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Geologic mapping—Where the rubber meets the road

By
W.G. Ernst
W.G. Ernst
Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA
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Published:
January 01, 2006

A geologic map represents the melding of field observations with various types of analytical data and earth science concepts. Choosing features to be portrayed is a reflection of the questions posed. Some would claim that in the mapping process, theory meets reality. However, a map is a more subjective product based on the sum of the geologist's prior training, aggregate field experience, and the stage of development of scientific concepts, the complexity of the mapped units, the extent and quality of exposures, the wealth of constraining ancillary data, and the time and thought expended in the mapping. The published map also reflects accommodations to the scientific reviewers' knowledge, and to technical compromises required by the printer-publisher. Because mapping style depends on a geologist's prior experience, it is necessarily a somewhat idiosyncratic process.

My own field research has focused chiefly on the petrologic-structural development of Mesozoic and younger contractional orogenic belts, and through them, the tectonic evolution of continental margins. Mapping has been an essential step enhancing my understanding of processes that have shaped convergent portions of Earth's crust. (1) For instance, field relations combined with mineralogic analysis in the Panoche Pass area, southern Diablo Range, central California, indicated relatively high pressure–low temperature recrystallization and postmetamorphic, low-angle faulting of the Franciscan Complex. (2) Mapping of a similar Franciscan terrane in the central Diablo Range identified imbricate, subhorizontal, syn- to postmetamorphic bedding-plane thrust faults and implied accretionary growth in the Pacheco Pass quadrangle. (3) Field study of a structural inversion of high-grade metamorphic rocks tectonically overlying low-grade equivalents in the Sanbagawa belt, central Shikoku, Japan, led to the interpretation of postrecrystallization, ductile nappe emplacement and, as in California and the Western and Eastern Alps, (4) a progressive, relatively high P–low T metamorphism-exhumation subduction-zone model. (5) Mapping the interstratified distal turbidites and mafic lavas, and the discovery of pillow tops in the Sawyers Bar area, documented in situ stages of oceanic-island arc development in the North Fork terrane, central Klamath Mountains, northwestern California. Bulk-rock compositions of interlayered ocean-island basalts and island-arc tholeiites supported this interpretation. (6) Detailed geologic mapping combined with remote sensing in the central White Mountains, easternmost California, demonstrated that the Middle Jurassic Barcroft granodioritic complex is a steeply southeast-dipping slab that intruded previously deformed mid-Mesozoic arc volcanic rocks and Neoproterozoic–Lower Cambrian platform strata along a high-angle reverse fault. Conclusions derived from these studies, as well as more general plate-tectonic syntheses, depended on geologic mapping, and, for me, the field mapping was an enjoyable and scientifically fulfilling experience.

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GSA Special Papers

Earth and Mind: How Geologists Think and Learn about the Earth

Cathryn A. Manduca
Cathryn A. Manduca
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David W. Mogk
David W. Mogk
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Geological Society of America
Volume
413
ISBN print:
9780813724133
Publication date:
January 01, 2006

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