STRUCTURAL ANALYSIS OF FOLDED PALEOPROTEROZOIC GNEISS NEAR WEST FORK- NORTH FORK SAN GABRIEL RIVER CONFLUENCE, CALIFORNIA

We studied a section of complexly folded Paleoproterozoic gneiss exposed in pristine river-washed outcrops near the junction of the West Fork and North Fork San Gabriel Canyon. Folded rock types include banded quartzofeldspathic gneiss, amphibolite and augen gneiss (1.69 Ga; Premo et al., 2007), intruded by leucogratic granitic and pegmatite dikes. Most folds are tight to isoclinal with moderately dipping axial planes that appear to be uniformly oriented at outcrop scale. However, observed gross differences in foliation orientation between the south and north areas suggest that the section may be folded at map scale. We took systematic measurements of foliation, axial plane orientation and fold hinges to test a hypothesis that two episodes of folding are recorded in this area.

The gneisses display a profound flattening fabric, developed under amphibolite grade, in which various marker units and granitic dikes form tight to isoclinal, recumbent folds. We compiled the structural data on Stereonets to deduce patterns and to compare orientations of structural elements between the southern and northern areas. Foliation measurements generally correspond to sheared limbs of tight or isoclinal folds, and have a moderate degree of scatter. Axial plane orientations are more tightly clustered and reflect average foliation trends in a given outcrop area. In the southern area, axial planes and foliations dip moderately SE and fold hinges plunge gently SE or E. Farther north the structural elements appear to be shifted systematically in map view such that axial planes and foliation dip moderately E or NE and fold hinges plunge ENE. This pattern is consistent with map-scale open folding of the older flattening fabric along a hinge that plunges gently east.

The earlier folding event associated with the flattening fabric post-dated 1.69 Ga intrusion of the granite protolith to the augen gneiss. Leucogranite veins intruded across augen gneiss-banded gneiss contacts share similar fold geometries. Two generations of Tertiary dikes (Late Oligocene(?) rhyolite porphyry and Middle Miocene(?) basaltic andesite) crosscut the complexly folded Precambrian section. These dikes do not appear to be systematically rotated during the second folding event which we speculate may be driven by convergence related to the Paleocene Vincent thrust.