Subduction Style Research Articles

Thermal models of flat subduction and the rupture zone of great subduction earthquakes

In subduction zones, the size of the seismogenic zone that ruptures during a great thrust earthquake may be thermally controlled. We have constructed finite element thermal models of six transects across three different subduction zones in order to determine the temperature distribution along the plate interface and to predict the size of the seismogenic zone. These models incorporate the complex plate geometries (variable dip downsection) necessary to model a flat slab subduction style. We focus on the rupture zones of the great earthquakes of Nankai (SW Japan) 1946 M8.3 and Alaska 1964 M9.2 as well as on the Cascadia margin. Subduction zone segments with moderate to steep dips exhibit rupture zones of 100–150 km downdip width, consistent with earlier elastic dislocation models and thermal models. For shallow dipping flat slab segments, our models predict larger locked zones, of 150–250 km width, in good agreement with aftershock and geodetic studies. The wider seismogenic zone predicted for flat slab segments results from an uncommonly wide, cold, forearc region, as corroborated by surface heat flow observations. A global analysis of great M > 8 interplate earthquakes of the 20th century reveals that more than a third of these events occurred in flat slab segments, whereas these segments represent only 10% of modern convergent margins. This implies substantially higher interplate coupling for flat slab segments, likely due to the increased downdip extent of the seismogenic zone, and suggests that the seismic risk near such regions may be higher than previously thought.

On the role of subducting oceanic plateaus in the development of shallow flat subduction

Oceanic plateaus, aseismic ridges or seamount chains all have a thickened crust and their subduction has been proposed as a possible mechanism to explain the occurrence of flat subduction and related absence of arc magmatism below Peru, Central Chile and at the Nankai Trough (Japan). Their extra compositional buoyancy could prohibit the slab from sinking into the mantle. With a numerical thermochemical convection model, we simulated the subduction of an oceanic lithosphere that contains an oceanic crustal plateau of 18-km thickness. With a systematic variation, we examined the required physical parameters to obtain shallow flat subduction. Metastability of the basaltic crust in the eclogite stability field is of crucial importance for the slab to remain buoyant throughout the subduction process. In a 44-Ma-old subducting plate, basalt must be able to survive a temperature of 600–700 °C to keep the plate buoyant sufficiently long to cause a flat-slab segment. We found that the maximum yield stress in the slab must be limited to about 600 MPa to allow for the necessary bending to the horizontal. Young slabs show flat subduction for larger parameter ranges than old slabs, since they are less gravitationally unstable and show less resistance against bending. Hydrous weakening of the mantle wedge area and lowermost continent are required to allow for the necessary deformation of a change in subduction style from steep to flat. The maximum flat slab extent is about 300 km, which is sufficient to explain the observed shallow flat subduction near the Nankai Trough (Japan). However, additional mechanisms, such as active overthrusting by an overriding continental plate, need to be invoked to explain the flat-slab segments up to 500 km long below Peru and Central Chile.

Andean subduction styles and their effect on thermal structure and interplate coupling

The variability in subduction styles along the Andean margin is investigated with regard to its probable causes and likely effects. It is shown that there is no single Andean or Chilean subduction style. The alternation of segments of flat-slab subduction and steep subduction argue against mechanisms involving continental scale boundary conditions. The best candidate for explaining flat subduction style appears to be buoyancy related to thickened oceanic crust of moderate to young age. A high degree of interplate coupling and great subduction zone earthquakes can occur along both flat-slab segments and steeply subducting segments. In general interplate coupling is higher for flat-slab segments as demonstrated by a statistical analysis of deformation in the upper plate. A flat subduction style is shown to have an important thermal impact resulting in a much colder plate interface and may succeed in explaining the apparent increase in coupling.

The tectono-metamorphic history of the Valaisan domain from the Western to the Central Alps: New constraints on the evolution of the Alps

Petrographic and structural studies on metasedimentary rocks of the Valaisan domain from the Engadine window (Switzerland) to the Petit St. Bernard (France) show that this entire domain was subjected to high pressure–low temperature conditions during the Alpine orogeny. The pressure and temperature conditions increased from east to west. The metasedimentary rocks of the Engadine window and the Grisons area are characterized by blueschist-facies conditions (12–13 kbar, 350–400 °C, defined by Mg-carpholite and chloritoid occurrences). Toward the west, in the Petit St. Bernard and Versoyen units, the metamorphic conditions reached the eclogite facies (15– 16 kbar, 500 °C). The shortening direction associated with the high-pressure metamorphic event was oriented northwest- southeast all over the Valaisan domain. This direction is compatible with the direction of convergence between the European and Apulian plates in early Tertiary time (65–35 Ma). In the Eastern and Central Alps, blueschist-facies rocks occur below the Austroalpine nappes over an area of 300 × 20 km 2 (from the Tauern window to the Grisons area) and have a thickness around 10 km. This volume of blueschist-facies rocks is in contrast with eclogite-facies rocks of Western Alps that form only a small slice with a thickness of 2 to 5 km. We interpret the difference in volume and metamorphic conditions from east to west as being due to a change of the subduction style. In the east, we assume the formation of a large wedge with a thickness of 40–50 km, such that rocks underwent blueschist metamorphism and were exhumed before collision between the European and Apulian plates. In the west, the Petit St. Bernard and the Versoyen units were metamorphosed and extruded in a subduction channel above the subducting slab. The data further allow us to conclude that the Valaisan domain was an ocean that was independent of the Piemontese Ocean and that had an important place in the youngest phase of the evolution of the Alps before the collision.

Latent heat effects of the major mantle phase transitions on low-angle subduction

Very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate. We have investigated the effects of latent heat for this situation, where mantle material is pushed through the major mantle phase transitions during shallow low-angle subduction below the overriding plate. The significance of the buoyancy forces, arising from the latent heat effects, on the dynamics of the shallowly subducting slab is examined by numerical modeling. When a 32-Ma-old slab is overridden with 2.5 cm/yr by a continent, flat subduction occurs with a 4–5 cm/yr convergence rate. When latent heat is included in the model, forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces. Under these circumstances, the flat slab segment subducts horizontally about 350 km further and for about 11 Ma longer than in the case without latent heat, before it breaks through the 400-km phase transition. The style of subduction strongly depends on the mantle rheology: increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow. Similarly, an overriding velocity of less than 1 cm/yr leads to steep subduction, which gradually changes to flat subduction when increasing the overriding velocity. However, these model parameters do not change the aforementioned effect of the latent heat, provided that low-angle subduction occurs. In all models latent heat resulted in a substantial increase of the flat slab length by 300–400 km. Varying the olivine–spinel transition Clapeyron slope γ from 1 to 6 MPa/K reveals a roughly linear relation between γ and the horizontal length of the slab. Based on these results, we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low-angle subduction below an overriding plate.

Orogenic gold and geologic time: a global synthesis

AbstractOrogenic gold deposits have formed over more than 3 billion years of Earth’s history, episodically during the MiddleArchean to younger Precambrian, and continuously throughout the Phanerozoic. This class of gold deposit is characteristi-cally associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustalstructures. A consistent spatial and temporal association with granitoids of a variety of compositions indicates that melts andfluids were both inherent products of thermal events during orogenesis. Including placer accumulations, which arecommonly intimately associated with this mineral deposit type, recognized production and resources from economicPhanerozoic orogenic-gold deposits are estimated at just over one billion ounces gold. Exclusive of the still-controversialWitwatersrand ores, known Precambrian gold concentrations are about half this amount.The recent increased applicability of global paleo-reconstructions, coupled with improved geochronology from most ofthe world’s major gold camps, allows for an improved understanding of the distribution pattern of orogenic gold in spaceand time. There are few well-preserved blocks of Middle Archean mid-crustal rocks with gold-favorable, high-strain shearzones in generally low-strain belts. The exception is the Kaapvaal craton where a number of orogenic gold deposits arescattered through the Barberton greenstone belt. A few )3.0 Ga crustal fragments also contain smaller gold systems in theUkrainian shield and the Pilbara craton. If the placer model is correct for the Witwatersrand goldfields, then it is possiblethat an exceptional Middle Archean orogenic-gold lode-system existed in the Kaapvaal craton at one time. The latter half ofthe Late Archean ca. 2.8–2.55 Ga was an extremely favorable period for orogenic gold-vein formation, and resulting oresŽ.preserved in mid-crustal rocks contain a high percentage of the world’s gold resource. Preserved major goldfields occur ingreenstone belts of the Yilgarn craton e.g., Kalgoorlie , Superior province e.g., Timmins , Dharwar craton e.g., Kolar ,Ž. Ž . Ž.Zimbabwe craton e.g., Kwekwe , Slave craton e.g., Yellowknife , Sao Francisco craton e.g., Quadrilatero Ferrifero , andŽ. Ž . Ž .Tanzania craton e.g., Bulyanhulu , with smaller deposits exposed in the Wyoming craton and Fennoscandian shield. SomeŽ.workers also suggest that the Witwatersrand ores were formed from hydrothermal fluids in this period.The third global episode of orogenic gold-vein formation occurred at ca. 2.1–1.8 Ga, as supracrustal sedimentary rocksequences became as significant hosts as greenstones for the gold ores. Greenstone–sedimentary rock sequences nowexposed in interior Australia, northwestern Africarnorthern South America, Svecofennia, and the Canadian shield were the

Crustal structure of the southernmost Chilean margin from seismic and gravity data

The southern Chilean convergent margin south of the Strait of Magellan, between 52 and 57°S, is undergoing a style of subduction where obliquity plays a significant role. Seismic reflection and gravity data collected during 1988 by LDEO (RC2902) across the study area image crustal structures from oceanic crust (seaward of the trench) to the continental shelf. Pre-stack and post-stack depth migration have been applied to MCS profile RC2902-790, from which we have obtained a depth section, with true geometries and an associated accurate velocity model. Additional velocity information from sonobuoys has been used to further constrain the depth-converted section. 2D gravity modelling has supported the deduced crustal structure. Four main domains are imaged, which, from SW to NE, are: (1) the oceanic domain, where the Chilean trench is buried with a clastic sedimentary wedge reaching maximum thickness of 4 km, lying on a 7–8-km-thick crystalline oceanic crust that gently dips landward (3–4°); (2) the accretionary prism domain, about 40 km wide, of highly deformed sediments with a clear Bottom Simulating Reflector (BSR); (3) the forearc basin domain, a 25-km-wide basin (nearly undeformed) with a maximum sediment thickness of 4.5 km, which is bordered along its seaward edge by a zone of outer-arc structural highs and the accretionary prism forming the so-called ‘Fuegian terrace’; and (4) the continental domain which consists of an 8° dip continental slope and the continental shelf. Seismic reflection data along the continental shelf reveal that between 5 and 8 s twt, there is package of reflections that could mark the presence of a high reflective lower crust. Results from the corresponding sonobuoy are not conclusive enough to confirm such a hypothesis, but the brightness of these reflectors suggests the presence of a 10-km-thick reflective lower crust between 14 and 24 km depth. At greater depths (between 11 and 14 s twt), there is a thin package of bright landward-dipping reflectors, which may indicate the top of the subducting slab as deduced from gravity data. Furthermore, gravity modelling shows that the slab subducts at a very low angle of about 7–8°. In spite of the lack of seismicity, our results favour the conclusion that subduction still occurs in this region of the Chilean margin or has recently ceased.

‘Subduction Style’ Magmatism in a Non-subduction Setting: the Colville Igneous Complex, NE Washington State, USA

The Colville Igneous Complex is located within the Eocene Magmatic Belt of the North American Cordilleran interior. It straddles the US–Canadian border in northeast Washington and southern British Columbia. The complex consists of three intrusive and two extrusive phases, the first extrusive phase being contemporaneous with the latter two intrusive phases. As a consequence of sub-solidus re-equilibration in the plutonic rocks, this study concentrates on the two extrusive phases, the Sanpoil Volcanic Formation and the Klondike Mountain Formation. The Sanpoil Volcanic Formation consists of andesites, dacites and rare trachyandesites (SiO2 = 55–70 wt %) exhibiting a slight decrease in total alkalis (Na2O + K2O) with increasing silica. The Klondike Mountain Formation consists of basalts, basaltic andesites, andesites, dacites and rhyolites (SiO2 = 51–75 wt %) with total alkalis increasing with increasing silica. The calc-alkaline affinity of the rocks of the Colville Igneous Complex, coupled with the presence of a ‘subduction signature’ of enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), has traditionally been attributed to petrogenesis in a subduction-related magmatic arc, the ‘Challis Arc’. New trace and rare earth element and isotopic data (87Sr/86Sri, ϵNdi, δ18O), however, suggest that this explanation is no longer tenable. We propose that the magmas of the Sanpoil Volcanic Formation were generated by mid-crustal partial melting of a mid-Proterozoic source and that the Klondike Mountain Formation was formed by varying degrees of mixing between two distinct late-Proterozoic lower-crustal sourced magmas and a mantle-derived magma. In all cases, the subduction or calc-alkaline signature was inherited from the Proterozoic crustal sources. The only magmas that can confidently be attributed to a mantle source are the basalts of the Klondike Mountain Formation, which show no decoupling of the LILE and HFSE, i.e. no subduction signature, precluding the presence of a subduction slab beneath this part of the North American Cordillera during the Eocene. We propose the alternative model: that the Colville Igneous Complex formed as a result of decompression melting of crust and mantle during post-Laramide orogenic collapse of an overthickened crust.

The development of slabs in the upper mantle: Insights from numerical and laboratory experiments

We have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab. A two‐dimensional finite element code is applied to model Newtonian creep in the numerical experiments. Scaled analog media are used in the laboratory, a sand mixture models the brittle crust, silicone putty simulates creep in the lower crust and mantle lithosphere, and glucose syrup is the asthenosphere analog. Both model approaches show similar results and reproduce first‐order observations of the subduction process in nature based on density and viscosity heterogeneities in a Stokes flow model. Subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold. We find that the subduction velocity and angle are time‐dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670‐km discontinuity. The style of subduction is controlled by the prescribed velocity of convergence, the density contrast between the plates, and the viscosity contrast between the oceanic plate and the mantle. These factors can be combined in the buoyancy number F which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate. Variations in F control the stress in the plates, the speed and the dip of subduction, and the rate of trench retreat, reproducing the contrasting styles of subduction observed in nature. The subduction rate is strongly influenced by the work of bending the lithosphere as it subducts.

PROVENANCE FROM OPHIOLITES AND OCEANIC ALLOCHTONS: MODERN BEACH AND RIVER SANDS FROM LIGURIA AND THE NORTHERN APENNINES (ITALY)

The Northern Apennine arc consists of allochtonous oceanic units (Ligurids) and synorogenic terrigenous wedges, and includes both alpine (Voltri Massif) and apenninic (Bracco Unit) ophiolitic complexes and isolated slabs of oceanic lithosphere (Casanova Complex). Detritus transported by streams in this vast region, comprised between Bologna and Alessandria, and sand of Liguria beaches from Savona to La Spezia can be distinguished - through analysis of framework composition and transparent heavy mineral suites - in four provinces, further subdivided in sub-provinces and variants. The Apennine Province and the Antola Province are carbonaticlastic and characterized by recycling of Ligurid flysches, whereas the Voltri Province and the Bracco Province are dominated by ophioliticlastic detritus. Along the boundaries of major ophiolitic complexes various sub-provinces with mixed provenances characerized by significant ophioliticlastic detritus are recognized (Bormida di Spigno and Polcevera river sands, and Celle-Varazze beach sands for the Voltri Massif; Vara, Lavagna and Ghiaiaro river sands for the Bracco Unit). Ophiolite-derived material dominates the heavy mineral fraction also in the sand transported by major streams draining the Emilia Apennines (Trebbia, Nure, Taro). Framework composition and heavy mineral suites clearly differentiate the serpentineschist- and amphibole-rich (glaucophane, actinolite) detritus derived from alpine ophiolites involved in thick-skinned deformation and high-pressure metamorphism (Voltri Massif), from the serpentinite- and pyroxene-rich (diallage) detritus derived from oceanic lithosphere involved in thin-skinned deformation within the Apennine belt (Bracco Unit; Casanova Complex). In the study of ancient perisutural-basin clastic wedges, these criteria can help reconstructing subduction style and paleogeodynamic evolution of associated suture belts.

Styles of back‐arc extension in the Central Mediterranean

In the Central Mediterranean two back‐arc basins, the Liguro‐Provençal (LPb) and the Tyrrhenian basin (Tb), opened progressively and consecutively from the late Eocene–Oligocene to the present. Evolution in space and time of rifting and drifting processes, along three different transects across these basins, shows differences in the style of extension: LPb opened with the formation of a narrow, single rift, while in the Tb deformation and magmatism is spread over a wide area. Moreover at the Northern end of the Tb the locus of extension progressively migrated towards the east whereas in the Southern Tb the locus of extension and magmatism migrated inside the basin, inducing continental break‐up and drifting of the previously formed older conjugate basins. We propose that these different styles of back‐arc extension depend upon internal conditions, such as prerift rheology linked with its geological heritage, and external conditions, e.g. the style of subduction.

The style of the Tyrrhenian subduction

Momentum and energy equations are solved simultaneously for an incompressible viscous fluid in order to model the changes in the shape of a subducted slab when active convergence between the subducting and overriding plates comes to the end and slab pull becomes the dominant tectonic mechanism. This model can be applied to the Tyrrhenian domain, where it has been suggested that active convergence terminated about 7–9 Myr ago. During the active phase the angle of immersion of the slab at intermediate depths between 100 and 270 km is small, about 45°–50°, and large, about 80°–90°, at depths greater than 300 km. The phase of passive gravitational sinking is characterized by a substantial modification in the shape of the slab, with a large angle of immersion of 70° now at intermediate depths, decreasing to 50° in proximity of the tip of the slab. When the shape of the modelled slab is compared with the seismogenic portion of the subducted Ionian lithosphere in the Tyrrhenian, our results are consistent with subduction driven by slab pull and with cessation of active convergence between 7–9 Myr before present.

Flat to steep transition in subduction style

Research Article| October 01, 1994 Flat to steep transition in subduction style Dallas Abbott; Dallas Abbott 1Lamont-Doherty Earth Observatory, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Rebecca Drury; Rebecca Drury 1Lamont-Doherty Earth Observatory, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Walter H. F. Smith Walter H. F. Smith 2NOAA, Silver Spring, Maryland 20910 Search for other works by this author on: GSW Google Scholar Author and Article Information Dallas Abbott 1Lamont-Doherty Earth Observatory, Palisades, New York 10964 Rebecca Drury 1Lamont-Doherty Earth Observatory, Palisades, New York 10964 Walter H. F. Smith 2NOAA, Silver Spring, Maryland 20910 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1994) 22 (10): 937–940. https://doi.org/10.1130/0091-7613(1994)022<0937:FTSTIS>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Dallas Abbott, Rebecca Drury, Walter H. F. Smith; Flat to steep transition in subduction style. Geology 1994;; 22 (10): 937–940. doi: https://doi.org/10.1130/0091-7613(1994)022<0937:FTSTIS>2.3.CO;2 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 SocietyGeology Search Advanced Search Abstract We use a thermal history of the mantle to calculate the evolution of oceanic crustal thickness over Earth history and use residual depth anomalies from the present-day Pacific to find the crustal thickness range (9.2-11.6 km) where the subduction style changes from mainly flat to mainly steep. We find that steep subduction was well developed by 2.5 Ga, which coincides with a major change in sedimentary rare-earth element patterns. Over 50% of all oceanic crust subducted steeply by 2.0-1.6 Ga, the same interval over which the average thickness of continental plates declined rapidly. Because producing thick plates requires many episodes of flat subduction, our model can explain why there are no known thick plates <1.6 Ga. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Correspondence of ores of silver and gold with basement terranes in the American southwest

The ratios of silver to gold produced from epigenetic ore districts of the American southwest reveal a consistency of value ranges, differing by an order of magnitude, that may be identified with either one or the other of two geologic terranes in which the ores occur. A discriminating value of the ratio is about 17.5∶1, the ratio of crustal abundance given by Ahrens (1965). (No further significance is attributed to this value, at this time, beyond the fact that it appears to establish a reasonable separation of values on the basis of geographic occurrence.) Ores relatively enriched in Ag occur in terranes floored by thick Proterozoic clastic and Paleozoic marine successions, and ores relatively enriched in Au lie above or within a Proterozoic basement dominated by maficfelsic volcanic (arc) successions. Proterozoic granites occur in each region. The values of the ratio are broadly consistent within each terrane, irrespective of the age of ore formation, the ore deposit style, associated igneous rocks, structural control, differing interpreted styles of subduction, and weathering histories. These characteristics and associations support a hypothesis that metallogenic signatures of ore districts in this region are fundamentally related to the crust in which the ores occur.

前弧海盆の形態と沈み込み様式

Structural characteristics of Recent forearc regions can be divided into three types (shelf type, terrace type, slope type) in terms of the morphology of forearc basins and the topography of basements. The differences in forearc structure seems to correspond with the difference in deformation style of forearc lithospher by subduction. Supply and/or subduction of sediments at trench are secondary factor of forearc deformation. High plate convergence rate leads to widely uplifting of island arc less subsidence of forearc basin (shelf type). Conversely, low plate convergence rate leads to one-sided subsidence of trench side and increasing in undulation of forearc topography (slope type).Sediments from forearc basin can reconstruct the subsidence rate of forearc basin and the vertical movement of island arc and by use of planktonic microfossils and benthic for eminifera which determine the changes in sedimentation rate and paleobathymetry. Thus, forearc basins are tectonic recorder to estimete the arc evolution and the change in subduction styles.

A plate-kinematic framework for models of Caribbean evolution

We define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the Caribbean region. Newly determined poles of rotation defining the approximate spreading histories of the central North and the South Atlantic oceans are given. For the late Jurassic-Early Cretaceous anomaly sequence of the central North Atlantic, we have used previously published ∗ ∗ Klitgord and Schouten (1987). definitions of fracture-zone traces and magnetic anomaly picks, redetermining the pole positions and angular rotations for various isochrons on an Evans and Sutherland interactive graphics system. For magnetic anomalies younger than the Cretaceous Quiet Period in both oceans, we (1) used Seasat altimeter data to help define fracture-zone traces, and (2) identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones. By the finite difference method, the relative paleopositions and the relative motion history between North and South America were computed. This analysis defines the size and shape (and the rate at which the size and shape changed) of the interplate region between North and South America since the Middle Jurassic. Thus, a plate-kinematic framework is provided for the larger plates pertaining to the Caribbean region, in which can be derived more detailed scenarios for Gulf of Mexico and Caribbean evolution. North and South America diverged to approximately their present relative positions from Late Triassic? to Early Campanian (about 84 m.y. ago) time. This is the period during which the Gulf of Mexico and a Proto-Caribbean seaway were formed. Since the Campanian, only minor relative motion has occurred; from Early Campanian through to Middle Eocene times. South America diverged only another 200 km, and since the Middle Eocene, minor N-S convergence has occurred. These very minor post-Early Campanian motions have probably been accommodated by imperfect shear and compression along the Atlantic fracture zones to the east of the Lesser Antilles, and along the northern and southern borders of the Caribbean Plate. Accordingly, it is suggested that from Campanian time to the present, the relative motions between the North and South American plates have had only minor effects on the structural development of the Caribbean region. Primarily using the data of Engebretson et al. ∗∗ ∗∗ Engebretson (1982); Engebretson et al. (1985). , the convergence history of Pacific plates with North America was calculated for two points near the western Caribbean. By completing finite difference solutions, the convergence history of the Pacific plates with the Caribbean and South American plates can be approximated. The direction and rate of convergence of the Pacific plates with the Americas may have controlled the style of subduction and possible microplate migration along the North American, South American and western Caribbean boundaries that define the eastern Pacific plate margin.

Accurate source depths and focal mechanisms of shallow earthquakes in western South America and in the New Hebrides Island Arc

Synthetic seismograms are matched to long‐period P waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms. Accurate depths are obtained for a large sample of moderate‐sized (6 &lt; Ms &lt; 7) events which produced relatively simple P waveforms and do not require intensive analysis. The estimated error in the determination of depth is about ±5 km or less for most of the events considered. The procedure is applied to two suites of events, one in South America (83 events) and one in the New Hebrides (61 events). In these two areas of contrasting styles of subduction, the accurate depths provide new information on the bending of descending plates near trenches, the seismicity and tectonics of the upper plate, and the geometry and structure of the zone of contact between the upper and descending plates. Depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with Chapple and Forsyth's (1979) preferred model of a bending elastic‐perfectly plastic plate. However, an unusually deep event occurred beneath the Fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late Miocene time. The calculated depth of the event, 48 km, places it below the lithosphere‐asthenosphere boundary derived from thermal models. Vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin (&lt;10 km thick) curved zone of contact dipping arcward or landward from the trench axis. However, a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact. These events would be accommodated by an interplate zone 15 km thick. In both subduction zones, interplate events occur at depths between about 15 and 50 km. However, the downdip width of the South American interplate zone is 2 to 3 times larger than that in the New Hebrides. Most of the intraplate events near the interplate boundary are located in the descending plate. In the upper plate, seismicity is more pronounced in the “back arc” regions of the two subduction zones. Thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc. Depths of shallow earthquakes in the eastern Andes and sub‐Andean regions of South America range between about 9 and 36 km, indicating that both the upper and middle crust are seismically active; about 75% of the events occurred in the upper 25 km. Furthermore, the focal mechanisms indicate that from Ecuador to Argentina the crust of the eastern side of the Andean Cordilleran is in a state of east‐west compression oriented nearly parallel to the direction of convergence between the Nazca and South American plates. The depths, the orientations of the earthquake nodal planes, and the close relationships to Quaternary tectonics imply basement‐involved crustal shortening in much of the Eastern Andes and sub‐Andean zones of Peru and Argentina.

Sonobuoy and teleseismic study of Gulf of California transform fault earthquake sequences : REICHLE M. S., G. F. SHARMAN and J. N. BRUNE, 1976. Bull. seism. Soc. Am., 66 (5): 1623–1641

The southern Chilean convergent margin south of the Strait of Magellan, between 52 and 57°S, is undergoing a style of subduction where obliquity plays a significant role. Seismic reflection and gravity data collected during 1988 by LDEO (RC2902) across the study area image crustal structures from oceanic crust (seaward of the trench) to the continental shelf. Pre-stack and post-stack depth migration have been applied to MCS profile RC2902-790, from which we have obtained a depth section, with true geometries and an associated accurate velocity model. Additional velocity information from sonobuoys has been used to further constrain the depth-converted section. 2D gravity modelling has supported the deduced crustal structure. Four main domains are imaged, which, from SW to NE, are: (1) the oceanic domain, where the Chilean trench is buried with a clastic sedimentary wedge reaching maximum thickness of 4 km, lying on a 7–8-km-thick crystalline oceanic crust that gently dips landward (3–4°); (2) the accretionary prism domain, about 40 km wide, of highly deformed sediments with a clear Bottom Simulating Reflector (BSR); (3) the forearc basin domain, a 25-km-wide basin (nearly undeformed) with a maximum sediment thickness of 4.5 km, which is bordered along its seaward edge by a zone of outer-arc structural highs and the accretionary prism forming the so-called ‘Fuegian terrace’; and (4) the continental domain which consists of an 8° dip continental slope and the continental shelf. Seismic reflection data along the continental shelf reveal that between 5 and 8 s twt, there is package of reflections that could mark the presence of a high reflective lower crust. Results from the corresponding sonobuoy are not conclusive enough to confirm such a hypothesis, but the brightness of these reflectors suggests the presence of a 10-km-thick reflective lower crust between 14 and 24 km depth. At greater depths (between 11 and 14 s twt), there is a thin package of bright landward-dipping reflectors, which may indicate the top of the subducting slab as deduced from gravity data. Furthermore, gravity modelling shows that the slab subducts at a very low angle of about 7–8°. In spite of the lack of seismicity, our results favour the conclusion that subduction still occurs in this region of the Chilean margin or has recently ceased.