Quantitative Analysis of a Transpressional System
, El Biod
Arch, Ghadames Basin, Algeria
Steven R. Moore, Robert W. Krantz, Kaci Akkache, and
Mohamed Messaoudi
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