Dec 4, 2009 - tional or Rayleigh-Taylor instabilities from a ... and plugs by gravitational instability. ... crust of the Colorado Plateau, San Rafael Desert, Utah.
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Lithosphere Evidence of small-volume igneous diapirism in the shallow crust of the Colorado Plateau, San Rafael Desert, Utah M. Díez, C.B. Connor, S.E. Kruse, L. Connor and I.P. Savov Lithosphere 2009;1;328-336 doi: 10.1130/L61.1
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Downloaded from lithosphere.gsapubs.org on December 4, 2009
Evidence of small-volume igneous diapirism in the shallow crust of the Colorado Plateau, San Rafael Desert, Utah M. Díez*,1, C.B. Connor1, S.E. Kruse1, L. Connor1, and I.P. Savov2 1
DEPARTMENT OF GEOLOGY, UNIVERSITY OF SOUTH FLORIDA, 4202 EAST FOWLER AVE., SCA 528, TAMPA, FLORIDA 33620, USA SCHOOL OF EARTH AND ENVIRONMENT, INSTITUTE FOR GEOPHYSICS AND TECTONICS, UNIVERSITY OF LEEDS, LEEDS, LS2-9JT, UK
2
ABSTRACT Magma is transported through Earth’s solid crust by two different processes, diking and diapirism, although other mechanisms, such as porous and channeled flow, can transport melt through partially molten crustal areas. Dikes are ubiquitous indicators of the transport of magma in the shallow crust by brittle fracture, and there is ample geological and geophysical evidence supporting diking as a magmaascent mechanism through the crust. On the other hand, igneous diapirism, involving magma ascent by gravitational instability and requiring viscous or plastic flow of country rock (“hot Stokes” diapirs), is often invoked as a magma-transport mechanism restricted to the ductile upper mantle or lower crust. However, unequivocal geological field evidence for igneous diapirism has proven elusive and has been a matter of considerable debate. We report geological and geophysical evidence showing that Pliocene sills emplaced in the upper levels of brittle continental crust of the Colorado Plateau in the San Rafael subvolcanic field (Utah) became gravitationally unstable by mechanically altering the overlying sedimentary rocks. These sills grew into structures that we recognize as domes and plugs at the current level of exposure. Some of these plugs continued to transport magma to shallower levels of the continental crust and eventually acted as conduits feeding volcanic eruptions. Our geological and geophysical findings indicate that gravitational instability is a viable mechanism for the initiation of magma ascent in the upper continental crust for small volumes of basaltic magma under specific conditions.
LITHOSPHERE; v. 1; no. 6; p. 328–336.
INTRODUCTION
The initiation of magma ascent as gravitational or Rayleigh-Taylor instabilities from a source region, and subsequent transport through the lithosphere, has been explored through several experimental studies (Ramberg, 1970; Marsh and Carmichael, 1974; Marsh, 1979; Kerr and Lister, 1988; de Bremond d’Ars and Jaupart, 1995). The mechanics of igneous diapirism, particularly within the context of granitic magmas, has also been studied from theoretical perspectives (Marsh, 1982; Schmeling et al., 1988; Rubin, 1993; Podladchikov et al., 1993; Weinberg and Podladchikov, 1994; Burov et al., 2003). In the field, magmatic diapirs and their mode of emplacement are usually inferred from large igneous intrusive bodies or plutons and their contacts with host country rocks (e.g., Bateman, 2002; Paterson and Fowler, 1993; Paterson and Farris, 2006; He et al., 2009). Due to the large vertical dimension of these intrusive bodies, only a fraction of their structure is usually exposed, and therefore the link with their magmatic source remains largely undetected. Thus, the mechanisms that govern their origin *Present address: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ, UK.
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doi: 10.1130/L61.1
and ascent are currently widely debated (Petford, 1996; Weinberg, 1996; Miller and Paterson, 1999; Petford and Clemens, 2000). Substantial erosion associated with the uplift of the Colorado Plateau has exposed low-volume basaltic intrusions, such as dikes, plugs, and domes, in a small area (200 × 100 m), which we refer to as the Carmel outcrop, in the San Rafael Desert, Utah. This locality (Figs. 1A and 1B) provides a unique opportunity to explore the mechanisms that govern the initiation of magma ascent from sill-like intrusions in the shallow continental crust. Here, we report geological and geophysical observations that show that the small mafic intrusions, generally 100 m in length, far larger than individual outcrops. Short-wavelength magnetic anomalies associated with the N-S– trending dike segments, in contrast, are on the order of
