Arc-continent Collision Zone Research Articles

Retro‐Foredeep Basin Evolution in Taiwan: Zircon U‐Pb and Hf Isotope Constraints From the Coastal Range

AbstractThe evolution of the Taiwan orogen, an active arc–continent collision zone between the North Luzon Arc (NLA) and the Eurasian continent, is recorded in arc volcanic and sedimentary rocks from the Taiyuan retro‐foredeep basin of the Coastal Range, eastern Taiwan. Arc volcanism in the northern NLA experienced a magmatic culmination at 9–7 Ma and a cessation at 5.3–4.3 Ma, based on new U‐Pb zircon ages. Young zircon U‐Pb ages (1.74, 1.58, and <1.2 Ma) and low Hf isotope signatures also indicate that three biotite‐bearing lapilli tuff layers deposited in the retro‐foredeep basin are associated with eruptions of the Lutao volcano in the southern NLA. NLA volcanism terminated progressively from ∼5.3 to <1.2 Ma in response to a southward‐propagating collision zone that followed the uplift of the orogenic wedge by <3 myr. The detrital zircon U‐Pb age spectra suggest no significant shift in provenance during the Plio‐Pleistocene period and indicate that the synorogenic sediments in the Taiyuan Basin were mainly derived from the Yuli Belt, a Miocene accretionary complex in the eastern Taiwan orogen. The accumulation rate of synorogenic deposits varies from 0.53 to 5.14 km/myr, with an abrupt increase at ∼2 Ma. This new rate is higher than the previously proposed rates and that of the pro‐foreland basin to the west, suggesting asymmetric and rapid uplift of the Taiwan orogen.

Dynamic processes of upper plate deformation at the New Guinea Trench

The interactions between arc-continent collision and subduction zones are still poorly understood. Here, we use 2D seismic data to document upper plate deformational features and identify different structural styles along the New Guinea Trench, western Pacific, and discuss their broader significance. In the western section, normal faults and negative flower structures occur within the strata of the upper plate. An unconformity represents the signal of tectonically driven subsidence. In the eastern section, both thick-skinned and thin-skinned thrusting are found within the frontal upper plate. In the central section, rare extensional or shortening deformation is identified within the forearc basement and overlying strata. Horizontal, thick sedimentary sequences within the Caroline subbasin have been subducted beneath the upper plate. This suggests that Miocene subduction in the eastern section led to the formation of a thrust-deformed ophiolitic basement in the upper plate. Since the Miocene, strike-slip tectonic processes dominated in the upper plate, causing compressional deformation in the strata in the eastern section and extensional to transtensional deformation in the strata in the western section. The strike-slip tectonic processes might have been weakened in the upper plate in central section due to the incoming Eauripik Rise. Over geologic time, arc-continent collision processes and related changing of regional tectonic forces can have a long-lived influence on shaping and deforming subduction systems.

Geochemical classification of Palaeoproterozoic oceanic volcanic front and backarc basin remnants of the Granites-Tanami Orogen, Australia

The Granites-Tanami Orogen (GTO) is a lower-greenschist to amphibolite facies succession of >1840 Ma backarc basalts and volcaniclastic-sedimentary rocks that was metamorphosed (~1850–1840 Ma), and later intruded by granites and granodiorites (~1825–1791 Ma) during assembly of the Kimberley Craton, proto-Northern Australia Craton, and the Arunta Region.The data presented here show geochronological, sedimentological, petrographic, and geochemical evidence for the existence of a volcanic arc (‘Tanami Arc’) in the GTO at ~1912 Ma. Furthermore, the combination of volcanic structures, including fiamme and hyaloclastites, and a mafic HFSE geochemical signature of the pyroclastic and volcaniclastic-sedimentary turbidite successions suggests a syndepositional volcanic front within an oceanic arc-backarc system during deposition of the Dead Bullock Formation (and equivalents). Geochemical vectors applied to existing stratigraphic interpretations indicate a trend towards more felsic composition with younging, providing an alternative proxy for relative stratigraphy and a tool for exploration and correlation across the GTO. The localisation of the volcanic arc within the N-NNE dipping arc-continent collision zone has important implications for the debated Palaeoproterozoic amalgamation of the North Australia Craton.

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

AbstractContinents grow mainly through magmatism, relamination, accretionary prism development, sediment underplating, tectonic accretion of seamounts, oceanic plateaus and oceanic lithosphere, and collisions of island arcs at convergent margins. The modern Pacific–Rim subduction zone environments present a natural laboratory to examine the nature of these processes. The papers in this special issue focus on the: (1) modern and ancient accretionary margins of Japan; (2) arc–continent collision zone in the Taiwan orogenic belt; (3) accreting versus non-accreting convergent margins of the Americas; and (4) several examples of ancient convergent margins of East Asia. Subduction erosion and sediment underplating are important processes, affecting the melt evolution of arc magmas by giving them special crustal isotopic characteristics. Oblique arc–continent collisions cause strong deformation partitioning that results in orogen-parallel extension, crustal exhumation and wrench faulting in the hinterland, and thrust faulting–folding in the foreland. Trench-parallel widths of subducting slabs exert major control on slab geometries, the degree of coupling–decoupling between the lower and upper plates, and subduction velocity partitioning. An initially large width of the subducting Palaeo-Pacific Plate against East Asia caused flat subduction and resistance to slab rollback during the Triassic Period. These conditions resulted in shortening across SE China. Foundering and delamination of the flat slab during the Early Jurassic Epoch led to slab segmentation and reduced slab widths, followed by slab steepening and rollback. This pull-away tectonics induced lithospheric extension and magmatism in SE China during Late Jurassic – Cretaceous time. Melting of subducted carbonaceous sediments commonly produces networks of silicate veins in CLM that may subsequently undergo partial melting, producing ultrapotassic magmas.

Early extensional detachments in a contractional orogen: coherent, map-scale, submarine slides (mass transport complexes) on the outer slope of an Ediacaran collisional foredeep, eastern Kaoko belt, Namibia

The existence of coherent, large-scale, submarine landslides on modern continental margins implies that their apparent rarity in ancient orogenic belts is due to non-recognition. Two map-scale, coherent, pre-orogenic, normal-sense detachment structures of Ediacaran age are present in the Kaoko belt, a well-exposed arc–continent collision zone in northwestern Namibia. The structures occur within the Otavi Group, a Neoproterozoic carbonate shelf succession. They are brittle structures, evident only through stratigraphic omissions of 400 m or more, that ramp down to the west with overall ramp angles of 1.1° and 1.3° with respect to stratigraphic horizons. The separations of matching footwall and hangingwall stratigraphic cut-offs require horizontal translations >20 km for each detachment. One of the detachments is remarkably narrow (5 km) in the up-dip direction, just one fourth of its translation. The other detachment is stratigraphically dated at the shelf–foredeep transition, when the passive margin was abortively subducted westward, in the direction of submarine sliding. Trenchward sliding on the foreslope occurred concurrently with deep karstification of the autochthonous carbonate succession to the east, presumably due to forebulge uplift and (or) conjectural basin-scale base-level fall. We expect that similar detachments exist in other orogenic belts, and failure to recognize them can lead to misinterpretations of stratigraphy, sedimentary facies, and paleogeography.

Anatomy of the Alto Alegre gneiss dome, São Francisco Craton, Brazil: A geological record of transpression along a Palaeoproterozoic arc-continent collision zone

Gneiss domes are crustal-scale structures that can be formed in different tectonic settings. In recent collisional orogens they occur along terrane boundaries either within, or on the edge of deformation belts. The Alto Alegre gneiss dome was recognized in the northern São Francisco Craton along the limit of Archaean-Palaeoproterozoic terranes. It consists of a large N-S-elongated elliptic structure that is composed of migmatite-gneisses and concentric, metre-sized amphibolite sheets. In its central zone, the structure is intruded by irregular pink granite stocks. Geochemical data show that the amphibolites are similar in composition to mafic dykes that crosscut the Archaean Santa Luz high-grade Complex (paleocontinent) and unlike basalts of the Palaeoproterozoic Rio Itapicuru greenstone belt (arc). SHRIMP U–Pb zircon geochronology reveals ages of 3012±25Ma and 3073±51Ma for gneisses and 2082±5Ma for pink granite within the Alto Alegre dome, and 3061±44Ma for Santa Luz gneiss. Field, structural, geochemical, and geochronological data support a model in which the Alto Alegre dome is part of the Santa Luz gneiss complex. The dome was formed by large-scale folding and transpression during the late stage of Palaeoproterozoic arc-continent collision.

Structural relationships along a Neoarchean arc-continent collision zone, North China craton

The Archean North China craton is composed of the Western block, Eastern block, and the intervening Central orogenic belt. A 4–10-km-wide and 85-km-long tectonic melange belt informally called the Zanhuang tectonic melange is documented in the Zanhuang Massif of the Central orogenic belt, separating the Eastern block from an Archean arc terrane in the Central orogenic belt. The melange belt contains a structurally complex tectonic mixture of metapelites, metapsammites, marbles, and quartzites mixed with exotic tectonic blocks of volcanic, mafic, and ultramafic rocks, metabasalts that locally include relict pillow structures, and tonalite-trondhjemite-granodiorite (TTG) gneisses. The Zanhuang tectonic melange marks the suture of an arc-continent collisional zone between the Western Zanhuang Massif in the Central orogenic belt and the Eastern block of the North China craton, and it is one of the best-preserved Archean tectonic melanges in the world. Here, using zircon U-Pb dating of various types of blocks from the Zanhuang melange, we show that the formation and associated deformation of the Zanhuang melange occurred in the Neoarchean (ca. 2.5 Ga). High-precision (1:20–1:200) lithostructural mapping of three key outcrops reveals details of the internal fabrics and kinematics of the melange and regional structural relationships along the arc-continent collisional zone. A synthesis of studies on the tectonic evolution of the North China craton, coupled with our new fabric and kinematic analysis of the Zanhuang melange, further constrains the initial amalgamation timing and geometry of the arc-continent collision between the Fuping arc terrane in the Central orogenic belt and the Eastern block with a northwest-dipping subduction polarity. The asymmetric structures and mixture of different blocks and matrices with folding and thrusting events in the Zanhuang melange record kinematic information that is consistent with the tectonic setting of an accretionary wedge that was thrust over the passive margin of the Eastern block by 2.5 Ga. Lithostructural mapping shows that the classic melange and fold-and-thrust structures along the Neoarchean arc-continent collisional zone are broadly similar to Phanerozoic collisional belts.

Magnetic Signatures and Curie Surface Trend Across an Arc–Continent Collision Zone: An Example from Central Philippines

Ground and aeromagnetic data are combined to characterize the onshore and offshore magnetic properties of the central Philippines, whose tectonic setting is complicated by opposing subduction zones, large-scale strike-slip faulting and arc–continent collision. The striking difference between the magnetic signatures of the islands with established continental affinity and those of the islands belonging to the island arc terrane is observed. Negative magnetic anomalies are registered over the continental terrane, while positive magnetic anomalies are observed over the Philippine Mobile Belt. Several linear features in the magnetic anomaly map coincide with the trace of the Philippine Fault and its splays. Power spectral analysis of the magnetic data reveals that the Curie depth across the central Philippines varies. The deepest point of the magnetic crust is beneath Mindoro Island at 32 km. The Curie surface shallows toward the east: the Curie surface is 21 km deep between the islands of Sibuyan and Masbate, and 18 km deep at the junction of Buruanga Peninsula and Panay Island. The shallowest Curie surface (18 km) coincides with the boundary of the arc–continent collision, signifying the obduction of mantle rocks over the continental basement. Comparison of the calculated Curie depth with recent crustal thickness models reveals the same eastwards thinning trend and range of depths. The coincidence of the magnetic boundary and the density boundary may support the existence of a compositional boundary that reflects the crust–mantle interface.

South Ilan Plain High-Resolution 3-D S-Wave Velocity from Ambient Noise Tomography

The Ilan Plain in northeastern Taiwan is located at a pivotal point where the Ryukyu trench subduction zone, the northern Taiwan crustal stretching zone, and the ongoing arc-continent collision zone converge. In contrast to the North Ilan Plain, the South Ilan Plain exhibits a thin unconsolidated sedimentary layer with depths ranging from 0 - 1 km, high on-land seismicity and significant SE movements relative to Penghu island. We deployed a dense network of 43 short-period vertical component Texan instruments from June to November 2013 in this study, covering most of the South Ilan Plain and its vicinity. We then used the ambient noise tomography method for simultaneous phase and group Rayleigh wave velocity measurements to invert a high-resolution 3-D S-wave for shallow structures (up to a depth of 2.5 km) in the South Ilan Plain. We used the fast marching method for ray tracing to deal with ray bending in an inhomogeneous medium. The resulting rays gradually bend toward high velocity zones with increasing number of iterations. The high velocity zone results are modified by more iterations and the resolutions become higher because ray crossings are proportional to ray densities for evenly distributed stations. The final results agreed well with known sedimentary basement thickness patterns. We observed nearly EW trending fast anomalies beneath the mountainous terrain abutting to the South Ilan Plain. The Chingshui location consistently exhibited a low S-wave velocity zone to a depth of 1.5 km.

Silica Geothermometry Applications in the Taiwan Orogenic Belt

The silica concentrations from 130 hot spring measurements were calculated and used to indicate the heat flow distribution in the active Taiwan orgenic belt. The heat flow spatial distribution seems to be controlled by the Taiwan arc-continent collision zone tectonic development. Two abnormally high silica heat flow regions, 160 and 190 mW m^(-2), are identified in northeastern and southeastern Taiwan, respectively. In northeastern Taiwan, the Chingshui area, the data suggests that the anomalous heat flow distribution may be controlled mainly by hot fluids and faults. The highest heat flow in southeastern Taiwan may be generated by heat advection caused by a rapidly uplifting hot crust with a decreasing mature arc-continent collision zone exhumation rate. In addition, we estimated the exhumation rate in different tectonic zones in Southern Taiwan by applying fission track ages and silica heat flow values. The exhumation rates are 1.33, 1.72-3.87, and 0.51-1.74 mm yr^(-1) in the tectonic zones around Taiwan for advanced, initial arc-continent collision and accretionary wedge deformation, respectively.

Generation of calc-alkaline andesite of the Tatun volcanic group (Taiwan) within an extensional environment by crystal fractionation

The Pliocene–Pleistocene northern Taiwan volcanic zone (NTVZ) is located within a trench-arc–back-arc basin and oblique arc–continent collision zone. Consequently the origin and tectonic setting of the andesitic rocks within the NTVZ and their relation to other circum-Pacific volcanic island-arc systems is uncertain. Rocks collected from the Tatun volcanic group (TTVG) include basaltic to andesitic rocks. The basalt is compositionally similar to within-plate continental tholeiites whereas the basaltic andesite and andesite are calc-alkaline; however, all rocks show a distinct depletion of Nb-Ta in their normalized incompatible element diagrams. The Sr-Nd isotope compositions of the TTVG rocks are very similar and have a relatively restricted range (i.e. ISr = 0.70417–0.70488; εNd(T) = +2.2 to +3.1), suggesting that they are derived directly or indirectly from the same mantle source. The basalts are likely derived by mixing between melts from the asthenosphere and a subduction-modified subcontinental lithospheric mantle (SCLM) source, whereas the basaltic andesites may be derived by partial melting of pyroxenitic lenses within the SCLM and mixing with asthenospheric melts. MELTS modelling using a starting composition equal to the most primitive basaltic andesite, shallow-pressure (i.e. ≤1 kbar), oxidizing conditions (i.e. FMQ +1), and near water saturation will produce compositions similar to the andesites observed in this study. Petrological modelling and the Sr-Nd isotope results indicate that the volcanic rocks from TTVG, including the spatially and temporally associated Kuanyinshan volcanic rocks, are derived from the same mantle source and that the andesites are the product of fractional crystallization of a parental magma similar in composition to the basaltic andesites. Furthermore, our results indicate that, in some cases, calc-alkaline andesites may be generated by crystal fractionation of mafic magmas derived in an extensional back-arc setting rather than a subduction zone setting.

Diverse Origins of Earth's Youngest Ophiolitic Rocks in the Active Banda Arc-Continent Collision Zone

Diverse Origins of Earth's Youngest Ophiolitic Rocks in the Active Banda Arc-Continent Collision Zone

Coseismic indentor-related deformation during the termination of subduction and its associated geophysical characteristics: An example from Taiwan

Collision is among the important processes for the growth of continents, and the way in which subduction becomes a collision is still an active research topic. Here, I examine the seismogenic structures of southern and central Taiwan where the subduction along the Manila Trench has terminated and given way to an arc-continent collision on land. Based on focal mechanisms and seven finite-fault slip models, coseismic tectonic extrusion is active in this region, in which the basement highs on the incoming passive margin are acting as indentors and strongly modifying the seismic moment release patterns in the collision zone. At least three magnitude 7 earthquakes have ruptured both north and south of an indentor called the Peikang high in the past hundred years. After examination, the basement highs show little global positioning system (GPS)–recorded relative motion with respect to the incoming passive margin; high Bouguer gravity anomalies associated with denser materials of the basement; and low heat flow due to less dewatering and exhumation. With regard to seismogenic structures, faster GPS relative motions, lower Bouguer gravity anomalies, and higher heat flow characterize the regions surrounding the indentors. Similar processes might be operating in other arc-continent collision zones. For other regions where there are fewer seismic instruments to monitor earthquakes, it might be helpful to combine a geological survey with gravity and other geophysical data sets to help identify such potential seismogenic structures.

Crust–mantle boundaries in the Taiwan–Luzon arc-continent collision system determined from local earthquake tomography and 1D models: Implications for the mode of subduction polarity reversal

In order to better understand the mode of subduction polarity reversal in the Taiwan–Luzon arc-continent collision zone, we mapped its crust–mantle boundaries using local earthquake tomography. By contouring surfaces of constant Vp=7.5kms−1, we identified three Moho discontinuities and the plate interface that juxtaposes Eurasian lower crust against mantle lithosphere of the Philippine Sea plate. The plate interface dips to the east under southeastern Taiwan and steepens progressively towards north until it becomes vertical at 23.7°N. From there it continues northward in a vertical orientation, until the limit of the tomographic model inhibited further mapping. For the Moho, additional depth constraints were derived from 1D models using P-wave arrivals of local earthquakes. The Mohos of the Eurasian and Philippine Sea plates are disconnected across the plate interface. Beneath southern Taiwan, the Eurasian Moho dips to the east at 50–60°, following the orientation of the plate boundary and continuous with the Benioff zone. Towards north, the Eurasian Moho steepens into subvertical, again together with the plate boundary. The Philippine Sea plate Moho exhibits a synform-like crustal root, interpreted as the base of the magmatic Luzon arc. Towards the north, this root deepens from 30 to 70km underneath the Ryukyu trench. In northernmost Taiwan, the hinge of the vertically subducting Eurasian slab steps westward out of the thrust belt, leaving the deformation front to the east inactive and giving way to the north-dipping Philippine Sea plate. A subhorizontal Moho at 30–35km depth overlies the north-dipping Philippine Sea slab and is interpreted as a newly formed Moho, established after westward rollback and delamination of the subducting Eurasian slab. In combination, these data support a model of progressive subduction polarity reversal, in which a tear within the Eurasian lithosphere propagated southwestward, deactivating the deformation front.

インドネシア非火山性外弧のオフィライト―世界最若オフィオライトの産状と岩石学的多様性―

The western Pacific region, where the Eurasia, Australia, and Pacific plates currently interact, has been recognized as an important site for constraining the origins and emplacement of ophiolites (particularly for island-arc or supra-subduction zone types), because the spatial distribution of oceanic micro-plates and numerous ophiolitic rocks along their convergent margins infers possible genetic linkages among them. Mafic-ultramafic rocks distributed in the Timor-Tanimbar island chain, eastern Indonesia may be a good example of the on-going emplacement of the marginal basin lithosphere on the continental margin in the arc-continent collision zone, and are recognized as a possible modern analogue for Mesozoic Tethyan-type ophiolites (e.g. Troodos and Oman) in the Alpine-Himalayan orogenic system. Geological occurrence suggests that the buoyant subduction of the Australian continent uplifted fragments of newly formed mantle-crust section, which extends to neighboring pre-emplaced forearc marginal basins. However, from petrological and geochemical points of view, young pillowed basalt, dolerite, and gabbroic cumulate commonly possess island-arc signatures, whereas structurally underlying peridotites are mostly fertile (lherzolitic) in composition. This suggests that the crustal section is not linked to the underlying mantle by a genetic melt-and-residua relationship, as inferred from the lack of complete succession and the presence of abundant crosscutting structures. This inconsistency leads to the emergence of two contrasting models accounting for the unusual occurrence of a fertile mantle in the forearc setting of the Timor-Tanimbar region: (1) thrust-stacked fragments of the subcontinental mantle originally exhumed in the rifting stage of Australia; (2) depth-related heterogeneities in the lithospheric part of the mantle wedge. We note that the current debates on the origins of fertile lherzolites found throughout the Tethyan sutures and western Pacific regions can be settled through a better understanding of Timor-Tanimbar peridotite masses by age-dating studies employing several radiogenic isotope systematics.

Buruanga peninsula and Antique Range: Two contrasting terranes in Northwest Panay, Philippines featuring an arc–continent collision zone

AbstractAlternating chert–clastic sequences juxtaposed with limestone blocks, which are units typical of accretionary complexes, constitute the Buruanga peninsula. New lithostratigraphic units are proposed in this study: the Unidos Formation (Jurassic chert sequence), the Saboncogon Formation (Jurassic siliceous mudstone–terrigenous mudstone and quartz‐rich sandstone), the Gibon Formation (Jurassic(?) bedded pelagic limestone), the Libertad Metamorphics (Jurassic–Cretaceous slate, phyllite, and schist) and the Buruanga Formation (Pliocene–Pleistocene reefal limestone). The first three sedimentary sequences in the Buruanga peninsula show close affinity with the ocean plate stratigraphy of the North Palawan terrane in Busuanga Island: Lower–Middle Jurassic chert sequences overlain by Middle–Upper Jurassic clastics, juxtaposed with pelagic limestone. Moreover, the JR5–JR6 (Callovian to Oxfordian) siliceous mudstone of the Saboncogon Formation in the Buruanga peninsula correlates with the JR5–JR6 siliceous mudstone of the Guinlo Formation in the Middle Busuanga Belt. These findings suggest that the Buruanga peninsula may be part of the North Palawan terrane. The rocks of the Buruanga peninsula completely differ from the Middle Miocene basaltic to andesitic pyroclastic and lava flow deposits with reefal limestone and arkosic sandstone of the Antique Range. Thus, the previously suggested boundary between the Palawan microcontinental block and the Philippine Mobile Belt in the central Philippines, which is the suture zone between the Buruanga peninsula and the Antique Range, is confirmed. This boundary is similarly considered as the collision zone between them.

Geological controls on BSR occurrences in the incipient arc-continent collision zone off southwest Taiwan

Bottom simulating reflectors (BSRs) observed on seismic sections are often considered as indicators for the existence of free gas, delineating the base of the gas hydrate stability zone. Abundant BSRs seen on seismic sections acquired off the SW coast of Taiwan indicate the likely and prevalent existence of gas hydrates in the study area. This study aims to characterize the occurrence of BSRs off SW Taiwan and to understand their relationship to topography, tectonic activity, and possible migration paths of gas-bearing fluids in this area. The tectonic setting off SW Taiwan is during the initial stage of arc-continent collision between the Luzon arc and the northeastern continental margin of the South China Sea. A series of west-vergent, imbricated folds and emergent thrusts develop in the accretionary wedge. Each fold-and-thrust sequence corresponds to an elongated submarine ridge if its crest is not buried by flat-lying sediments. By contrast, normal faulting prevails in the northeastern margin of the South China Sea. A correlation between distribution of BSRs, topography, and tectonic features can be observed. Four major occurrences of BSR types of ridge type, basin type, submarine-canyon type, and continental slope type, are recognized on the basis of the relationship of BSRs to topographic and structural features. Main characteristics of BSRs in the study area can be described as: (1) they occur mostly beneath topographic highs; (2) a discordant relationship between surfaces of the seafloor and underlying strata where BSRs are present; (3) BSRs are prevalent especially beneath the crest and flank of the upthrusting, large and inclined slope basins; and (4) in general, a series of high-amplitude dipping reflectors beneath BSRs can be found. These features indicate that gas hydrate may accumulate preferably beneath topographic ridges especially underneath four-way-dip topographic closures. This effect may exist because the buoyancy-driven, gas-bearing fluids tend to migrate upward and laterally toward structural highs and their corresponding topographic ridges. The distribution of BSRs indicates that gas hydrates occur more commonly in the accretionary wedge than in the South China continental margin. We suggest that the more widespread occurrence of gas hydrates in the accretionary wedge is due to the existence of multiple fault zones, which may help to tap more deep-seated gas-bearing fluids, in addition to the shallow biogenic gas, in this region.

Tectonic features of the incipient arc-continent collision zone of Taiwan: Implications for seismicity

Southern Taiwan and its offshore area lie in the region where the Luzon volcanic arc initially collides with the rifted China continental margin. Because of the incipient arc-continent collision, the structures vary markedly along-strike the collision zone so as the patterns of seismicity. We use new seismic reflection profiles and integrate existing data to reveal major tectonic features and potential seismogenic faults of the study area. The accretionary wedge in the incipient arc-continent zone can be divided into the lower slope, upper slope, and backthrust domains, respectively. These structural domains reflect different aspects of wedge deformation, and exhibit significant structural variations along-strike. Reflection seismic data show that the prominent seismogenic structures in the Taiwan incipient collisional wedge include: (1) frontal decollement beneath the lower-slope domain, (2) out-of-sequence thrusts bordering the lower-slope and upper-slope domains, (3) megathrust that cuts into the oceanic (?) basement beneath the upper-slope domain, and (4) the Chaochou-Hengchun faults in the onshore upper-slope domain. Thermal regime for those structures indicates that the megathrust and part of frontal decollement are seismogenic. The geometry of the frontal decollement, out-of-sequence thrusts and megathrust is analogous to those observed along the Nankai prism of Japan, so that they are possibly capable of generating great earthquakes as shown in the Nankai Trough. Beneath the lower and upper-slope domains off SW Taiwan, the seismicity is characterized by mantle earthquakes with the accretionary wedge being largely aseismic. We interpret the lack of prominent seismicity within the accreted wedge to result from excess fluid pressure that has significantly weakened the wedge materials and fault zones and therefore results in less seismicity. The predominant mantle earthquakes beneath the accretionary wedge, however, may result from water-enriched mantle materials infiltrated during previous Mesozoic subduction event and later rift events. The volatile contents may have significantly reduced the rigidity of the mantle, leading to the mantle being more susceptible for brittle deformation and hence anomalously high seismicity.

Tectonic features associated with the overriding of an accretionary wedge on top of a rifted continental margin: An example from Taiwan

Off southwest Taiwan, a west-advancing orogenic wedge has obliquely impinged on the northern continental slope of the South China Sea (SCS) margin. We analyzed a dense grid of multi-channel seismic profiles to reveal the tectonic features in this oblique collision setting. In the upper SCS slope and adjacent to the accretionary wedge, the rifted continental margin is characterized by a deep-seated, oceanward-dipping, listric and active normal fault with rotated hangingwall strata beneath the slope. To the west of this slope segment, the mid-Oligocene to Recent post-breakup sediments show prograding and aggrading shelf-margin clinoforms that cover a series of small, possibly Paleogene, rift basins. In the submarine accretionary wedge, a series of west-vergent, NNW-striking fold-and-thrust structures characterizes the lower wedge, which can be further divided into frontal and rear segments with distinct structural features. The frontal segment is characterized by four west-vergent blind thrusts with gently folded limbs. In contrast, the rear, arcward segment is characterized by west-vergent, emergent and imbricate thrusts with tilted beds truncated at the hangingwalls. Strata within slope basins are tilted arcward, with dips that increase with depth, indicating continued relative uplift along thrust planes during sedimentation. Pulsed thrust activity is further evidenced by an array of arcward-dipping unconformable surfaces with westward onlapping strata in the basins. Longitudinal sedimentary tapering of pre-orogenic sediments correlates strongly with curvature of the submarine frontal accretionary belt, suggesting that pre-orogenic sediment thickness is the major control on the geometry of frontal structures. The preexisting SCS slope that lies obliquely in front of the advancing accretionary wedge has impeded the advancing of frontal folds resulting in a successive termination of folds against and along strike of the SCS slope. The existence of the SCS slope also leads the strike of impinging folds with NNW-trend to turn more sharply to a NE-strike, parallel to strike of the SCS slope. Our analysis shows that the pre-orogenic mechanical/crustal heterogeneities and seafloor morphology exert strong controls on the thrust-belt development in the incipient Taiwan arc-continent collision zone.

Zircon U–Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China

The crustal structure of the Dabie orogen was reconstructed by a combined study of U–Pb ages, Hf and O isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith origin with respect to continental subduction and exhumation. Zircon U–Pb dating provides consistent ages of 751 ± 7 Ma for protolith crystallization, and two group ages of 213 ± 4 to 245 ± 17 Ma and 126 ± 4 to 131 ± 36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative ε Hf( t) values of − 5.1 to − 2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive ε Hf( t) values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile crust. Zircon O isotope analyses yield δ 18O values of − 3.26 to 2.79‰, indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low δ 18O rocks. North Dabie shares the same age of Neoproterozoic low δ 18O protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement. The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower.