Description
The rocks of the Oriflmmne Mountains of southeastern San Diego County are metamorphosed quartz sandstones, siltstones, and calcareous argillaceous rock (?) intruded by the Bonsall tonalite (mid-Cretaceous) and pegmatite of presumed mid-Cretaceous age. Mineral assemblages indicate metamorphism of high grade amphibolite facies. The metamorphic rocks exhibit appressed cylindroidal folds with monoclinic symmetry in bedding (S1) and transposed bedding surfaces (S2). The axes of these folds trend north-northwest with average plunge of 25°. The poles to the axial planes (S3) of the mesoscopic folds are spread on a great circle arc, the normal to which coincides with the regional megascopic fold axis BS1S2S2 . A slip cleavage (S4) is developed subparallel to the axial planes of the megascopic folds. S4, which strikes north-northwest with an average dip of 80° east, parallels S1S2 in some subfields. The symmetry of both field and microscopic structures is monoclinic (C1H). Deformation was probably accomplished by early flexural slip folding of S1 and S2 about an axis parallel to BS1 S2 with development of S3 (kinemetically active folding stage) . Coaxial refolding followed and as slip could no longer be accommodated on S1-S2, S4 was developed accompanied by rotation of S3 about BS1 S2S3 (passive folding stage). Slip on S4 is thought to have been normal to the axis S4.
