Newly identified strike-slip plate boundary in the northeastern Arabian Sea Nina Kukowski* Thies Schillhorn Ernst R. Flueh Katrin Huhn* GEOMAR Forschungszentrum für Marine Geowissenschaften der Christian-Albrechts-Universität zu Kiel, Wischhofstrasse 1-3, D-24148 Kiel, Germany ABSTRACT The first regional swath-bathymetry survey of the Makran accretionary wedge revealed a sinistral strike-slip fault, named herein the Sonne fault, obliquely crossing the wedge and contin- uing into the abyssal plane. This fault separates the western part of the Makran subduction zone where plate boundary events are absent from the eastern part that does show plate boundary seismicity; most events are concentrated along the Sonne fault. Little Murray Ridge (a basement high) and related magnetic anomalies are offset along the Sonne fault. Together, these observa- tions identify the newly discovered Sonne strike-slip fault as a plate boundary that has been active ~2 m.y. This finding suggests that what has been considered the northeasternmost part of the Arabian plate is actually a separate microplate, named herein the Ormara plate, the formation of which resulted from tearing of the Arabian plate along the Sonne fault. With this concept, the different dips of the downgoing plate below the western and eastern parts of the Makran margin and the related different distances between the trench and Quaternary arc volcanic centers can be unequivocally explained. Keywords: plate tectonics, northeastern Arabian Sea, Makran convergent margin, swath bathymetry. INTRODUCTION front occurs along the right-lateral Minab tear fault (Jacob and Quittmeyer, Convergence between the Arabian plate and the Eurasian plate causes 1979). To the east, the Makran subduction zone passes into the sinistral subduction at the Makran convergent margin, offshore Iran and Pakistan. Ornach Nal fault, the boundary between the Eurasian and the Indian plates, Among the Earth’s subduction zones, Makran has the thickest incoming which continues toward the Himalayan suture. The present-day regional sediment pile, as much as 7 km at the deformation front at long 62°E to plate tectonic situation is the result of a complex plate interaction in the past, 63°E. Reflection seismic profiles acquired during the 1970s and 1980s including several stages of convergence, separation, and reaccretion of revealed a huge accretionary wedge formed from a sediment pile of ~3.4 km microplates (Powell, 1979). A convergence zone east of Arabia and north of thickness scraped off from the Arabian plate while the lower sediment sec- the (proto) Owen Fracture Zone was already active prior to the collision tion is underplated or subducted (White and Louden, 1982; Platt et al., between India and Eurasia and probably represented the most continuous 1985; Minshull and White, 1989; Kopp et al., 2000). Early descriptions of tectonic element in the Makran region (Powell, 1979). the Makran prism assumed laterally continuous structures built of imbricate thrust slices (Platt, 1988). A detailed study of the Makran accretionary Zagros wedge—including multibeam swath mapping, wide-angle seismic surveys, 30˚N Thrust EURASIAN PLATE Sultan single-channel reflection seismic surveys, and gravity and magnetic sur- Minab Fault veys—was accomplished during cruise SO 123 (MAMUT: Makran Murray Taftan Bazman Pakistan India Traverse) of the German RV Sonne in autumn 1997 (Flueh et al., 1997). Iran Between the deformation front and landward of the mid-slope terrace, an Fault Nal Ornach Pasni Ormara area of more than 10 000 km2 was systematically mapped. The results Gwadar INDIAN PLATE 26˚N Makran revealed a more complicated image with a complex pattern of accretionary Subduction Zone 42 ridges of limited length and a sinistral strike-slip fault cutting across the 36.5 Murray wedge. In this paper we describe this strike-slip fault and present a thorough Ridge discussion of its regional tectonic significance, as we identify it as the upper relative plate motion (mm/yr) ARABIAN PLATE convergent plate boundary plate expression of a tear fault in the downgoing plate. 22˚N Oman divergent plate boundary PLATE AND REGIONAL TECTONIC FRAME transform fault The Makran subduction zone results from the convergence of the OFZ Quaternary volcanic centers Eurasian plate and the Arabian plate (Fig. 1). West of Makran, the Zagros 54˚E 58˚E 62˚E 66˚E 70˚E thrust marks the continental collision between the Arabian and Eurasian plate. The transition between the Zagros thrust and the Makran deformation Figure 1. Simplified map of present-day plate tectonic framework of Arabian-Indian-Eurasian convergence zone. Dark-shaded rectangle is area covered with swath mapping during cruise SO 123 (cf. Fig. 2). *Present address: GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Arrows indicate rate and direction of convergence. For details and refer- Potsdam, Germany. E-mail: [email protected]. ences see text; for all individual Quaternary volcanoes, see Figure 3. Geology; April 2000; v. 28; no. 4; p. 355–358; 5 figures. 355 Lut Helmand 24˚48'N 24˚42'N 24˚36'N 24˚30'N 24˚24'N 24˚18'N 63˚36'E 24˚12'N Sultan erosive canyons 29˚N Taftan 63˚24'E Bazman N 63˚12'E 63˚00'E Pakistan 27˚N Iran 62˚48'E 63˚36'E Pasni Ormara 62˚36'E 63˚24'E Gwadar 0 62˚24'E 63˚12'E -300 25˚N Sonne fault -600 62˚12'E 63˚00'E 0 -900 62˚48'E -1200 Little Murray Ridge -1000 segments strike lines Northern -1500 W 23˚N 62˚36'E a -2000 -1800 t e Dalrymple TroughSouthernMurray Ridge r 62˚24'E -2200 59˚E 61˚E 63˚E 65˚E 67˚E -3000 D e -2500 p Figure 3. Regional tectonic map of Arabian-Indian-Eurasian convergence 62˚12'E 24˚48'N 24˚42'N t 24˚36'N -2800 zone showing distribution of earthquakes (circles; compiled from Byrne 24˚30'N 24˚24'N h 24˚18'N 24˚12'N et al., 1992; Jackson et al., 1995; and U.S. National Earthquake Infor- -3100 A accretionary ridges [m] mation Center data set) and Quaternary onshore volcanoes (triangles; on lower slope -3400 compiled from White, 1984; Dykstra and Birnie, 1979; Afa–ghi and Sa–lek, 1977). Onshore topography is from satellite altimetry; submarine topog- raphy is combined from satellite altimetry and RV Sonne cruise data set. 25˚ 00'N meandering canyons Son 280 Iran Pakistan n e most landward fa 26˚N Gwadar ult accretionary ridge ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ 140 ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ [nT] ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ 0 ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ Little Murray Ridge ▲ ▲ ▲ ▲ ▲ ▲ 24˚ 30'N ▲ 24˚N ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ magnetic anomaly ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲▲ ▲▲ ▲ ▲ ▲▲ ▲▲ ▲ ▲▲ ▲▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ -140 ▲ ▲ ▲ ▲ ▲ deformation front ▲ 1. accretionary ridge ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲▲ ▲ mid slope terrace deformation front lower slope B 60˚E 62˚E 64˚E 66˚E -280 62˚30'E 63˚00'E 63˚30'E Figure 4. Map of magnetic anomalies in northeastern Arabian Sea. Note Figure 2. A:Three-dimensional perspective, shaded bathymetric image offset in linear anomaly caused by Little Murray Ridge (after Edwards of Makran accretionary wedge, showing Sonne strike-slip fault and ero- et al., 2000). Symbols as in Figure 1. sive canyons crossing wedge. B:Tectonic interpretation showing offset of accretionary ridges and extensional jog of Sonne fault, which may result from some rotation of Ormara plate. constrained. The available seismological data (Byrne et al., 1992) and the different trench to volcano distances in the western and eastern parts of the The age of the subducting oceanic crust facing the Makran wedge is Makran subduction zone imply that the western part of the Arabian slab dips not well determined. Alternative plate reconstructions assume it to be at a steeper angle than its eastern part. Jurassic or older (Whitmarsh, 1979), or of Eocene age (Mountain and Prell, 1990). Ongoing subduction at least since the Late Cretaceous, probably MORPHOLOGY OF THE MAKRAN ACCRETIONARY WEDGE with a continuously thick sediment input, has led to the formation of an AND IDENTIFICATION OF A STRIKE-SLIP FAULT accretionary prism more than 350 km wide with a small taper of about 5° in The Makran accretionary wedge is built by a succession of remarkably the offshore part of the wedge. Currently, only the frontal 100–150 km are steep accretionary ridges of limited length (Fig. 2). Because of the thick sedi- submarine, while the largest part is subaerial. The recent convergence rate ment cover, there is no distinct trench. In the western part of the surveyed area, is about 40 mm/yr, showing a west to east increase of about 15% from a smooth bulge in the western part of the surveyed area is a newly forming ac- 36.5 mm/yr close to the Minab fault to 42 mm/yr in easternmost Makran cretionary ridge in a nascent stage. Its seaward edge marks the actual defor- (DeMets et al., 1990, Fig. 1). The deformation front extends approximately mation front. The first accretionary ridge has a throw of as much as 1200 m. perpendicular to the direction of convergence. Commonly, all following ridges have remarkably steep flanks with slopes of Seismicity in the Makran subduction zone is surprisingly low com- between 9° and 14°, whereas the regional slope is very small (1.5°–2.1°). pared to that at other active margins. However, the Mw 8.1 event in 1945 Actual deformation is taking place along steeply dipping thrusts at the seaward near Pasni revealed the potential for very large interplate thrust earthquakes flanks of all ridges. In between, continuous sedimentation is indicated by (Byrne et al., 1992). Thus, the geometry of the downgoing slab is not well undisturbed layering observed in 3.5 kHz echo soundings (Flueh et al., 1997). 356 GEOLOGY,April 2000 The most prominent tectonic feature, however, is a sinistral strike-slip tion of the offset; farther southeast, the strike line intersects Murray Ridge fault (named the Sonne fault after the research vessel surveying the area) just at the transition between the southern and northern segments.