Leland Timothy Long

The fractal character of fracture spacing and RQD

Abstract Fracturing and fragmentation processes occur on all scales and, hence, may be described in terms of fractals. We examined the fractal dimensions of discontinuity spacing measurements in a fractured rock mass and observed a relation between the fractal dimension and the quality of the rock mass in terms of RQD. RQD tends to increase with a decrease infractal dimension for a given scanline length. Thus a relatively more-competent fractured rock mass will tend to have a lower fractal dimension and vice versa.

Statistical distribution of natural fractures and the possible physical generating mechanism

We have fitted field measurements of fracture spacings (from the vicinity of Lake Strom Thurmond, Georgia, U.S.A.) to the Weibull, Schuhmann and fractal distributions. The fracture spacings follow a fractal and Weibull distribution which implies that they were formed as a result of a repetitive fragmentation process. The limited variation of the fracture density with orientation in the study area suggests that the stress distribution generating these fractures may be uniform.

The Carolina slate belt—Evidence of a continental rift zone

Near the South Carolina and Georgia border, the southeastern parts of the Charlotte belt and Carolina slate belt near the Coastal Plain overlap are coincident with the northwestern edge of an isolated negative Bouguer gravity anomaly. This anomaly is interpreted as evidence for a detached fragment of continental crust. The pattern of the magnetic and gravity anomaly contours for the northwestern edge of this fragment is similar to the pattern of magnetic anomaly contours and the Piedmont gravity gradient in central Georgia. This geometric similarity suggests that the fragment may have been separated from a larger continental block by a rift zone nearly 160 km wide. The Carolina slate belt in Georgia and South Carolina may delineate the axis of this continental rift or rift system and may represent remnants of rift-derived volcanic and sedimentary rocks. Gravity profiles across the edges of the hypothesized rift indicate the existence of anomalous high-density rocks in the upper 5 km of the crust in the proposed rift and low-density rocks at the base of the adjacent continental crust. Seismic data in Georgia indicate a typical rift-zone velocity structure that consists of a high-velocity (6.3 km/s) discontinuous surface layer 0.0 to 5.0 km thick over a low-velocity (6.0 km/s) crustal layer which extends to depths of at least 30 km. Magnetic lineations provide evidence for the faults and/or depositional layering of the rift and outline the structures and margins of the rift. The rift developed from late Precambrian through Cambrian time (650 to 520 m.y. B.P.), as determined from radiometric dating of Carolina slate belt rocks. In North Carolina the rift was reactivated during the Triassic, but in Georgia and South Carolina, Triassic extension was restricted to the southeastern edges of the detached crustal fragment.

A local weakening of the brittle-ductile transition can explain some intraplate seismic zones

Abstract A decrease in the strength of a localized area of the lower crust would decrease the depth to the brittle-ductile transition and concentrate stress in the stronger elastic crust around and above the zone of decreased strength. Deformation within the zone of weakened lower crust would occur through viscous or dislocation creep in response to regional plate stress as well as by elastic failure in small earthquakes. Two-dimensional finite-element models of zones of weakness subjected to a regional plate stress predict stress amplification of 10–100% surrounding the local decrease in strength. A finite-element model for an irregular zone of weakness in the lower crust can generate stress directions and relative magnitudes that satisfy the focal mechanisms and seismicity of southeastern Tennessee. An analysis of the displacements and stresses in the central area of decreased crustal strength suggests that strike-slip faulting should dominate. Above this zone the strike-slip faulting should exhibit a strong thrust component. The compression and extension of the crust surrounding the weakened zone at the level of the brittle-ductile transition predicts that the dominant strike-slip faulting should exhibit components of normal faulting on the edges of the weak zone which are near a line through the weak zone parallel to the regional stress and reverse faulting on the edges which are near a line transverse to the regional stress. The seismicity of southeastern Tennessee is diffused over a narrow elliptical zone trending NE. The greatest concentration of activity is near the center. The central zone is characterized by deep-focus strike-slip events with predominantly N or E striking nodal planes. These events are responding directly to compression in the direction of the regional compressive stress and extension perpendicular to the regional compressive stress within the weak zone. The area surrounding the central zone is characterized by focal mechanisms with larger components of reverse or normal fault movements. Events with normal components are dominant on the edges in line with the regional stress as predicted by the stress model for a weak central zone under stress. The agreement between observed earthquake focal mechanisms in southeastern Tennessee and models of crustal stress surrounding a zone of weakness suggests that these events may be caused by a zone of weakness in the lower crust.

Intraplate seismicity and stress in the southeastern United States

Abstract The stress induced by topography and by density heterogeneities in the lithosphere has been computed for two seismic regions in the southeastern United States: the Southern Appalachian mountains and the South Carolina Coastal Plain. The lithosphere was assumed to be a three-dimensional layered elastic slab overlying an inviscid fluid. The calculations indicate that the local stress is of the same magnitude as the tectonic stress (tens of MPa); but contradictory conclusions are suggested by the comparison of stress differences and principal directions with earthquake locations and focal mechanisms. In the Southern Appalachians, the seismicity correlates well with the stress maximum and focal mechanisms agree with the calculations where regional stress is combined with locally induced stress. In South Carolina, a combination of regional and local stress can explain the orientation of focal mechanisms, but stress does not concentrate in the region of Charleston, and, thus, other factors must play a role in the Charleston seismicity.

Thermal Model for Some Continental Margin Sedimentary Basins and Uplift Zones

The process of sea-floor spreading has caused some fragments of continental material to be separated from the main continental mass. The heat production in the fragments is greater than that in the surrounding oceanic or transitional crust; consequently, the rate of crustal cooling with time is diminished, and the rate of subsidence of the crust is retarded near the continental fragment. Such dislocated fragments will exhibit sustained topographic prominence over geologic time. A numerical thermal model applied to the Ocala Uplift in northern Florida compares well with the observed elevations of the pre-Cretaceous surface. Other available geophysical data support the existence of zones of anomalous crust north and south of the uplift.