IS THERE A CAROLINE PLATE? 1. Introduction - Science Direct

Earth and Planetary Science Letters, 4l (1978) 143-158 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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IS THERE A CAROLINE PLATE? J E F F R E Y K. WEISSEL and ROGER N. ANDERSON Lamont-Doherty Geological Observatory of Columbia University, Palisades, N.Y. 10964 (U.S.A.)

Received February 10, 1978 Revised version received July 18, 1978

We examine available marine geophysical and seismological data from the Caroline Sea region and conclude that a separate Caroline plate currently exists. The Caroline plate is moving relative to the Pacific plate on its northern and eastern boundaries, the Philippine plate on its western margin, and the Indian and smaller plates along its southern side in New Guinea and the Bismarck Sea area. The southern Yap Trench, the Palau Trench, and an accreting plate boundary within the Ayu Trough manifested as an axial rift valley comprise the Caroline-Philippine plate boundary. On the basis of sediment thickness and subsidence of basement away from the rift, we estimate that the Ayu Trough started to open during the Miocene. The northern section of Pacific-Carolineplate boundary coincides with the Sorol Trough which exhibits both strike-slip and extensional characteristics. The southeastern section of this boundary occurs along the Mussau Trench where Caroline plate underthrusts the Pacific plate. The section of plate boundary between the Sorol Trough and Mussau Trench is characterized by highly unusual deformational tectonics. Convergence between the Pacific and Caroline plates is apparently accommodated here by overthrusting of small slivers of sea floor towards the northeast. The intensity of deformation appears to increase southward towards the Mussau Trench. Our calculated instantaneous angular rotation vector for the Pacific-Carolineplates predicts that convergence rates increase uniformly south along the overthrust and underthrust sections of plate boundary. The transition in tectonic style from overthrusting to underthrusting occurs between 3° and 4°N.

1. Introduction A major difficulty in applying plate tectonics to the geologic history of east and southeast Asia is that the region is primarily one of plate convergence involving major (Pacific, Indian, Philippine, and Asian) and minor lithospheric plates (Fig. 1). Whereas directions of present-day plate convergence are often obtainable from focal mechanisms for shallow earthquakes, no reliable measurements of rates of relative motion can be obtained as they can from magnetic lineations associated with mid-ocean ridges. This means that present relative motions between plates must be calculated by vector addition around a closed circuit of plate boundaries (e.g. [ 1 - 3 ] ) and uncerContribution No. 2691 of Lamont-Doherty Geological Observatory.

tainties are compounded by this procedure. Convergence between the Pacific and Philippine plates presently occurs at the Izu-Bonin-Mariana island a r c - t r e n c h system with complications along the Mariana and possibly the Bonin sections due to back-arc extension. South of the Mariana Trench the seismicity trends are less distinct and it becomes difficult to locate the southward continuation of the Pacific-Philippine plate boundary solely on the basis of shallow earthquakes. Katsumata and Sykes [4] noted that the pattern of seismicity divides into two branches: one along the Yap and Palau trenches and southward through the Ayu Trough to northwest New Guinea, and another, a more diffuse band, along the Sorol Trough-Caroline Islands ridge (Fig. 2). These two branches, defined by infrequent events, together with the substantial seismicity across northern New Guinea, provide evidence for current activity

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Fig. 1. Tectonic setting of the Caroline plate. Magnetic lineations ([6] and this study) are dashed and fracture zones are finely hatched. DSDP sites are numbered solid circles. Present motion of the Pacific relative to the Caroline plate is shown by the arrows. These vectors follow small circles about the pole of relative motion between the Pacific and Caroline plates (enclosed circle 3). This pole and its rotation rate was obtained by adding the Philippine-Pacific ([25 ], enclosed circle 1) and Philippine-Pacific (enclosed circle 2) relative motion vectors. Bathymetric contours are in kilometers.

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from the earthquake first motion study (Fig. 12B), (2) structure and morphology observed from seismic reflection and echosounder data (Figs. 7, 9, and 11), (3) calculated relative motion between the Pacific and Caroline plates (Fig. 1), and (4) the correlation of magnetic lineations at least partially across the zone of deformation (Fig. 10)which argues against large strike-slip movements;we believe that interplate convergence characterizes the sections of Caroline-Pacific plate boundary in the eastern Caroline Sea. At the Mussau Trench, except for the lack of associated teleseismicity, a regular subduction situ-

ation has developed. The trench characteristics disappear north of 3°N (Fig. 7) and the inferred convergence seems accommodated by an unusual mode of crustal deformation. In the absence of direct evidence to the contrary, we propose that the local morphology within the zone of deformation that is particularly conspicuous in the top four profiles of Fig. 9 (also Fig. 11) represents slivers of oceanic crust overthrust towards the northeast. In Fig. 13 we show an interpreted cross section from profile 6 (Fig. 11) in the northern part of the disrupted section of plate boundary. The section is drawn without vertical

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20/08/68 5.6"N 146.9"E 15/10/71 2.9"N 1~2.8*E Fig. 12. Focal mechanisms for shallow earthquakes (A) near the axis of the Ayu Trough, and (B) within the disrupted (overthrust) section of Caroline-Pacific plate boundary. Solid circles are compressions, open circles are dilatations, and crosses are nodal. For Fig. 12B, we show possible thrust (solid nodal planes) and strike-slip (dashed nodal planes) mechanisms. Bold arrows in Fig. 12B indicate the horizontal projections of the axis of maximum compression.

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Fig. 13. Interpreted structural section corresponding to the 3.5-kHz record profile in Fig. 11. Since the Pacific-Caroline plate boundary is probably convergent in this vicinity, our preferred interpretation is the thrust faulting (solid lines). A weaker, although geometrically possible interpretation, involves normal faulting (dashed lines). Section is drawn without vertical exaggeration. Upper part of the oceanic crust is denoted by the diagonal hatching.

157 exaggeration and shows both a preferred and a possible alternative interpretation for the observed morphology. Our working hypothesis is that the morphology is due to overthrusting of slivers of sea floor towards the northeast (heavy thrust faults in Fig. 13). Although an alternative set of normal faults is also a geometric possibility, we have presented evidence above to suggest that this section of Caroline-Pacific plate boundary is under compression, not extension.

6. Discussion and summary Two areas of possible tectonic complications in our preferred model for the Pacific-Caroline plate boundary can be discerned at present. First, the area between 3 ° and 4°N, where the trench to no-trench transition occurs, also marks the intersection of the Kiilsgaard Trough and the Lyra Trough with the Caroline-Pacific plate boundary (Fig. 1). These older plate boundaries are probably sites of inherent lithospheric weakness and they may, therefore, influence the tectonic regime on the present plate boundary. We are convinced that the Kiilsgaard Trough is an extinct spreading center of Oligocene age (Fig. 3). However, the age and tectonic nature of the Lyra Trough is poorly known at present. The second complication arises from the occurrence of some teleseismic events along the Caroline Islands ridge to the north (Fig. 2). Available marine geophysical data are insufficient to determine the tectonic processes that these events represent. Incipient plate boundary activity cannot be ruled out. Jarrard and Clague [27] argue that the Caroline Islands represent a "hort spot" trail similar to the generally accepted origin for the Hawaiian Island ridge. Age data obtained along the Caroline Islands ridge, Miocene and older at Truk [28] and Oligocene at DSDP sites 57 and 58 to the west [29], are not convincing evidence for an age progression from east two west along the ridge. It is possible that the recorded small shocks are associated with submarine volcanic activity, but this volcanism would be located between Truk and the DSDP site contradicting the "hot spot" model [27]. Recent VEMA seismic profiler data show some evidence of deformation of sediments in the vicinity of the earthquake activity. However, we are not certain whether the observed deformation is still occurring or is

extinct. We are left with ambiguous interpretations for the earthquake activity on the Caroline Islands ridge which are not resolvable with the available marine geophysical data or island geology. Until the tectonic processes reflected by the seismicity are understood, our preferred interpretation is that the Caroline-Pacific plate boundary coincides with obliquely extensional Sorol Trough, the compressional Mussau Trench, and the zone of unusual deformation striking N35°W that links these features. In summary, we conclude from a study of seismologic and marine geophysical data from the Caroline Sea region that a separate Caroline plate currently exists. Interaction with the Philippine plate occurs along the southern Yap Trench, the Palau Trench, and a rift system in the Ayu Trough. This plate boundary links a Pacific-Caroline-Philippine triple junction at the Yap Trench-Sorol Trough intersection with the New Guinea "tectonic zone" in Vogelkop peninsula. The Caroline plate is in motion with the Indian plate along northern New Guinea and may interact with smaller plates in the Bismarck Sea region. The Pacific-Caroline plate boundary comprises the Sorol Trough on the north which shows strike-slip and extensional characteristics, the Mussau Trench on the southeast at which Caroline plate is being subducted, and the unusual zone of dominantly overthrusting which links these two sections. Our conclusion is also consistent with a previous suggestion (e.g. [5]) that the present Caroline plate is a "transient" feature resulting resulting from an ongoing relocation of the Pacific-Philippine plate boundary from the west to the east.

Acknowledgements We thank Dennis Hayes, Brian Taylor, and Tony Watts for their helpful criticism and suggestions. This study was supported through Office of Naval Research contract N00014-75-C-0210 and National Science Foundation grants OCE 75-16027 and OCE 76-22034.

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