Intra-arc extension in Central America: Links between plate motions, tectonics, volcanism, and geochemistry

This study revisits the kinematics and tectonics of Central America subduction, synthesizing observations of marine bathymetry, high-resolution land topography, current plate motions, and the recent seismotectonic and magmatic history in this region. The inferred tectonic history implies that the Guatemala–El Salvador and Nicaraguan segments of this volcanic arc have been a region of significant arc tectonic extension; extension arising from the interplay between subduction roll-back of the Cocos Plate and the ~ 10–15mm/yr slower westward drift of the Caribbean plate relative to the North American Plate. The ages of belts of magmatic rocks paralleling both sides of the current Nicaraguan arc are consistent with long-term arc-normal extension in Nicaragua at the rate of ~ 5–10mm/yr, in agreement with rates predicted by plate kinematics. Significant arc-normal extension can ‘hide’ a very large intrusive arc-magma flux; we suggest that Nicaragua is, in fact, the most magmatically robust section of the Central American arc, and that the volume of intrusive volcanism here has been previously greatly underestimated. Yet, this flux is hidden by the persistent extension and sediment infill of the rifting basin in which the current arc sits. Observed geochemical differences between the Nicaraguan arc and its neighbors which suggest that Nicaragua has a higher rate of arc-magmatism are consistent with this interpretation. Smaller-amplitude, but similar systematic geochemical correlations between arc-chemistry and arc-extension in Guatemala show the same pattern as the even larger variations between the Nicaragua arc and its neighbors.     

Slip partitioning along major convergent plate boundaries

Along plate boundaries characterized by oblique convergence, earthquake slip vectors are commonly rotated toward the normal of the trench with respect to predicted plate motion vectors. Consequently, relative plate motion along such convergent margins must be partitioned between displacements along the thrust plate interface and deformation within the forearc and back-arc regions. The deformation behind the trench may take the form of strike-slip motion, back-arc extension, or some combination of both. We observe from our analysis of the Harvard Moment Tensor Catalog that convergent arcs characterized by back-arc spreading, specifically the Marianas and New Hebrides, are characterized by a large degree of slip partitioning. However, the observed rates, directions, and location of back-arc spreading are not sufficient to account for degree of partitioning observed along the respective arcs, implying that the oblique component of subduction is also accommodated in part by shearing of the overriding plate. In the case of the Sumatran arc, where partitioning is accommodated by strike-slip faulting in the overriding plate, the degree of partitioning is similar to that observed along the Marianas, but the result is viewed with caution because it is based on a predicted plate motion vector that is based on locally derived earthquake slip vectors. In the case of the Alaskan-Aleutian arc, where back-arc spreading is also absent, the degree of partitioning is less and rotation of slip vectors toward the trench normal appears to increase linearly as a function of the obliquity of convergence. If partitioning in the Alaskan-Aleutian arc is accommodated by strike-slip faulting within the upper plate, the positive relationship between obliquity of convergence and the rotation of earthquake slip vectors to the trench normal may reflect that either (1) the ratio of the depth extent of strike-slip faults behind the trenchZ _s to the subduction thrustZ _t increases westward along the arc, (2) the dip of the subduction thrust increases westward along the arc, or (3) the strength of the subduction thrust decreases westward along the arc.     

Global variability in subduction thrust zone-forearc systems

Deviations of slip vector azimuths of interplate thrust earthquakes from expected plate convergence directions at oblique subduction zones provide kinematic information about the deformation of forearcs and indirect evidence on the dynamics of the plate boundary. A global survey of slip vectors at major trenches of the world reveals a large variability in the kinematic response of forearcs to shear produced by oblique convergence. The variability in forearc deformation inferred from slip vector deflections is suggested to be caused by variations in forearc rheology rather than in the stresses acting on subduction zone thrust faults. Estimated apparent macroscopic rheologies range from elastic to perfectly plastic (or viscous). Forearc rheologies inferred from slip vectors do not correlate with age of the subducting lithosphere, but continental forearcs or old arcs appear to deform less than oceanic or young arcs. The inferred absence of forearc deformation at continental arcs from this study is counter to inferences drawn from compiled geologic information on forearc faults. Correlations of the apparent forearc rheology with backarc spreading, convergence rate, slab dip, arc curvature, and downdip length of the thrust contact are poor. However, great subduction zone earthquakes occur where forearcs are apparently more elastic (i.e., less deformed by oblique convergence), which suggests that the mechanical properties of forearcs rather than stress magnitude on thrust faults control both the kinematic behavior of forearcs and where great subduction zone earthquakes occur.     

Volcanoes of the southwestern extension of the active Mariana island arc: New swath-mapping and geochemical studies

Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at ∼ 14°N immediately west of Guam. Sea floor mapping in 1995 shows a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequence of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate margin are becoming increasingly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back-arc basin intersects the line of arc volcanoes. They develop well-defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal arc volcanoes and tend to form along lineaments. Compositions of lavas from these intersection volcanoes bear some similarities to back-arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subducts nearly orthogonal to the strike of the trench along the southern part of the Mariana system and the distance to the arc line from the trench axis is only ∼ 150 km. Several deep fault-controlled canyons on the inner slope of the southern Mariana trench indicate an enhanced tectonic extension of this plate margin. The presence of these active arc volcanoes and the existence of the orthogonal normal faulting along the southern Mariana forearc supports a model of radial extension for formation of the Mariana Trough, a model previously dismissed because of the lack of evidence of these two major geological features.     

Cenozoic geodynamic evolution of the Andaman–Sumatra subduction margin: Current understanding

The Andaman–Sumatra margin displays a unique set‐up of extensional subduction–accretion complexes, which are the Java Trench, a tectonic (outer arc) prism, a sliver plate, a forearc, oceanic rises, inner‐arc volcanoes, and an extensional back‐arc with active spreading. Existing knowledge is reviewed in this paper, and some new data on the surface and subsurface signatures for operative geotectonics of this margin is analyzed. Subduction‐related deformation along the trench has been operating either continuously or intermittently since the Cretaceous. The oblique subduction has initiated strike–slip motion in the northern Sumatra–Andaman sector, and has formed a sliver plate between the subduction zone and a complex, right‐lateral fault system. The sliver fault, initiated in the Eocene, extended through the outer‐arc ridge offshore from Sumatra, and continued through the Andaman Sea connecting the Sagaing Fault in the north. Dominance of regional plate dynamics over simple subduction‐related accretionary processes led to the development and evolution of sedimentary basins of widely varied tectonic character along this margin. A number of north–south‐trending dismembered ophiolite slices of Cretaceous age, occurring at different structural levels with Eocene trench‐slope sediments, were uplifted and emplaced by a series of east‐dipping thrusts to shape the outer‐arc prism. North–south and east–west strike–slip faults controlled the subsidence, resulting in the development of a forearc basins and record Oligocene to Miocene–Pliocene sedimentation within mixed siliciclastic–carbonate systems. The opening of the Andaman Sea back‐arc occurred in two phases: an early (～11 Ma) stretching and rifting, followed by spreading since 4–5 Ma. The history of inner‐arc volcanic activity in the Andaman region extends to the early Miocene, and since the Miocene arc volcanism has been associated with an evolution from felsic to basaltic composition.     

Mechanisms of Cenozoic deformation in the Bohai Basin, Northeast China: Physical modelling and discussions

The Bohai Basin is a petroliferous Cenozoic basin in northeast China (Fig. 1(a)) and has apparent geo- metrical and kinematic similarities with the other Meso-Cenozoic extensional basins located along the eastern margin of the Eurasian Plate[1,2]. Its architec- ture and Cenozoic stratigraphy have been well under- stood from several decades of petroleum exploration. Previous studies have suggested that the Bohai Basinis a typical extensional basin and has two tectonic evolution phases, rift…     

Three-dimensional mantle lithosphere deformation at collisional plate boundaries: A subduction scissor across the South Island of New Zealand

The continental plate collision across the South Island of New Zealand is highly oblique (dextral) and bounded by oppositely verging ocean plate subduction zones. As such, the region can be considered as a type of ‘subduction scissor’. Within this tectonic context, we use three-dimensional computational geodynamic models to consider how convergent mantle lithosphere can be modified by scissor and strike–slip effects. Bounding subduction at both ends of the continental collision causes flow of the descending mantle lithosphere in the direction along strike of the model plate boundary, with thinning in the centre and thickening towards the subduction zones that bifurcates the continental mantle lithosphere root. With dipping bounding subduction, the mantle lithosphere root takes on a more complex morphology that folds over from one subduction polarity to the other, but remains as a continuous feature as it folds under the collision zone. In the absence of bounding subduction, the plate convergence causes a linear (along strike) mantle lithosphere root to develop. A rapid strike–slip motion between the converging plates transfers material in the plate boundary-parallel direction and tends to blur out features that develop in this direction—such as descending viscous instabilities. The along-strike variations in the morphology of the mantle lithosphere root that develop in the models—viz., thickening of the root towards the subduction edges, thinning in the centre—are consistent with recent, albeit poorly constrained, geophysical interpretations of the large-scale lithospheric structure of the South Island. We speculate that this reflects the nature of the evolution of the South Island collision as a limited continental segment of the plate boundary that it is dominated and guided by adjacent well-developed/developing ocean plate subduction.     

The Epi－continental arc of Southeast China and relevant earthquakes

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Origin of the Kunlun Mountains by arc-arc and arc-continent collisions

Abstract The Kunlun Mountains were formed by early Mesozoic arc-arc and arc-continent collisions. The Middle Kunlun Are was the outer volcanic arc of the Paleozoic Asiatic continent, and the arc-related magmatic activities from the Proterozoic to Mesozoic are recorded by numerous volcanic and plutonic rocks of the area. Several back-arc basins and relic arcs exist north of the arc and the north Kunlun arc is one of these. The Kudi mélange of Kunlun was formed in a south-dipping subduction zone when the basin between the north and middle Kunlun arcs was consumed by the process of back-arc basin collapse, and the ophiolite mélange marked the suture zone where the two arcs collided. The Mazar mélange was formed in the north-dipping subduction zone under the middle Kunlun arc, and the mélange marks the main Paleotethys suture where the Qogir-Karamilan rocks of the Qangtang block (a fragment of Gondwanaland) is sutured on to Laurentia. The geology of Kunlun emphasizes the importance of arc-arc and arc-continent collisions in mountain-building processes.     

An evaluation of the global variations in the major element chemistry of arc basalts

Arc volcanoes occur at convergent margins with a wide range in subduction parameters, and variations in these parameters might be expected to lead to variations in the chemistry of magmas parental to arcs. Major element analyses from approximately 100 volcanic centers within 30 arcs, normalized to 6% MgO to minimize the effects of crystal fractionation, display wide variations. Na₂O and CaO at 6% MgO (Na_6.0 and Ca_6.0) correlate remarkably well with the thickness of the overlying crust. These systematics are consistent with two possible models. In the first model, the crust behaves as a chemical filter; where the crust is thick, magmas crystallize at higher pressure and interact more extensively with the arc crust. Modeling of high-pressure crystallization and assimilation, however, does not reproduce the associated variations in Na_6.0 and Ca_6.0 without calling upon complicated combinations of fractionating phases and assimilants. In the second model, crustal thickness determines the height of the mantle column available for melting beneath arc volcanoes. If melting begins beneath arcs at similar depths, then the column of mantle that undergoes decompression melting is much shorter beneath the thickest arc crust. The shorter mantle column for arcs built on thick crust will lead to smaller extents of melting in the mantle, and hence higher Na_6.0 and lower Ca_6.0 in the parental magmas. Modeling shows that variations in the extent of melting in the mantle can easily account for the associated variations in Ca_6.0 and Na_6.0. The abundances of the other major elements at 6% MgO do not correlate well with crustal thickness, or any other subduction parameter. Co-variation of some of these other major elements (e.g., Si_6.0 and Fe_6.0) within individual arcs suggests that they are strongly influenced by local crustal level processes that obscure partial melting systematics. Correction for the crustal processes improves the relationship between Na_6.0 and Ca_6.0 that is so readily explained by partial melting. The extents of melting in the mantle beneath arc volcanoes estimated from the ranges in Na_6.0 and Ca_6.0 are remarkably similar to those estimated beneath mid-ocean ridges. This observation provides further evidence that the mantle wedge, and not the slab, melts beneath arc volcanic fronts.     

The South Connemara Group reinterpreted: a subduction-accretion complex in the Caledonides of Galway Bay, western Ireland

The Caledonian geology of western Ireland records the collision of two arc complexes with the Laurentian Margin during the closure of the Iapetus Ocean. An earlier complex collided with this hitherto passive margin in the mid-Ordovician during the Grampian Orogeny. Subsequently, arc magmatism developed along the Laurentian margin and continued until the late Silurian collision between Laurentian and Avalonia. The Ordovician volcanic and sedimentary rocks comprising the South Connemara Group lie along the Southern Uplands Fault, the terrane boundary separating these two arc complexes. Palaeontological dating indicates an Arenig-Llanvirn age for part of this complex (Williams, Armstrong and Harper, 1988), making it contemporaneous with the earlier arcs. However, most authors correlate this complex with the northern belt of the Southern Uplands (Morris, 1983; Williams, D.M., 1984. The stratigraphy and sedimentology of the Ordovician Party Group, south-eastern Murrisk, Ireland. Geological Journal, 19, 173–186; Williams et al., 1988), associated with post-Grampian subduction of north directed polarity. We present new field evidence that the South Connemara Group is tectonically disrupted by bedding parallel shear zones and that contacts previously interpreted as conformable are marked by units of tectonic mélange. We present structural and provenance arguments consistent with the mélanges forming above a north-dipping subduction zone after 463Ma. This Group is reinterpreted as occurring within a subduction–accretion complex that was generated by the accretion of early Ordovician mafic seamounts into a post-Grampian trench, thus reconciling the age of the Group with its generally accepted tectonic setting. We discuss the regional significance of this finding with respect to the Caledonide-Appalachian orogeny and argue that this is the site along which the Iapetus Ocean closed.     

Volcanoes as possible indicators of tectonic stress orientation — Aleutians and Alaska

A new method for obtaining from volcanic surface features the orientations of the principal tectonic stresses is applied to Aleutian and Alaskan volcanoes. The underlying concept for this method is that flank eruptions for polygenetic volcanoes can be regarded as the result of a large-scale natural magmafracturing experiment. The method essentially relies on the recognition of the preferred orientation of radial and parallel dike swarms, primarily using the distribution of monogenetic craters including flank volcanoes. Since dikes tend to propagate in a direction normal to the minimum principal stress (T-axis), the method primarily yields the direction of the maximum horizontal compression (MHC) of regional origin. The direction of the MHC may correspond to either the maximum (P-axis) or intermediate (B-axis) principal stress.The direction of MHC obtained at 20 volcanoes in the Aleutian arc coincides well with the direction of convergence between the Pacific and North American plates. This result provides evidence that in the island arc the inferred direction of MHC is parallel to the maximum principal tectonic stress. In the back-arc region, general E-W trends of MHC are obtained from seven volcanic fields on islands on the Bering Sea shelf and the mainland coast of Alaska. These volcanic fields consist mostly of clusters of monogenetic volcanoes of alkali basalt. In the back-arc region, the trends of MHC may correspond to an E-W intermediate, a vertical maximum, and a N-S minimum principal stress.Implications for the tectonics of island arcs and back-arc regions are: (1) volcanic belts of some island arcs, including the Aleutian arc, are under compressional deviatoric stress in the direction of plate convergence. It is improbable that such arcs would split along the volcanic axis to form actively spreading marginal basins. (2) This compressional stress at the arc, probably generated by underthrusting, appears to be transmitted across the entire arc structure, but is apparently replaced within several hundred kilometers by a stress system characterized by horizontal extension (tensional deviatoric stress) in the back-arc region. (3) The volcanoes associated with these two stress systems differ in type (polygenetic vs. monogenetic) and in the chemistry of their magmas (andesitic vs. basaltic). These differences and the regional differences in orientation of the principal tectonic stresses suggest that the back-arc stress system has its own source at considerable depth beneath the crust.Lamont-Doherty Geological Observatory Contribution No. 2503.     

Slab melt as metasomatic agent in island arc magma mantle sources, Negros and Batan (Philippines)

Abstract Two new cases of association of adakites with ‘normal’ island arc lavas and transitional adakites are recognized in the islands of Batan and Negros in northern and central Philippines, respectively. The Batan lavas are related to the subduction of the middle Miocene portion of the South China Sea basin along the Manila trench; those of Negros come from the almost aseismic subduction of the middle Miocene Sulu Sea crust along the Negros trench. The occurrence of the Batan adakites is consistent with previous findings showing adakitic glass inclusions within minerals of mantle xenoliths associated with Batan arc lavas. The similarity of adakite ages (1.09 Ma) and that of the metasomatized xenoliths (1 Ma) suggests that both are linked to the same slab‐melting and metasomatic event. Earlier Sr, Pb and Nd‐isotopic studies, however, also reveal the presence of an important sediment contribution to the Batan lava geochemistry. Thus, the role played by slab melts, assumed to have mid‐ocean ridge basalts‐like (MORB) isotopic characteristics, in enriching the Batan subarc mantle is largely masked by the sediment input. The Negros adakites are present only in Mount Cuernos, the volcanic center nearest to the Negros trench. Batch partial melting calculations show that the Negros adakites could be derived from a garnet amphibolitic source with normal‐MORB (N‐MORB) geochemistry. This is supported by the MORB‐like isotopic characteristics of the Mount Cuernos lavas. The volcanic rocks from the other volcanoes consist of normal arc and transitional adakitic lavas that have slightly higher Sr‐ and Pb‐isotopic ratios, probably due to slight sediment input. Mixing of adakites and normal arc lavas to produce transitional adakites is only partly supported by trace element geochemistry and not by field evidence. The transitional adakites can be modeled as partial melts of an adakite‐enriched mantle. Trace element enrichment of non‐adakitic lavas could reflect the interaction of their mantle source with uprising slab melts, as metasomatic mantle minerals scavenge certain trace elements from the adakitic fluids. Therefore, in arcs beneath which thick (up to 2 km) continent‐derived detrital sediments are involved in subduction, like in Batan, the sediment signature can overwhelm the slab melt input. In arcs like Negros where slow subduction could cause a more efficient scraping of thinner (approximately 1 km) detrital sediments, the contribution of slab melts is easier to detect.     

Morphologic and geologic effects of the subduction of bathymetric highs

Morphologic and geologic observations suggest that subduction of bathymetric highs, such as aseismic ridges, chains of seamounts, and fracture zones, are important in the development of many forearc features and that those features form during relatively brief episodes of intense tectonism. A bathymetric high obliquely entering a subduction zone tends to compress sediments along its leading edge, resulting in arcward compression of the accretionary wedge. A landward deflection of the trench axis and a steepened inner wall result from this deformation. If a significant component of oblique slip occurs along the subduction zone, then along-strike movement of the accretionary wedge may also occur. Stresses resulting from subduction of bathymetric features with sufficient buoyancy or high relief extend farther landward than in the case of smaller, less buoyant features, inducing uplift of the leading edge of the overriding plate. Tectonic erosion of the base of the overriding plate and along-strike transport of are material may also occur. The accelerated tectonism observed along several convergent margins can be attributed to the consumption of bathymetric irregularities on the seafloor rather than temporally abrupt changes in rates and directions of plate motions or other episodic events in the accretionary prism.     

Collision tectonics in the New Hebrides arc (Vanuatu)

Abstract The New Hebrides island arc in Vanuatu has been significantly modified by collision with several major submarine ridges and plateaux. Bathymetric sections taken at intervals along the arc, perpendicular to the trench, show that prior to collision at 3 Ma the morphology was typical of modern intraoceanic island arcs. Collision has caused uplift of the trench and forearc (up to 6000 m), subsidence around the arc volcanic edifices (up to 2500 m), forming a large intra-arc basin and uplift of the arc-backarc transition (up to 2000 m). In the transition zone between collisional and non-collisional sections of the arc, subsidence occurs in the forearc and uplift occurs around the arc volcanoes. Many of these characteristics are typical of collisions in other Western Pacific island arcs such as the Tonga–Kermadec and Izu–Bonin arcs. The pattern of uplift and subsidence has important implications for the tectonic history of the New Hebrides system. The morphology of the arc shows that collision of the West Torres Massif probably accounts for at least half the uplift. Arrival at 0.7 Ma of the West Torres Massif in the trench may have caused the slowing of subduction in the entire northern half of the arc and not just in the central segment as previously suggested. Re-equilibration of the arc following collision probably masks any evidence of collision prior to 3 Ma. For example, the Efate re-entrant, a large indentation in the arc immediately to the south of the collision zone, probably originated as a result of erosion during collision followed by subsidence after collision. The Vanuatu collision shows that the subduction of seamounts and ridges in an intraoceanic arc temporarily changes the arc morphology, allowing the development of angular unconformities and changing the pattern of sedimentation. This provides information which can be used to facilitate recognition of these events in ancient arc-related sequences.     

Geochemistry of volcanic glass from Oligocene detrital sediments at DSDP Site 296, Kyushu Palau Ridge: Interpreting the magmatic evolution of the early northern Izu–Bonin–Mariana Island Arc

Understanding the petrologic and geochemical evolution of island arcs is important for interpreting the timing and impacts of subduction and processes leading to the formation of a continental crust. The Izu–Bonin–Mariana (IBM) Arc, western Pacific, is an outstanding location to study arc evolution. The IBM first arc (45–25 Ma) followed a period of forearc basalt and boninite formation associated with subduction initiation (52–45 Ma). In this study, we present new major and trace element data for the IBM first arc from detrital glass shards and clasts from DSDP Site 296, located on the northernmost Kyushu Palau Ridge (KPR). We synthesize these data with published literature for contemporaneous airfall ash and tephra from the Izu–Bonin forearc, dredge and piston core samples from the KPR, and plutonic rocks from the rifted eastern KPR escarpment, locations which lie within or correlate with KPR Segment 1 of Ishizuka, Taylor, Yuasa, and Ohara (2011). Our objective is to test ways in which petrologic and chemical data for diverse igneous materials can be used to construct a complete picture of this section of the Oligocene first arc and to draw conclusions about its evolution. Important findings reveal that widely varying primary magmas formed and differentiated at various depths at this location during this period. Changes in key trace element ratios such as La/Sm, Nb/Yb, and Ba/Th show that mantle sources varied in fertility and in the inputs of subducted sediment and fluids over time and space. Plutonic rocks appear to be related to early K‐poor dacitic liquids represented by glasses sampled both in the forearc and volcanic fronts. An interesting observation is that the variation in magma compositions in this relatively small segment encompasses that inferred for the IBM Arc as a whole, suggesting that sampling is a key factor in inferring temporal, across‐arc, and along‐strike geochemical trends.     

Active tectonics around the junction of Southwest Japan and Ryukyu arcs: Control by subducting plate geometry and pre‐Quaternary geologic structure

The origin of active faults in the Inner zone of the western part of Southwest Japan was explained by a decrease of the minimum principal stress and reactivation of ancient geologic structures. Although the E–W maximum principal stress in Southwest Japan due to the collision of the Southwest and Northeast Japan arcs along the Itoigawa–Shizuoka Tectonic Line is assumed to decrease westward, the density of active strike‐slip faults increases in the western margin of the Southwest Japan Arc (western Chugoku and northern Kyushu) where the subducting Philippine Sea Plate dips steeply. The E–W maximum compressional stress is predominant throughout Southwest Japan, while the N–S minimum principal stress that is presumably caused by coupling between Southwest Japan arc and Philippine Sea Plate decreases due to the weak plate coupling as the plate inclination increases under the western margin of Southwest Japan. The increase of the fault density in the western margin of the arc is attributed to a decrease of the minimum principal stress and consequent increase of shear stress. Low slip rates of the active faults in this region support the view that the westward increase of fault density is not a response to increasing maximum stress. These faults of onshore and offshore lie in three distinct domains defined on the basis of fault strike. They are defined domains I, II, and III which are composed of active faults striking ENE–WSW, NW–SE, and NE–SW, respectively. Faulting in domains I, II, and III is related to Miocene rift basins, Eocene normal faults, and Mesozoic strike‐slip faults, respectively. Although these active faults are strike‐slip faults due to E–W maximum stress, it is unclear whether their fault planes are the same as those of pre‐Quaternary dip‐slip faults.     

Curvature of convergent boundaries

Abstract It is well known that knowledge about convergent boundary geometry is crucial for global geodynamical reconstructions. At the same time, views related to the subject are controversial and speculative. For instance, island arcs, which are considered to be the most important convergent boundary structural elements, are thought to be either the parts of the small circles on the Earth's surface, or adjacent to each other as straight-line segments. Accordingly, one view says that island arcs are rectilinear rather than arched, and there are no reasons to consider their arched shape to be a rule. Is the island arc curvature invariant in time and space? General considerations, palaeomagnetic data and structural features such as strike-slip displacements of the opposite sign along the longitudinal faults on the island arc flanks, prompt the author to have a similar view to others, that is, that island arc curvature varies with time. It is worthy to note, nevertheless, that this statement could hardly be referred to as generally adopted, and it is not necessarily true. One can try to answer the second part of the question by means of mathematical statistics. Points of an island arc characterizing the geometry, such as volcanic centres or deep-trench axial line points, were led to the stereographical projection centre, where distortions are minimal. By the least squares method these points have been approximated with straight lines, circumferences and lines with variable curvature. Then by means of variance analysis the competing hypotheses have been compared. The results of the present study show that the curvature of the island arcs in the north-western Pacific varies with location. It is possible to search for relationships between island arc curvature as a measure of its bending, various physical parameters of island arc lithosphere and such recent island arc processes as volcanism, seismicity etc. Progress in this direction has already been made; namely a direct relationship between trench depth and island arc curvature has been established.     

Crustal heat transfer in the Taupo Volcanic Zone (New Zealand): comparison with other volcanic arcs and explanatory heat source models

The Taupo Volcanic Zone (TVZ) is a 200-km-long volcanic arc segment which developed ≤2 Ma ago within the continental crust of the North Island of New Zealand and lies at the southern end of the much larger Tonga-Kermadec arc system. The total crustal heat transfer of the TVZ is at present c. 2600 MW/100 km, most of the heat being transferred by convective geothermal systems. The rate of transfer is anomalously high in comparison to that of other active arcs, and arguably the highest world wide for such a setting. Heat transfer of other active arcs appear to vary almost linearly with subduction speed (about 150 MW/100 km for 10 mm/yr). The mass rate of common type arc extrusions (basalts, andesites, dacites) also increases almost linearly with subduction speed. This allows separation of the TVZ heat transfer into a “normal” component, associated with extrusions and intrusions of andesites and dacites (about 600 MW/100 km), and an “anomalous” component of about 2000 MW/100 km, related to extrusions and intrusions of rhyolitic melts whose generation is not directly controlled by subduction processes.Rhyolitic melts in the TVZ are partial melts of dominantly crustal origin. Comparison with other arcs indicates that the long-term extrusion rate of TVZ rhyolites (about 400 kg/s per 100 km) is also the highest world wide for this setting. The occurrence of voluminous Quaternary rhyolitic pyroclastics is a rare phenomenon and appears to be associated with a few arc segments (TVZ, Sumatra, Kyushu) that undergo significant crustal deformation.Various models have been proposed to explain the phenomenon of the anomalously high heat transfer within the TVZ. Models which require only heat transfer from plumes and subcrustal melts, either ponded at the crust/mantle boundary or intruding a spreading crust, are not suitable because the associated heat transfer at the contact is too low by a factor 2 to explain the required transfer rate of about 0.8 W/m² representing the “anomalous” crustal heat component of the TVZ. Heat generation by focussed plastic deformation within the ductile lithosphere is an alternative mechanism to explain “endogenous crustal heating” which yields heating rates that are also too low by a factor of two, although important parameters (average yield strength of lithosphere and opening rate of the TVZ) are not well known. A further search for a suitable combination of heat source models is required.     

平行走滑断层相互作用的粘弹模型和减震作用

研究了二维粘弹性介质中平行走滑断层的相互力学作用及其地震活动性的影响。当一条断层发生滑动,将导致平行断层面上剪切应力减小,因此,可能推迟平行断层发生滑动,推迟时间在几年至几百年的范围内,这取决于发生滑动的断层与平行断层之间的距离,以及平行断层自身应力积累孕育地震过程经历的时间。