Forearc Sliver Research Articles

Relationships Between Upper-Plate Structure, Mantle Wedge Melting, and Fore-Arc Sliver Transport in the Nicaraguan Subduction Zone

Abstract This study images the base of the upper-plate lithosphere in the Nicaraguan subduction zone using common conversion point stacking of Sp phases and explores the relationships between deep upper-plate structure, subduction zone melting processes, and the arc-parallel transport of the Central American fore-arc sliver. We observe the negative velocity gradient associated with the upper-plate lithosphere–asthenosphere boundary in the Nicaraguan back-arc at depths of 60–80 km. However, the amplitude of the lithosphere–asthenosphere velocity gradient diminishes beneath the arc, consistent with a reduction in the shear-wave velocity of the deep upper-plate lithosphere. This zone lies above mantle wedge velocity anomalies which indicate upward-migrating partial melt, suggesting that ascending melt has altered and weakened the lithospheric mantle of the upper plate. In northwestern Nicaragua, the boundary of the Central American fore-arc sliver and the Caribbean plate, which is marked by a northeast decrease in geodetically measured arc-parallel surface velocities, lies above the northeast increase in the amplitude and localization of the lithosphere–asthenosphere velocity gradient. This correlation indicates that the kinematic margin of the Central American fore-arc sliver corresponds to a structural boundary that extends throughout the upper plate, and that is influenced by melt ascending from the mantle wedge.

Strain accumulation in the Mentawai Forearc Sliver, Indonesia, inferred from continuous GNSS-derived strain rate

The Mentawai Forearc Sliver (MFS) is characterized by oblique deformation formed as slip partitioned between normal and parallel trench plate convergence. The surge of great earthquakes from 2004 to 2012 along the adjacent Sunda trench left a large unbroken segment known as the Mentawai Seismic Gap. Here, we adopted continuous Global Navigation Satellite System (GNSS) observation data to identify the present regional crustal deformation using geodetic strain rates. The principal strain rate, dilatation rate, and maximum shear strain rate are about 0.13 microstrain/yr, 0.2 microstrain/yr, and 0.1 microstrain/yr, respectively, with the range of its uncertainties between 0.01 and 0.04 microstrain/yr. The dilatation and maximum shear strain rates reveal the spatial coverage of strike-slip duplex and backthrust tectonics along the Mentawai Forearc Sliver.

Exhumation and topographic evolution of the Chiapas Massif Complex (southern Mexico) constrained by thermochronologic data modeling along vertical profiles

Thermochronometry is used to better understand the processes responsible for Cenozoic magmatism and exhumation of the Chiapas Massif Complex that spans a diffuse triple junction between the Caribbean, North American and Cocos plates. A combination of zircon U-Pb, apatite fission tracks, (U-Th)/He as well as numerical modeling show contrasting histories. Exhumation started earlier in the south (~16 Ma) relative to the north (~9 Ma). Northern exhumation is related to activity on the Tonalá fault system, while to the south it may be correlated with transpressive deformation in Chiapas fold-and-thrust belt. The southern block also experienced significant topographic growth from ~5 Ma to ~1 Ma followed by intense erosion. Overall, the pattern of uplift is in agreement with the ‘closing zipper’ model in which a forearc sliver is progressively incorporated to the North American plate. Thermal models also support a Pleistocene decrease in topography consistent with independent paleoenvironmental and geomorphologic evidences.

Paleomagnetic Rotations in the Northeastern Caribbean Region Reveal Major Intraplate Deformation Since the Eocene

AbstractRelative Caribbean‐North American plate motion is partitioned over the trench and intra‐Caribbean plate faults that bound large scale tectonic blocks. Quantifying the kinematic evolution of this tectonic corridor is challenging because much of the region is submarine. We present an extensive regional paleomagnetic data set (1,330 cores from 136 sampling locations) from Eocene and younger rocks of the northern Lesser Antilles, the Virgin Islands, and Puerto Rico, and use a statistical bootstrapping approach to quantify vertical axis block rotations. Our results show that the Puerto Rico–Virgin Island (PRVI) block and the Northern Lesser Antilles (NoLA) block formed two coherently rotating domains that both underwent at least 45° counterclockwise rotation since the Eocene. The first ∼20° occurred in tandem in late Eocene and Oligocene time, after which the blocks were separated in the Miocene by the opening of the Anegada Passage. The last 25° of rotation of the PRVI block ended in the middle Miocene, whereas the NoLA block rotated slower, until the latest Miocene. The boundary between the NoLA block and a non‐rotated Southern Lesser Antilles was likely the Monserrat‐Harvers fault zone. These results require hundreds of kilometers of intra‐Caribbean motions with oroclinal bending of the trench or forearc sliver motion along the curved plate boundary as endmembers. These data invite a critical re‐evaluation of the kinematic reconstruction of Caribbean‐North American plate motion. The consequent changes in paleogeography may provide a new view on the enigmatic eastern Caribbean paleo‐biogeography and the Paleogene dispersal of South American mammals toward the Greater Antilles.

Stress accumulation and earthquake activity on the Great Sumatran Fault, Indonesia

The seismically active Sumatra subduction zone has generated some of the largest earthquakes in the instrumental record, and both historical accounts and paleogeodetic coral studies suggest these were large enough to transfer stress to the surrounding region, including the Great Sumatran Fault (GSF). Therefore, evaluating the stress transfer from these large subduction earthquakes could delineate segments of elevated stress along the GSF where large earthquakes may potentially occur. In this study, we investigated eight megathrust earthquakes from 1797 to 2010 and resolved the accumulated Coulomb stress changes onto 18 segments along the GSF. Additionally, we also estimated the rate of tectonic stress on the GSF segments which experienced large earthquakes. We considered two cases, with: (1) no forearc sliver movement, and (2) the forearc sliver movement suggested by recent studies. Based on the historical stress changes of large earthquakes and the increase in tectonic stress rate, we analyzed the time evolution of stress changes on the GSF. The Coulomb stress changes on the GSF due to megathrust earthquakes between 1797 and 1907 increased the Coulomb stress mainly on the southern part of GSF, which was followed by four major GSF events during 1890–1943. The estimation of tectonic stress rates using case (1) produces a low rate of stress accumulation and long recurrence intervals, which would imply that megathrust earthquakes play an important role in promoting the occurrence of GSF earthquakes. When implementing the arc-parallel sliver movement of case (2), the tectonic stress rates are much higher than case (1), with an observed slip rate of 15–16 mm/yr at the GSF consistent with a recurrence interval for full-segment rupture of 100–200 years. The case (2) result suggests that the occurrence of GSF earthquakes is dominantly controlled by the rapid arc-parallel forearc sliver motion. Furthermore, the analysis of the evolution of stress changes with time shows that some segments such as Tripa (North and South), Angkola, Musi and Manna, which have experienced full-segment rupture and are therefore likely locked, appear to have returned to stress levels similar to those prior to previous historical events, suggesting elevated earthquake hazard along these GSF segments.

Role of convergence obliquity and inheritance on sliver tectonics: Insights from 3-D subduction experiments

The subduction dynamics and deformation style have been classically understood under a vision of trench-orthogonal plate convergence. We perform 3-D upper mantle-scale analog models to further understand the dynamics of subduction systems and overriding plate strain distribution under varying convergence angles and in the presence or absence of intraplate inherited weak zones. The laboratory experiments show that whatever the obliquity of convergence, the subduction dynamics is largely influenced by the interaction of the slab with the 660 km discontinuity. The slab geometry alternates between stages of slab shallowing and steepening, which are accompanied by alternating periods of shortening and stretching (or moderate shortening) of the overriding plate, respectively. As convergence departs from being orthogonal and in the absence of an intraplate inherited weak zone, strain within the overriding plate shifts from pure shear to sub-simple shear. When a lithospheric-scale weak zone is introduced along the forearc-arc interface and under trench-oblique convergence, the strain is partitioned and the forearc develops sliver motion - and independent deformation - from the rest of the overriding plate. The forearc evolution is characterized by a three-step history: (i) detachment and oceanward motion, (ii) advancing motion toward the continent, and, (iii) accretion that can ultimately lead to underthrusting of the forearc. Decreasing slab dip increases the interplate force, resulting in larger stresses applied on the intraplate fault separating the forearc sliver from the continent. Hence, models show that because larger compression increases the coupling between the forearc and the plate, it does not favor sliver motion.

Active Triclinic Transtension in a Volcanic Arc: A Case of the El Salvador Fault Zone in Central America

The El Salvador Fault Zone (ESFZ) is part of the Central American Volcanic Arc and accommodates the oblique separation movement between the forearc sliver and the Chortis block (Caribbean Plate). In this work, a triclinic transtension model was applied to geological (fault-slip inversion, shape of volcanic calderas), seismic (focal mechanisms) and geodetic (GPS displacements) data to evaluate the characteristics of the last stages of the kinematic evolution of the arc. The El Salvador Fault Zone constitutes a large band of transtensional deformation whose direction varies between N90° E and N110° E. Its dip is about 70° S because it comes from the reactivation of a previous extensional stage. A protocol consisting of three successive steps was followed to compare the predictions of the model with the natural data. The results show a simple shear direction plunging between 20° and 50° W (triclinic flow) and a kinematic vorticity number that is mostly higher than 0.81 (simple-shearing-dominated flow). The direction of shortening of the coaxial component would be located according to the dip of the deformation band. It was concluded that this type of analytical model could be very useful in the kinematic study of active volcanic arcs, even though only information on small deformation increments is available.

Deformation in Western Guatemala Associated With the NAFCA (North America‐Central American Forearc‐Caribbean) Triple Junction: Neotectonic Strain Localization Into the Guatemala City Graben

AbstractRecent structural and geodetic data define the Guatemala City graben region as the continental triple junction between the North American plate, Caribbean plate, and the Central American Forearc sliver. We present minor fault analysis, geochronological and geochemical analyses, and newly updated GPS velocities in western Guatemala, west of the Guatemala City graben, to characterize the magnitude and timing of extensional deformation in this poorly understood area. Elongations estimated from fault data are parallel (∼east‐west) and perpendicular to the Polochic‐Motagua fault system to the north, similar to geodetically measured active deformation observed east of the Guatemala City graben. Four new 40Ar/39Ar dates and correlation of tephra deposits suggest that faulting was active during the Pliocene, but ceased eastward toward the Guatemala City graben over time. From west to east, fault cessation occurred before the deposition of the Los Chocoyos ash (75 ka) and E tephra (51 ka). Faulting just west of the Guatemala City graben appears to be active, where a major fault cuts the most recent Amatitlán tephras. Based on this data, we propose a time‐progressive strain model for deformation related to North America‐Caribbean plate interactions, whereby distributed elongation of the westernmost Caribbean plate occurred during the Pliocene but localized mostly within the Guatemala City graben and nearby faults during the Pleistocene. Our model supports that: (a) The Guatemala City graben is effectively the western limit of the Caribbean plate; and (b) Western Guatemala, which was the trailing edge of the Caribbean plate, has been transferred to the forearc region.

Active faults of El Salvador

In this work we present a review of the current state of knowledge of the active faults in El Salvador and its seismo-tectonic implications. An updated map of active fault traces is combined with the spatial analysis of shallow seismicity, focal mechanisms, recent geodetic GPS velocities, local strain estimations and morpho-tectonic features to provide a synoptic view of the active tectonics of the El Salvador. The major faults selected as potential seismic sources bound tectonic blocks or regions that include GPS sites with consistent relative velocity vectors. We propose several active tectonic domains along El Salvador controlled by three current deformation regimes: a crustal block in the forearc sliver dominated by rigid westward translation with the faster and more homogeneous GPS velocities; three zones dominated by E-W distributed extensional coaxial deformation; and two bands with a deformation compatible with transtensional regime. In the Western sector of the ESFZ, GPS velocities and local structure suggest that E-W extension concentrates along the NNW-SSE oriented Santa Ana volcanic axis and it could connect to the north with the extensional region of the Ipala Graben. This is consistent with to the eastward shift of the North America-Cocos-Caribbean triple junction and “the closing of the zipper” proposed in recent models that progressively slows down the strike-slip movement along the northern limit of the forearc sliver. In the central sector large GPS velocity gradient parallel to the volcanic arc may be associated to complex and discontinuous structure of the ESFZ driving slowdown of the westward movement of local tectonic blocks. The southeaster sector of the ESFZ is an incipient large pull-apart structure affecting a pre-existing extensional N-S oriented fabric that induces two tectonic subdomains, the eastern one undergo E-W coaxial extension and the western one characterized by transtensional strain regime. We propose a new structural segmentation of the ESFZ considering that this fault zone includes all those structures that accommodate the relative velocity between the forearc sliver and the Chortís block. The 38 active faults with surface traces that are mapped for more than 5 km long proposed in this work, with maximum potential magnitude Mw ranging from 5.98 to 7.94, will contribute to improve regional and local seismic sources databases and seismic hazard assessment.

Active 650-km Long Fault System and Xolapa Sliver in Southern Mexico

New estimates of long-term velocities of permanent GPS stations in Southern Mexico reveal that the geologically discernible ~650 km long shear zone, which strikes parallel to the Middle America trench, is active. This left-lateral strike-slip La Venta-Chacalapa (LVC) fault system is apparently associated with a motion of the Xolapa terrain, and at the present time is the northern boundary of a ~110 – 160 km wide forearc sliver with a sinistral motion of 3 – 6 mm/yr with respect to the North America plate. This sliver is the major tectonic feature in the Guerrero and Oaxaca regions, which accommodates most of the oblique component of the convergence between the Cocos and North America plates. Previous studies based purely on the moment tensor coseismic slips exceedingly overestimated the sliver inland extent and allocated its northern margin on or to the north of the Trans-Mexican Volcanic Belt. While the LVC fault system probably slips slowly over geologic scale time, and there is not any historic evidence of large earthquakes on the fault so far, its seismic potential could be very high assuming a feasible order of ~103 years recurrence cycle. A detailed analysis of long-term position time series of permanent GPS stations in the Guerrero and Oaxaca states, Southern Mexico discards previous models and provides clear evidence of an active LVC fault zone bounding the Xolapa forearc sliver. The southeastward motion of this sliver may have persisted for the last ~8-10 Myr and played an important role in the tectonic evolution of the region.

Neotectonic structures and stress fields associated with oblique collision and forearc sliver formation in northern Hispaniola: Implications for the seismic hazard assessment

Active oblique collision between the Caribbean and North America plates has led to the formation of the Septentrional forearc sliver, which is a wedge-shaped crustal domain limited to the north by the North Hispaniola subduction thrust and to the south by the Septentrional Fault Zone (SFZ). In northern Hispaniola, oblique collision of the Bahamas Platform began in the uppermost Pliocene to Early Pleistocene and continues today. Collision-related tectono-stratigraphic processes include destruction of the shallow-water carbonate forearc platform, uplift and erosion of the Cordillera Septentrional and Samaná Peninsula, emergence of the Cibao Basin and syn-tectonic deposition of alluvial fan systems. The geometry and kinematics of neotectonics structures at all scales, together with stress field solutions obtained from late Cenozoic fault-slip data, show that the ENE-directed (far-) stress field associated with collision triggered the formation of the WNW-trending, left-lateral strike-slip SFZ. The SFZ thus represents the inland, concave master fault bounding the Septentrional forearc sliver. Subsidiary ENE to NE-trending left-lateral strike-slip faults (R-type) and NNE to NE-trending normal and oblique faults (T-type) controlled the internal deformation and subdivision into smaller tectonic blocks of the sliver. The highly oblique convergence and the increase in frictional plate coupling associated with the underthrusting of the Bahamas Platform are the likely causes of the formation of this forearc sliver. In the Western Cordillera Septentrional and Cibao Basin, an extensional regime is locally produced by the transfer of the left-lateral movement to a southern splay of the SFZ with the incorporation of a detached block to the Septentrional forearc sliver during the Middle Pleistocene-Holocene. The regional distribution of the modeled Peak Ground Acceleration values indicates that the SFZ is the greatest seismic hazard in northern Hispaniola.

Fate of Forearc Lithosphere at Arc‐Continent Collision Zones: Evidence From Local Earthquake Tomography of the Sunda‐Banda Arc Transition, Indonesia

AbstractA new 3‐D seismic P wave velocity model of the Sunda‐Banda Arc Transition from local earthquake tomography reveals (i) northward subduction of oceanic lithosphere, associated with the convergence of Australia and Sundaland, as a high‐velocity zone extending down to ~200 km depth; (ii) two distinct low‐velocity zones, one immediately above the slab, which is likely a zone of partial melt, and one in the 0–40 km depth range, which is probably a magma chamber associated with active volcanoes above; and (iii) a northerly dipping high‐velocity zone that bisects the two low‐velocity anomalies, which we interpret as an underthrust forearc sliver of continental origin. Based on He3/He4 measurements from volcanic outflows, it is possible that the magma supply is contaminated by interaction with this forearc sliver as it ascends from the melt region below.

Evidence of giant earthquakes and tsunamis of the seventeenth-century type along the southern Kuril subduction zone, eastern Hokkaido, northern Japan: a review

Abstract Large earthquakes of magnitude 7–8 occur at intervals of about 70–100 years in the southern Kuril Trench off eastern Hokkaido, northern Japan. However, especially large giant earthquakes (M w c. 8.8) have recurred at 300–500 year intervals, and the most recent of these was in the early seventeenth century. Surveys on the Pacific coast of eastern Hokkaido have documented field evidence that includes sharp contacts of peat and mud layers, indicative of coseismic coastal uplift, and large tsunami sand layers. Several proposed fault models of the seventeenth-century giant earthquake have been tested with numerical tsunami simulations. The best of these models incorporates a multi-segment interplate earthquake (M w 8.8) with large slip near the trench axis. The seventeenth-century giant earthquake and the 1611 Keicho Sanriku tsunami appear to have had different sources because varve counts from Lake Harutori suggest that the seventeenth-century giant earthquake occurred at c. 1637. Moreover, a consideration of regional tectonics suggests that convergence of the Kuril forearc sliver controls the current topography of eastern Hokkaido. Further progress in understanding the seismicity of this region will benefit from the combined insights of both geologists and seismologists.

Deformation associated with sliver transport in Costa Rica: seismic and geodetic observations of the July 2016 Bijagua earthquake sequence

SUMMARYTectonic slivers form in the overriding plate in regions of oblique subduction. The inner boundaries of the sliver are often poorly defined and can consist of well-defined faults, rotating blocks or diffuse fault systems, which pass through or near the volcanic arc. The Guanacaste Volcanic Arc Sliver (GVAS) as defined by Montero et al., is a segment of the Central American Forearc Sliver, whose inner boundary is the ∼87-km-long Haciendas-Chiripa fault system (HCFS), which is located ∼10 km behind the volcanic arc and consists of strike slip faults and pull apart steps. We characterize the current ground motion on this boundary by combining earthquake locations and focal mechanisms of the 2016 Bijagua earthquake sequence, with the surface ground deformation obtained from Interferometric Synthetic Aperture Radar (InSAR) images from the ALOS-2 satellite. The coseismic stack of interferograms show ∼6 cm of displacement towards the line of sight of the satellite between the Caño Negro fault and the Upala fault, indicating uplift or SE horizontal surface displacement. The largest recorded earthquake of the sequence was Mw 5.4, and the observed deformation is one of the smallest earthquakes yet detected by InSAR in the Central American region. Forward and inverse models show the surface deformation can be partially explained by slip on a single fault, but it can be better explained by slip along two faults linked at depth. The best-fitting model consists of 0.33 m of right lateral slip on the Caño Negro fault and 0.35 m of reverse slip on the Upala fault, forming a positive flower structure. As no reverse seismicity was recorded, we infer the slip on the Upala fault occurred aseismically. Observations of the Bijagua earthquake sequence suggests the forearc sliver boundary is a complex and diffuse fault system. There are localized zones of transpression and transtension and areas where there is no surface expression suggesting the fault system is not yet mature. Although aseismic slip is common on subduction interfaces and mature strike-slip faults, this is the first study to document aseismic slip on a continental tectonic sliver boundary fault.

Activity of crustal faults and the Xolapa sliver motion in Guerrero\u2013Oaxaca forearc of Mexico, from seismic data

Oblique convergent margins often host forearc slivers separated by the subduction interface and a trench parallel strike-slip fault system in the overriding plate. Mexican oblique subduction setting led to the formation of a forearc sliver and accomodation of part of the slip at the bounding system of strike-slip faults. The Xolapa sliver is, on average, a sim 105-km-wide crustal block located along the coast of Guerrero and Oaxaca states of Mexico, and is limited by a sim 650-km-long La Venta-Chacalapa fault zone. Two types of datasets, local catalog and Global CMT compilation, are used to estimate the motion of the Xolapa sliver using the rigid block model that describes the phenomenon of slip partitioning. According to the results obtained from local and Global CMT catalogs for selected subduction thrust earthquakes, the forearc sliver moves southeastwards with respect to the fixed North America plate at the rate of 10 ± 1 mm/year and 5.6 ± 0.8 mm/year, respectively. These velocities in general agree with the values obtained from long-term GPS observations (5–6 mm/year). The origin of the inconsistency between local and teleseismic estimates is attributed to a difference in the double couple focal mechanism parameters for two types of datasets. Convergence obliquity changes from 10.42{^circ } and the rate of 58 mm/year to 13.29{^circ } at the rate of 68 mm/year along the Guerrero and Oaxaca coast increasing from northwest to southeast; therefore, the Xolapa sliver is supposed to be stretched. However, the slip vector azimuths of thrust subduction earthquakes tend to approach plate convergence vectors southeastwards along the coast; so, we assume that this may produce the forearc block compression.

Push-pull driving of the Central America Forearc in the context of the Cocos-Caribbean-North America triple junction

Different kinematic models have been proposed for the triple junction between the North American, Cocos and Caribbean plates. The two most commonly accepted hypotheses on its driving mechanism are (a) the North American drag of the forearc and (b) the Cocos Ridge subduction push. We present an updated GPS velocity field which is analyzed together with earthquake focal mechanisms and regional relief. The two hypotheses have been used to make kinematic predictions that are tested against the available data. An obliquity analysis is also presented to discuss the potential role of slip partitioning as driving mechanism. The North American drag model presents a better fit to the observations, although the Cocos Ridge push model explains the data in Costa Rica and Southern Nicaragua. Both mechanisms must be active, being the driving of the Central American forearc towards the NW analogous to a push-pull train. The forearc sliver moves towards the west-northwest at a rate of 12–14 mm/yr, being pinned to the North American plate in Chiapas and western Guatemala, where the strike-slip motion on the volcanic arc must be very small.

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.

Chapter 4 Geological and tectonic evolution of the Indo-Myanmar Ranges (IMR) in the Myanmar region

The Indo-Myanmar Ranges (IMR) of Myanmar, also known as the Indo-Burman Ranges (IBR) or the Western Ranges, extend from the East Himalayan Syntaxis (EHS) southwards along the eastern side of the Bay of Bengal to the Andaman Sea, comprising the Naga Hills Tract in the north, the Chin Hills in the middle and the Rakhine (Arakan) Yoma in the south. The IMR is economically important; major discoveries of oil and gas have been made in the Bay of Bengal to the west of the Rakhine Yoma, and there are several occurrences of chromite and nickel deposits (e.g. Webula, Mwetaung in Chin State) and submarine volcanic-hosted massive sulphide deposits (e.g. Laymyetna in Ayerwaddy Region). The IMR occupies a complex tectonic zone as the southeastwards continuation of the Indian–Asian collision belt in Tibet and Assam, and lies north of the active subduction zone of the Sunda–Andaman arc (Figs 4.1 & 4.2). The IMR occurs along the western margin of the Myanmar Microplate, also known as the Burmese Platelet or the West Myanmar Terrane or Block, situated between the Eurasian Plate to the east and the Indian Plate to the west (e.g. Fitch 1972; Curray et al. 1979; Mukhopadhyay & Dasgupta 1988; Pivnik et al. 1998; Bertrand & Rangin 2003; Shi et al. 2009; Baxter et al. 2011; Garzanti et al. 2013; Soibam et al. 2015). The West Myanmar Block has been also described as a forearc sliver, bounded on the west by a subduction zone and a strike-slip margin, on the east by a strike-slip fault (Sagaing Fault), on the south by a spreading centre and on the north by a compressional plate boundary (Curray et al. 1979; Pivnik et al. 1998; Nielsen et al. 2004). Fig. 4.1. Tectonic map of Myanmar region …

Chapter 6 The mafic–ultramafic (ophiolitic) rocks of Myanmar

Mafic and ultramafic rocks are widely distributed in Myanmar and form major tectonic belts and suture zones representing remnants of the Mesotethyan Ocean. They occur as deformed and disrupted ophiolitic sequences and in forearc slivers as part of an accretionary wedge, intimately associated with siliciclastic and limestone units. They commonly host chromite–nickel–manganese, copper (gold) and jadeite mineralization. In this chapter, the nature, distribution, geological setting, petrological, mineralogical and ore characteristics of the mafic and ultramafic rocks in Myanmar are reviewed and described, and their tectonic setting and mineralization potential are discussed. Myanmar is characterized by north–south-trending major tectonic domains. From west to east, these include: the Rakhine coastal strip, an ensimatic foredeep; the Indo-Myanmar Ranges, an outer arc or forearc; the Western Inner-Myanmar Tertiary Basin, an inter-arc basin; the Central Volcanic Belt (Central Volcanic Line), an inner magmatic-volcanic arc; the Eastern Inner-Myanmar Tertiary Basin, a back-arc basin; and the Shan-Tenasserim massif, the ensialic Sino-Myanmar Ranges (Bender 1983; Khin Zaw 1989, 1990, 2017; Khin Zaw et al. 2015). Among these tectonic domains, ophiolitic rock associations occur in three belts trending nearly north–south parallel with each other (from west to east): the Western Ophiolitic Belt (WOB); the Central Ophiolitic Belt (COB); and the Eastern Ophiolitic Belt (EOB) (Fig. 6.1). The narrow WOB forms the longest belt, extending from the north to the southern part of Myanmar covering the eastern hills of the Naga, Chin and Rakhine ranges. According to Hutchison (1975), there are two ophiolite belts in Myanmar: the Naga Hill Line and the Mandalay Line. In this chapter the Naga Hill Line is termed the WOB. The Mandalay Line is the combination of the COB and the EOB. The southern continuation of the Mandalay Line is uncertain, due to a lack of comprehensive geological information. These …

Rupture area analysis of the Ecuador (Musine) Mw = 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients

The Ecuador Mw = 7.8 earthquake on April 16, 2016, ruptured a nearly 200 km long zone along the plate interface between Nazca and South American plates which is coincident with a seismic gap since 1942, when a Mw = 7.8 earthquake happened. This earthquake occurred at a margin characterized by moderately big to giant earthquakes such as the 1906 (Mw = 8.8). A heavily sedimented trench explains the abnormal lengths of the rupture zones in this system as inhibits the role of natural barriers on the propagation of rupture zones. High amount of sediment thickness is associated with tropical climates, high erosion rates and eastward Pacific dominant winds that provoke orographic rainfalls over the Pacific slope of the Ecuatorian Andes. Offshore sediment dispersion off the oceanic trench is controlled by a close arrangement of two aseismic ridges that hit the Costa Rica and South Ecuador margin respectively and a mid ocean ridge that separates the Cocos and Nazca plate trapping sediments. Gravity field and Ocean Circulation Explorer (GOCE) satellite data are used in this work to test the possible relationship between gravity signal and earthquake rupture structure as well as registered aftershock seismic activity. Reduced vertical gravity gradient shows a good correlation with rupture structure for certain degrees of the harmonic expansion and related depth of the causative mass; indicating, such as in other analyzed cases along the subduction margin, that fore-arc structure derived from density heterogeneities explains at a certain extent propagation of the rupture zones. In this analysis the rupture zone of the April 2016 Ecuador earthquake developed through a relatively low density zone of the fore-arc sliver. Finally, aftershock sequence nucleated around the area of maximum slips in the rupture zone, suggesting that heterogeneous density structure of the fore-arc determined from gravity data could be used in forecasting potential damaged zones associated with big ruptures along the subduction border.