Section I. An Analytical and Experimental Study of Some Simple Geologic Structures. Section II. Gravity Survey of a Part of the Raymond and San Gabriel Basins, Southern California

SECTION I: The geologic structures produced by two distributions of applied vertical displacement along the base of a homogeneous layer, (1) a broad curve and (2) a sharp step, were investigated analytically and experimentally. A special form of the general theory of elasticity and scale models (with layers of dry sands and clay) were used for a two dimensional analytical and experimental study of these structures. Each of the two distributions of applied vertical displacement produce a characteristic fracture pattern in the model experiments. For a broad curve, the fracture pattern is a complex zone of normal faults which taper inward to the axis of the fold and die out at depth. For a sharp step, the fracture pattern is (1) a series of curved reverse faults which start steeply at the base of the layer but intersect the upper surface at low angles (thrusts), and (2) a series of normal faults in the uplifted block near the reverse faults. The initial fractures in the model experiments, for example the curved reverse faults, can be predicted on the basis of the Mohr fracture criterion and the stress distributions found in the elastic analyses. In addition, the displacement fields obtained in the elastic analyses are good first order approximations of the displacement fields observed in the model experiments. SECTION II: A gravity survey (640 stations) was conducted over thirty-six square miles of the alluvium covered portion of the Raymond and San Gabriel basins. Corrections for latitude, elevation, and topography were applied to the gravity data. On the basis of the known surface and sub-surface geology, regional gravity due to deep crustal structure, and gravity due to near surface structure were separated and interpreted independently. The regional gravity due to deep crustal structure indicates a uniform thickening of the earth's crust to the northeast. If all of the regional gravity is attributed to a density difference of 0.5 c.g.s. at the Mohorovicic discontinuity, the rate of thickening is about 100 meters per kilometer. The distribution of gravity due to near surface structure shows two steep gradient valleys in the bedrock surface beneath the alluvium in the northern part of the area. The elevation to the floor of these valleys indicates at least 1000 feet of subsidence since the deposition of the alluvium (Upper Pleistocene). The bedrock relief is fairly gentle in the southern part of the area where Tertiary rock lies between bedrock and alluvium. This intermediate layer of Tertiary rock may extend more than a mile north of the Raymond fault. The vertical displacement on the Raymond fault may be 600 feet along one section of the fault in east Arcadia.