Structural Interpretation of the Zallah Trough and Adjacent Basement Highs, Western Sirte Basin, Libya: Insights from EGM2008 Satellite Gravity Data

Abstract

The Zallah Trough is a structurally significant depocentre in the western Sirte Basin, Libya, whose geometry reflects the interaction between inherited basement fabric and later extensional reactivation. This study develops an integrated structural model of the Zallah Trough and its bounding highs — the Waddan Uplift, Gattar Ridge, Al Qarqaf Arch, and Al Halayq Ridge — within a window strictly bounded by 17°–18° E and 28°–29° N, using the EGM2008 global satellite gravity model at a working grid spacing of 1000 m. Bouguer and residual anomaly maps were interpreted jointly with edge-enhancing derivatives (total horizontal gradient, tilt-angle derivative, and analytic signal amplitude), radially averaged power-spectrum depth analysis, three-dimensional Euler deconvolution (structural index 0.0), and a constrained two-dimensional forward-modelled profile (WZG-02) extending from the Waddan Uplift, across the Zallah Trough, to Al Halayq Ridge.
The Bouguer anomaly field is dominated by a NW–SE-elongated low over the Zallah Trough flanked by positive responses over the surrounding highs. Edge-detection derivatives resolve a dominant NW–SE structural grain with subordinate NE–SW and N–S lineaments, and reveal that the trough itself is internally compartmentalised by at least two discrete sub-depocentres separated by a basement saddle, bounded by two throughgoing marginal faults whose Euler-derived depths deepen systematically from north to south. Power-spectrum analysis yields two well-resolved characteristic depths (7.62 ± 0.22 km, R² = 0.987, for a deep source population, and 2.07 ± 0.15 km, R² = 0.912, for a shallow population). The forward-modelled profile, calibrated against a density of 2.5 g/cm³ derived from well C5-NC74B (7,560 ft / 2,304 m total depth, selected from three candidate wells) and a basement density of 2.7 g/cm³, converges to an RMS misfit of 0.188 mGal after iterative refinement, with a maximum basement depth of 5,050 m beneath the Zallah Trough axis and a minimum of 2,070 m beneath Al Halayq Ridge; the two largest local misfit zones coincide with independently mapped tilt-derivative zero-crossings at the Waddan Uplift and Al Halayq Ridge margins, while a residual misfit at the profile's south-eastern end remains unexplained by the mapped lineaments and is attributed to a probable basement density contrast within Al Halayq Ridge. The results support a fault-controlled, internally segmented horst–graben architecture for the Zallah region and provide a quantitatively constrained regional framework intended to guide, and to be tested against, subsequent seismic and well-based studies.