4 MARCH 2010, GSA TODAY Evaluating lateral compaction in deepwater fold and thrust belts: How much are we missing from “nature’s sandbox”? R.W.H. Butler, Geology and Petroleum Geology, School of Geosciences, University of Aberdeen AB24 3UE, UK, rob. butler@abdn.ac.uk; and D.A. Paton, Institute of Geophysics and Tectonics, School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK, d.a.paton@leeds.ac.uk ABSTRACT Deepwater fold and thrust belts offer unique opportunities for evaluating deformation in sedimentary successions with un- rivalled seismic imaging of fold-thrust structures. A regional seismic line through the Orange Basin, offshore Namibia, re- veals a classic paired, gravity-driven deformation system, over 100 km across, with extension high on the submarine slope and contraction toward the toe of slope. A mismatch between the minimum estimate of extension (44 km) and slip on thrusts (18–25 km) requires an additional longitudinal strain compo- nent of 18%–25% to be distributed across the system, most plausibly as lateral compaction and volume loss. Strains of this magnitude raise issues for understanding deformation in par- tially lithiied strata, with implications for the applicability of theoretical fold-thrust models and the development of hydro- carbon resources in deepwater settings. INTRODUCTION The thick sedimentary sequences that characterize many of the world’s continental margins hold an unstable secret of grav- itational collapse, the scale of which has only become apparent through exploration for hydrocarbons in the past 20 years or so (e.g., Rowan et al., 2004). Although inaccessible, submerged beneath many kilometers of water, these structures are revolu- tionizing the understanding of the geometry of contractional deformation in sedimentary successions. They are nature’s sandbox, large-scale versions of the laboratory physical models currently in vogue in some parts of the structural geology com- munity (e.g., Adam et al., 2005). Here we examine one well- imaged system to learn more about the large-scale deformation GSA Today, v. 20, no. 3, doi: 10.1130/GSATG77A.1 of poorly lithiied sedimentary rocks. Our case study comes from the continental margin of southwest Africa, offshore Na- mibia. By making independent estimates of the extension and the contraction, we show that there is a considerable amount of deformation that is not accounted for in the imaged thrust belt structures. We discuss how these inferred strains might be accommodated and outline the implications. Deepwater fold and thrust belts are the down-slope expres- sion of large-scale gravitational failure of submarine slopes (Rowan et al., 2004). They are paired with extensional struc- tures higher on the slope (Fig. 1), with the two domains con- nected by a detachment preferentially located along a weak formation (salt, or, as in our study, over-pressured mudstone). There are two principal attractions for studying thrust systems created by gravity tectonics in deepwater systems. The irst is that seismic relection methods yield images of unrivalled clarity (Fig. 2). This means many of the ambiguities in struc- tural interpretation are reduced, especially in deining stratal terminations against faults and thereby deducing the geome- try and extent of thrust ramps and lats. Kinematic models that describe the relationship between folding, the geometry of stratal surfaces, and the displacement patterns on faults have been reined (e.g., Shaw et al., 2005) and reapplied (e.g., Briggs et al., 2006) in these settings. Second, purely gravity-driven systems are kinematically self-contained. The stratal shortening represented by the contractional structures, including the thrust and folds developed on the lower slopes, must balance the net extension accommodated higher on slopes. This attribute means estimates of the extensional mo- tion can be used to constrain structural interpretations of the contractional domain. Our speciic concern is whether the thrust and folds that can be interpreted from the seismic data are suficient alone to balance the extension. If not, a further strain component is required, the value of which can be esti- mated. Distributed strains, long known as a component of foreland fold and thrust belts (e.g., Coward, 1988) and re- cently recognized in physical deformation models of granular Figure 1. Architecture of gravity-driven thrust systems, based on the Pará-Maranhão basin, offshore Brazil (Zalan, 2005). The pre-kinematic section is shown in green and has been stacked up into a deepwater thrust belt on the lower slope. Sedimentation has continued (tan-yellow tones) and eventually buried the thrust belt. These syn-kinematic deposits are ponded in fault-related basins within the extensional domain, upslope from the thrust belt. The sediments show characteristic geometry of growth strata, thickening toward the main extensional faults and forming off-lapping depositional wedges that become progressively younger up the slope. Cumulative stratal shortening in the contractional domain should balance the net extension, here shown by the separation of pre-kinematic strata along the main detachment.