Quantitative Analysis of a Transpressional System, El Biod Arch, Ghadames Basin, Algeria
Steven R. Moore, Robert W. Krantz, Kaci Akkache, and
Trap definition within the northern extension of the Hassi Touareg - Rhourde El Baguel fault zone in the western Ghadames Basin of Algeria is difficult due to complex structural geometries. The fault zone consists of a narrow system of discontinuous, locally en echelon faults. Although north-trending to the south, the zone curves to a northeast trend to the north. Reserves associated with the southern portion of the system total 1500 MMBOR and 2 TCFG.
Several lines of evidence support a strike-slip component of motion for the northern segment. Horizontal slickensides have been described in cores taken from wells within the fault trend. Fracture patterns measured from logs taken within the NE-SW fault trend show clusters expected for right-lateral Reidel shears. Although complicated by an evaporite sequence at mid-level in the stratigraphic section, we interpret downward converging faults imaged on recent 2D seismic as positive flower profiles. Map patterns are also interpreted as right-lateral, recognizing that the 2D grid cannot resolve all of the structural complexity.
To confirm the component of strike-slip fault displacement, we applied a new quantitative method relating map view structural orientations to the shear magnitude, the degree of convergence or divergence, and the magnitudes of horizontal and vertical strains. Strike-slip to convergence ratios ranging from 2:1 to 3:1 were measured in the study area. Higher ratios (10:1) measured above the salt may indicate a detachment.
These ratios also fit the regional tectonic pattern: to the south, where the fault zone trends due north, structural geometries support dip-slip inversion indicative of east-west shortening. Applying the same shortening vector to the northeast-trending part of the zone suggests oblique right-lateral motion, with a strike-slip to convergence ratio of 2:1. Horizontal shortening measured from seismic near Hassi Touareg combined with the above ratios indicates 1 to 2 kilometers of strike-slip motion occurred in the study area. The quantitative agreement of the different approaches supports the oblique strike-slip interpretation as well as the detailed structural interpretation within the fault zone.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California