Sediment subduction, subduction erosion, and strain regime in the northern South Sandwich forearc
The first swath bathymetry and side-scan sonar imagery from the South Sandwich forearc reveals detailed seafloor morphology, tectonic fabric, and sedimentary features in an area where plate convergence is approximately normal to the trench. Simultaneously collected seismic reflection, gravity, and magnetic data provide information about the structure and composition of the forearc crust. Interpretation of these data together with the marine magnetic record of seafloor spreading in the east Scotia Sea back arc basin since 15 Ma has enabled quantitative estimation of sediment subduction and subduction erosion rates. Any accretionary prism present is constrained to be very small, extending 6 km or less from the trench. The contrast between this result and a time-integrated estimate of potential accretionary prism volume suggests that >95% of the sediment which has entered the trench since 15 Ma has been subducted. The position of the South Sandwich island arc in relation to back arc magnetic lineations implies that the arc has migrated ∼70 km westward relative to the Sandwich plate since 15 Ma. Taking account of published observations concerning the typical geometric evolution of arc-trench systems, the average rate of forearc slope retreat during this interval is inferred to be 3.1–4.7 km Myr−1. Gravity modeling suggests that the forearc crust averages ≤10 km in thickness within 110 km of the trench. For 10-km-thick forearc crust the rate of slope retreat implies an average rate of subduction erosion of forearc crust of 31–47 km3 km−1 Myr−1. At the present convergence rate of 74 km Myr−1 this erosion rate requires a 420–635 m thick layer of subducting material derived from the forearc crust. A time-integrated estimate of the potential volume of forearc-derived trench fill sediments subducted since 15 Ma implies that frontal erosion can only account for <40% of total subduction erosion. Therefore basal erosion must significantly exceed frontal erosion. The new data also provide insights concerning forearc strain regime and forearc basin evolution in the area studied. Extensional faulting near the trench slope break is probably caused by gravitational instability of the steep lower forearc slope. A lack of extensional strain indicators in the upper forearc and arc may be a consequence of the fact that the east Scotia Sea is a mature back arc basin contributing significant “ridge push” in the vicinity of the arc. The data reveal no evidence of arc-parallel extension or of large serpentinite seamounts, as found in the Mariana forearc, and a causal link between these two observations is proposed. Seismic profiles across the forearc basin suggest that the balance between sediment accumulation and erosion is sensitive to changes in the elevation of the trench slope break. It is hypothesized that the basin is a dynamic feature which goes through repeated cycles of growth and destruction controlled by cyclic uplift and collapse of the trench slope break.