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 revealed a sinistral strike-slip , 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 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 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: , 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 causes 1979). To the east, the Makran subduction zone passes into the sinistral subduction at the Makran convergent margin, offshore and . Ornach Nal fault, the boundary between the Eurasian and the Indian plates, Among the ’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 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 Ormara area of more than 10 000 km2 was systematically mapped. The results Gwadar

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 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 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. cutting the entire wedge striking southeastward (Fig. 2). The offset of the The same displacement found for the fourth accretionary ridge and the accretionary ridges caused by the left-lateral motion along the Sonne fault Little Murray Ridge along the Sonne fault provides the possibility to estimate increases with distance from the trench. The first ridge shows an offset of the age of the strike-slip motion and slip rate with the assumption that slip was <2 km, whereas the most landward one is displaced by ~10 km along the steady and uniform. The fourth accretionary ridge probably has formed since Sonne fault. As it can be followed in the , it is a feature of both 2 Ma, as inferred from the length of the accreted thrust slices, the convergence the upper and the lower plate, which gives evidence for strong coupling be- rate, and the assumption of only moderate shortening (Kopp et al., 2000; Platt tween the plates (McCaffrey and Goldfinger, 1995). et al., 1988), which gives an average slip rate of 5–7 mm/yr. This assumed age for the fourth accretionary ridge implies that motion along the Sonne fault was SONNE FAULT AND ITS REGIONAL TECTONIC SIGNIFICANCE also initiated ca. 2 Ma. A slip rate of several meters per million years is of the The nature of the Sonne fault can be identified by taking into account same order as slip rates on other strike-slip plate boundaries or strike-slip the regional plate tectonic setting, the different patterns of seismicity in west faults in other convergent margin settings and matches the difference in con- and east Makran, and offset of Little Murray Ridge, an old basement high in vergence rates between the West and East Makran subduction zones. Recent the abyssal plain (White, 1983) (Fig. 3). These factors have led us to iden- motion along the Sonne fault is able to explain the observed pattern of seis- tify the Sonne fault as a plate boundary separating the Arabian plate from an micity (Fig. 4). The sinistral sense of motion along the Sonne fault is consist- individual microplate that we henceforth call the Ormara plate after the ent with faster convergence above the East Makran subduction zone (Fig. 1). town of Ormara on the Pakistani coast. Therefore, the Sonne fault is sug- The plate tectonic concept following from the identification and verifi- gested to be a strike-slip plate boundary; it forms the western edge of the cation of the Sonne strike-slip plate boundary (Fig. 5A) builds upon the model Ormara microplate, which has a triangular shape with the northern Murray presented by Quittmeyer and Kafka (1984). In our concept, the Sonne fault Ridge and East Makran subduction zone being its other edges and a triple and southern Murray Ridge segment (Dalrymple trough) are assumed to be junction at each of the corners. pure transform faults. Like Quittmeyer and Kafka (1984), we make northern The northern and southern segments of the Murray Ridge differ signifi- Murray Ridge a boundary of short transforms and ridges, the direction of cantly in morphology, extension style, and spreading rate, and have a different spreading (extension) being a few degrees east of north-south. Then, the triple strike (Edwards et al., 2000). The northern, more easterly striking full-graben junction among the Arabian, Indian, and Ormara plates is stable and fixed to segment shows right-lateral slip at a rate of 0.2–4 mm/yr at the 95% confi- the Arabian plate (Fig. 5B). If convergence between the Ormara plate (and dence level and oblique extension on the northern segment of the Murray Arabian plate) and the Eurasian plate will remain to be faster than oblique Ridge at a similar rate (DeMets et al., 1994). The southern segment, however, extension along the Murray Ridge, the triple junction among the Arabian, including the Dalrymple Trough (the deepest sea floor in the Arabian Sea), Ormara, and Eurasian plates will move along the trench toward the east. resembles a half graben and has probably undergone significantly less exten- sion (Edwards et al., 2000). Recent deformation is mostly strike-slip motion. ORIGIN OF THE ORMARA PLATE AND IMPLICATIONS FOR These observations are compatible with interpreting the segments of the THE REGIONAL TECTONICS OF THE NORTHEASTERN Murray Ridge as belonging to different plates (Figs. 3 and 4). ARABIAN SEA On the basis of the pattern of seismicity, Makran can be subdivided As the strike of the northern segment of the Murray Ridge is about per- into a western and eastern region, the boundary of those coinciding very pendicular to the strike of the Sonne fault, oblique extension along the well with the Sonne strike-slip fault (Fig. 3). The western part is character- northern Murray Ridge is suggested to have caused the generation of the ized by the absence of interplate events. East of the Sonne fault and west of Sonne fault when the Arabian plate failed and tore to accommodate the long 64°E is the only region with a clustering of events within the submarine

and southernmost onshore part of the accretionary wedge, also including the ONF

Mw 8.1 event of 1945 (Byrne et al., 1992) (Fig. 3). Most events in the wedge Sonne

MF EURASIAN PLATE fault appear to be pure-thrust earthquakes and are interpreted as plate boundary TJ events (Quittmeyer and Kafka, 1984; Byrne et al., 1992). The earthquake of August 12, 1963, a few tens of kilometers east of the Sonne fault, had a large strike-slip component and its depth was estimated to be only 5 km (Quittmeyer and Kafka, 1984). Taking into account the uncertainties of TJ focal estimation, we speculate that this event may have occurred in connec- ARABIAN PLATE ORMARA PLATE tion with motion along the Sonne fault. In the Murray Ridge region, seismicity is nearly exclusively restricted LMR to its northern segment (Fig. 3). The distribution of earthquakes shows a TJ concentration in the Ormara plate toward the Sonne fault. We suggest that Dalrymple INDIAN PLATE Trough this is a consequence of the fact that the convergence rate between the OFZ Ormara plate and the Eurasian plate is higher than that between the Arabian A and Eurasian plate; typically, the higher the convergence rate, the more O ia stress can be accumulated and therefore also released. The Little Murray Ridge, a linear succession of topographic highs that A oi are supposed to be the morphologic expressions of a buried basement high, extends subparallel to Murray Ridge and enters the at about oa long 64°30′E (White, 1983) (Figs. 3 and 4). Its northern part is offset to the I B north by about 10 km, suggesting offset along a strike-slip fault. A strong linear magnetic anomaly (Fig. 4), which is suggested to be caused by the Figure 5. A: Plate tectonic sketch indicating position and framework of newly identified Ormara plate. ONF is Ornach Little Murray Ridge, shows the same offset (Edwards et al., 2000). If the Nal fault; OFZ is Owen Fracture Zone;TJ is triple junction. For strike line of the Sonne fault is continued toward the southeast to the Murray symbols see Figure 1 and text. B: Velocity diagram for triple Ridge, as suggested here, it meets the Little Murray Ridge just at the loca- junction among Arabian, Ormara, and Indian plates.

GEOLOGY,April 2000 357 additional forces and motion resulting from extension and plate motion on DeMets, C., Gordon, R. G., and Vogt,P., 1994, Location of the Africa-Australia-India a sphere. This scenario leads us to suggest that the Ormara plate was for- triple junction and motion between the Australian and Indian plates: Results from an aeromagnetic investigation of the Central Indian and Carlsberg ridges: merly part of the Arabian plate. Oblique extension along the Murray Ridge Geophysical Journal International, v. 119, p. 893–930. may have originated to accommodate the transition from right-lateral slip Dykstra, J. D., and Birnie, R. W., 1979, Segmentation of the Quaternary subduction along the Owen Fracture Zone to left-lateral slip along the Ornach Nal fault. zone under the Baluchistan region of Pakistan and Iran, in Farah, A., and These tectonic changes therefore allowed the beginning of the generation of de Jong, K. A., eds., Geodynamics of Pakistan: Quetta, Geological Survey of the Ormara plate, which is pushed ahead of the Arabian plate. A similar Pakistan, p. 319–323. Edwards, R. A., Minshull, T. A., and White, R. S., 2000, Extension across the Indian- concept was suggested for the Nootka fault plate boundary between the Arabian plate boundary: The Murray Ridge: Geophysical Journal International and the (Hyndman et al., 1979). (in press). The Sonne fault dips to the east (Kopp et al., 2000) and the convergence Flueh, E. R., Kukowski, N., and Reichert, C., eds., 1997, RV Sonne, cruise report direction between Eurasia and India changes around the northern edge of SO 123 “MAMUT” (Makran Murray Traverse): Kiel, Germany, GEOMAR Report 62, 291 p. the Indian plate (Nakata et al., 1990). Additional westward compression re- Haq, S. S. B., and Davis, D. M., 1997, Oblique convergence and the lobate mountain sults, pushing the Ormara plate against the Arabian plate and dovetailing belts of western Pakistan: Geology, v. 25, p. 23–26. both plates so that the Ormara plate slab dips less than the Arabian plate Hyndman, R. D., Riddihough, R. P., and Herzer, R., 1979, The Nootka fault zone— slab. This is a straightforward explanation for the different distance of Qua- A new plate boundary off western Canada: Royal Astronomical Society Geo- ternary volcanoes from the deformation front (Fig. 3) and for the observed physical Journal, v. 58, p. 667–683. Jackson, J., Haines, J., and Holt, W., 1995, The accommodation of Arabia-Eurasia plate pattern of plate boundary seismicity (Byrne et al., 1992). convergence in Iran: Journal of Geophysical Research, v. 100, p. 15,205–15,219. In addition, a tectonic sliver between the transform boundaries at the Jacob, K. H., and Quittmeyer, R. L., 1979, The Makran region of Pakistan and Iran: eastern edge of the Eurasian plate and the margin of the Indian plate is being Trench-arc system with active plate subduction, in Farah, A., and de Jong, pushed northward owing to the oblique convergence of India (Nakata et al., K. A., eds., Geodynamics of Pakistan: Quetta, Geological Survey of Pakistan, p. 305–317. 1990), and at least one rigid block that is part of this sliver is moving with Kopp, C., Fruehn, J., Flueh, E. R., Reichert, C., Kukowski, N., Bialas, J., and respect to the Eurasian and Indian plates (Haq and Davis, 1997). Together Klaeschen, D., 2000, Structure of the Makran subduction zone from wide-angle with our new data and interpretation, this presence of the sliver implies that and reflection seismic data: Tectonophysics (in press). the triple junction between the Eurasian, Arabian, and Indian plates may be McCaffrey, R., and Goldfinger, C., 1995, Forearc deformation and great subduction a complex region of blocks and microplates moving with respect to each earthquakes: Implications for Cascadia offshore earthquake potential: Science, v. 267, p. 856–859. other. Such a situation would also imply that collision may be responsible Minshull, T. A., and White, R. S., 1989, Sediment compaction and fluid migration for the breaking off of microplates from big colliding plates and that in the Makran accretionary prism: Journal of Geophysical Research, v. 94, motions at the Eurasian-Arabian-Indian convergence zone are much more p. 7387–7402. complicated than previously suggested. Mountain, G., and Prell, W. I., 1990, A multiphase plate tectonic history of the south- east continental margin of Oman, in Robertson, A. H. F., et al., eds., The geol- ogy and tectonics of the Oman region: Geological Society [London] Special CONCLUSIONS Publication 49, p. 725–743. New geophysical data from the northeastern Arabian Sea region reveal Nakata, T., Otsuki, K., and Khan, S. H., 1990, Active faults, stress field, and plate mo- a strike-slip plate boundary, the Sonne fault, cutting the Makran accre- tion along the Indo-Eurasian plate boundary: Tectonophysics, v. 181, p. 83–95. tionary wedge. This boundary formed since 2 Ma as the Arabian plate failed Platt, J. P., 1988, The mechanics of frontal imbrication: A first order analysis: Geolo- gische Rundschau, v. 77, p. 577–589. by tear faulting as a consequence of the higher northward convergence rate Platt, J. P., Leggett, J. K., Young, J., Raza, H., and Alam, S., 1985, Large-scale sedi- of the northern segment of the Murray Ridge. The Sonne fault separates a ment underplating in the Makran accretionary prism, southwest Pakistan: Geol- microplate, the Ormara plate, from the Arabian plate. The Ormara plate fits ogy, v. 13, p. 507–511. into the regional tectonic framework unequivocally, and its presence allows Platt, J. P., Leggett, J. K., and Alam, S., 1988, Slip vectors and fault mechanics in the Makran accretionary wedge, southwest Pakistan: Journal of Geophysical Re- reasonable explanations for the distribution of seismicity and strike-slip search, v. 93, p. 7955–7973. components in the August 12, 1963, seismic event. Powell, C. M., 1979, A speculative history of Pakistan and surroundings: Some constraints from the , in Farah, A., and de Jong, K. A., eds., Geo- ACKNOWLEDGMENTS dynamics of Pakistan: Quetta, Geological Survey of Pakistan, p. 5–24. We thank U. von Rad, H. Roeser, and H. Villinger for valuable discussions, and Quittmeyer, R. C., and Kafka, A. L., 1984, Constraints on plate motions in southern R. Edwards for sharing her illustration of the magnetic anomalies in the Arabian Sea. Pakistan and the northern Arabian Sea from the focal mechanisms of small Discussions with A. Kopf, S. Husen, and O. Oncken made our hypothesis much earthquakes: Journal of Geophysical Research, v. 89, p. 2444–2458. clearer to us. We thank R. Larson, J. Platt, and C. Goldfinger for thoughtful and con- Wessel, P., and Smith, W. H. F., 1991, Free software helps map and display data: Eos structive reviews of an earlier version of the manuscript. The excellent support by (Transactions, American Geophysical Union), v. 72, p. 445–446. master H. A. Andresen and his crew on board RV Sonne is gratefully acknowledged. White, R. S., 1983, The Little Murray Ridge, in Bally, A. W., ed., Seismic expression The enthusiasm of the MAMUT working group scientists and technicians on cruise of structural styles: American Association of Petroleum Geologists Studies in SO 123 is heartily acknowledged. We used the mbsystem software (Caress and Geology, v. 15, p. 1.3.10–1.3.23. Chayes, 1996) to process the swath data and GMT package (Wessel and Smith, 1991) White, R. S., 1984, Active and passive plate boundaries around the Gulf of Oman, for Figures 1 to 4. The MAMUT project was funded by the German Ministery of north-west Indian Ocean: Deep-Sea Research, v. 31, p. 731–745. Education, Research, and Science (BMBF project 03 G 0123 A). White, R. S., and Louden, K. E., 1982, The Makran continental margin: Structure of a thickly sedimented convergent plate boundary, in Watkins, J. S., and Drake, REFERENCES CITED C. L., eds., Studies in continental margin geology: American Association of – – Afaghi, A., and Salek, M. M., eds., 1977, Geological map of Iran: Teheran, National Petroleum Geologists Memoir 34, p. 499–518. Iranian Oil Company sheet no. 6, scale 1:1000000. Whitmarsh, R. B., 1979, The Owen basin off the south-east margin of Arabia and the Byrne, D. E., Sykes, L. R., and Davis, D. M., 1992, Great thrust earthquakes and evolution of the Owen fracture zone: Royal Astronomical Society Geophysical aseismic slip along the plate boundary of the Makran subduction zone: Journal Journal, v. 58, p. 441–470. of Geophysical Research, v. 97, p. 449–478. Caress, D. W., and Chayes, D. N., 1996, Improved processing of hydrosweep DS Manuscript received October 6, 1999 multibeam data on the R/V Maurice Ewing: Marine Geophysical Research, Revised manuscript received January 12, 2000 v. 18, p. 631–650. Manuscript accepted January 26, 2000 DeMets, C., Gordon, R. G., Argus, D. F., and Stein, S., 1990, Current plate motions: Geophysical Journal International, v. 101, p. 425–478.

358 Printed in U.S.A. GEOLOGY,April 2000