Distribution of faults and extensional strain in fractured carbonates of the North Malta Graben

Abstract

Faults in carbonate rocks occur over a wide range of scales, some of which lie below the resolution of seismic reflection surveys. For this reason, our knowledge of the spatial and size distributions of subseismic faults is very limited. In this study, we address this issue by directly measuring the positions and displacements of extensional faults along a total of 56 km (35 mi) of outcrop and map-derived cross sections from the pelagic carbonates of the Maltese Islands. The data cover close to four orders of displacement magnitude and provide insight into the organization of the fault system associated with the North Malta Graben system. Virtually all the brittle extension in the Maltese Islands is accommodated by faults, with displacements ranging from centimeters to greater than 100 m (328 ft). This extension is not homogeneously distributed across the islands but is localized in a few kilometer-wide higher strain zones, which are separated by virtually undeformed lower strain regions. At the centimeter scale, both regions have an average fault density of about 101 m1 (spacing 10 m [33 ft]). In the lower strain areas, few of these faults attain displacements of greater than 10 m (33 ft) and would be below the resolution of most commercial seismic surveys. In the higher strain regions, however, faults with displacements of greater than10 m (33 ft) occur at spacings of about 1 km (0.6 mi). This difference is reflected in the scaling exponents of D = 0.84 (higher strain) and D = 1.26 (lower strain), with larger faults dominating the deformation when D is greater than 1. Higher strain zones are more prominent in the North Malta Graben and produce an overall extension of approximately 3.3%, whereas they occur only locally in the surrounding horst regions where the extension is about 1.8%. These results indicate that high-quality seismic reflection surveys across the Malta platform might miss between 40 and 45% of the total extension and that estimates of subseismic faulting need to consider differences in the scaling of the higher and lower strain zones.