Mann, P., Coffin, M., Shipley, T., Cowley, S., Phinney, E., Teagan, A., Suyehiro, K., Takahashi, N., Araki, E., and Miura, S., 1996, Researchers investigate fate of oceanic plateaus at subduction zones: EOS, Transactions of the American Geophysical Union, v. 77, no. 30, July 23, 1996, p. 282-3.

Copyright 1996 American Geophysical Union. Further electronic distribution is not allowed.

Researchers investigate fate of oceanic plateaus at subduction zones

During a 32 day marine geophysical expedition on board the RV Maurice Ewing, scientists investigated the fate of the Ontong Java oceanic plateau (OJP) at the Solomon island arc (Fig. 1). U.S. and Japanese researchers acquired 4050 km of multichannel seismic data in transects that include the Early Cretaceous-Tertiary OJP, the Tertiary Solomon arc, Neogene oceanic crust of the Woodlark basin, and continental or oceanic plateau crust of unknown age beneath the Louisiade Plateau. A 8510 in³, twenty airgun array and a ~3 km long seismic streamer comprised the acquisition system. In addition, a 500 km long coincident, wide-angle seismic line constrained by 17 ocean bottom seismometers (OBS) was acquired along a central transect that spans all geologic provinces.

The Solomon Islands were selected to examine the question of whether oceanic plateaus subduct, obduct, or partially obduct at subduction zones. The OJP is the largest (1.86 X 106 km²), thickest (25-43 km crust) oceanic plateau on Earth and one of the few Pacific oceanic plateaus actively converging on an island arc. Obducted remnants of the OJP have been proposed to occur in a Neogene accretionary prism, the Malaita anticlinorium, that separates the Solomon Island volcanic arc from the OJP (Fig. 1). The dominantly basaltic, pre-Middle Eocene section of the Malaita anticlinorium shows striking similarities in age, lithology and geochemical character to basalts drilled on the OJP. Previous workers have suggested that collision between the OJP and the ENE-facing Solomon arc occurred about 10 m.y. ago at the North Solomon trench and produced the 50 to 120 km-wide Malaita anticlinorium. The inability of the Solomon arc to consume the OJP resulted in backthrusting and subduction polarity reversal along the length of the Solomon arc with subduction thought to have initiated in the past 10 m.y. along the San Cristobal trench. Similar scenarios of eastern Pacific oceanic plateau obduction are thought to have preserved the Wrangellia terrane of the western Cordillera and smaller, dominantly basaltic terranes of the circum-Caribbean.

Earthquakes and new seismic reflection data suggest continued subduction at both the North Solomon and San Cristobal trenches. On line Y-Y', roughly parallel to our central OBS line, alignment of deep hypocenters suggests that the the OJP, or oceanic crust that formerly flanked the OJP, subducts to a depth of about 500 km (Fig. 2B). Curiously, deep intermediate and deep earthquakes are less apparent on line X-X' (Fig. 2A). Localized convergence between the OJP and oceanic plateau or continental crust of the Louisiade Plateau may result in the larger and more intense seismic activity on line Z-Z' along with the narrow width, greater topography, deeper erosional level, and more rapid Quaternary uplift of the southwestern Solomon arc. This style of closure may be proceeding in a zipper-like fashion from southeast to northwest along the length of the arc. Folds within the Malaita anticlinorium become more open and less numerous to the northwest and support the idea of OJP collision beginning in the southeast and progressing to the northwest.

Preliminary, single trace plots from this cruise also support the idea of an actively subducting OJP at the North Solomon trench (Fig. 2D). Thrust faulting and offscraping of a post-Middle Eocene pelagic carbonate section at the toe of slope indicates convergence is ongoing and did not end 10 m.y. as assumed in previous studies. The Malaita "anticlinorium", or accretionary prism, continues to form by accretion of shallow Tertiary pelagic sediment deposited on the OJP. The Malaita prism is unusual compared to other actively forming prisms in that it is composed almost entirely of pelagic carbonate deposited in an intraoceanic setting on the incoming OJP. Details of the deformational features of the prism along with answers to more complex questions - such as how did crystalline basement of the OJP become obducted to form the cores of large folds in the prism - will require detailed processing and integration of the MCS data at the University of Texas Institute for Geophysics (UTIG) and the OBS data at the Ocean Research Institute of the University of Tokyo (ORI) and Chiba University. We plan to organize a special session on the tectonics and geochemistry of the Solomon Islands and Ontong Java Plateau at the Fall AGU meeting.

A cruise report for EW95-11 is available through Mann (email: paulm@ig.utexas.edu).