The relationship between crustal density and volcanic rocks in the Western United States including a comparison of isostatic compensation techniques for gravity data correction

Abstract

Processes and properties controlling magma ascent may include magma viscosity, the stress regime in surrounding country rock, geometric relationship between the rising magma and connected source area (if any), and density contrast between the magma and country rock. The purpose of this study was to determine the role upper crustal density plays in the control of magma ascent through the use of gravity modeling to obtain estimates of crustal density and then relating that density to the type and location of late Tertiary and Quaternary volcanics. A comparison of commonly used isostatic correction techniques was conducted to assess the impact the differences in correction technique have on isostatic gravity anomaly values, and thus on the results of gravity modeling. The choice of isostatic correction method was found to change the isostatic correction value by as much as 30 mgal over 36 km in areas of rapidly changing elevation. However, in most areas the difference in isostatic correction values are long in wavelength and will not greatly impact gravity modeling. A new idea for preparing an isostatic correction is introduced in preliminary form. Models of crustal density contrasts were prepared for three study areas in California and Nevada through the use of forward gravity modeling on isostatic gravity anomaly datasets. The results of the gravity models were gridded layers of crustal density contrasts that would reproduce the value of the isostatic gravity anomaly in that area. The location, type and quantity of late Tertiary and Quaternary volcanics were evaluated with respect to the value of crustal density in the underlying "basement" (lowest layer). Approximately half of the late Tertiary and Quaternary basalt in the study areas has erupted through crust that is less dense than the magma was. It is likely that in these cases that a conduit exists connecting the erupting magma to a deep source to provide sufficient buoyancy force for eruption.