Nathan W. Onderdonk , California State University (Long Beach)
During the extended third 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 was done primarily by Todd Tyler as part of his Masters thesis.
Five different terrace levels were mapped in detail and have been surveyed with GPS techniques in the field that are accurate to within 30cm. These surveys show that the Baseline-Los Alamos Fault has offset all three terraces that cross it and is most likely active. 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. The terraces are being dated using OSL (Optically Stimulated Luminescence). OSL is a geologic dating method that estimates the age since sand grains in the terraces were last exposure to sunlight. The results of this study will be presented by the student at the Annual American Geophysical Union meeting this coming December. An abstract was submitted in August.
2. Using the sedimentological data from Plio-Pleistocene Paso Robles Formation to evaluate the structural history of the northern boundary of the basin-
In addition to the stratigraphic data previously gathered in order to determine depositional and sedimentological history, a percent composition study and a DEM (digital elevation model) study were used to gather further information about the recent activity along the Little Pine Fault. The percent composition study was aimed at determining the percent of Franciscan-type clasts within three different depositional settings which were then compared to illuminate some of the temporal constraints regarding the exhumation of the Franciscan rocks within the present-day San Rafael Mountains. Since these rocks are believed to have been brought to the surface by movement along the Little Pine Fault system, the appearance (and percentage) of their components within the Paso Robles Formation, recent alluvium, and active drainages can be compared, and the data can be used to determine relative timing of the most recent episodes of slip along the Little Pine Fault. The data suggest that exhumation of the Franciscan Formation did not occur until Pleistocene time and that the Little Pine Fault has experienced significant reverse displacement since cessation of Paso Robles Formation deposition.
A DEM-based examination of the drainage patterns adjacent and across the Little Pine fault shows no evidence of stream deflection or redirection due to fault movement. This suggests that strike-slip displacement on the fault has not occurred during Holocene time or that the slip-rate is very low.
3. The kinematic history of the Little Pine Fault and its relation to folding within the basin-
Evaluating the structural kinematics of the Little Pine Fault Zone (LPFZ) 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 basin inversion. To do this, we have digitized fold axes from previously published maps and compared the average orientation of each fold hinge with the adjacent segment of the Little Pine Fault trend. The fold patterns consistently support a combination of north-south shortening along with a component of right-lateral shear along the fault zone. Kinematic data collected at a number of locations, both along the LPFZ and at varying distances from the fault zone, also indicate right-lateral reverse displacement across the fault zone, but show an additional component of sinistral strike-slip in the southeastern part of the fault zone. These data confirm earlier ideas that the Little Pine Fault has experienced a combination of reverse and strike-slip motion and continued work is underway to determine the timing relationship between the two modes of faulting, as well as the temporal relationship between faulting and folding within the basin.