Reports: GB8
48329-GB8 Sedimentary Basin Inversion in Rotational Tectonics
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 first year of this grant, progress was made on three fronts:
1. Theory of rotational folding-
A significant amount of shortening that occurs during sedimentary basin inversion is in the form of folding. The shape of these folds influences the development, migration, and storage of the hydrocarbons. It is therefore important to understand the geometric characteristics of folds that develop in rotation and how to identify this style of folding. Work in this area over the past year resulted in: a set of fold characteristics that can be used to identify rotational folding, an analytic solution that allows the determination of rotation amount from geometric characteristics of folds, and descriptions of how to interpret paleomagnetic data collected from rotational folds. The analytic solution is:
R= 2ø – 4Cos-1 [ Cos(.5ø)/ Cos(.5Dp)]
where R= rotation amount across one fold wavelength, ø = the angle between syncline and anticline axes in the plane of the adjoining fold limb, and p= the plunge of the fold (or half the difference in plunge angles of the anticline and syncline axes). This solution was tested on several folds in the area by comparing calculated rotation amounts to measured rotation amounts from previously reported paleomagnetic data. The solution can also be used inversely to approximate the geometry of a fold from paleomagnetic data collected in the field. The results of this work were presented last May in the form of a talk at the American Association of Petroleum Geologist meeting in Ventura, California.
2. Kinematics of Little Pine Fault and folding within the Santa Maria Basin-
To evaluate the influence of tectonic rotation on the development of the Santa Maria Basin, the kinematics and amounts of folding and faulting in the basin must be understood. Two graduate students have been focusing on this aspect of the project under the supervision of Dr. Onderdonk.
Over the past year, three preliminary cross-sections across the basin have been made from previously published surficial mapping to evaluate the mode and degree of folding and look for variations from east to west. This work is ongoing and will be suplimented by the incorporation of field observation and subsurface data to add details to the cross-sections over the coming year.
Field investigations and data collection have also been started along the basin-bounding Little Pine Fault to see how this structure has evolved over time. Kinematic data collected thus far along the Little Pine Fault is inconclusive with conflicting data depending on sampled location. This may be an issue of measurement errors or an indication of changes in the fault kinematics through time. Both will be evaluated in the coming year as more data is collected in other locations along the fault.
3. Active deformation within the basin- An important question regarding the rotational formation of the Santa Maria Basin is whether the rotation is still occurring and influencing ongoing deformation within the basin. Understanding the mechanisms of Quaternary faulting and folding in the basin is also necessary to determine whether the shortening structures we see today are due to rotation or are a separate mode of deformation that began after rotation ceased. Detailed geomorphic mapping of Quaternary fluvial terraces was started this year to evaluate the mode and amount of active deformation in the basin. Mapping by a graduate student within the Santa Cruz watershed in the eastern Santa Maria Basin has resulted in the identification and description of a flight of unpaired strath terraces that record Quaternary uplift within the basin. Ongoing work will involve surveying these terraces to detect Quaternary folding and dating these terraces using Optically Stimulated Luminescence of terrace sediments to calculate timing and rates of deformation. Similar work with then be done along other drainages to the west so the degree of shortening and uplift can be compared to see if rotation is ongoing within the basin.
