Cruise SO190-Leg1 SINDBAD

Report of the project:

• SINDBAD - Seismic and Geoacoustic Investigations along the Sunda-Banda Arc Transition
• SINDBAD - Seismische und geoakustische Untersuchungen entlang des Übergangs vom Sunda- zum Banda-Bogen

Map of geophysical profiles during cruise SO190-1, SINDBAD Source: BGR

Within the framework of the research project SINDBAD (Seismic and Geoacoustic Investigations Along the Sunda-Banda Arc Transition) marine geophysical investigations have been carried out with RV SONNE from October 9th, 2006, to November 9th, 2006, off the eastern Sunda Arc and at the transition to the Banda Arc in Indonesia. The research cruise SO190 Leg 1 started in Jakarta, Indonesia and ended in Darwin, Australia. During this cruise, multichannel seismics (MCS), magnetics (M), and gravimetry (G) measurements have been carried out. Simultaneously, SIMRAD (multibeam echosounder) and PARASOUND (sediment echosounder) data have been collected using RV SONNEs onboard systems.

During the expedition, a total of 4933 km of profiles with MCS, M, and G have been acquired. Six of the 20 profiles are long overview profiles perpendicular to the deformation front and cover the entire forearc from the forearc basin across the outer arc high, the deformation front onto the oceanic lithosphere. Additional profiles have been acquired along strike in the Lombok forearc basin and in the Savu Basin.

The main goal of the project SINDBAD is to investigate the relation between the variability of the lower plate and the tectonic evolution of the overriding plate (formation of an outer arc high, development of forearc basins, and accretion and erosion processes of the overriding plate). The "raw materials" – seafloor sediments, oceanic crust (at the Banda Arc also continental crust) and mantel lithosphere – are carried into the subduction system at the trench. The influence of these "raw materials" on the overriding plate is controlled by a number of factors: E.g. the convergence rate, the obliqueness of convergence and the physical and chemical properties of the lower plate (e.g. its age, its sediment-cover and –thickness, its fluid content and the composition of the crust).

Forearc basins are today attracting increased attention because of their hydrocarbon potential. The forearc basins of the eastern Sunda Arc are still frontier areas which are almost unexplored. An additional goal of this project is therefore the assessment of the hydrocarbon potential of the Lombok Basin.

In contrast to the Sumatra subduction zone, only a small amount of pelagic sediment is carried into the subduction system offshore East Java, Bali, Lombok, Sumbawa and Sumba. This results e.g. in a less pronounced development of the outer arc high, which is subaerial off Sumatra, but entirely below the sea surface in the eastern Sunda Arc. The Roo Rise, which is subducting off East Java, is a morphological high that lies about 1500 m higher than the Argo Abyssal Plain which is subducting further to the east. Despite of these pronounced differences, the deformation front in both areas shows similarities. While the foot of the slope shows lower dip than the upper slope, both areas are characterized by landward dipping thrust sheets. In both areas the outer arc high is characterized by active faults (the recent activity is indicated by deformed basin sediments on the outer arc high) and therfore no indications for a static backstop have been found. The accretionary character of the deformation front is clearly indicated in both areas, while subrosion in association with the subsidence of the Lombok Basin can not be excluded based on the preliminary interpretations. The trench in both areas is devoid of sediments, which indicates erosional processes caused by currents along the trench strike. However, a depocenter for these sediments could not be localized yet.

While a forearc basin is not clearly developed off East Java, the Lombok forearc basin with water depths of more than 4400 m extends from off Bali to off Sumbawa. On the southern slope of the basin prograding sedimentary sequences indicate uplift, probably caused by the subducting Roo Rise or a growth of the outer arc high. Additionally, carbonate platforms on the acoustic basement indicate phases of rapid subsidence of the basin. The sediment thickness reaches a total of about 3.5 sec TWT. A few seismic "bright spots", but no bottom simulating reflectors (BSRs) have been identified in the basin. The profiles striking along the basin axis indicate paleo-depocenters in the western part of the profile, while the recent depocenter is located in the eastern part of the basin. On the northern flank of the Lombok basin, indications for submarine volcanism (recent activity is unknown) are indicated by a seamount reaching above the seafloor associated with a clear magnetic anomaly.

East of the Lombok Basin the island of Sumba is located, which is regarded as a microcontinent that has been attached to the island arc during the Late Oligocene. Sumbas geographical location in front of the island arc is usually characterized by the location of a forearc basin and correlates with the seaward displacement of the deformation front (Roti Basin) at the transition from ocean/island arc subduction of the Sunda Arc to continent/island arc collision of the Banda Arc. An uplift of about 0.5 cm/a is reported for Sumba, associated with the underplating of the continental Scott Plateau. The uplift is especially evident in the MCS data. To the east of the Lombok Basin depocenter, a transition zone with deep reaching faults is observed, associated with eastward dipping sedimentary and basement structures. This transition zone is also indicated by anomalies in the magnetic and gravity data, the latter indicating isostatic undercompensation. On the western flank of Sumba, deformed sedimentary sequences indicate gravitational gliding in association with the uplift of Sumba. East of Sumba, two profiles into the Savu Basin have been acquired. Here the uplift of Sumba is indicated by the erosion of sedimentary sequences which have been deposited in the basin followed by uplift und subsequent erosion. Further indications of "inversion structures" are given by a reactivated thrust fault that in the past has served as the southern boundary of the Savu Basin und indicates recent activity by associated deformed basin sediments.

The oceanic crust of the Argo Abyssal Plain and the Roo Rise is characterized by thin sediments. On a connection profile between two long profiles on the Argo Abyssal Plain a basin with about 1.4 sec TWT of sediment has been observed, that, indicated by a magnetic anomaly, can be correlated with an age jump of about 15 Ma, thereby indicating a paleo plate boundary