GRAVITY LOW-HIGH PAIRS IN OROGENS: THICKENED-THINNED CRUST, SUTURES, OR HYBRID MECHANISMS?

Keller and Cebull in 1973 formulated one of the first plate tectonic models for the Ouachitas in AR and OK, based partly on a N-to-S gravity low-high pair. The prominent Bouguer gravity low-high pair on the foreland side of orogens has been known for several decades. These features are not attributes of topography, despite occurring in the Alps, Canadian Cordillera, and elsewhere, because they are also attributes of orogens in Precambrian shields where relief is low. The low-to-high gradient in the Alps has been correlated with a suture between lithosphere of two contrasting densities, with lower crust-mantle brought up beneath the high, an explanation that works well for the foreland and root zone-Insubric line there. The Appalachian low-high pair has been interpreted as thickened crust above a crustal root and a feature related to rifting and crustal thinning, with the high produced by high-density materials in the upper or lower crust. Regardless of the origin of the low-high pair, crustal thickness is a critical variable in gravity signatures.

Modern seismic reflection, wide-angle reflection, and refraction data provide independent crustal thickness data. Seismic data are mostly lines, whereas, gravity data are acquired over wide areas. Southern Appalachian wide-angle reflection and broadband data provide confirmation of the correlation between crustal thickness and the gravity low-high pair here. Thicknesses >50 km occur in the gravity low, but decrease to ~35 km east of the gradient in the associated high. The southern-central Appalachian gravity low may map thick crust; thinned crust east of the gradient is probably related to Mesozoic rifting and crustal extension during the breakup of Pangea and opening of the Atlantic. This explanation may not work for similar low-high pairs in Precambrian shields and in all other orogens, e.g., the Alps. Before invoking this explanation, gravity data should be compared with other data sets that independently yield crustal thicknesses.