ADVANTAGES AND LIMITATIONS OF LONG-TERM PLATE MOTION CALCULATION BASED ON INTEGRATED STRAIN AND VORTICITY ANALYSIS: EXAMPLES FROM BOTH ACTIVE (CENTRAL TRINIDAD) AND ANCIENT (WESTERN IDAHO) PLATE BOUNDARIES

Estimates of the amount and orientation of the plate motion are critical to understanding ancient and modern plate boundaries. GPS data from active plate boundaries can supply this information over geologically short intervals of time (decades). The long-term kinematics of those plate boundaries still attached to oceanic crust can be extracted from fracture zones and magnetic strips. However, at those active plate boundaries that lack a spreading center (Central Range fault zone, Trinidad) or ancient boundaries lacking oceanic crust (western Idaho shear zone, ID) the long-term kinematic history of the zone can only be revealed by a structural analysis. The Central Range fault zone is a broad region of unmetamorphosed, gently lithified sedimentary rock actively deformed in transpression along the Caribbean-South American Plate boundary. In contrast, the western Idaho shear zone is a narrow belt of highly strained and metamorphosed granites that record Late Cretaceous transpressional deformation. Different rock types and grades of metamorphism require various vorticity (e.g., fold geometry vs. critical aspect ratio, Rf-theta) and strain gages (e.g., fold geometry vs. grain shape preferred orientation). Application of these techniques to very different plate boundaries yields similar lessons about the extraction of plate motion history from the geologic record. The advantage of an integrated vorticity and strain analysis is that it provides quantitative constraints on the long-term average orientation of plate motion and the amount of plate motion. However, there are major limitations to the interpretation of these results: (1) Most strain analysis techniques provide only a minimum estimate of the total strain and (2) vorticity estimates most likely a bulk average orientation of plate motion and cannot provide information about variation throughout the history of the plate boundary. Additionally, there are large uncertainties on the strain and vorticity estimates. Despite these formidable limitations, an integrated strain and vorticity analysis is valuable because it allows for the evaluation of strain partitioning across a plate boundary or throughout the lithosphere. Moreover, these values are critical for evaluating GPS data in the context of the earthquake cycle.