Analysis of gravity data in volcanic terrain and gravity anomalies and subvolcanic intrusions in the Cascade Range, U.S.A., and at other selected volcanoes
Published:January 01, 1985
David L. Williams, Carol Finn, 1985. "Analysis of gravity data in volcanic terrain and gravity anomalies and subvolcanic intrusions in the Cascade Range, U.S.A., and at other selected volcanoes", The Utility of Regional Gravity and Magnetic Anomaly Maps, William J. Hinze
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Gravity data were investigated to reveal the presence of subvolcanic intrusions. With few exceptions, these intrusions produce a detectable gravity anomaly. In the past, these gravity anomalies have often been overlooked or misinterpreted because the data reduction procedure was inadequate. A pragmatic method for reducing and interpreting reconnaissance gravity data from volcanoes as well as gravity models of a variety of volcanoes is developed.
Large calderas (diameters greater than 15 km) have relatively low-density intrusions beneath them. All other large volcanic systems that would include small calderas (diameters less than 15 km) have relatively high-density intrusions beneath them. The density contrasts that produce the observed anomalies occur between the intrusion, whose density is usually greater than 2.6 g/cm3, and the country rock. Commonly, the shallow country rock is an older volcanic layer with a density less than 2.5 g/cm3. The result of the contrast is a positive anomaly over the intrusion. For larger calderas, the surrounding volcanic layer is usually thin and overlies dense metamorphic and plutonic country rocks. In this case, we find the intrusion commonly less dense than country rock. The result is a negative anomaly.
In modeling volcanoes of the Cascade Range, gravity data and geologic considerations required a bottom on the intrusion. This may be an actual bottom or the depth at which the density contrast between the intrusion and the country rock disappears. The tops of the intrusions are usually shallow and are significantly wider than overlying craters or calderas. Calderas are associated with wider intrusions. Some intrusions are single cooling units, but more commonly they are an accumulation of the unerupted portions of individual magmatic injections. These injections could occur periodically throughout the life of the volcano, and would generally be accompanied by eruption. Comparing the volume of the intrusion and the volume of the volcanic edifice indicates that only a small part of a magma injection erupts, although some of the apparent intrusive material may be reworked older volcan-ics. Exceptions to the general discussion presented tend to be related to the nature of the country rock.
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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.