Graduation Year

2010

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Paul Wetmore, Ph.D

Keywords

Gravity survey, Structure, Microplate motion, Agua Blanca fault, Basin geometry

Abstract

The Maneadero Basin is identified as a transtensional sedimentary basin along the Agua Blanca Fault (ABF) in the southern limit to the "Big Bend" Domain of the North American-Pacific plate boundary zone. The ABF exhibits both the dextral and normal components of slip. This creates an interesting setting for the formation of the Maneadero Basin because structures with orientations similar to the ABF are typically contractional (e.g.: Puente Hills Fault, Whittier Fault, and Rancho Cucamonga Fault). The question if this basin is evidence of plate-scale transtension or local extension associated with bends/stopovers along the ABF is addressed by this study with three working hypotheses.

The hypotheses presented by this study are: 1) the basins formed by a dip-slip component on the ABF and truly are an expression of regional transtension, 2) the basins formed at right steps along the dextral ABF, or 3) the basins formed as a result of juxtaposing basement blocks with disparate topographies. Each of these hypotheses would produce unique basin geometries and structures within and around the Maneadero Basin. To test these aforementioned hypotheses, a multi-disciplined study was conducted in the basin. A structural dataset was collected to identify kinematics and offsets of faults both within and bounding the basin. A gravity survey was also conducted to image the basin geometry. The results of the study show an asymmetrical gravity anomaly that closely follows the trace of the ABF. The amplitude of the anomaly is 54 mGal, the gradient of which is steepest around the ABF and shallows away from the fault to the north and east.

Forward models of this anomaly indicate the ABF is a steeply north dipping fault. The gravity anomaly also indicates that the deepest part of the basin is located close to, but not coincident with the ABF and the basin gradually shallows to the northeast. This geometry is consistent with the hypothesis that the basin results from dip-slip on the ABF. This idea is also supported by the structural data, which includes fluvial terraces that have been uplifted and offset by faulting on the ABF, and by the presence of a normal fault on the ABF in the center of the basin. The third hypothesis is also supported by models of the gravity data, which suggest a deep (~900m) bowl shaped erosional feature in the bedrock. Dextral slip on the ABF juxtaposes the topographically high Punta Banda Ridge with this topographically low feature.

Overall, the data presented in this study suggest the formation of the Maneadero Basin results from is a combination of the dip-slip component on the ABF and juxtaposition of the topographically elevated Punta Banda Ridge with a topographically lower basin of Bahia Todo Santos and Valle Maneadero. Geodetic data strongly suggest that the difference in motion of the Baja Microplate (south of the ABF) to the disrupted southern California Block (north of the ABF), and the orientation of the ABF relative to that motion, is causing transtension in the Maneadero Basin. This combined with strike-slip juxtaposition of different topographies allowed for the formation and evolution of the Maneadero Basin.

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