Slab Window Research Articles

Geology, Geochronology, and Geodynamics of the Khan Bogd alkali granite pluton in southern Mongolia

The Khan Bogd alkali granite pluton, one of the world’s largest, is situated in the southern Gobi Desert, being localized in the core of the Late Paleozoic Syncline, where island-arc calc-alkaline differentiated volcanics (of variable alkalinity) give way to the rift-related bimodal basalt-comendite-alkali granite association. The tectonic setting of the Khan Bogd pluton is controlled by intersection of the near-latitudinal Gobi-Tien Shan Rift Zone with an oblique transverse fault, which, as the rift zone, controls bimodal magmatism. The pluton consists of the eastern and the western ring bodies and comes into sharp intrusive contact with rocks of the island-arc complex and tectonic contact with rocks of the bimodal complex. The inner ring structure is particularly typical of the western body and accentuated by ring dikes and roof pendants of the country island-arc complex. According to preliminary gravity measurements, the pluton is a flattened intrusive body (laccolith) with its base subsiding in stepwise manner northwestward. Reliable geochronologic data have been obtained for both plutonic and country rocks: the U-Pb zircon age of alkali granite belonging to the main intrusive phase is 290 ± 1 Ma, the 40Ar/39Ar ages of amphibole and polylithionite are 283 ± 4 and 285 ± 7 Ma, and the Rb-Sr isochron yields 287 ± 3 Ma; i.e., all these estimates are close to 290 Ma. Furthermore, the U-Pb zircon age of red normal biotite granite (290 ± 1 Ma) and the Rb-Sr age of the bimodal complex in the southern framework of the pluton are the same. The older igneous rocks of the island-arc complex in the framework and roof pendants of the pluton are dated at 330 Ma. The geodynamic model of the Khan Bogd pluton formation suggests collision of the Hercynian continent with a hot spot in the paleoocean; two variants of this model are proposed. According to the first variant, the mantle plume, after collision with the margin of the North Asian paleocontinent, reworked the subducted lithosphere and formed a structure similar to an asthenospheric window, which served as a source of rift-related magmatism and the Khan Bogd pluton proper. In compliance with the second variant, the emergence of hot mantle plume resulted in flattening of the subducted plate; cessation of the island-arc magmatism; and probably in origin of a local convective system in the asthenosphere of the mantle wedge, which gave rise to the formation of a magma source. The huge body of the Khan Bogd alkali granite pluton and related volcanic rocks, as well as its ring structure, resulted from the caldera mechanism of the emplacement and evolution of magmatic melts.

Emplacement of the Kodiak batholith and slab-window migration

Research Article| November 01, 2006 Emplacement of the Kodiak batholith and slab-window migration David W. Farris; David W. Farris 1Department of Earth Sciences, University of Southern California, 3651 Trousdale Avenue, Los Angeles, California 90089, USA Search for other works by this author on: GSW Google Scholar Peter Haeussler; Peter Haeussler 2U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA Search for other works by this author on: GSW Google Scholar Richard Friedman; Richard Friedman 3Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada Search for other works by this author on: GSW Google Scholar Scott R. Paterson; Scott R. Paterson 4Department of Earth Sciences, University of Southern California, 3651 Trousdale Avenue, Los Angeles, California 90089, USA Search for other works by this author on: GSW Google Scholar R.W. Saltus; R.W. Saltus 5U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, Colorado 80225, USA Search for other works by this author on: GSW Google Scholar Robert Ayuso Robert Ayuso 6U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA Search for other works by this author on: GSW Google Scholar Author and Article Information David W. Farris 1Department of Earth Sciences, University of Southern California, 3651 Trousdale Avenue, Los Angeles, California 90089, USA Peter Haeussler 2U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA Richard Friedman 3Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada Scott R. Paterson 4Department of Earth Sciences, University of Southern California, 3651 Trousdale Avenue, Los Angeles, California 90089, USA R.W. Saltus 5U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, Colorado 80225, USA Robert Ayuso 6U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA Publisher: Geological Society of America Received: 07 Sep 2004 Revision Received: 18 Apr 2006 Accepted: 30 Apr 2006 First Online: 03 Oct 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2006) 118 (11-12): 1360–1376. https://doi.org/10.1130/B25718.1 Article history Received: 07 Sep 2004 Revision Received: 18 Apr 2006 Accepted: 30 Apr 2006 First Online: 03 Oct 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David W. Farris, Peter Haeussler, Richard Friedman, Scott R. Paterson, R.W. Saltus, Robert Ayuso; Emplacement of the Kodiak batholith and slab-window migration. GSA Bulletin 2006;; 118 (11-12): 1360–1376. doi: https://doi.org/10.1130/B25718.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Kodiak batholith is one of the largest, most elongate intrusive bodies in the forearc Sanak-Baranof plutonic belt located in southern Alaska. This belt is interpreted to have formed during the subduction of an oceanic spreading center and the associated migration of a slab window. Individual plutons of the Kodiak batholith track the location and evolution of the underlying slab window. Six U/Pb zircon ages from the axis of the batholith exhibit a northeastward-decreasing age progression of 59.2 ± 0.2 Ma at the southwest end to 58.4 ± 0.2 Ma at the northeast tip. The trench-parallel rate of age progression is within error of the average slab-window migration rate for the entire Sanak-Baranof belt (∼19 cm/yr).Structural relationships, U/Pb ages, and a model of new gravity data indicate that magma from the Kodiak batholith ascended 5–10 km as a northeastward-younging series of 1–8-km-diameter viscoelastic diapirs. Individual plutons ascended by multiple emplacement mechanisms including downward flow, collapse of wall rock, stoping, and diking. Stokes flow xenolith calculations suggest ascent rates of 5–100 m/yr and an effective magmatic viscosity of ≈107–108 Pa s. Pre-existing structural or lithologic heterogeneities did not dominantly control the location of the main batholith. Instead, its location was determined by migration of the slab window at depth. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Continental response to active ridge subduction

Apatite fission track ages from a ∼2000 m elevation transect from the Patagonian fold and thrust belt (47.5°S) allow us to quantify the denudational and orographic response of the upper plate to active ridge subduction. Accelerated cooling started at 17 Ma, predating the onset of ridge collision (14–10 Ma), and was followed by reheating between 10 and 6 Ma. Thermal modeling favors reheating on the order of 60°C at ∼28°C/Ma due to east‐migration of a slab window after the ridge‐trench collision. Final rapid cooling since 4 Ma of ∼18°C/Ma (geothermal gradient of 14°C/km) correlates with the presence of an orographic barrier and >1 km rock uplift in this region between 17.1 and 6.3 Ma. Increased precipitation and erosion since 4 Ma caused asymmetric exhumation, with 3–4 km on the leeside. Repeated crustal unroofing in response to active ridge subduction can explain the positive gravity anomaly south of the Chile Triple Junction.

Evidence for melt injection in the crust of northern California?

The occurrence of an earthquake sequence near Lake Pillsbury, California during a 6-month period in mid-2000 can be linked to magma transport in the crust; this activity is potentially a pre-cursor to future volcanism. The position of Cenozoic volcanism in the crust of the northern California Coast Ranges is directly linked to the northward migration of the Mendocino triple junction (MTJ) and the formation of a slab window beneath North American crust. The most recent manifestation of this is the Clear Lake volcanics, which began erupting ∼ 2 Ma. With the continued migration of the triple junction, volcanic activity is expected to occur to the north of Clear Lake in the future. These volcanic centers erupt in areas where the crust is undergoing significant modification as a result of slab window processes, and reflect important components in the development of the San Andreas plate boundary system. When precisely relocated, the earthquake sequence defines a streak of seismicity at ∼ 8 km depth. This sequence demonstrates a local time–space migration over a distance of ∼ 10 km in two major pulses of activity in the 6-month time period. Stress modeling allows us to infer that these earthquakes may be the result of several dike injections. In combination with evidence for lower crustal melts in the same region from other seismological studies, we propose that these earthquakes help map the occurrence of melt injection in the mid-crust to depths of ∼ 10 km, and present evidence that MTJ-related melt in the crust of northern California has migrated north of the Clear Lake volcanic center.

Late-stage evolution of the Chemehuevi and Sacramento detachment faults from apatite (U-Th)/He thermochronometry--Evidence for mid-Miocene accelerated slip

Research Article| May 01, 2006 Late-stage evolution of the Chemehuevi and Sacramento detachment faults from apatite (U-Th)/He thermochronometry—Evidence for mid-Miocene accelerated slip Timothy J. Carter; Timothy J. Carter 1School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia Search for other works by this author on: GSW Google Scholar Barry P. Kohn; Barry P. Kohn 1School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia Search for other works by this author on: GSW Google Scholar David A. Foster; David A. Foster 2Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA Search for other works by this author on: GSW Google Scholar Andrew J.W. Gleadow; Andrew J.W. Gleadow 3School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia Search for other works by this author on: GSW Google Scholar Jon D. Woodhead Jon D. Woodhead 3School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Timothy J. Carter 1School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia Barry P. Kohn 1School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia David A. Foster 2Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA Andrew J.W. Gleadow 3School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia Jon D. Woodhead 3School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia Publisher: Geological Society of America Received: 27 Sep 2004 Revision Received: 03 Nov 2005 Accepted: 25 Nov 2005 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2006) 118 (5-6): 689–709. https://doi.org/10.1130/B25736.1 Article history Received: 27 Sep 2004 Revision Received: 03 Nov 2005 Accepted: 25 Nov 2005 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Timothy J. Carter, Barry P. Kohn, David A. Foster, Andrew J.W. Gleadow, Jon D. Woodhead; Late-stage evolution of the Chemehuevi and Sacramento detachment faults from apatite (U-Th)/He thermochronometry—Evidence for mid-Miocene accelerated slip. GSA Bulletin 2006;; 118 (5-6): 689–709. doi: https://doi.org/10.1130/B25736.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The unique low-temperature sensitivity of the apatite (U-Th)/He (AHe) technique provides valuable new information relating to the late-stage thermal evolution of metamorphic core complexes in the southern Basin and Range Province. In all the ranges studied, the data show greater complexity than previously seen from other thermochronometers. In the Chemehuevi and northern Sacramento Mountains, a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall. This is followed by essentially age-invariant segments of 15 ± 1 Ma, reflecting an increase in the rate of detachment fault movement. The timing of the change suggested by the data agrees with recently published AHe results from the nearby Harcuvar Mountains, located ∼125 km to the southeast, suggesting that an underlying regional mechanism triggered this change. Plate reconstructions indicate that the Pioneer-Mendocino fracture zone migrated northward through the area at this time, leaving a slab window in its wake. We suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this, combined with the extensional strain regime present, resulted in an increased rate of slip within the actively extending metamorphic core complexes. This agrees well with timing constraints for extension onset in the central Basin and Range Province of southern Nevada, and suggests that weakening of the southern Basin and Range Province at 15 ± 1 Ma was partially responsible for the onset of extension in the central Basin and Range. These findings demonstrate that improved constraints on changes in extension rate within the Basin and Range Province should be possible through the application of the AHe technique to other metamorphic core complex footwalls. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

La Purísima volcanic field, Baja California Sur (Mexico): Miocene to Quaternary volcanism related to subduction and opening of an asthenospheric window

Geological mapping and geochemical analyses combined with 40K– 40Ar ages for lavas from the Late Miocene to Quaternary La Purísima volcanic field (Baja California Sur) provide evidence for five volcanic events. These, in turn, may reflect plate interactions in the region. The oldest event (event 1), prior to 11 Ma, corresponds to the emission of normal to K-rich calc-alkaline lavas, exposed as large mesas in the eastern part of the studied area and as pyroclastic breccias and volcaniclastic sediments to the west. It is associated with the end of the Comondú arc activity resulting from subduction of the Farallon and Guadalupe plates. Between 10.6 and 8.8 Ma (event 2), magnesian andesites and tholeiites were emplaced. At 5.5 Ma (event 3) and 2.5 Ma (event 4) small volumes of magnesian andesites erupted in the central and southern parts of the volcanic field. Finally, between 1.2 Ma and Holocene (event 5), numerous basaltic and magnesian andesitic fissural and central emissions resulted in the formation of strombolian cones and associated lava flows, mainly distributed within a NNW–SSE trending graben located SE of the town of La Purísima. Magmatic events 2 to 5 occurred well after the supposed end of the subduction event. Their geochemical characteristics are still typical of subduction-modified sources and possibly indicate partial melting of hot slab and formation of an asthenospheric window due to a slab rupture event which followed ridge–trench collision, prior to the continental breakup of the Gulf of California extensional province.

Eocene volcanism above a depleted mantle slab window in southern Alaska

The Caribou Creek volcanic fi eld lies along the continent-side edge of forearc basin rocks in south-central Alaska and consists of over 1000 m of shallow-dipping basalt and andesite lavas with minor mafi c pyroclastic deposits. Dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas. The basalts are tholeiitic and strongly depleted in the light rare earth elements (La/ Yb = 0.18–1.5), with concentrations of high fi eld strength elements (e.g., Zr, Hf, Ti, Y) similar to mid-ocean-ridge basalt and with variable enrichment in fl uid-mobile elements (e.g., Cs, Ba, and Pb). Intermediate and felsic rocks show enrichment in the rare earth elements and fl uid-mobile elements plus Rb and K, but retain low La/Yb ratios (0.48–3.6). A few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites (e.g., Sr/Y up to 52). Ten 40 Ar/ 39 Ar ages for the Caribou Creek volcanic rocks range from 49.4 ± 2.2 to 35.6 ± 0.2 Ma. An adakite-like tuff beneath the other volcanic rocks yields an age of 59.0 ± 0.4 Ma. Caribou Creek basalts were derived from mid-ocean-ridge–like depleted mantle that was emplaced beneath the southern margin of Alaska through a slab window following spreading ridge subduction. Caribou Creek volcanism was coeval with oblique subduction, oroclinal bending, and right-lateral strike-slip faulting in south-central Alaska, all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas. The basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of Jurassic arc basement rocks to form the intermediate and felsic magmas. Enrichment of the basaltic parent magmas in fl uid-mobile elements occurred by contamination from the Jurassic arc rocks and/or by contamination with metasomatic mantle remnant from preceding subduction. High heat fl ow through the slab window induced partial melting of garnet-bearing mafi c parts of the Jurassic arc basement to form the adakite-like rocks. The Caribou Creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime (e.g., crustal extension) and that the infl uence of a slab window on continental margin magmatism can be long-lived (>20 m.y.).

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

The Bohai Basin is a Cenozoic petroliferous extensional basin in China and has apparent geometrical and kinematic similarities with the other Meso-Cenozoic extensional basins located along the eastern margin of Eurasian Plate. However, the deformation mechanisms of the basin are still in dispute. Physcial modelling referring to the Huanghua Depression, located in the central part of the Bohai Basin was conducted employing four sets of planar sandbox experimental models with different extension directions. Only experimental results of the model with N-S extension show good structural similarity with the depression. The results also indicate that complex variations of fault strike in a rift basin are not necessarily the results of complex kinematic mechanisms or polyphase deformation. Based on comparison of experimental results with the actual structures and the good structural simi-larity between Huanghua Depression and the whole Bohai Basin, it is concluded that the Bohai Basin was formed by the N-S extension. The strike slip deformation along the NNE-trending border faults of the basin resulted from the N-S extension and played the role of lateral transformation for the N-S extension. In addition, according to the apparent geometrical and kinematic similarities among the Bohai Basin and other Meso-Cenozoic extensional basins located along the eastern margin of the Eurasian Plate, it is proposed that: (1) this “N-S extension” model provides a better kinematic inter-pretation for the formation of Bohai Basin and the other adjacent basins located along the eastern margin of the Eurasian Plate; and (2) the N-S extension was probably the effect of the “slab window” formed by the subduction of the nearly E-W trending oceanic ridge between the Kula and Pacific Plates. The “slab window” effect can also provide reasonable explanations for the phenomena that initial rifting ages of basins become progressively younger westwards along the eastern margin of the Eurasian Plate and that these basins changed gradually from rifting to thermal subduction.

Geochronology and Geochemistry of the Francisco Gneiss: Triassic Continental Rift Tholeiites on the Mexican Margin of Pangea Metamorphosed and Exhumed in a Tertiary Core Complex

Migmatized amphibolite-facies gneisses and amphibolites of the Francisco Gneiss exposed in the northern part of the Guerrero composite arc terrane have been interpreted as either Precambrian basement, a Triassic metamorphic complex, or a distinct terrane. Field observations suggest exposure in a metamorphic core complex of a protolith composed of interleaved bimodal igneous and sedimentary rocks. Geochemical data indicate that the amphibolites are within-plate, continental tholeiites. Recalculated TDM ages for the rhyolites are consistent with partial melting of the Grenvillian basement of North America projected beneath the area. U-Pb isotopic analyses of zircon from two felsic rocks yielded concordant ages between 216 and 197 Ma due to a combination of inheritance and Pb-loss: the best estimate of protolith age is ∼206 Ma—i.e., Norian, Late Triassic. Concordant U-Pb titanite ages range from 112 to 98 Ma, whereas nearly concordant U-Pb xenotime ranges from 91 to 51 Ma. These are inferred to result from partial-complete resetting during the high-grade metamorphic event. 40Ar/39 Ar analyses from the gneisses yielded plateau ages of 16.5 ± 1 Ma (muscovite) and 13 ± 1 Ma (biotite), which date cooling through ∼370°C and ∼300°C, respectively (early Middle Miocene). Biotite from the granitic sheet yielded a plateau age of 13 ± 2 Ma. These data are interpreted in terms of Miocene exhumation in a core complex of high-grade metamorphic rocks developed either over a slab window or as a result of tectonic burial during the Laramide orogeny. The extrusion of Upper Triassic, continental rift tholeiites is consistent with emplacement in a back-arc environment.

Cenozoic to Recent plate configurations in the Pacific Basin: Ridge subduction and slab window magmatism in western North America

Forearc magmatic rocks were emplaced in a semicontinuous belt from Alaska to Oregon from 62 to 11 Ma. U-Pb and 40 Ar39 Ar dating indicates that the magmatism was concurrent in widely separated areas. Eight new conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from forearc intrusions on Vancouver Island (51.2 ± 0.4, 48.8 ± 0.5 Ma, 38.6 ± 0.1, 38.6 ± 0.2, 37.4 ± 0.2, 36.9 ± 0.2, 35.4 ± 0.2, and 35.3 ± 0.3 Ma), together with previous dates, indicate that southwestern British Columbia was a particularly active part of the forearc. The forearc magmatic belt has been largely attributed to ridge-trench intersection and slab window formation involving subduction of the Kula-Farallon ridge. Integration of the new and previous ages reveals shortcomings of the Kula-Farallon ridge explanation, and supports the hypothesis of two additional plates, the Resurrection plate (recently proposed) and the Eshamy plate (introduced herein) in the Pacifi c basin during Paleocene and Eocene time. We present a quantitative geometric plate-tectonic model that was constructed from 53 Ma to present to best account for the forearc magmatic record using ridge-trench intersection and slab window formation as the main causes of magmatism. The model is also in accord with Tertiary to present inboard magmatic and structural features.

Oceanic-ridge subduction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental margin since 15 Ma

Research Article| January 01, 2006 Oceanic-ridge subduction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental margin since 15 Ma F. Michaud; F. Michaud 1Géosciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France Search for other works by this author on: GSW Google Scholar J.Y. Royer; J.Y. Royer 2CNRS-UBO Domaines Océaniques, Place Copernic, 29280 Plouzané, France Search for other works by this author on: GSW Google Scholar J. Bourgois; J. Bourgois 3Géosciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France Search for other works by this author on: GSW Google Scholar J. Dyment; J. Dyment 4Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris, France Search for other works by this author on: GSW Google Scholar T. Calmus; T. Calmus 5Instituto de Geología, Universidad Nacional Autónoma de México, AP 1039, 83000, Hermosillo, Sonora, Mexico Search for other works by this author on: GSW Google Scholar W. Bandy; W. Bandy 6Instituto de Geofisica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Mexico D.F Search for other works by this author on: GSW Google Scholar M. Sosson; M. Sosson 7Géosciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France Search for other works by this author on: GSW Google Scholar C. Mortera-Gutiérrez; C. Mortera-Gutiérrez 8Instituto de Geofisica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Mexico D.F Search for other works by this author on: GSW Google Scholar B. Sichler; B. Sichler 9Ifremer, BP 70, 29280 Plouzané, France Search for other works by this author on: GSW Google Scholar M. Rebolledo-Viera; M. Rebolledo-Viera 10Instituto de Geofisica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Mexico D.F Search for other works by this author on: GSW Google Scholar B. Pontoise B. Pontoise 11Géosciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France Search for other works by this author on: GSW Google Scholar Geology (2006) 34 (1): 13–16. https://doi.org/10.1130/g22050.1 Article history received: 12 Jul 2005 rev-recd: 26 Aug 2005 accepted: 30 Aug 2005 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 F. Michaud, J.Y. Royer, J. Bourgois, J. Dyment, T. Calmus, W. Bandy, M. Sosson, C. Mortera-Gutiérrez, B. Sichler, M. Rebolledo-Viera, B. Pontoise; Oceanic-ridge subduction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental margin since 15 Ma . Geology 2006;; 34 (1): 13–16. doi: https://doi.org/10.1130/g22050.1 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 SocietyGeology Search Advanced Search Abstract The interaction of the Pacific-Farallon spreading centers with the North American convergent margin off Baja California, Mexico, supposedly ceased at 12 Ma, when plate convergence and seafloor spreading stopped. We propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23°N to 27°N (Famex cruise of the R/V L'Atalante, April 2002). The data unveil a major clockwise rotation of the Pacific-Farallon spreading direction, starting ca. 14 Ma, that formed a series of short spreading centers that became extinct ca. 8–7 Ma. We suggest that the transcurrent motion between the Pacific and North America along Baja California was accommodated by seafloor spreading and oblique convergence along the trench. This change in spreading direction was followed by a concomitant progressive demise of both Pacific-Farallon seafloor spreading and Farallon–North America subduction that are attributed to the break-off of the Farallon slab. This also resulted in the opening of a trench-parallel slab window beneath Baja California. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Somma–Vesuvius volcano (Southern Italy): Structure, dynamics and hazard evaluation

We review the main results, with several new analyses, obtained in recent times about the structure, present dynamics and hazard evaluation at Somma–Vesuvius volcanic complex. We present a global review and interpretation of structural features, both at local and regional scale, constrained both by seismic and petrological data. The local structure of Somma–Vesuvius is reviewed in three depth ranges, shallow, intermediate and deep. The shallow velocity structure is inferred by the joint inversion of shot and local earthquake arrival time data. The main feature pointed out at shallow depth is a high velocity anomaly at the crater axis, extending down to about 5 km of depth. Such an anomaly, first observed at Vesuvius, seems to be common to many other volcanoes. It can be interpreted in terms of the presence of solidified residual magma in the shallow conduits, accumulated in last eruptive cycles. The local seismicity is strongly clustered around this anomaly, due to the focusing effect of the rigidity contrast. The seismic occurrence appears as a result of the superposition of a background level, mainly due to gravitational instability of the Vesuvius cone, and of intense activity episodes, which possibly reflect episodic internal activity. Two main zones of magma accumulation in the upper crust are evidenced by the joint interpretation of seismic and petrological data. The first one, located in the depth range 4–6 km, is mainly constrained by the crystallisation depth of phonolitic magmas which fed Plinian and sub-Plinian eruptions; the second one, around 11–15 km of depth, is mainly constrained by reflected–converted seismic waves, and in agreement with crystallization depths inferred for the moderate eruptions. The study of the deep structure, performed by regional tomography with teleseisms, further points out magma roots at higher depths (15–30 km). An additional result for the deep structure, studied at regional scale and very important for geodynamic interpretations of the Tyrrhenian volcanisms, has been the evidence for a subducting slab under the Apennines, in an area where previous models hypothesised a slab window. New original studies of crystal growth (phenocrystals and microlites) on the eruptive products allow to infer typical times of magma rising from such reservoirs, which appear very low, on the order of minutes to tens of minutes. Static deformation at this volcano, in the last 30 yr, has been detected by the joint use of levelling, GPS and DIFSAR techniques. It indicates subsidence, very concentrated in the crater area and in a narrow strip all around the volcanic edifice, with maximum rates less than 0.01 m/yr. Static deformation in the crater area appears in agreement with the mechanism of gravitational instability generating local volcano-tectonic seismicity, while the peculiar pattern around the volcanic edifice is probably due to the combination of extensional stress and volcanic loading, generating a ring normal fault-like structure. While the key results about structure and dynamics help to define pre-eruptive scenarios, a new probabilistic procedure to combine volcanological data and computer simulations has been used, in this paper, to build hazard maps giving the probability, at each location in the area, to be hit by a pyroclastic flow or to experience a destructive fall-out deposit. The review and new results of this work give then the first complete picture of the state of the art in our knowledge about Somma–Vesuvius volcano.

Miocene to Late Quaternary Patagonian basalts (46–47°S): Geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction

Miocene to Quaternary large basaltic plateaus occur in the back-arc domain of the Andean chain in Patagonia. They are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the South Chile Ridge (SCR). We have investigated three volcanic centres from the Lago General Carrera–Buenos Aires area (46–47°S) located above the inferred position of the slab window corresponding to a segment subducted 6 Ma ago. (1) The Quaternary Río Murta transitional basalts display major, trace elements, and Sr and Nd isotopic features similar to those of oceanic basalts from the SCR and from the Chile Triple Junction near Taitao Peninsula (e.g., ( 87Sr/ 86Sr) o = 0.70396–0.70346 and εNd = + 5.5 − + 3.0). We consider them as derived from the melting of a Chile Ridge asthenospheric mantle source containing a weak subduction component. (2) The Plio-Quaternary (< 3.3 Ma) post-plateau basanites from Meseta del Lago Buenos Aires (MLBA), Argentina, likely derive from small degrees of melting of OIB-type mantle sources involving the subslab asthenosphere and the enriched subcontinental lithospheric mantle. (3) The main plateau basaltic volcanism in this region is represented by the 12.4–3.3-Ma-old MLBA basalts and the 8.2–4.4-Ma-old basalts from Meseta Chile Chico (MCC), Chile. Two groups can be distinguished among these main plateau basalts. The first group includes alkali basalts and trachybasalts displaying typical OIB signatures and thought to derive from predominantly asthenospheric mantle sources similar to those of the post-plateau MLBA basalts, but through slightly larger degrees of melting. The second one, although still dominantly alkalic, displays incompatible element signatures intermediate between those of OIB and arc magmas (e.g., La/Nb > 1 and TiO 2 < 2 wt.%). These intermediate basalts differ from their strictly alkalic equivalents by having lower High Field Strength Element (HFSE) and higher εNd (up to + 5.4). These features are consistent with their derivation from an enriched mantle source contaminated by ca. 10% rutile-bearing restite of altered oceanic crust. The petrogenesis of the studied Mio-Pliocene basalts from MLBA and MCC is consistent with contributions of the subslab asthenosphere, the South American subcontinental lithospheric mantle and the subducted Pacific oceanic crust to their sources. However, their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment SCR-1, which entered the trench at 6 Ma. Indeed, magmatic activity was already important between 12 and 8 Ma in MLBA and MCC as well as in southernmost plateaus, i.e., 6 Ma before the subduction of the SCR-1 segment. We propose a geodynamic model in which OIB and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear-in-the-slab, which formed when the southernmost segments of the SCR collided with the Chile Trench around 15 Ma. During their ascent, they interacted with the Patagonian supraslab mantle and, locally, with slivers of subducted Pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts.

1: Subduction, slab detachment and mineralization: The Neogene in the Apuseni Mountains and Carpathians

The Inner Carpathians comprise several distinct Neogene late-stage orogenic Pb–Zn–Cu–Ag–Au ore districts. The mineral deposits in these districts are closely related to volcanic and subvolcanic rocks, and represent mainly porphyry and epithermal vein deposits, which formed within short periods of time in each district. Here, we discuss possible geodynamic and structural controls that suggest why some of the Neogene volcanic districts within the Carpathians comprise abundant mineralization, while others are barren. The Neogene period has been characterized by an overall geodynamic regime of subduction, where primary roll-back of the subducted slab and secondary phenomena, like slab break-off and the development of slab windows, could have contributed to the evolution, location and type of volcanic activity. Structural features developing in the overlying lithosphere and visible in the Carpathian crust, such as transtensional wrench corridors, block rotation and relay structures due to extrusion tectonics, have probably acted in focusing hydrothermal activity. As a result of particular events in the geodynamic evolution and the development of specific structural features, mineralization formed during fluid channelling within transtensional wrench settings and during periods of extension related to block rotation. In the Slovakian ore district of the Western Carpathians, Neogene volcanism and associated mineralization were localized by sinistral, NE-trending wrench corridors, which formed part of the extruding Alcapa block. The Baia Mare ore district, in the Eastern Carpathians, reflects a transtensional wrench setting on distributed oversteps close to the termination of the Dragos Voda fault. There, mineralization was spatially controlled by the transtensional Dragos Voda master fault and associated cross-fault systems. The Golden Quadrangle Cu–Au ore district of the Southern Apuseni Mountains reflects an unusual rotated transtensional/extensional setting close to the termination of a graben system. There, fluid flow was probably localized by fault propagation at the inner tip of the graben system. The spatial and temporal evolution of the magmatism and its changing geochemical signature from (N)W to (S)E strongly suggests a link with the contemporaneous northeastward roll-back of the subducted slab and a progressive southeastward detachment during accelerating roll-back. This geodynamic evolution is further supported by the present-day overall and detailed mantle lithospheric density images, the present-day heat flow patterns, the crustal architecture and its interpreted evolution, and the spatial and temporal evolution of depocentres around the Carpathian arc. In contrast to all these features, the mineral deposits in the West Carpathians, East Carpathians and Apuseni Mountains are too synchronous with respect to their individual volcanic history and contrast too much with younger volcanics of similar style, but barren, in southeastern parts of the Carpathians to simply link them directly to the slab evolution. In all three districts, the presence of magmatic fluids released from shallow plutons and their mixing with meteoric water were critical for mineralization, requiring transtensional or extensional local regimes at the time of mineralization, possibly following initial compressional regimes. These three systems show that mineralization was probably controlled by the superposition of favourable mantle lithospheric conditions and partly independent, evolving upper crustal deformation conditions. In the 13 to 11 Ma period the dominant mineralization formed all across the Carpathians, and was superimposed on structurally favourable crustal areas with, at that time, volcanic–hydrothermal activity. The period may reflect the moment when the (upper part of the) crust failed under lithospheric extension imposed by the slab evolution. This crustal failure would have fragmented the overriding plate, possibly breaking up the thermal lid, to provoke intensive fluid flow in specific areas, and allowed subsequent accelerated tectonic development, block rotation and extrusion of a “family of sub-blocks” that are arbitrarily regarded as the Tisia–Dacia or Alcapa blocks, even though they have lost their internal entity.

Upper mantle structure of the South American continent and neighboring oceans from surface wave tomography

We present a new three-dimensional SV-wave velocity model for the upper mantle beneath South America and the surrounding oceans, built from the waveform inversion of 5850 Rayleigh wave seismograms. The dense path coverage and the use of higher modes to supplement the fundamental mode of surface waves allow us to constrain seismic heterogeneities with horizontal wavelengths of a few hundred kilometres in the uppermost 400 km of the mantle. The large scale features of our tomographic model confirm previous results from global and regional tomographic studies (e.g. the depth extent of the high velocity cratonic roots down to about 200–250 km). Several new features are highlighted in our model. Down to 100 km depth, the high velocity lid beneath the Amazonian craton is separated in two parts associated with the Guyana and Guapore shields, suggesting that the rifting episode responsible for the formation of the Amazon basin has involved a significant part of the lithosphere. Along the Andean subduction belt, the structure of the high velocity anomaly associated with the sudbduction of the Nazca plate beneath the South American plate reflects the along-strike variation in dip of the subducting plate. Slow velocities are observed down to about 100 km and 150 km at the intersection of the Carnegie and Chile ridges with the continent and are likely to represent the thermal anomalies associated with the subducted ridges. These lowered velocities might correspond to zones of weakness in the subducted plate and may have led to the formation of “slab windows” developed through unzipping of the subducted ridges; these windows might accommodate a transfer of asthenospheric mantle from the Pacific to the Atlantic ocean. From 150 to 250 km depth, the subducting Nazca plate is associated with high seismic velocities between 5°S and 37°S. We find high seismic velocities beneath the Paraná basin down to about 200 km depth, underlain by a low velocity anomaly in the depth range 200–400 km located beneath the Ponta Grossa arc at the southern tip of the basin. This high velocity anomaly is located southward of a narrow S-wave low velocity structure observed between 200 and 500–600 km depth in body wave studies, but irresolvable with our long period datasets. Both anomalies point to a model in which several, possibly diachronous, plumes have risen to the surface to generate the Paraná large igneous province (LIP).

The Hudson Volcano and surrounding monogenetic centres (Chilean Patagonia): An example of volcanism associated with ridge–trench collision environment

Hudson Volcano (45°54′S, 72°58′W) represents the southernmost central volcano of the South Andean Volcanic Zone (SVZ). It lies about 280 km east of Nazca–Antarctic–South American Plate Triple Junction, where the Chile spreading ridge enters the Chile Trench. Hudson is located on the border of a volcanic gap separating the Southern from the Austral Volcanic Zone (AVZ), which is commonly interpreted as linked to a slab window. The volcano is characterised by a volcano-tectonic depression about 10 km wide, set in a morphological high of the Patagonian Batholith. The scarce pre-caldera products are made up of mafic lavas interbedded with hyaloclastite. The majority of the volcanic products out from the caldera rims consist of thick deposits of lahars or hyaloclastitic breccias, subordinate lava flows and pyroclastic deposits linked to historical large explosive eruptions. Rough volume calculations on pre-caldera and post-caldera products suggest that the caldera formed in steps in the lifetime of the volcano and that it is mainly linked to a volcano-tectonic collapse during partial emptying of the magma chamber. The monogenetic cones surrounding Hudson (Río Ibanez and Murta) erupted primitive basalts that gave important clues on magma sources. The volcanic products of Hudson, with composition ranging from basalts to dacites, have a calc-alkaline signature. Nevertheless, they have distinctly higher K 2O, TiO 2, Na 2O, LILE, HFSE and REE than the calc-alkaline rocks of SVZ. The basaltic lavas from the monogenetic cones show La/Nb, Ba/Nb and Th/Ta significantly lower than the calc-alkaline lavas. Furthermore, they have some ratios as Th/Yb and La/Yb remarkably low, with values approaching those of MORB-like basalts. All the geochemical characteristics of Río Ibanez and Murta cones are very similar to those of the basalts dredged in the Chile Ridge. Geochemical modeling suggests that the Hudson basalts may derive from the basalts erupted at the Río Ibanez centre through an assimilation plus fractional crystallisation process. On the basis of these data, we propose a geodynamical model. In correspondence to the monogenetic centres, close to the border of the incipient opening of the slab window, the magma features are still those of the depleted asthenosphere under the subducting ridge. In this zone, remnant fragments of subducted oceanic crust undergo partial melting processes, while the mantle wedge is deeply modified by subduction-related components. In correspondence to Hudson and the nearby monogenetic centres, the MORB-type primary source component is modified by altered oceanic crust and variably mixed with andesitic magmas having a calc-alkaline signature. A contribution from partial melting of the continental lithosphere may be present in the complex source mixing process at the border of the slab window.

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex (SFVC) comprise three distinct petrological groups based on their whole-rock geochemistry: (1) oceanic tholeiites; (2) transitional alkali basalts and glasses; (3) high-Al, low-Ti tholeiites. Major and trace element, and Sr–Nd–Pb isotopic data indicate that the oceanic tholeiites formed as low-degree partial melts of normal mid-ocean ridge basalt (N-MORB)-source asthenosphere similar in isotope composition to the East Pacific Rise today; the alkalic lavas were derived from an enriched source similar to that of E-MORB. The high-Al, low-Ti lavas resemble second-stage melts of a depleted MORB-source asthenosphere that formed by melting spinel lherzolite at low pressures. Trace element systematics of the high-Al, low-Ti basalts show the influence of an enriched component, which overprints generally depleted trace element characteristics. Tectonic discrimination diagrams show that the oceanic tholeiite and alkali suites are similar to present-day basalts generated at mid-oceanic ridges. The high-Al, low-Ti suite resembles primitive arc basalts with an enriched, alkali basalt-like overprint. Isotopic data show the influence of recycled components in all three suites. The SFVC was constructed on a substrate of normal Coast Range ophiolite in an extensional forearc setting. The close juxtaposition of the MORB-like olivine tholeiites with alkali and high-Al, low-Ti basalts suggests derivation from a hybrid mantle source region that included MORB-source asthenosphere, enriched oceanic asthenosphere, and the depleted supra-subduction zone mantle wedge. We propose that the SFVC formed in response to collision of a mid-ocean ridge spreading center with the Coast Range ophiolite subduction zone. Formation of a slab window beneath the forearc during collision allowed the influx of ridge-derived magmas or the mantle source of these magmas. Continued melting of the previously depleted mantle wedge above the now defunct subduction zone produced strongly depleted high-Al, low-Ti basalts that were partially fertilized with enriched, alkali basalt-type melts and slab-derived fluids.

Implications of volcanism in coastal California for the Neogene deformation history of western North America

The geologic record of coastal California includes evidence of numerous volcanic centers younger than 30 Ma that do not appear to have erupted in an arc setting. By correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the Pacific plate, we add new constraints to tectonic reconstructions since 30 Ma. Our correlations, such as erupting the Morro Rock–Islay Hill complex south of the Pioneer fracture zone and the Iversen Basalt south of the Mendocino fracture zone, require larger displacements within western North America than advocated by most previous authors. Specifically, we infer at least 315 km of motion between the Sierra Nevada and rigid North America at an azimuth of about N60°W and at least 515 km between Baja California and rigid North America in a similar direction. A consequence of inferring a large displacement of Baja California is that the Pacific–North American plate boundary must have developed most of its current form prior to 10 Ma. We interpret a slab window developing between Cocos and Monterey plates after 19 Ma that reconstructs under nearly all of the southern California volcanic centers dated at 18–14 Ma. Most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism, followed by slow subsidence of variable but often extended duration, consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere.

The origin of early Archean banded iron formations and of continental crust, Isua, southern West Greenland

A long-standing geological debate is whether the Fe component of Precambrian banded iron formations (BIFs) was derived from continental sources, or had its origin in hydrothermal alteration of the oceanic crust. Recent REE, and Nd and Fe isotope studies of iron-rich layers in BIFs have provided strong supporting evidence for the hydrothermal origin of the Fe component. Pillow basalts from the 3800–3700 Ma Isua greenstone belt, West Greenland, underwent significant enrichment of Fe (up to 21 wt.% Fe 2O 3) and some chalcophile elements (e.g., Pb = 5.4–7.2 ppm) prior to a late Archean tectonothermal metamorphic event recorded in the region. Published and new Pb-Pb isotopic data from the Isua pillow basalts and the spatially associated BIFs suggest that the enrichment of Fe and Pb and deposition of the BIFs took place between 3800 and 3600 Ma. Similar Pb isotopic compositions in pillow basalts and BIFs are consistent with a common source of Pb for both supracrustal rocks. Iron and Pb were probably scavenged from the oceanic crust and were re-deposited on the ocean floor by hydrothermal fluids, suggesting that high-temperature hydrothermal alteration of early Archean oceanic crust played an important role in the deposition of the Isua BIFs. Geochemical, lithological, and structural data from the Isua greenstone belt are all collectively consistent with a convergent margin geodynamic setting. The Isua BIFs are spatially and temporally associated with boninitic and island arc picritic pillow basalts. Given the observations that Cenozoic boninites and picrites tend to form in an intra-oceanic arc-forearc setting, we suggest that the Isua BIF-boninite and BIF-picrite associations were also deposited in a similar geodynamic setting. The original pillow basalt-BIF stratigraphy was dismembered and transposed throughout the belt during early to late Archean tectonothermal events. The high-temperature hydrothermal alteration of the oceanic crust produced significant hydrothermal discharge with large quantities of Fe necessary to contribute to the deposition of the Isua BIFs. The hydrothermal alteration may have resulted from the opening of an asthenospheric window developed as a consequence of ridge subduction beneath an early Archean arc-forearc region. The ridge subduction model can also explain the origin of the contemporaneous tonalite-trondhjemite-granodiorite (TTG) intrusions in the Isua region. Partial melting of laterally accreted and thickened oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of asthenospheric windows may have produced TTG melts. The diapirically upwelling TTG melts formed the early Archean continental crust and the complementary eclogitic residues contributed to the sub-continental lithospheric mantle.

Petrogenesis of the Eocene and Mio–Pliocene alkaline basaltic magmatism in Meseta Chile Chico, southern Patagonia, Chile: Evidence for the participation of two slab windows

The Meseta Chile Chico (MCC, 46.4°S) is the westernmost exposure of Eocene (lower basaltic sequence, LBS; 55–40 Ma, K–Ar ages) and Mio–Pliocene (upper basaltic sequence, UBS; 16–4 Ma, K–Ar ages) flood basalt volcanism in Patagonia. The MCC is located south of the Lago General Carrera-Buenos Aires (LGCBA), southeast from the present day Chile Triple Junction (CTJ), east of the actual volcanic gap between Southern South Volcanic Zone and Austral Volcanic Zone (SSVZ and AVZ, respectively) and just above the inferred location of the South Chile Ridge segment subducted at ∼6 Ma (SCR-1). Erupted products consist of mainly ne-normative olivine basalt with minor hy-normative tholeiites basalt, trachybasalt and basanite. MCC lavas are alkaline (42.7–53.1 wt.% SiO 2, 3–8 wt.% Na 2O+K 2O) and relatively primitive (Ni: 133–360 ppm, Cr: 161–193 ppm, Co: 35–72 ppm, 4–16.5 MgO wt.%). They have a marked OIB-like signature, as shown by their isotopic compositions ( 87Sr/ 86Sr o=0.70311–0.70414 and εNd=+4.7–+5.1) and their incompatible trace elements ratios (Ba/La=10–20, La/Nb=0.46–1.09, Ce/Pb=15.52–27.5, Sr/La<25), reflecting deep mantle origin. UBS-primitive lavas have characteristics similar to those of the Eocene LBS basalts, while UBS-intermediate lavas show geochemical imprints (La/Nb>1, Sr/La>25, low Ce/Pb, Nb/U) compatible with contamination by arc/slab-derived and/or crustal components. We propose that the genesis and extrusion of magmas is related to the opening of two slab windows due to the subduction of two active ridge segments beneath Patagonia during Eocene and Mio–Pliocene.