An isostatic residual gravity map of California—A residual map for interpretation of anomalies from intracrustal sources
Published:January 01, 1985
Robert C. Jachens, Andrew Griscom, 1985. "An isostatic residual gravity map of California—A residual map for interpretation of anomalies from intracrustal sources", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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An isostatic residual gravity map of California effectively separates gravity anomalies caused by intracrustal and near-surface density inhomogeneities from the large Bouguer gravity anomalies that result from isostatic compensation of the topography. This regional-residual separation reveals some anomalies that are not easily recognized on the Bouguer gravity map and converts others that are difficult to interpret quantitatively on the basis of the Bouguer gravity data into anomalies that can readily be analyzed. Major residual anomalies in the first group include (1) a gravity anomaly caused by the Gorda plate subducted beneath northern California, and (2) a pattern of linear gravity highs along the western margins and gravity lows in the eastern parts of both the Sierra Nevada and Peninsular Ranges batholiths. Two examples of prominent anomalies in the second group are (1) gravity highs defining a major detached thrust sheet within the western Klamath Mountains, and (2) a gravity low caused by a combination of low-density sedimentary rocks in the Ventura basin and the associated mantle upwarp accompanying isostatic compensation of the basin fill.
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The Utility of Regional Gravity and Magnetic Anomaly Maps
The first composite magnetic-anomaly map of the conterminous United States and adjacent offshore areas has been published at a color-contour interval of 200 gammas and at the scale and projection of other national geologic and geophysical maps for easy comparison. This map, despite the inconsistent characteristics of the surveys from which it was compiled, is useful in providing a regional framework for the interpretation of magnetic studies of limited areas, in selecting areas for more detailed magnetic investigations, and in studying the distribution and character of regional geologic features.
The map has a wide variation of magnetic-anomaly patterns, trends, and types, thus reflecting the diversity of the geologic terranes of the United States. In general, the anomaly pattern east of the Cordillera in the craton and in the Appalachian Mountains consists of more and greater intensity anomalies. The muted nature of the anomalies of much of the Cordillera is a result of several factors but appears to be primarily related to a decreased crustal magnetization caused by an abnormally shallow Curie isotherm. The anomalies of the Appalachian Mountains and the Cordilleran system primarily reflect the major structural patterns of the orogens, but important exceptions occur, such as those associated with rocks underlying thrust sheets in the Appalachian Mountains and westerly-striking anomaly trends in the Cordillera, which are correlated with igneous intrusives, faults, and mineral deposits.
The buried southern and eastern edges of the Pre-cambrian craton are indicated by changes in the magnetic anomalies and their dominant trends. Within the central United States, numerous regional magnetic-anomaly provinces are observed that reflect the long, complex history of the Precambrian basement rocks of the craton. These provinces are transected by conspicuous, intense, long, generally linear anomalies that originate from mafic extrusive or shallow intrusive igneous bodies within failed rifts, such as the Midcontinent rift system, the Southern Oklahoma aulacogen, and the Reelfoot rift buried beneath the Mississippi embayment. These are only a few of the many interesting regional geologic features that are observed on the composite magnetic-anomaly map of the United States.