly on horizontal sections. Here we present data collected from an exceptional and accessible outcrop of the IDS along the South Fork of Big Pine Creek, where 3D sections of the subvertical intrusions can be measured along rock drumlins produced by the last glaciation.

Field observations reveal several types of dikes, which can be placed in chronological order using crosscutting relations: intermediate coarse-grained dikes, composite dikes with intermediate fine-grained centers and mafic fine-grained borders, mafic fine-grained dikes, and a composite of xenoliths of mafic material and wall rocks. The dikes are subparallel with a mean strike of ~095. We used the Jolly and Sanderson (1997) method to obtain the Mohr construction and the three paleostress components. The calculated minimum stress axis (σ₃) is oriented NNE-SSW, consistent with the opening direction presented in other works. The maximum stress axis (σ₁) is subvertical and the intermediate stress axis (σ₂) is subhorizontal with a WNW-ESE orientation.

Pre-dike aplite veins in the host rock and jogs in the dike margins reveal the net opening direction, although a later set of aplite veins cuts the dikes. Offset veins are separated sinistrally across ~E-W oriented dikes. The measured offset vectors do not agree with opening vectors predicted by the stress analysis. We infer that markers in the host rock were displaced from their original positions by dike-parallel shear, recorded by tectonic fabrics in some dikes and by dm- to cm-scale mylonite zones in the granodioritic host rock. Some mylonite bands formed at dike margins but others are subparallel to or oblique to the dikes. The mylonite zones range widely in orientation but most commonly they strike ~100º, ~080º or ~135º.

The structural relationships indicate that tectonic stresses changed from dike emplacement to later sinistral shear and mylonitization. This history is consistent with relations in nearby areas (e.g., Split Mountain area, Bartley et al., 2012) where overprinting mylonitization appears to be mid-Cretaceous in age (~100 Ma).