Tectonics and the gravity field in the continental interior

Abstract

Gravity fields in the continental interior reveal little relation to late Paleozoic structure and often show an inverse relation to early Paleozoic structure. Because of the inversion, displacements have been interpreted by several investigators as being caused by an isostatic mechanism; however, structural development recorded by geologic evidence indicates a more complex history than simple isostasy would allow. Detailed gravity surveys covering large areas in Illinois are used to determine mechanisms of crustal movements more definitively. From these detailed studies interpretations are extended to other areas having similar geologic‐gravity relationships within the midcontinent region. In the interior, a mosaic pattern of gravity highs and lows outlines mass concentrations that formed during the evolution of the craton. The midcontinent gravity high, the central anomaly lying within a linear, symmetrical belt of gravity anomalies, is superimposed over the mosaic pattern and trends through the Lake Superior‐Keweenawan rift system. Similar anomalies, although of smaller intensity, trend through centers of intracratonic basins. From the relation between linear, symmetrical anomalies and structure displayed by the Lake Superior rift system, intracratonic basins are interpreted as representing the terminal stage of more subdued rifting. Initial depression of basins is believed to be caused by the collapse of incipient rift systems. The presence of numerous ancient rift systems in the craton suggests that mantle convection played an important part in the deformation of the youthful continental crust. Erosion from uplifts and sedimentation in depressions resulted in further deflections, but of an isostatic nature, with subsequent faulting and plutonism. Subcrustal convection continued throughout Paleozoic time, but its effects on crustal bending became less pronounced. Examples of these structures and related gravity fields are presented.

Structure and evolution of the Amadeus, officer and Ngalia basins of central Australia

“…Alternatively, the subsidence may be attributed to the penetration of dense basic and ultrabasic intrusives into the lower crust (e.g. McGinnis 1970) or by metamorphism of the lower crust to denser granulite or eclogite facies (Falvey 1974) or by a combination of the two (Haxby et al 1976).…”

Thermal Contraction and Flexure of Mid-Continent and Atlantic Marginal Basins

“…* The qualitative reason for including this coupling of adjacent points can be seen as follows (McGinnis 1970;Walcott 1972a;Sleep 1971Sleep , 1973. If more subsidence during an earlier period (say the Cretaceous) occurred at point A than at nearby point B, point A is buoyed up and point B dragged down by the amount indicated by dotted lines (Fig.…”

Sinking Mafic Body in a Reactivated Lower Crust: A Mechanism for Stress Concentration at the New Madrid Seismic Zone

“…The weaker layer possesses a Maxwell viscoelastic fluid rheology, and the stronger layer possesses a standard linear solid rheology (Cohen, 1982) with long-term strength lЈ ‫ס‬ 2.6 GPa. This assumed long-term strength is based on the existence of a positive free-air gravity anomaly of greater than about 20 mgal (McGinnis, 1970), suggesting that the dense rift pillow is at least partially supported by the upper and lower crust. The extreme age of the rift pillow requires that, if the proposed mechanism is operable, lЈ in part or all of the lower crust has never diminished below a certain minimum value, that is, if a Maxwell rheology had existed in the entire lower crust for any significant period of time since about 600 m.y.…”