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The primary goal of this research is to evaluate how tectonic rotation affects the structural development and inversion of sedimentary basins and the distribution of hydrocarbons in these rotated basins.  To do this, we are investigating the fault and fold characteristics of an oil-bearing basin in California (Santa Maria Basin) to develop a general model of sedimentary basin inversion in rotational tectonic regimes.

During the second year of research progress was made in the following areas:

1. Using fluvial terraces to evaluate ongoing rotational deformation-

The folds and faults that deform the rock units in the Santa Maria Basin formed as a result of rotation of the South side of the basin relative to the Northeast side that started approximately 18 Million years ago. To evaluate whether this rotation and folding is still occurring today, we have identified and mapped fluvial terraces along a major drainage that cuts perpendicular through the dominant structural trend. These terraces are the youngest geologic deposits in the area, and can thus serve as markers for recent or active deformation in the area. This work is being done primarily by Todd Tyler as part of his Masters Thesis.

Five different terrace levels have now been mapped in detail and have been surveyed with GPS techniques in the field that are accurate to within 30cm. These surveys are being used to document the amount of folding or faulting the terraces have been subjected to. A transect survey was also performed along the active creek channel that can be compared to an expected graded stream profile to highlight any active deformation. Sample collection for OSL (Optically Stimulated Luminescence) and C¹⁴ dating of the terraces is in progress. OSL is a geologic dating method that estimates the age since sand grains in the terraces were last exposure to sunlight.

2. Using the sedimentological data from Plio-Pleistocene Paso Robles Formation to evaluate the structural history of the northern boundary of the basin-

Stratigraphic data has been collected from the Paso Robles formation at 10 locations along the Little Pine Fault. This data will be used to look for changes in sedimentary environment and source areas along the fault, and at different distances away from the fault. This will allow us to determine how the two sides of the fault have moved relative to each other as sediment eroded from one side of the fault moves across and is deposited on the other side. This data is being collected, compiled, and analyzed mainly by Rick Lee as part of his Masters thesis.

3. The kinematic history of the Little Pine Fault and its relation to folding within the basin-

Documenting the structural kinematics of the Little Pine Fault and its relationship to the folds within the basin is needed in order to understand how the northern boundary of the Santa Maria Basin has developed and influenced the inversion of the basin. To do this, we have been digitizing fold axes from previously published maps and comparing the fold patterns with the Little Pine Fault trend. A large amount of kinematic data has also been collected along the Little Pine Fault to see how the fault has moved. Preliminary analysis suggests that the fold patterns describe 3 separate domains that may point to a new interpretation of the structural framework of the eastern part of the basin. The kinematic data roughly confirm earlier ideas that the Little Pine Fault has experienced a combination of reverse and strike-slip motion, and further work is needed to determine the timing relationship between these two modes of faulting.

4. Using paleomagnetic data to look for vertical-axis rotation-

Samples for paleomagnetic analysis are being collected from suitable locations within the Santa Maria Basin to evaluate the amount of vertical-rotation across individual structures. Paleomagnetic techniques involve analyzing the magnetic direction recorded in rocks to determine how much rotation their have experienced with respect to north since the time of their formation. A lot of effort has been spent locating suitable rocks for this application and sample collection has only just begun at this time.

These various approaches are complimenting one another and leading to a more comprehensive view of basin inversion in the study area. There has been constant collaboration between all the students involved in this project and myself. As the work continues throughout the next year, the new data will be synthesized and used to develop a model of basin inversion in rotational folding. This grant has provided funding for 3 graduate students, who will all be completing and defending their theses within the next year. Although the funding period has ended, this work will continue and we expect to present the results in the form of meeting presentations in the Spring of 2011, and in manuscripts submitted to international journals.  A manuscript on the theory of rotational folding, mainly the result of last years work, has been started and will be submitted to the Journal of Structural Geology.