East-dipping Slab Research Articles

Geodynamic evaluation of the pacific tectonic model for chortis block evolution using 3D numerical models of subduction

Past position and subsequent tectonic evolution of the Chortis Block represents one of the major uncertainties related to the geodynamic evolution of southern Mexico and northern Central America. This uncertainty has led to different hypotheses and evolutionary models based on tectonic, geological and paleographic studies that attempt to explain the origin and paleoposition of the Chortis Block. Among these models, a challenging hypothesis suggests the Chortis Block originates from a west-southwest position in the Pacific rather than attached to the Mexican margin. Additionally, this model proposes a high rate (200 km/20 Myr) of trench erosion along the Mexican margin just north of the present-day Chortis Block. In the Pacific model, the Chortis Block is being pushed eastward by the subduction movement of the Farallon and Cocos oceanic plates, and is finally captured by the Caribbean plate. In this study, the evolution of subduction along the Mexican and Central America margins is investigated numerically taking into account kinematic reconstruction of the Pacific model. The main goal is to investigate validity of the Pacific model for Chortis Block evolution by comparing our predicted slab geometries with present-day tomographic images. The kinematic 3D numerical models are integrated 45 Myr in time and use a mantle reference frame for tectonic plate evolution of the study area. These models generate subducting plate geometries that are only partial consistent with what is observed from seismic tomography, they reproduce realistic slab geometries only for the region located beneath the North America Plate, where the Cocos slab is flat at shallow depths and also stagnates along the mantle transition zone. On the other hand, for the region located beneath the Chortis Block the slab penetrates vertically through the upper mantle and even curls back in the upper portion of the lower mantle, a geometry that is inconsistent with seismic tomography images which shows the presence of east-dipping slab that tends to stagnate at 1000 km depth. This study indicate that the Pacific model is not an appropriate hypothesis for explaining the Chortis Block origin, and future studies should focus on alternative tectonic models.

Evidence of an east-dipping slab beneath the southern end of the Philippine Trench (1°N–6°N) as revealed by ISC-EHB

We examine recently released global seismic datasets in 3D visualization to study slab configurations in the Mindanao-Molucca region, where the present-day arc-arc collision of divergent double subduction propagates northward and attains completion on Mindanao Island, Philippines. The activity of inter-plate thrust earthquakes in the Philippine Trench is not significant until Mindanao (6°N) is reached, while shallow earthquakes on the island exhibit predominantly strike-slip movements on the Cotabato and Philippine Faults. The spatial distributions of GCMT thrust-type earthquakes shallower than 60 km reveal that current collisions mostly occur along the Central and Talaud-Miangas Ridges. ISC-EHB events deeper than 80 km, as well as Slab2 contours, show the eastward-dipping trench of the Halmahera slab flipping to the westward-dipping Philippine Trench northward. We identify a zone of ISC-EHB earthquakes steeply dipping to the east beneath the southern end of the Philippine Trench (1°N–6°N) that is not modeled by Slab2. This feature of a steeply east-dipping slab is explained by collision of the subduction arc and is consistent with steeply plunging T-axes of earthquakes. The identification of the east-dipping slab and estimation of its extent are crucial for understanding geodynamic and plate-boundary evolution in the Mindanao-Molucca region.

Bridging the gap between the foreland and hinterland II: Geochronology and tectonic setting of Ordovician magmatism and basin formation on the Laurentian margin of New England and Newfoundland

Ordovician strata of the Mohawk Valley and Taconic allochthon of New York and the Humber margin of Newfoundland record multiple magmatic and basin-forming episodes associated with the Taconic orogeny. Here we present new U-Pb zircon geochronology and whole rock geochemistry and neodymium isotopes from Early Paleozoic volcanic ashes and siliciclastic units on the northern Appalachian margin of Laurentia. Volcanic ashes in the Table Point Formation of Newfoundland and the Indian River Formation of the Taconic allochthon in New York yield dates between 466.16 ± 0.12 and 464.20 ± 0.13 Ma. Red, bioturbated slate of the Indian River Formation record a shift to more juvenile neodymium isotope values suggesting sedimentary contributions from the Taconic arc-system by 466 Ma. Eight ashes within the Trenton Group in the Mohawk Valley were dated between 452.63 ± 0.06 and 450.68 ± 0.12 Ma. These ashes contain zircon with Late Ordovician magmatic rims and 1.4 to 1.0 Ga xenocrystic cores that were inherited from Grenville basement, suggesting that the parent magmas erupted through the Laurentian margin. The new geochronological and geochemical data are integrated with a subsidence model and data from the hinterland to refine the tectonic model of the Taconic orogeny. Closure of the Iapetus Ocean by 475 Ma via collision of the peri-Gondwanan Moretown terrane with hyperextended distal fragments of the Laurentian margin is not clearly manifested on the autochthon or the Taconic allochthon other than an increase in sediment accumulation. Pro-foreland basins formed during the Middle Ordovician when these terranes were obducted onto the Laurentian margin. 466 to 464 Ma ashes on the Laurentian margin coincide with a late pulse of magmatism in both the Notre Dame arc in Newfoundland and the Shelburne Falls arc of New England that is potentially related to break-off of an east-dipping slab. Following slab reversal, by 455 Ma, the Bronson Hill arc was established on the new composite Laurentian margin. Thus, we conclude that Late Ordovician strata in the Mohawk Valley and Taconic allochthon of New York and on the Humber margin of Newfoundland were deposited in retro-foreland basins.

Assimilation, differentiation, and thickening during formation of arc crust in space and time: The Jurassic Bonanza arc, Vancouver Island, Canada

Research Article| March 01, 2016 Assimilation, differentiation, and thickening during formation of arc crust in space and time: The Jurassic Bonanza arc, Vancouver Island, Canada Rameses J. D’Souza; Rameses J. D’Souza † 1School of Earth and Ocean Science, University of Victoria, Victoria, British Colombia V8W 3P6, Canada †rdsouza@uvic.ca Search for other works by this author on: GSW Google Scholar Dante Canil; Dante Canil 1School of Earth and Ocean Science, University of Victoria, Victoria, British Colombia V8W 3P6, Canada Search for other works by this author on: GSW Google Scholar Robert A. Creaser Robert A. Creaser 2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2016) 128 (3-4): 543–557. https://doi.org/10.1130/B31289.1 Article history received: 30 Jan 2015 rev-recd: 21 Aug 2015 accepted: 06 Oct 2015 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rameses J. D’Souza, Dante Canil, Robert A. Creaser; Assimilation, differentiation, and thickening during formation of arc crust in space and time: The Jurassic Bonanza arc, Vancouver Island, Canada. GSA Bulletin 2016;; 128 (3-4): 543–557. doi: https://doi.org/10.1130/B31289.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Continental arcs and island arcs, eventually accreted to continental margins, are thought to have been the locus of continental growth since at least the Proterozoic Eon. The Jurassic Bonanza arc, part of the Wrangellia terrane on Vancouver Island, British Columbia, exposes the stratigraphy of an island arc emplaced between 203 and 164 Ma on a thick preexisting substrate of noncontinental origin. We measured the bulk major- and trace-element geochemistry, and Rb-Sr and Sm-Nd isotope compositions of 18 plutonic samples to establish if differentiation involved contamination of the Bonanza arc magmas by the pre-Jurassic basement rocks. The 87Sr/86Sr and 143Nd/144Nd isotope ratios of the plutonic rocks at 180 Ma vary from 0.70253 to 0.7066 and 0.512594 to 0.512717, respectively. Assimilation–fractional crystallization modeling using trace-element concentrations and Nd and Sr isotope ratios indicates that contamination by a Devonian island arc in the Wrangellia basement is less than 10%. Rare earth element modeling indicates that the observed geochemistry of Bonanza arc rocks represents two lineages, each defined by two stages of fractionation that implicate removal of garnet, varying in modal proportion up to 15%. Garnet-bearing cumulate rocks have not been reported from the Bonanza arc, but their inference is consistent with our crustal thickness estimates from geological mapping and geobarometry, indicating that the arc grew to at least 23 km total thickness. The inference of garnet-bearing cumulate rocks in the Bonanza arc is a previously unsuspected similarity with the coeval Talkeetna arc (Alaska), where garnet-bearing cumulate rocks have been described. Geochronological data from the Bonanza arc show a continuum in plutonic ages from 203 to 164 Ma, whereas the volcanic rocks show a bimodal age distribution over the same span of time with modes at 198 and 171 Ma. We argue that the bimodal volcanic age distribution is likely due to sampling or preservation bias. East-west separation of regions of young and old volcanism could be produced by rollback of a west-dipping slab, forearc erosion by an east-dipping slab, or juxtaposition of two arcs along arc-parallel strike-slip faults. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemical constraints on the relationship between the Miocene–Pliocene volcanism and tectonics in the Palaoco and Fortunoso volcanic fields, Mendoza Region, Argentina: New insights from 40Ar/39Ar dating, Sr–Nd–Pb isotopes and trace elements

New 40Ar/39Ar analyses constrain the formation of the volcanic succession of Sierra de Palaoco in the present back-arc of the Andean Southern Volcanic Zone (SVZ), near 36°S, to the Late Miocene and assigns them to the Huincán II Formation. The composition of major and trace elements, Sr, Nd and Pb isotopes of the Palaoco and nearby Río Grande rocks require a strong arc-like component in the mantle that is absent or weak in both Early Miocene (Fortunoso Group) and Pleistocene alkaline lavas (Llancanelo Group) erupted in the same area. We evaluate the relative roles of varying mantle source compositions and crustal contamination in the generation of geochemically very different lavas from the Palaoco, Fortunoso and Río Grande volcanic fields, north of the Payún Matrú Volcano. The source for the Early Miocene Fortunoso(I) basalts was a OIB-type mantle devoid of subduction zone input. This type of OIB-like volcanic activity terminated due to a change from an extensional to a compressional tectonic regime. Towards the end of the Miocene renewed alkaline volcanism at Fortunoso (II) display a transition to arc-type incompatible element enrichment. Shortly after the calc-alkaline Palaoco volcanism started with a very strong geochemical arc-signature including Ba/La≈60 and La/Nb=2–3. After a quiesence of 1Ma the major part of the voluminous Late Palaoco basalts were erupted around 7.5Ma over a few hundred ka. These are less enriched in Ba and Sr and have compositions like many Holocene rocks of the Southern Volcanic Zone. Isotopically the Fortunoso I and Palaoco rocks are distinct. Regional volcanism of the Charilehue, Huincán I and II mostly has a moderate arc-type enrichment indicating incipient arc developments. However, Palaoco and La Brea at (c. 35°S) show full geochemical arc-signature, and we infer that a frontal arc was established. The subsequent development in the Palaoco-Río Grande area encompasses renewed late Pliocene calc-alkaline low volume volcanic eruptions (Río Grande group) succeeded in the Late Pleistocene by alkaline OIB-type eruptions (Llancanelo group). In the light of the course of volcanism to the east, in the Nevado area, where late Miocene–Pliocene calc-alkaline volcanism was followed by Late Pliocene–Pleistocene alkaline volcanism. We propose a scenario where the Nazca plate developed an eastwards widening flat slab from which the east dipping slab before the Late Pliocene translated from Palaoco to Nevado and subsequently retreated passing Río Grande in the Late Pliocene. Alkaline back-arc volcanism was active east of the arc-volcanism and expanded westwards during the Late Pliocene and Pleistocene.

Age, construction, and exhumation of the midcrust of the Jurassic Bonanza arc, Vancouver Island, Canada

We present new U-Pb zircon ages for heterogeneous mafic-felsic rocks of the West Coast complex, a midcrustal plutonic component of exposed Jurassic arc-crustal section on Vancouver Island. We examine the timing of juvenile plutonic crust production, and consanguinity of volcanism and plutonism in this arc. The midcrustal plutons were emplaced between 193 Ma and 174 Ma, contemporaneous with the upper-crustal ( 3 yr −1 . Deformation and emplacement were contemporaneous, and there are no correlations among age, differentiation (peridotite to granite), or structural level of plutons in the arc. The age range and a weak eastward younging age polarity of the Jurassic arc section on Vancouver Island match that of the Talkeetna arc-crustal section in Alaska, suggesting that the two arcs are correlative and evolved by either forearc erosion above of an east-dipping slab, or slab rollback during west-dipping subduction.

Source-side shear-wave splitting and upper-mantle flow beneath the Arakan slab, India-Asia-Sundalind triple junction

Shear-wave splitting of S waves from earthquakes in the Arakan slab is consistent with strong asthenospheric anisotropy that developed as a consequence of the India-Asia collision. Global positioning system (GPS) site velocities in the India-Asia-Sundaland triple junction region show that deformation along the Arakan subduction zone is partitioned into dextral strike-slip motion, as India moves northwards with respect to Asia, and contraction across the Arakan trench and Chittagong-Tripura fold belt. Indian Ocean lithosphere comprising the Arakan slab is dismembered into three segments as a result of its collision with Asian lithosphere at the East Himalayan syntaxis. Offsets of intermediate-depth earthquake hypocenters at two locations delineate slab segments that form a left-stepping en echelon structure. Arakan slab focal mechanisms are consistent with slab sinking and along-strike compression and bending, and, south of 25°N, dextral strike-slip along the slab. Two regions of N-S contraction within the slab appear to be localized at the slab segment offsets. Teleseismically recorded S waves from earthquakes within the three slab segments, and surroundings, are split systematically: once corrected for receiver-side splitting, fast shear trends are predominantly trench-parallel beneath the east-dipping slab segments; are more nearly trench-normal on the Sundaland (east) side of the Arakan lithosphere; parallel the southern ∼E-W gap between Arakan slab segments; and turn sharply around the extreme northern and southern edges of subducted Arakan lithosphere. Source-side shear-wave splitting beneath India is consistent with observed ∼E-W–trending fast shear polarizations of SK(K)S splitting in northeastern India. The general pattern of both surface site velocities from GPS and shear-wave splitting studies is consistent with material flow around the eastern Himalayan syntaxis and into the mantle wedge above the Arakan slab, and around the northern terminus of the Arakan slab. The upper mantle may also flow through the gap between the central and southern Arakan slab segments.

Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina

Recent studies of the upper mantle above the flat slab in central Chile and Argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with “normal,” steeper slab geometries. The mantle above the horizontal section of the flat slab is characterized by low P-wave velocities, high S-wave velocities, and low V p / V s ratios. As the slab begins to transition to a more normal dip to the south, the mantle above it changes as well. Above this “transition zone”, the mantle is characterized by high P-wave velocities, high S-wave velocities, and generally high V p / V s ratios. Above this high velocity anomaly, in the corner between the south-dipping slab of the transition zone and the east-dipping slab of the normal subduction zone, lies a small shallow anomaly distinguished by its very low P- and S-wave velocities and its moderately high V p / V s ratio. We interpret these anomalies to represent very depleted cold lithosphere, cooled trapped asthenosphere, and frozen pooled melt, respectively. Recent research looking at the elastic properties of cratonic xenoliths indicate that a low V p, high V s, and low V p / V s signature is indicative of cratonic lithosphere. This suggests that the material above the flat slab may be ancient Laurentian lithosphere, or other melt-depleted, dry lithospheric material. The presence of frozen asthenosphere to the south of the flat slab may indicate that large volumes of asthenosphere were displaced in advance of the eastward progression of the flattening slab between 10 and 2 Ma, forcing a change in flow patterns from trench normal to SSW around the south eastern corner of the flattening slab. As flattening progressed this asthenosphere became trapped above the transition zone. Eventually, temperatures dropped sufficiently to freeze the last remaining pocket of extracted melt which had collected in the corner of the descending slab.