Thomas W.C. Hilde

Origin and evolution of the West Philippine Basin: a new interpretation

Abstract Combined analyses of the magnetic lineations and seafloor structures of the West Philippine Basin show that it was formed by spreading from the Central Basin Spreading Center in two distinctly different spreading phases, before and after subduction was initiated along the Palau-Kyushu trend at ~ 45 Ma B.P. These episodes are recognized by differences in the strike of the magnetic lineations, spreading rates, density and strike of fracture zones, and basement relief. From 60-45 Ma B.P. spreading was NE-SW, relative to present orientations, at a half-rate of 44 mm/a. At about 45 Ma B.P. the spreading direction changed to a more N-S direction with a reconfiguration of the Central Basin Spreading Center into numerous, short E-W segments offset by closely spaced N-S transform faults. Spreading slowed to a half-rate of 18 mm/a, and ceased at 35 Ma B.P. Thus, the West Philippine Basin originated at 45 Ma B.P. by the trapping of normal ocean crust west of the initial subduction along the Palau-Kyushu trend. The 45-35 Ma B.P. period represents the dying phase of spreading on the Central Basin Spreading Center following isolation of the West Philippine Basin from the plate driving forces of the Pacific.

OKINAWA TROUGH: ORIGIN OF A BACK-ARC BASIN*

Abstract The sequence of back-arc basin development in the southwestern part of Okinawa Trough has apparently been crustal thinning, involving normal faulting and fault block rotation, rifting, and crustal separation with associated magmatic intrusion. Seismic-refraction data shows a mantle depth of about 15 km beneath the central rift of Okinawa Trough while gravity data suggests that the crust thickens away from the central region of Okinawa Trough towards both the Ryukyu Arc and the continental shelf. Seismic-reflection profiles across Okinawa Trough reveal a generally continuous, undisturbed upper sediment section beneath the trough floor, unconformably overlying a highly faulted lower section which has a velocity of 4.9 km/sec. Exceptions to this structural relationship are fault grabens cutting the upper section along the axial and southeastern regions of the trough and normal faults at the extreme margins of the trough. Correlation of similar structural relationships on Taiwan suggests that extension produced normal faults in the lower section during Miocene time creating the initial expression of Okinawa Trough, and that the unconformity between the upper and lower sections is of Miocene age. Linear magnetic anomalies and high-velocity crustal rocks associated with the central trough region are interpreted as due to Pliocene to Recent magmatic intrusion associated with crustal separation. Offsets in the magnetic lineations indicate that the crust is separating along a series of short spreading centers and transform offsets.

Sediment subduction versus accretion around the pacific

Abstract Subducting oceanic plates are typically broken by normal faults as they bend downward into subduction zones, usually forming regular patterns of grabens. The faults strike parallel or subparallel to the trench axes and are most commonly 5–10 km in spacing and width. Rupture occurs initially near the outer topographic high and vertical displacement or graben depth increases as the plate descends, the 400 m or more at many trench axes. It is suggested that the grabens provide void spaces within the surface of the subducting plate, below the plane of subduction, into which the trench sediments are tectonically displaced and thus subducted. Around the Pacific, the only regions of apparent fore-arc sediment accretion are where the graben structures are missing or masked by thick sediment deposits. Even in these cases sediment subduction, by inclusion in subducting plate grabens or by other mechanisms, must be invoked to explain the relatively small fore-arc sediment volumes compared to calculated accretion volumes based on historical convergence. Where trench sediment volumes are small compared to the graben volumes the grabens may abrade the leading edge and underside of the overriding plate and subduct the eroded material. It is concluded that sediment subduction is dominant around the Circum-Pacific and that the bending-induced graben structures of the subducting plates are a major factor for sediment subduction and tectonic erosion.

Pre-Cretaceous tectonic evolution of the Pacific plate and extension of the geomagnetic polarity reversal time scale with implications for the origin of the Jurassic “Quiet Zone”☆

Abstract Linear magnetic anomalies resulting from seafloor spreading were mapped in the vicinity of the magnetic bight in the western Pacific Ocean. New aeromagnetic data allowed the magnetic bight to be more accurately mapped from M21 to M28 and enabled the identification of low-amplitude magnetic lineations in the Jurassic “Quiet Zone”. These lineations were formed by magnetic field reversals prior to M29. A revised Jurassic geomagnetic polarity reversal time scale was constructed with nineteen reversals older than M29, numbered M30-M38. These reversals extend the record of geomagnetic polarity back in time by approximately 8 Ma and are important constraints on the origin of the quiet zone. In particular, they imply that the Jurassic was not a period of constant normal polarity, an explanation offered by some authors. Further, they cast doubt on a model of systematically decreasing geomagnetic field strength with increasing age during this period. The early history of the northern Pacific plate and Pacific-Farallon-Izanagi (P-F-I) triple junction was traced by mapped magnetic isochrons. The Pacific plate seems to have evolved from a small plate that formed near the Phoenix-Farallon-Izanagi triple junction about 180–188 m.y. ago at approximately 17°N, 160°E in present coordinates. Until M21 time the evolution of the northern Pacific plate was relatively simple and the P-F-I triple junction migrated north-northwest with respect to the Pacific.

The subducted Rivera‐Cocos Plate Boundary: Where is it, what is it, and what is its relationship to the Colima Rift?

Gravity data, the geometry of the Wadati-Benioff zone beneath western Mexico, and the seafloor morphology of the Rivera-Cocos plate boundary west of the Middle America trench suggest that the subducted part of this boundary lies directly beneath and is oriented parallel to the Southern Colima rift. Thus, the Southern Colima rift likely formed in response to divergence between the subducting Rivera and Cocos plates due to direct coupling between these two plates and the overriding North American plate. In contrast, the subducted plate boundary lies east of and oblique to the Northern Colima and Central Colima grabens. East of the Central Colima graben a low density zone overlies the boundary and underlies surface exposures of Cretaceous granitoids and associated thermal springs and shallow focus earthquakes; characteristics that are explained by thermal convection induced in the upper mantle by divergence between the subducted Rivera and Cocos plates. These characteristics along with the adjacent locations of the low density upper mantle and the Central Colima graben are consistent with crustal extension produced by the uniform-sense normal simple shear mechanism.

Tectonic evolution of the northern Pacific plate and Pacific-Farallon Izanagi triple junction in the Late Jurassic and Early Cretaceous (M21-M10)

Abstract Magnetic lineations in the northwestern Pacific, near the Shatsky Plateau, were interpreted to trace the tectonic history of the northern Pacific plate from the time of anomalies M21 to M10. At M22 time the Pacific-Farallon-Izanagi (P-F-I) triple junction was located southwest of the plateau and migrating north-northwest with respect to the Pacific plate. Shortly thereafter the stress field acting on the three plates changed significantly and the Pacific-lzanagi ridge reacted by pivoting 24° clockwise from M21 to M19 time. Several ridge propagation events occurred while the ridge adjusted to the new spreading regime. To the east of the M22 location of the junction it appears that this reorganization triggered a series of at least three ridge jumps along the Pacific-Farallon ridge. The geometry of magnetic isochron M21 suggests that several triple junctions existed simultaneously during the reorganization bordering one or more microplates. Differences in the spreading rate of the Japanese magnetic lineations north of the plateau imply that one microplate, which we call the Shatsky microplate, existed until about M15-M13 time. It was probably captured by either the Farallon or the Izanagi plate and subsequently destroyed by subduction. After M15 the P-F-I triple junction was reformed on seafloor that is now beneath the Shatsky Plateau. The junction drifted generally northeast, either in a ridge-ridge-ridge or in a ridge-fault-fault configuration, perhaps alternating between the two. From M11-M10 time the stress field on the three plates changed again and the Pacific-Farallon ridge southeast of the plateau reoriented 15° clockwise. At this time the P-F-I junction assumed a ridge-ridge-ridge geometry at the northeast end of the plateau.

Geodynamics of the Western Pacific-Indonesian region

The contributions to this volume have been organized into two categories: General Studies, which deal with the dynamic processes, and the systematic geological and geophysical relationships of importance to the region as a whole; and Regional Studies, which focus on specific areas and features of the Western Pacific. The composition of the volume reflects the research efforts under taken in the Western Pacific within the framework of the Geodynamics Project, by the countries of the region and by other countries concerned with the geodynamics of the Western Pacific. The new understanding of the dynamics and development of the trench-arc-backarc systems, and the dynamic relationship of the Western Pacific basin lithosphere and ancient continental margins with trench-arc-backarc development, brought about through the Geodynamics Project, is a major advancement in the geosciences. Much of these new findings is contained in this volume. The editors wish to thank all of the authors for their outstanding contributions, and for their patience with the editing process and assembly of the publication.

Structural features of the bonin arc: Implications for its tectonic history☆

Abstract The Bonin Arc, from the Shikoku Basin to the trench axis, is broken into four crustal blocks, each separated by major right-lateral strike-slip faults. From north to south, they are termed the Sumisu Jima, the Sofu Gan and the Central Bonin Dislocations. These dislocations are delineated by 1. (1) changes in trends of en-echelon ridge and trough structures of the main arc. 2. (2) right-lateral offsets of the bathymetry and structure of the arcs outer ridge. 3. (3) right-lateral offsets in the free-air gravity contours. The most likely time of formation of the en-echelon structures and fracturing of the arc into the four crustal blocks is between 27 and 17 m.y. B.P. (Miocene). During this time, Japan was moving southeastward relative to Asia, and the Philippine Sea plate (including the Bonin Arc) was moving northward, subducting beneath southwest Japan. Also, convergence and subduction of the Pacific Plate was taking place along the east side of the Bonin Arc, which involved the collision of thick buoyant crustal blocks of the Marcus Necker Ridge near the south end of the Arc. These movements resulted in compressive forces acting on the Bonin Arc from the northwest and produced a clockwise torque within the arc. The response to these forces included development of NE-NNE structural trends within the arc and fracturing of the arc into four crustal blocks, offset from each other by NE-NNE right lateral displacements.

Kuril (South Okhotsk) Backarc Basin

The Kuril backarc basin is a 3300-m-deep basin located behind the Kuril island arc and subduction zone. It is underlain by oceanic crust, the top of which is 7–8 km deep, and it contains 4 to 5+ km of technically undisturbed sedimentary fill. The oldest sedimentary rocks sampled from the basin margin are Miocene/Oligocene. Although no seafloor spreading magnetic lineations exist, we conclude that the basin has formed by backarc extension and spreading. Elongated in the NE-SW direction, parallel to the Kuril arc, the basin is ~250 km wide at its SW end, narrowing to closure at its NE termination at the southern tip of Kamchatka. Opening of the basin was apparently related to a relative motion pole at or near the south end of Kamchatka, consistent with Miocene/Oligocene lateral fault trends between Sakhalin and Hokkaido and the strike of transverse basement ridges extending across the basin. The age of the basin may be as young as Miocene or as old as late Cretaceous. Ocean drilling is needed to establish its precise age.

Convergence structures of the Peru trench between 10°S and 14°S☆

Abstract High resolution seafloor studies of the Peru Trench between 10°S and 14°S with the GLORIA long-range side-scan sonar system show that the Nazca plate is broken by numerous normal faults as it bends into the trench. These bending-induced faults strike subparallel to the trench axis and overprint and cut across spreading fabric structures of the plate. They commonly form grabens having widths and spacings of 3–5 km and extend for as much as 100 km along strike. Vertical displacements are generally 200 m or more by the time they reach the trench axis. Turbidite deposits are found in the trench north of 11.5°S. Both turbidite and pelagic sediments are folded and temporarily accreted to the base of the overriding plate along the length of the trench axis. They are apparently subsequently implaced in the grabens by slumping and subducted with the Nazca plate. The Mendana Fracture Zone, which intersects the trench between 9°40′S and 10°35′S, appears to be the locus of a seaward propagating rift that is forming in response to subduction-induced extensional stresses in the Nazca plate.