Donald L. Reed

East Asia plate tectonics since 15 Ma: constraints from the Taiwan region

Abstract 15 Ma ago, a major plate reorganization occurred in East Asia. Seafloor spreading ceased in the South China Sea, Japan Sea, Taiwan Sea, Sulu Sea, and Shikoku and Parece Vela basins. Simultaneously, shear motions also ceased along the Taiwan–Sinzi zone, the Gagua ridge and the Luzon–Ryukyu transform plate boundary. The complex system of thirteen plates suddenly evolved in a simple three-plate system (EU, PH and PA). Beneath the Manila accretionary prism and in the Huatung basin, we have determined magnetic lineation patterns as well as spreading rates deduced from the identification of magnetic lineations. These two patterns are rotated by 15°. They were formed by seafloor spreading before 15 Ma and belonged to the same ocean named the Taiwan Sea. Half-spreading rate in the Taiwan Sea was 2 cm/year from chron 23 to 20 (51 to 43 Ma) and 1 cm/year from chron 20 (43 Ma) to 5b (15 Ma). Five-plate kinematic reconstructions spanning from 15 Ma to Present show implications concerning the geodynamic evolution of East Asia. Amongst them, the 1000-km-long linear Gagua ridge was a major plate boundary which accommodated the northwestward shear motion of the PH Sea plate; the formation of Taiwan was driven by two simple lithospheric motions: (i) the subduction of the PH Sea plate beneath Eurasia with a relative westward motion of the western end (A) of the Ryukyu subduction zone; (ii) the subduction of Eurasia beneath the Philippine Sea plate with a relative southwestward motion of the northern end (B) of the Manila subduction zone. The Luzon arc only formed south of B. The collision of the Luzon arc with Eurasia occurred between A and B. East of A, the Luzon arc probably accreted against the Ryukyu forearc.

Forearc-basin closure and arc accretion in the submarine suture zone south of Taiwan

Abstract The obliquely propagating Taiwan collision provides an active example of an intraoceanic arc being accreted to a young rifted continental margin. The actual accretion of the exotic arc is taking place immediately south of Taiwan, in a complex area of rapid uplift and shortening between the emerging crest of the submarine accretionary prism and the extinct northernmost segment of the Luzon volcanic arc. The northern part of this region accommodates over half of the convergence between the Philippine Sea and Eurasian plates, based on recent results of triangulation and Global Positioning System studies. Assuming that nearly all the plate convergence to the south, in the region of normal intraoceanic subduction, is concentrated near the active trench, as is true in most subduction zones, this region of arc accretion is a zone across which approximately 60% of the total plate convergence, amounting to about 50 mm/yr, is being actively transferred. This transfer of slip is presumably caused by the buoyant nature of continental crust that has been subducting beneath the Taiwan orogen. This arcward transfer of plate convergence has strongly affected development of the suture between the Luzon arc and the continental margin, represented by the Taiwan mountain belt. Backthrusting of the accretionary prism in this region is accommodated on east-vergent thrust faults, which locally reach the surface and deform the entire forearc sequences, thereby building the Huatung Ridge, a distinct structural and bathymetric ridge east of the main accretionary prism (the Hengchun Ridge). The Huatung Ridge dams orogenic sediment from the emergent collision in the Southern Longitudinal Trough, a suture basin that projects directly northward to the Longitudinal Valley of eastern Taiwan. Growth strata in the Southern Longitudinal Trough document progressive uplift of the Huatung Ridge to the east, apparently along east-verging thrust systems. Seismic reflection and sidescan sonar data south of about 23°N provide no evidence of back-arc thrusting along the eastern margin of the Luzon arc, as has been hypothesized in order to transfer shortening to the Philippine Sea plate. Neither do these data show clear evidence of west-vergent thrusting of the Luzon arc over adjacent elements of the forearc, in contrast to the very active thrusting documented onland to the north, along the Longitudinal Valley of eastern Taiwan. The arcward-vergent structures in the region of arc accretion have closed the North Luzon Trough, the major forearc basin. These structures have also built the Huatung Ridge as a compressional ridge of orogenic strata that serves to broaden the accretionary prism toward the arc (eastward) and, in so doing, have formed small collisional or suture basins and redirected orogenic sedimentation patterns throughout this key area. Thus the arcward flank of the collision has evolved in a much more complicated fashion than the relatively smooth progression followed by the western, frontal slope of the submarine accretionary prism as it evolves northward to the fold-and-thrust belt exposed along strike in western Taiwan. This complexity on the arcward flank of the collision zone is likely a response of the collision complex to continued plate convergence in the face of increasing resistance, to the north, to the subduction of continental crust.

Relations between mud volcanoes, thrust deformation, slope sedimentation, and gas hydrate, offshore north Panama

Abstract SeaMARC II swath-mapping and migrated seismic reflection data show a high concentration of mud volcanoes in the primary sediment depocentre along the lower slope of a thrust belt, offshore north Panama. The mud volcanoes are 0.4–2.0 km wide, Sonar reflectivity (backscattering), sediment cores, and seismic stratigraphic relations indicate that the depocentre contains thick sequences of basinal turbidites which are ponded between the anticlinal ridges. The ridges are composed of the deformed turbidites of the Colombian basin and exhibit a strong bottom-simulating reflector (BSR), apparently associated with a gas hydrate layer. Based on the concentration of mud volcanoes along the crests of the anticlinal ridges in the depocentre and the structural position of the BSR, we suggest that folding along the deformation front, sediment ponding leading to differential loading, methane migration and accumulation in the anticlines, and gas hydrate formation are important factors in the development of mud volcanoes in this region.

STRUCTURAL INSIGHT INTO THE SOUTH RYUKYU MARGIN : EFFECTS OF THE SUBDUCTING GAGUA RIDGE

Abstract This study presents three multi-channel deep seismic reflection profiles located in the south Ryukyu margin between 122°30′E and 123°30′E, where a N-S-trending oceanic ridge, the Gagua Ridge, is entering the subduction zone, for the purpose of examining the effects of ridge subduction on structures of the forearc region. Structural features which correspond to different stages of the oblique ridge subduction are observed. East of 123°E, a short-lived sequence of indentation, tunneling, then resumption of frontal accretion occurred in the accretionary wedge (the Yaeyama Ridge) as the subducted portion of the Gagua Ridge swept the overriding Ryukyu margin from below along the northwesterly convergent direction. Under the forearc basin, the subducted portion of the Gagua Ridge is uplifting the arc basement to form the Nanao Basement Rise which separates the sedimentary strata of the Nanao and East Nanao forearc basins. Results from this study suggest that the oblique subduction of the Gagua Ridge has not only affected accretionary wedge structures but also the arc basement of the south Ryukyu margin.

Contourite sedimentation in an intraoceanic forearc system: eastern Sunda Arc, Indonesia

Abstract Combined analyses of bathymetry, piston cores, 3.5-kHz echograms, and seismic reflection data reveal that sedimentation patterns in the eastern Sunda forearc are strongly influenced by vigorous deep- and bottom-water circulation. The affected region is located at the intersection of the Sumba Ridge and the Sawu-Timor Ridge, which together form a barrier to the outflow of Pacific Ocean Deep Water from the Sawu Sea to the eastern Indian Ocean. Bottom currents associated with the outflow have eroded a gap in the sill at a water depth of 1150 m, between the islands of Sumba and Sawu. Southwest of the gap, the widespread exposure of well-consolidated, middle Miocene to Pliocene foraminiferal chalks and oozes along the Sumba Ridge suggests that up to 1 km of overburden has been removed by the deep (1–1.5 km) currents. The eroded sediments have been subsequently deposited as muddy contourites in a thick ( > 1 km ) sediment drift in the adjacent Sumba Basin. The drift consists of an elongated mound composed of reworked calcareous ooze and is bounded by moat-like channels. The influence of contour currents on trench-slope sedimentation can be significant and should be considered during studies of modern forearc systems and ancient subduction complexes on land.

Strike-slip faults offshore southern Taiwan: implications for the oblique arc-continent collision processes

Taiwan is the site of present-day oblique arc-continent collision between the Luzon arc of the Philippine Sea plate and the Chinese continental margin. The major structural pattern revealed from marine geophysical studies in the area offshore southern Taiwan is that of a doubly-vergent orogenic belt, bounded by significant zones of thrusting on the west and east of the submarine accretionary wedge. Due to the oblique collision process, strike-slip faults could play an important role in this convergent domain. Topographic lineaments revealed from new digital bathymetry data and seismic reflection profiles confirm the existence of three sets of strike-slip faults in the collision-subduction zone offshore southern Taiwan: the N-S-trending left-lateral strike-slip faults within the Luzon volcanic arc, the NE-SW-trending right-lateral strike-slip faults across the accretionary wedge, and the NNE-SSW-trending left-lateral strike-slip faults lie in the frontal portion of the accretionary wedge. These strike-slip faults overprint pre-existing folds and thrusts and may convert into oblique thrusts or thrusts as the forearc blocks accrete to the mountain belt. A bookshelf rotation model is used to explain the observed geometrical relationships of these strike-slip fault systems. Based on this model, the counter-clockwise rotation of the forearc blocks in the area offshore southern Taiwan could have caused extrusion of the accretionary wedge material into the forearc basin. The originally continuous forearc basin is thus deformed into several closed and separate proto-collisional basins such as the Southern Longitudinal Trough and Taitung Trough. A tectonic evolution model which emphasizes on the development of various structures at different stages of the oblique arc-continent collision for the Taiwan mountain belt is proposed.

Evolution of shallow, crustal thermal structure from subduction to collision: An example from Taiwan

We study crustal thermal evolution by examining heat flow patterns along a convergent boundary from a young subduction zone to a more structurally mature collision zone. More than 8000 km of seismic profiles covering an offshore region of 45,000 km 2 in southern Taiwan show widespread bottom-simulating reflectors (BSRs). We derived 1107 BSR-based heat flows before combining 42 additional, published, offshore thermal probe data and 86 on-land heat flow data to document the shallow forearc thermal structures from the subduction zone to the collision zone. In the subduction zone, the geothermal gradient ranges mostly within 30–80 °C/km, and decreases toward the arc due to slab cooling, intensive dewatering at the toe, sediment blanketing, topographic effects, and other processes. The geothermal gradient ranges mostly from 30 to 90 °C/km in the collision zone, and increases, instead of decreases, toward the arc, possibly caused by exhumation, erosion, topographically induced ground-water circulation, and some upper mantle processes related to collision. Heat flow in the collision zone ranges from 80 to 250 mW/m 2 . The high heat flow in the collision zone correlates with a shallower seismicity zone and high seismic attenuation, while the lower heat flow in the subduction zone might allow the earthquakes to rupture to greater depth. The heat flow increases along the topographic high from subduction to collision zone due to increasing geothermal gradients and higher thermal conductivities of the exhumed basement rocks. This heat flow variation may generate an artificial exhumation rate pattern, if a conventional 30 °C/km geothermal gradient was used in fission-track studies.

Deformation and sedimentation along a developing terrane suture: Eastern Sunda forearc, Indonesia

The collision of the eastern Sunda arc with northwest Australia has resulted in the development of a suture between the Sumba ridge and Sawu-Timor terranes along a zone of intraforearc convergence. The developing suture varies from the low-angle Sawu thrust, with attendant mud diapirs in the Sumba basin, to high-angle reverse faults near a basement high of the underthrust Sumba ridge terrane. Bottom currents, associated with the flow of Pacific Ocean deep water into the Indian Ocean, have eroded the terranes and subsequently deposited the detritus in an assemblage of contourites along the suture. This study reveals the high structural variability of a terrane suture and the oceanographic influence on the deposition of overlap assemblages.

Deep structure and structural inversion along the central California Continental Margin from edge seismic profile RU‐3

Deep-penetration seismic reflection profile RU-3 crosses the central California continental margin revealing a subducted oceanic plate, a modified accretionary prism, and complex structures of the overlying sedimentary basins. This structural framework was established by subduction processes during Paleogene and earlier time and subsequently was modified by Neogene transform motion combined with apparent components of extension and compression. Subducted rocks are indicated by deep, gently dipping reflectors that extend beneath the continental margin for at least 38 km at a depth of about 15 km. We interpret the subducted crust as either a part of the Pacific plate or, more likely, a subducted fragment derived from the Farallon plate. A set of more steeply dipping, deep events may indicate faulting within the subducted plate or its boundary with a no-slab zone. The overlying, largely nonreflective layer of accreted material rapidly reaches 10 km in thickness landward of the paleotrench and increases to 15 km in thickness near the coast. The Santa Lucia Basin, landward of the steep continental slope, originated as a slope basin during Paleogene subduction. The lower strata of this basin were deposited onto and partially incorporated into the accretionary complex. The offshore Santa Maria Basin exhibits a variety of compressional structures that formed in the last 3–5 m.y. and whose locations correspond to an earlier framework of extensional faults. Structural inversion has occurred in Miocene depocenters adjacent to the Santa Lucia Bank fault and at the Queenie structure. Miocene and lower Pliocene strata also thicken toward the Hosgri fault zone where subsequent compression is characterized by low-angle thrusts and folding.

Large-scale duplexes within the New Britain Accretionary Wedge: A possible example of accreted ophiolitic slivers

A multichannel seismic line acquired in the western Solomon Sea images reflectors within the New Britain accretionary wedge which we interpret as accreted duplexes of the downgoing oceanic plate. The seaward edge of the largest duplex is located just 6 km from the toe of the accretionary wedge suggesting recent incorporation into the wedge. These data therefore provide an excellent opportunity to determine the dynamics of oceanic basement accretion. The oceanic basement is represented by a high-amplitude, low-frequency reflector that can be traced up to 40 km arcward of the toe of the accretionary wedge on several seismic lines in this survey. Another prominent reflector, similar in character to the basement reflector, lies approximately one second higher in at least one seismic section. This reflector is likely to be the top of a basement duplex. Structures imaged above the basement duplex define a preduplex accretionary wedge, whereas those at the toe indicate a new episode of postduplex accretion. We have tested this interpretation using both magnetic modeling and interval velocity analysis. Magnetic models of the wedge that include basement duplexes provide a good match to the observed magnetic data and provide limits on the size and interval velocities of the slivers. Interval velocities between the two strong reflectors, calculated using stacking velocities and constrained through magnetic modeling, exceed 5000 m/s and are consistent with our interpretation of ophiolitic slivers within the accretionary wedge.