Gravity studies of the Grenville province: Significance for Precambrian plate collision and the origin of anorthosite
Published:January 01, 1985
M. D. Thomas, 1985. "Gravity studies of the Grenville province: Significance for Precambrian plate collision and the origin of anorthosite", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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One of the most prominent negative Bouguer gravity anomalies in the Canadian shield follows the northeastern part of the Grenville front for almost 1 200 km. It is attended along its southeastern flank, within the Grenville province, by a belt of positive anomalies that are significantly more positive than the general level of the gravity field along the opposite flank, over the older Superior, Churchill, and Nain provinces. Together, the belts of negative and positive anomalies constitute a typical example of a particular kind of paired gravity anomaly that signifies the presence of collisional sutures produced by convergent plate tectonics. The tectonic significance of the gravity signature first became apparent by way of the Grenville example when it was noted that the steep gradient separating negative and positive components, ascribed to a steep discontinuity penetrating the entire crust, was located within a zone defined on paleomagnetic grounds as one likely to contain a suture; the discontinuity was subsequently equated with a cryptic suture. Studies of other paired gravity anomalies in both Phanerozoic and Precambrian terranes have provided a variety of evidence supporting a collisional origin for the signature.
In the northeastern half of the Grenville province, gabbroic areas of terranes formed of rocks of the anorthositic suite correlate with large positive gravity anomalies. Gravity models suggest that these are related to saucer- or funnel-shaped gabbroic masses extending, in cases, to mid-crustal depths. These masses are interpreted as dense mafic cumulates produced by gravity-assisted differentiation of gabbroic magma, and as such represent the lower levels of former magma chambers. Associated anorthosites are believed to have developed by flotation of plagioclase to higher levels.
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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.