GEOLOGIC MAPPING—WHERE THE RUBBER MEETS THE ROAD

Construction of a geologic map represents the practical melding of geologic concepts, theories, and observations. Some would claim that here "theory meets reality." However, the resultant map actually is a far more subjective, intricate interweaving based on the sum of the mapmaker’s prior field experience and geologic prejudices, the extent and degree of complexity of the mapped units, the relationship of exposures to topography, and the quality time expended in the mapping effort. The published product also reflects accommodations to the scientific reviewers’ knowledge of the area, and technical compromises required by the printer-publisher. In my experience, mapping has been an essential, illuminating step toward increased geologic understanding. For instance, field relations combined with petrologic analysis in the Panoche Pass area, CA (Ernst, 1965) require high-pressure (HP) recrystallization, and the presence of low-angle faults (which I missed!), as well as imbricate bedding plane thrusts in the Pacheco Pass quadrangle, CA (Ernst, 1993). Our mapping of a metamorphic inversion in central Shikoku (Ernst et al., 1970) led to the interpretation of post-recrystallization nappe emplacement, and—as in CA and the Western and Eastern Alps—a progressive subduction-zone HP metamorphism-exhumation model (Ernst, 1971, 1973). Recognition of pillow tops in the Sawyers Bar area document in situ, immature oceanic arc development in the central Klamath Mountains, CA, as also mirrored by IAT geochemistry (Ernst et al., 1991; Ernst, 1998). Detailed on-the-ground mapping combined with remote-sensing of the Barcroft Granodiorite and its walls, White-Inyo Range, CA, show it to be a steeply SE-dipping slab intruded along a pre-existing high-angle reverse fault (Ernst et al., 1993, 2002). However, zircon U/Pb age data (Hanson et al., 1987) and unpublished magnetic susceptibility anisotropy measurements (Karen Michelsen, personal communication) suggest possible truncation of the northern pluton margin by post-intrusion slip. All these conclusions, and many more, depend on geologic mapping. Besides, it’s fun to do.