Pacific Section AAPG Convention

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Underestimation Of Shortening In Balanced Cross-Sections Across The Santa Maria Basin, California Revealed By Map-Scale And Outcrop-Scale Structural Analysis


The Santa Maria basin (SMB) in California underwent a complicated tectonic history from Miocene basin development to the formation of a fold and thrust belt by several phases of north-south shortening. Accurate quantification of structural deformation in the SMB is essential to unraveling the tectonic history of the California active plate margin and test models of major block movements and rotations. Namson and Davis (1988) quantified shortening across the basin by balancing regional-scale cross-sections. However, examination of outcrop-scale structures of Miocene rock units in the southern SMB and preliminary quantification of map-scale structures show that there is significant internal deformation and shortening within structural panels that were assumed to maintain constant volume in the reconstruction. Thus, an underestimation of shortening of regional-scale balanced cross-sections is hypothesized. The objectives of this study are to assess the quantitative contribution of map-scale and outcrop-scale structures at the southern boundary of the SMB to existing balanced cross-sections and evaluate how omission of these smaller scale structures affects the assessment of the regional deformation and tectonic history. The Monterey and Sisquoc formations are well suited for this study because (1) they formed during the Miocene - early Pliocene time period and display structural features from tectonic events related to the formation of the SMB fold and thrust belt, (2) their thinly bedded character and diverse rock mechanics provide excellent smaller structures that allow the quantification of tectonic shortening at outcrop-scale, and (3) they are well exposed and provide excellent and continuous structures at map-scale that are representative on a regional scale. Namson and Davis used surface mapping and well data to create a regional-scale balanced cross-section across the SMB. We conducted a preliminary test across a 7.4 km long north-south section of the southern SMB by making cross-sections from existing geologic maps. The cross-section by Namson and Davis imply 6.6% of total shortening across this area. However, our preliminary measurements of shortening across the same area contribute an additional 8.9% of shortening to the regional-scale cross-section suggesting a total deformation of 15.5%. Structural mapping and restoration of continuous outcrops will document how much deformation at outcrop-scale will be further additive to the map-scale.