Collision Of Seamounts Research Articles

Crustal structure of the Western Azuero Peninsula, Panama: Insights into the structure of accretionary complexes and forearc ophiolites

ABSTRACT Detailed geologic mapping (639 field stations in ~700 km2) and a ~50 km-long gravity survey (142 stations) in the western Azuero Peninsula revealed two faulted and folded slivers of oceanic crust attached to the trailing edge of the Caribbean Large Igneous Plateau (CLIP). Our new data, along with published geochronology, allowed us to reconstruct the Cretaceous forearc configuration of the trailing edge of the CLIP prior to seamount collision, ophiolite accretion, and whole-margin deformation. The ophiolite in western Azuero is composed of two tectonic slivers arranged in south-verging, imbricated thrust faults that stack a ~73 Ma pillow, flow, and picritic basalt and black chert, together with a ~ 89–93 Ma and older basalt flows and capping red chert sequences. Accretion of these slivers to form a supra-subduction zone ophiolite resulted from the middle Eocene collision and accretion of Galapagos seamounts against the trailing edge of the CLIP. Accreted seamounts are arranged in a north-verging antiformal stack duplex, and below the thrust sheets. Change in kinematics after fission of the Cocos-Nazca Plate during early Miocene times prompted the propagation of the Azuero-Sona fault zone flower structure, favouring the preservation of these slivers of oceanic crust.

Conjugate strike-slip faulting across a subduction front driven by incipient seamount subduction

Abstract The initial stages of seamount subduction and associated deformation in an overriding accretionary wedge is rarely documented. Initial subduction of Bennett Knoll seamount and faulting of the overlying strata along the Hikurangi subduction margin, New Zealand, are here studied using multibeam swath bathymetry, subbottom profiles, and regional seismic reflection lines. Our results provide new insights into the earliest stages of seamount collision at sediment-rich margins. Differential shortening along the subduction front induced by seamount subduction is initially accommodated in the accretionary wedge by conjugate strike-slip faults that straddle the buried flanks of the seamount and offset the frontal thrusts by as much as 5 km. The geometries of the strike-slip faults are controlled by the seamount’s dimensions and aspect, the obliquity of plate convergence, pore-fluid pressure, and the thickness and rheology of the incoming sedimentary section. Strike-slip faults in such settings are ephemeral and overprinted by the formation of new structures as seamount subduction advances.

Tectono‐stratigraphic response of the Sandino Forearc Basin (N‐Costa Rica and W‐Nicaragua) to episodes of rough crust and oblique subduction

AbstractThe southern Central American active margin is a world‐class site where past and present subduction processes have been extensively studied. Tectonic erosion/accretion and oblique/orthogonal subduction are thought to alternate in space and time along the Middle American Trench. These processes may cause various responses in the upper plate, such as uplift/subsidence, deformation, and volcanic arc migration/shut‐off. We present an updated stratigraphic framework of the Late Cretaceous–Cenozoic Sandino Forearc Basin (SFB) which provides evidence of sedimentary response to tectonic events. Since its inception, the basin was predominantly filled with deep‐water volcaniclastic deposits. In contrast, shallow‐water deposits appeared episodically in the basin record and are considered as tectonic event markers. The SFB stretches for about 300 km and varies in thickness from 5 km (southern part) to about 16 km (northern part). The drastic, along‐basin, thickness variation appears to be the result of (1) differential tectonic evolutions and (2) differential rates of sediment supply. (1) The northern SFB did not experience major tectonic events. In contrast, the reduced thickness of the southern SFB (5 km) is the result of at least four uplift phases related to the collision/accretion of bathymetric reliefs on the incoming plate: (i) the accretion of a buoyant oceanic plateau (Nicoya Complex) during the middle Campanian; (ii) the collision of an oceanic plateau (?) during the late Danian–Selandian; (iii) the collision/accretion of seamounts during the late Eocene–early Oligocene; (iv) the collision of seamounts and ridges during the Pliocene–Holocene. (2) The northwestward thickening of the SFB may have been enhanced by high sediment supply in the Fonseca Gulf area which reflects sourcing from wide, high relief drainage basins. In contrast, sedimentary input has possibly been lower along the southern SFB, due to the proximity of the narrow, lowland isthmus of southern Central America. Moreover, two phases of strongly oblique subduction affected the margin, producing strike‐slip faulting in the forearc basin: (1) prior to the Farallon Plate breakup, an Oligocene transpressional phase caused deformation and uplift of the basin depocenter, triggering shallowing‐upward of the Nicaraguan Isthmus in the central and northern SFB; (2) a Pleistocene–Holocene transtensional phase drives the NW‐directed motion of a forearc sliver and reactivation of the graben‐bounding faults of the late Neogene Nicaraguan Depression. We discuss arguments in favour of a Pliocene development of the Nicaraguan Depression and propose that the Nicaraguan Isthmus, which is the apparent rift shoulder of the depression, represents a structure inherited from the Oligocene transpressional phase.

Crustal deformation dynamics and stress evolution during seamount subduction: High‐resolution 3‐D numerical modeling

AbstractSeamounts or submarine volcanoes frequently collide with the overriding crust along presently active subduction zones locally modifying stress and permanent deformation patterns. Dynamics of this process is not fully understood, and several end‐member scenarios of seamount‐crust interaction are proposed. Here we use high‐resolution 3‐D numerical models to investigate evolution of crustal deformation and stress distribution within the upper plate induced by the underthrusting of subducting seamounts. The dynamical effects of the upper plate strength, subduction interface strength, and strain weakening of the crust are investigated. Experiment results demonstrate that characteristic crustal fracturing patterns formed in response to different seamount‐crust interaction scenarios. Indenting seamounts strongly deform the overriding plate along a corridor as wide as the underthrusting seamount by constantly shifting subvertical shear zones rooted at the seamount extensions. A reentrant develops during initial seamount collision. A topographic bulge atop the seamount and lateral ridges emerge from further seamount subduction. Obtained stress pattern shows areas of large overpressure above the rearward and large underpressure above the trenchward flank of the seamount. Results of numerical experiments are consistent with seismic reflection images and seismic velocity models of the upper plate in areas of seamount subduction along the Middle America Trench and give important insights into the long‐lasting question, whether subducting seamounts and rough seafloor act as barriers or asperities for megathrust earthquakes.

Restoring the Silurian to Carboniferous northern active continental margin of the Mongol–Okhotsk Ocean in Mongolia: Hangay–Hentey accretionary wedge and seamount collision

Trapped between the Siberian Craton to the north, and the Tarim and North China blocks to the south, the Mongol–Okhotsk Belt represents a young portion of the Central Asian Orogenic Belt. The Andean-type orogeny was caused by the closure of the Mongol–Okhotsk Ocean, whose timing and mode of geodynamic evolution still needs to be clarified in more detail. This paper investigates the lithostratigraphic units of the Gorkhi Formation, which are comprised in the Hangay–Hentey terrane representing the accretionary prism of the northern continental margin of the Mongol–Okhotsk Ocean. In the tectonically disrupted slices of the Gorkhi Formation we identify a typical ocean plate stratigraphy with pelagic to hemipelagic and turbiditic sequences. Locally, shallow marine limestones are found associated with mafic volcanic rocks. XRF and LA–ICPMS measurements of these volcanic rocks clearly indicate an OIB signature. The geochemistry together with lithostratigraphic observations permit us to reconstruct the evolution of a seamount, ranging from its growth in shallow water depths and drowning due to subduction-related subsidence linked with subsequent radiolarian chert, hemipelagic shale and turbiditic sandstone cover sedimentation. Calc-alkaline felsic dikes later intruded the accreted silt and sandstone deposits. Standard sandstone provenance analysis, detrital zircon U–Pb laser ablation ICP-MS dating and Hf isotope ratios in the sandstones document a prevailing continental volcanic arc sourcing during Carboniferous from the northern margin of the Mongol–Okhotsk Ocean.

Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska

Research Article| May 01, 2012 Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska Peter D. Clift; Peter D. Clift † 1School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK †E-mail: pclift@lsu.edu; current address: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA. Search for other works by this author on: GSW Google Scholar Nico M. Wares; Nico M. Wares 1School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK Search for other works by this author on: GSW Google Scholar Jeffrey M. Amato; Jeffrey M. Amato 2Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA Search for other works by this author on: GSW Google Scholar Terry L. Pavlis; Terry L. Pavlis 3Department of Geology and Geophysics, University of Texas, El Paso, Texas 79968, USA Search for other works by this author on: GSW Google Scholar Malcolm J. Hole; Malcolm J. Hole 1School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK Search for other works by this author on: GSW Google Scholar Caleb Worthman; Caleb Worthman 2Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA Search for other works by this author on: GSW Google Scholar Erik Day Erik Day 3Department of Geology and Geophysics, University of Texas, El Paso, Texas 79968, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Peter D. Clift † 1School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK Nico M. Wares 1School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK Jeffrey M. Amato 2Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA Terry L. Pavlis 3Department of Geology and Geophysics, University of Texas, El Paso, Texas 79968, USA Malcolm J. Hole 1School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK Caleb Worthman 2Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA Erik Day 3Department of Geology and Geophysics, University of Texas, El Paso, Texas 79968, USA †E-mail: pclift@lsu.edu; current address: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA. Publisher: Geological Society of America Received: 16 Aug 2011 Revision Received: 03 Nov 2011 Accepted: 29 Nov 2011 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2012 Geological Society of America GSA Bulletin (2012) 124 (5-6): 989–1006. https://doi.org/10.1130/B30594.1 Article history Received: 16 Aug 2011 Revision Received: 03 Nov 2011 Accepted: 29 Nov 2011 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter D. Clift, Nico M. Wares, Jeffrey M. Amato, Terry L. Pavlis, Malcolm J. Hole, Caleb Worthman, Erik Day; Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska. GSA Bulletin 2012;; 124 (5-6): 989–1006. doi: https://doi.org/10.1130/B30594.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Gulf of Alaska is one of the largest accretionary complexes on Earth. In this study, we examined the earliest phase of accretion in the Mesozoic McHugh Complex and Valdez Groups, exposed in SE Alaska. The oldest preserved fragment, the Mesomélange assemblage, is Jurassic (ca. 160–140 Ma) and consists of an ∼3-km-thick structural package of strongly deformed shaley materials with slices of oceanic cherts and basalts. Heavy minerals indicate dominant erosion from a magmatic arc source uplifted after the collision of the Wrangellia and the Talkeetna oceanic arc. A tectonic erosion event affected the forearc just prior to ca. 120 Ma and was likely caused by seamount collision, ridge subduction, or both. This was followed at 105 Ma by mass wasting of sandstone and conglomerates, preserved as the Graywacke-Conglomerate assemblage (ca. 105–83 Ma). Heavy minerals indicate continued flux from arc sources, but with significant changes suggesting a larger, more diverse catchment area. Erosion of deeper crustal sources provided high-Mg diopside and garnets to the trench. Faster sediment flux was caused by rock uplift triggered by final accretion of the Wrangellia-Peninsula terrane to North America. The start of large-scale accretion in Alaska roughly coincided with the initiation of Shimanto Complex accretion in Japan and can be understood as primarily linked to sediment supply driven by plate-margin tectonics rather than climatically induced erosion onshore. 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The Java margin revisited: Evidence for subduction erosion off Java

The eastern Sunda margin off Indonesia (from central Java to Sumba Island) remains a little investigated subduction zone, contrary to its well-studied northwestern segment. Whereas large portions of the Sunda margin are considered a classical accretionary zone, subduction characteristics along the central Java sector indicate erosive processes as the dominant mode of mass transfer. The tectonic framework of the central Java margin, with a convergence rate of 6.7 cm/yr, insignificant sediment input and a pronounced seafloor roughness where the oceanic Roo Rise is subducting underneath Java, facilitates subduction erosion. Evidence for erosion comes from newly acquired geophysical data off central Java: local erosive processes in the wake of seamount subduction are documented by a high-resolution bathymetric survey and result in an irregular trend of the deformation front sculpted by seamount collision scars. Subduction of oceanic basement relief leads to large-scale uplift of the forearc, as recorded on a reflection seismic profile, and to a dismemberment of the previous outer forearc high, giving way to isolated topographic elevations. The broad retreat of the Java Trench and deformation front above the leading edge of the Roo Rise has exposed an area of approximately 25,000 km2 of deeper seafloor formerly covered by the previous frontal prism. Frontal erosion coincides with a steepening of the lower slope angle in the central Java sector compared to the neighbouring segments. In global compilations, the key geological parameters of the central Java margin lie in the erosive regime, reflecting the interplay of basement relief subduction, negligible sediment supply and a high convergence rate on the evolution of the margin.

Tectonic setting of primitive magmas in volcanic arcs: an example from the Antarctic Peninsula

Abstract: Primitive magmas representing mantle partial melts minimally affected by fractionation and assimilation are rare in the magmatic arc environment. Most examples are either associated with high rates of arc-parallel extension, or occur along faults and dykes perpendicular to the trend of the arc and related to arc compression. In two cases, the Vanuatu and Solomon Islands arcs, such arc compression is being caused by collision of seamounts. In the Antarctic Peninsula, primitive mafic dykes were emplaced perpendicular to the continental arc. Ar–Ar and K–Ar data suggest intrusion of the dykes at c. 126–106 Ma, possibly during mid-Cretaceous regional compression of the arc. The dykes form two compositional groups. One group has low La N /Yb N ratios (0.31–0.49), lower Nb/Yb and higher Th/Nb than N-MORB, age-corrected ϵNd values of +7.3 to +7.9, and are interpreted as melts of subduction modified sub-arc asthenosphere. The other has high La N /Yb N ratios (3.86–8.92), higher Nb/Yb and Th/Nb than N-MORB, age-corrected ϵNd values of −2.8 to +3.4, and are interpreted as melts of sub-arc lithosphere. The absence of dykes compositionally between these groups suggests that the primitive magmas avoided storage and mixing in magma chambers.

Seismic images at the convergence zone from south of Cyprus to the Syrian coast, eastern Mediterranean

Multichannel seismic profiles from cruise 5 of the R/V Akademik Nikolaj Strakhov provide the first deep seismic reflection images and extensive coverage south and east of Cyprus. Five NW–SE-trending seismic lines cross an area of active continental collision. Main tectonic structures are the Eratosthenes Seamount collision zone, the Hecateaus Rise and the Latakia–Larnaca Ridge systems. The Levantine Basin extends to the south all over the area. The data required careful processing due to the low-fold coverage, the reverberatory character of the signal and the strong multiple energy. The seismic results clearly image the thick sedimentary sequence of the Levantine Basin. This basin is observed to terminate abruptly at the junction with the Hecateaus Rise south of Cyprus. To the east and north, the deformation appears to be partitioned along two separated structures next to the Latakia and Larnaca Ridges. They correspond to major oblique fault systems.

Accretion and tectonic erosion processes revealed by the mode of occurrence and geochemistry of greenstones in the Cretaceous accretionary complexes of the Idonnappu Zone, southern central Hokkaido, Japan

AbstractThe Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area are divided into five lithological units, four of which contain greenstone bodies. The Lower Cretaceous Naizawa Complex consists of two lithologic units. The Basaltic Unit (B‐Unit) is a large‐scale tectonic slab of greenstone, consisting of depleted tholeiite similar to that of the Lower Sorachi Ophiolite (basal forearc basin ophiolite) in the Sorachi‐Yezo Belt. The Mixed Unit of Naizawa Complex (MN‐Unit) contains oceanic island‐type alkaline greenstones which occur as slab‐like bodies and faulted blocks with tectonically dismembered trench‐fill sediments. Repeated alternations of the two units in the Naizawa Complex may have been formed by the collision of seamounts with forearc ophiolitic body (Lower Sorachi Ophiolite) in the trench. The Upper Cretaceous Horobetsugawa Complex structurally underlies the Naizawa Complex in its original configuration, and it also contains greenstone bodies. Greenstones in the MH‐Unit occur as blocks and sedimentary clasts in a clastic matrix, and exhibit depleted tholeiite and oceanic‐island alkaline basalt/tholeiite chemistry. This unit is interpreted as submarine slide and debris flow deposits. Greenstones in the PT‐Unit occur at the base of several chert‐clastic successions. Most of the greenstones are severely sheared and show normal‐type mid‐ocean ridge basalt composition. The PT‐Unit greenstones are considered to have been derived from abyssal basement peeled off during accretion. The different accretion mechanism of the greenstones in the Naizawa and Horobetsugawa complexes reflects temporal changes in subduction zone conditions. Seamount accretion and tectonic erosion were dominant in the Early Cretaceous, due to highly oblique subduction of the old oceanic crust and minimal sediment supply. Whereas, thick sediments with minor mid‐ocean ridge basalt and olistostrome accreted in the Late Cretaceous, due to near‐orthogonal subduction of young oceanic crust with voluminous sediment supply.

Tectonic controls on sedimentation and diagenesis in the Tonga Trench and forearc, southwest Pacific

Sedimentation in the Tonga forearc is dominated by the redeposition of volcaniclastic sediment from the arc volcanic front by mass flows and turbidity currents onto the adjacent Tonga Platform, the shallowest, flattest part of the forearc region. The greatest sediment thicknesses accumulate in debris aprons close to the volcanic front. Collision of seamounts, notably the Capricorn Seamount and the Louisville Ridge, with the forearc radically shortens and steepens the adjacent modern trench slope, allowing sediment to be redeposited deep into the trench. Rotation, usually arcward, of existing basins on the midslope during collision generates angular unconformities, while synchronous uplift of the outer forearc high results in canyon development and downcutting along the eastern edge of the Tonga Platform. Collision also reactivated major across-strike fault zones on the forearc; the zones are subsequently exploited by canyons depositing sediment into the trench. Collapse and renewed extension of the forearc in the wake of collision result in the development of small perched basins, measuring approximately 5 km by 15 km in the midslope area. This morphology is especially developed at 18°30′S to 20°S, implying a 2–3 m.y. interval for their formation following Louisville Ridge collision. Trenchward of these depocenters sedimentation is slow, resulting in manganese crust formation and localized mass wasting along fault scarps. Over longer periods of time (>5 m.y.) tectonic erosion reestablishes a wide, gently sloping forearc into which canyons incise the shallow Tonga Platform by headwall erosion and collapse.

Opening and Tectonic Evolution of the Paleo-Asian Ocean

The Paleo-Asian ocean is defined by units located between the Russian (East European), Siberian, Tarim, and Sino-Korean (North China) continents. The study of the composition, age, and structural position of island-arc magmatic rocks, ophiolites, and high-pressure meta-morphic assemblages and their mutual correlations made it possible to identify similarities and differences in the evolution of the Paleo-Asian and Paleo-Pacific oceans. The initial stage of the evolution of the Paleo-Asian ocean defined its opening at 900 Ma, whereas opening of the Paleo-Pacific took place at 750 to 700 Ma. Closing of the Paleo-Asian ocean in the Carboniferous (NE branch) and the Permian corresponds to the main stage of reorganization and reopening of the Paleo-Pacific. The maximal opening of the Paleo-Asian ocean occurred after or simultaneously with the first accretion-collision event at 600 to 700 Ma, resulting from the collision of microcontinents and the Siberian continent. Vendian-Early Cambrian boninite-bearing island-arc complexes occur as lavas, sheeted dikes, and sill-dikes associated with gabbro-pyroxenites and ultramafics. These complexes are widely distributed in the Gornyy Altay, East Sayan, and West Mongolian regions and can be considered fragments of a giant boninite-bearing belt. In the late Early Cambrian, collision of seamounts with an island arc caused the squeezing of the subduction zone and return flows within the accretionary wedge. Serpentinite melange within fragments of ophiolites and high-pressure rocks are typical components of the Late Paleozoic accretionary wedges. Because of Middle Cambrian-Early Ordovician collisional events, two new oceans (Junggar-Irtysh-Kazakhstan and Uralian-South Tien Shan-South Mongolian) were formed. The junction of both oceans in East Mongolia opened to the Paleo-Pacific.

Formation of forearc basins by collision between seamounts and accretionary wedges: An example from the New Hebrides subduction zone

Seabeam data reveal two deep subcircular reentrants in the lower are slope of the New Hebrides island arc that may illustrate two stages in the development of a novel type of forearc basin. The Malekula reentrant lies just south of the partly subducted Bougainville seamount. This proximity, as well as the similarity in morphology between the reentrant and an indentation in the lower arc slope off Japan, suggests that the Malekula reentrant formed by the collision of a seamount with the arc. An arcuate fold-thrust belt has formed across the mouth of the reentrant, forming the toe of a new accretionary wedge. The Efate reentrant may show the next stage in basin development. This reentrant lies landward of a lower-slope ridge that may have begun to form as an arcuate fold-thrust belt across the mouth of a reentrant. This belt may have grown by continued accretion at the toe of the wedge, by underplating beneath the reentrant, and by trapping of sediment shed from the island arc. These processes could result in a roughly circular forearc basin. Basins that may have formed by seamount collision lie within the accretionary wedge adjacent to the Aleutian trenches.

Spherical shell tectonics: on the buckling of the lithosphere at subduction zones

The buckling phenomena of the subducting lithosphere due to the sphericity of the earth are studied by a nonlinear finite element method (FEM), and the effects of geometrical nonlinearity are examined. The subduction of the lithosphere is modelled by a hemispherical shell squeezed at its circumferential edge. In scaling the geometry of the subducting lithosphere, two parameters are employed; one is the thickness of the hemispherical shell (i.e., the thickness of the lithosphere) and the other is the length of its deformable portion (i.e., the length of the descending slab). To simulate the bending of the lithosphere at the trench, uniform inward load is applied to the free circumferential edge of the deformable portion, which corresponds to the leading edge of the subducting lithosphere. At the opposite edge of the deformable portion (i.e., at the trench), a built-in boundary condition is imposed; no displacements and no rotations are allowed. With a load greater than some critical value, the deformable portion of the shell buckles and undulates azimuthally. The investigation of this buckling phenomenon can be summarized as follows: 1. (1) Buckling with a shorter wavelength is associated with a thinner and shorter shell. Mechanical irregularities simulating lithospheric tearing or seamount collision do not alter the buckling wavelength very much. After appropriate scaling, the lengths of subduction zones coincide with the buckling wavelength calculated by F.E.M. 2. (2) Sharp cusps pointing outward from the shell appear during the postbuckling undulation without any irregularities in shell property and in the applied load. The shape of this cuspidal undulation is quite similar to that actually observed at an arc-arc junction. 3. (3) Buckling can take place in the stress environment likely to exist in the earth. These results strongly suggest that the arcuate shapes of subduction zones are in fact generated by buckling of the subducting lithosphere.

Spherical shell tectonics: buckling of subducting lithosphere

The lithospheric plate is a spherical shell rather than a plane plate. A spherical shell must buckle when it is bent inward. We examined the possibility of lithospheric buckling upon subduction. The lithosphere with its subducted portion is simulated by a hemispherical elastic shell bent inward at its circumferential edge by a uniform radial load. The buoyancy force acting on the lithosphere seaward of the trench is simulated by clamping the shell along a parallel. The deformable portion between the loaded and clamped edges corresponds to the subducting slab of lithosphere. Buckling analyses were made using the techniques of a linear stability analysis and a finite element method. The wavelength of buckling depends on the thickness of the shell and the length of its deformable portion: a longer wavelength is associated with a thicker shell and a longer deformable portion. By scaling the shell thickness and the length of the deformable portion to the elastic thickness of lithosphere and the length of subducting slab, respectively, the wavelength of buckling can be compared favourably to the length of one unit of arcuate trench in a chain-like continuation of island arcs. The load to be applied for initiation of buckling is called the critical load: a lower critical load is associated with a thinner shell with a longer deformable portion. The excess weight of subducting slab provides a load large enough to initiate lithospheric buckling at a relatively early stage of subduction. Radial displacement of the circumferential edge at the critical state is an order of magnitude smaller than the depth of the leading edge of the Wadati-Benioff zone, indicating that most of the present subducting lithospheres are under a postbuckling state. Undulation of the shell in the postbuckling state is not purely sinusoidal but a successive continuation of arcs with cusps in between, invoking the continuation of arcuate deep-sea trenches with cusps at their junctions. The cuspate feature of island arc chains is thus a natural consequence of lithospheric buckling and does not require any such irregularities as seamounts colliding with a trench. Collision of seamounts, however, can aid buckling greatly if they are aligned along a trench at an interval not very different from the inherent buckling wavelength.

Why do island arcs form cusps at their junctions?

An interesting feature of an island-arc chain is a cusp of the arcuate trench at the arc-arc junction where the Wadati-Benioff zone and the volcanic front are also sharpened landward. This cusp has been interpreted as a result of the collision of an aseismic ridge with the trench. We propose a different mechanism, intrinsic to the subduction of a spherical shell of lithosphere. When the spherical lithosphere is bent inward at a subduction zone, buckling takes place, and the slab is deformed into a wavy configuration making the trench chainlike. We analyzed this buckling phenomenon of spherical shells by a finite element method, wliich has been developed in the fields of architecture and civil engineering. Our calculations show that the lithospheric shell buckles with a wavelength roughly consistent with the length of one unit of island arcs (i.e., the distance between the two neighboring junctions). The resultant undulation is not purely sinusoidal, but is a succession of arcs with cusps in between, very similar to the shape of the actual arc-trench system in the world. This agreement strongly suggests that the arcuateness of the trenches and the cuspate form of their junctions are the manifestation of buckling of the spherical lithosphere, although the collision of seamounts and aseismic ridges against the trench may promote the initiation of buckling.