POINT MARKERS: AN EMERGING APPROACH TO LARGE-SCALE RECONSTRUCTION OF CONTINENTAL LITHOSPHERE

Reconstruction of intracontinental deformation has traditionally been approached through mass balanced cross-sectional reconstructions of upper crustal fault systems in areas with appropriate pre-deformational strain markers such as platform or miogeoclinal sedimentary deposits. In reconstructing the Cenozoic history of southwestern North America (see McQuarrie and Wernicke, 2005, Geosphere, v. 1 n. 3), the most powerful constraints, both from a temporal and spatial perspective, are six “point markers,” here defined as geological feature with dimensions that are small relative to their tectonic offsets. Point markers may include (1) proximal sedimentary deposits with unambiguous sources; and (2) igneous centers. Both have dimensions an order of magnitude smaller than their net offsets, which in practice are of order 10's and 100's of km, respectively. Three type 1 point markers define 200 km of post-middle Miocene displacement of the Sierra Nevada relative to the Colorado Plateau (Gold Butte, Devil Peak and Hunter Mountain offsets). Two type 2 point markers define 255 km of post-6 Ma opening of the Gulf of California (Puertecitos volcanic province) and 315 km of post-22 Ma right-lateral shear on the central San Andreas (Pinnacles/Neenach volcanics). A type 1 marker defines an additional 250 km of rifting and shear of the Continental Borderlands west of the San Andreas system (Poway Group conglomerates). Although cross-section reconstruction, piercing lines offsets on discrete faults and paleomagnetic rotations also furnish key constraints, about 90% of the total displacement and the best constraints on timing are derived from point markers. None of these markers were conclusively documented until 1976 (Pinnacles/Neenach), and half of them (Hunter Mountain, Puertecitos and Devil Peak) have been described since 2001. Their relatively recent discovery is a function of a requirement of multidisciplinary, meticulous description of two small areas very far apart, and the difficulty of even suspecting correlation without a relatively advanced state of regional geological mapping. Nonetheless, a focus on hypothesis testing of suspected point markers in relatively poorly mapped regions (e.g. the Andes, Tibet, Iran) may provide the quickest path to accurate large-scale reconstructions.