Hot Mantle Research Articles

Teleseismic tomography reveals no signature of the Dead Sea Transform in the upper mantle structure

We present results of a tomographic inversion of teleseismic data recorded at 48 stations of a temporary network which was installed in the area of the Dead Sea Transform (DST) and operated for 1 yr in the framework of the multidisciplinary DESERT Project. The 3366 teleseismic P and PKP phases from 135 events were hand picked and corrected for surface topography and crustal thickness. The inversion shows pronounced low-velocity anomalies in the crust, beneath the DST, which are consistent with recent results from local-source tomography. These anomalies are likely related to the young sediments and fractured rocks in the fault zone. The deeper the retrieved anomalies are quite weak. Most prominent is the high-velocity strip-like anomaly striking SE–NW. We attribute this anomaly to the inherited heterogeneity of lithospheric structure, with a possible contribution by the shallow Precambrian basement east of the DST and to lower crustal heterogeneity reported in this region by other seismic studies. We do not observe reliable signature of the DST in the upper mantle structure. Some weak indications of low-velocity anomalies in the upper mantle beneath the DST may well result from the down-smearing of the strong upper crustal anomalies. We also see very little topography of the lithosphere–asthenosphere boundary beneath the DST, which would generate significant horizontal velocity variations. These results are consistent with predictions from a recent thermo-mechanical model of the DST. Our tomographic model provides some indication of hot mantle flow from the deeper upper mantle rooted in the region of the Red Sea. However, resolution tests show that this anomaly may well be beyond resolution of the model.

Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range

Late Miocene-Pliocene (8-3 Ma) olivine basalt lavas, dated in this study by the 40 Ar/ 39 Ar method, have been faulted and tilted on both the east and west sides of the Warner Range of NE California, which is itself a tilted block rising to 2960 m at its crest that is composed of Miocene-Oligocene lavas and volcaniclastic rocks. The late Miocene-Pliocene lavas, distinctively poor in K 2 O and rich in MgO, are called low-K olivine tholeiites and have a different mantle source region than that of the older subduction-related lavas of the main Warner Range. Hays Canyon Range (max. elev. 2400 m) lies to the east of the Wamer Range, and the broad Surprise Valley separates the two fault-bounded ranges. Middle Miocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap the Hays Canyon Range and overlie Oligocene silicic ash-flow deposits and a basaltic andesite spatter volcano. Middle Miocene basic lavas also form the crest of the Warner Range and its westerly dip slope (-15°). Nearly horizontal basic lavas of the same age are also found on both sides of the Warner Range, and it is a plausible conclusion that these middle Miocene basalts were a contiguous group before faulting and uplift of the Warner Range. Derived estimates of uplift rates (∼1 mm/ yr) of the Warner Range indicate that uplift could have been initiated at ca. 4 Ma, a period of the most voluminous eruption of low-K olivine tholeiite lavas. If the slower Cretaceous exhumation rate of the Sierra Nevada (0.5-1.0 mm/yr) is applied to the total offset of the Warner Range (4270 m), and it did not vary with time, then the uplift of the Warner Range was initiated at ca. 8 Ma, which coincides with the age of the oldest low-K olivine tholeiite lava (8 Ma). Low-K olivine tholeiites require a hot shallow asthenospheric source, and it is the rise of this hot mantle that is presumed to have caused the uplift of the Warner Range. Whether or not the widespread eruption of small volumes of Pliocene low-K olivine tholeiites in central and eastern Oregon is associated with crustal uplift is unknown.

Crustal Structure Features of the Leiqiong Depression in Hainan Province

AbstractSeismic data obtained from an about 210km long Baiyan‐Jianghong deep seismic sounding profile, which was completed in the Leiqiong area in 1984 and 1985, were processed by means of finite difference tomography, time term inversion and 2‐D dynamic ray tracing methods. And 2‐D inhomogeneous crustal structure images along the Baiyan‐Jianghong seismic line were determined. The results show that the crustal thickness in northern Hainan province and Leizhou Peninsula is about 25~26km. In northern Hainan Island embossment of the southern part of Qiongzhou Channel, the average crustal velocity is about 6.16km·s–1 and 5.70km·s–1 in the Leiqiong Depression area of northern segment of Qiongzhou Channel. The crustal velocity is particularly low and especially obvious in lower crust where it is only about 6.3~6.5km·s–1. The crystalline basement presents a velocity gradient belt, the thickness of which is about 2~4km and the velocity is approximately 5.5~6.0km·s–1. Moho, the velocities of which on its two sides are 6.5km·s–1 and 8.05km·s–1 respectively, shows one‐order velocity discontinuity, and there is no obvious crust‐mantle transitional zone to be found. Because the Baiyan‐Jianghong seismic line lies in the Leiqiong Depression area with basalt covering of the Pleistocene Period, it is suggested that the thinner crust and lower velocity in this area mainly reflect the multi‐erupting of volcanic magma from the Cenozoic till the late Quaternary as well as the underplating and delaminating of hot mantle to the crustal bottom which are interrelated with the crust being still in a comparatively hot state.

Upper mantle structure beneath the Azores hotspot from finite-frequency seismic tomography

The Azores archipelago is a classic expression of a hotspot that has interacted with a mid-ocean ridge. A variety of geochemical studies and geophysical observations point to the influence of some sort of mantle plume on the formation of the Azores Plateau and the adjacent Mid-Atlantic Ridge (MAR), but the mantle structure beneath the Azores has not heretofore been imaged tomographically at a sufficiently fine resolution to resolve a narrow plume. We present a P-wave velocity model of the mantle beneath the hotspot derived from teleseismic body waves recorded by six broadband seismic stations on the Azores Islands including the Global Seismographic Network station CMLA. Three-dimensional travel-time sensitivity kernels are used to account for the effects of wavefront healing and scattering of seismic waves. An inversion of 228 P-wave travel-time delays reveal a low-velocity anomaly in the shallow mantle (less than 200 km deep) along the Azores archipelago and beneath the Azores Plateau (centered approximately at 38.5°N, 28.5°W). The low-velocity anomaly extends northeastward and downward to connect to a plume-like column of low-velocity material from ∼ 250 km to at least 400 km depth centered northeast of Terceira. These results are consistent with other geophysical and geochemical observations and provide seismic evidence for a model of plume-ridge interaction in which the plume conduit is deflected to the southwest in the shallow mantle by asthenospheric flow and plate motion and contributes hot mantle material and excess melt to the MAR south of the Azores.

The influence of H 2O on mantle wedge melting

The solidus and near-solidus melting behavior of a primitive undepleted peridotite composition has been determined over a pressure range of 1.2–3.2 GPa at H 2O saturated conditions. Vapor-saturated melting (melting in the presence of an H 2O-rich supercritical fluid) begins at 940 °C at 1.2 GPa and the solidus temperature decreases continuously to 860 °C at 2 GPa and 800 °C at 3.2 GPa. This solidus is similar to the lower temperature results of previous investigations. The temperature discrepancies found in earlier studies could be a result of short run times used in several studies and the slower kinetics of olivine vs. diopside melting. The solidus phases include olivine, orthopyroxene, high-Ca clinopyroxene and Al-rich phases that change from spinel + amphibole (1.2–1.8 GPa) to spinel + chlorite over the pressure range of 2–2.4 GPa. Above 2.4 GPa garnet + chlorite + ilmenite are present along with olivine + orthopyroxene + clinopyroxene on the solidus. Chlorite may be a stable phase at the base of the mantle wedge and it may play a role in the onset of flux melting. Ilmenite might play a role in the development of HFSE depletions in arc magmas. Flux melting of the mantle wedge above the subducting oceanic lithosphere begins when an H 2O-rich component (either fluid or melt) released from the slab ascends within the overlying mantle. As it ascends into the mantle wedge the H 2O triggers melting at the vapor-saturated solidus at a depth shallower than the wedge–slab interface. Melting continues as the melt ascends into shallower, hotter overlying mantle. Melting in this part of the wedge occurs at vapor-undersaturated conditions because the H 2O content of the melt is continually diluted as the melt ascends through the wedge, dissolving and re-equilibrating with shallower, hotter mantle. Final equilibration with the mantle wedge occurs at shallow depths near the top of the wedge. A model of this process is developed using the vapor-saturated phase relations as a starting point.

Deep origin of the Hawaiian tilted plume conduit derived from receiver functions

We employ P to S converted waveforms to investigate effects of the hot mantle plume on seismic discontinuities of the crust and upper mantle. We observe the Moho at depths between 13 and 17 km, regionally covered by a strong shallow intracrustal converted phase. Coherent phases on the transverse component indicate either dipping interfaces, 3-D heterogeneities or lower crustal anisotropy. We find anomalies related to discontinuities in the upper mantle down to the transition zone evidently related to the hot mantle plume. Lithospheric thinning is confirmed in greater detail than previously reported by Li et al., and we determine the dimensions of the low-velocity zone within the asthenosphere with greater accuracy. Our study mainly focuses on the temperature-pressure dependent discontinuities of the upper mantle transition zone. Effects of the hot diapir on the depths of mineral phase transitions are verified at both major interfaces at 410 and 660 km. We determine a plume radius of about 200 km at the 660 km discontinuity with a core zone of about 120 km radius. The plume conduit is located southwest of Big Island. A conduit tilted in the northeast direction is required in the upper mantle to explain the observations. The determined positions of deflections of the discontinuities support the hypothesis of decoupled upper and lower mantle convection.

Seismic stratigraphy of the Adare Trough area, Antarctica

The Adare Trough, located 100 km NE of Cape Adare, Antarctica, is the extinct third arm of a Tertiary spreading ridge that separated East from West Antarctica. We use seismic reflection data, tied to DSDP Site 274, to link our seismic stratigraphic interpretation to changes in ocean-bottom currents, Ross Sea ice cover, and regional tectonics through time. Two extended unconformities are observed in the seismic profiles. We suggest that the earliest hiatus (early Oligocene to Mid-Miocene) is related to low sediment supply from the adjacent Ross Shelf, comprised of small, isolated basins. The later hiatus (mid-Miocene to late Miocene) is likely caused by strong bottom currents sourced from the open-marine Ross Sea due to increased Antarctic glaciation induced by mid-Miocene cooling (from Mi-3). Further global cooling during the Pliocene, causing changes in global ocean circulation patterns, correlates with Adare Basin sediments and indicate the continuing but weakened influence of bottom currents. The contourite/turbidite pattern present in the Adare Trough seismic data is consistent with the 3-phase contourite growth system proposed for the Weddell Sea and Antarctic Peninsula. Multibeam bathymetry and seismic reflection profiles show ubiquitous volcanic cones and intrusions throughout the Adare Basin that we interpret to have formed from the Oligocene to the present. Seismic reflection profiles reveal trans-tensional/strike-slip faults that indicate oblique extension dominated Adare Trough tectonics at 32–15 Ma. Observed volcanism patterns and anomalously shallow basement depth in the Adare Trough area are most likely caused by mantle upwelling, an explanation supported by mantle density reconstructions, which show anomalously hot mantle beneath the Adare Trough area forming in the Late Tertiary.

A very hydrous mantle under the western Pacific region: Implications for formation of marginal basins and style of Archean plate tectonics

The western Pacific region has been refrigerated by the subducting cold oceanic plates since 450 Ma. However, the region is also characterized by the presence of many oceanic microplates less than 1300 km across, as well as active magmatism; the Philippine Sea plate is representative. We have compiled and examined petrochemical characters of drilled basalts of DSDP from the Philippine Sea plate, and conclude that the source mantle for oceanic basalts is rich in water ca. 0.2 wt.%, and is 50–60 °C lower than that for MORB. The extensive melting is due to the high water content in the source mantle. It is well known that some marginal basins apparently have greater depths than the major oceans. We calculated the age–depth correlation based on a model of transient half-space cooling at given parameters of temperatures of mantle and surface, 1280 and 0 °C, and the thermal diffusivity, 1 mm 2 s − 1 . The calculation shows the correlation of age-residual depth from a mid-oceanic ridge is 367 age ( m ⁢ y ) for the Philippine Sea, consistent with the bathymetric data. Moreover, the mid-oceanic ridge may be relatively deep because this region is underlain by the cooler mantle. Addition of water to the mantle peridotite lowers the solidus temperature and viscosity. Melting experiments of hydrous peridotite show that addition of 0.2 wt.% H 2O content lowers the solidus temperature by 150 °C. As a result, the mantle under the region may practically correspond to a ca. 90 °C hotter mantle than normal MORB-source mantle in terms of magmatism and rheology. Numerical simulation for a hotter mantle suggests that many small plates should be formed because of extensive heat release by active magmatism, consistent with many microplates in this region. The presence of many oceanic microplates may be analogous to Archean plate tectonics, characterized by a hotter mantle.

Thermal structure of the Alboran Domain in the Rif (northern Morocco) and the Western Betics (southern Spain). Constraints from Raman spectroscopy of carbonaceous material

AbstractIn the Rif (northern Morocco) and the Western Betics (southern Spain), the Alboran Domain forms a complex stack of metamorphic nappes including mantle peridotites (Beni Bousera and Ronda). We present in this paper new temperature data obtained in the Alboran Domain based on Raman spectroscopy of carbonaceous material (RSCM thermometry). In the lower metamorphic nappes of the Alboran Domain (lower Sebtides–Alpujárrides) temperature ranges from > 640 °C at the base of the metapelitic sequence to 500 °C at the top. The relationships between field isotherms and nappe structure show that peak temperatures were reached during strong ductile thinning of these nappes whereas they partly postdate this main episode in the Rif. In the upper nappes of the Alboran Domain (Ghomarides–Maláguides), generally supposed to be only weakly metamorphosed, temperatures range from ∼500 °C at their base down to < 330 °C at the top. This temperature gradient is consistent with progressive Cenozoic resetting of K–Ar and 40Ar–39Ar ages. These nappes were thus affected by a significant thermal metamorphism, and the available age data in the underlying Sebtides–Alpujárrides show that this metamorphism is related to the metamorphic evolution of the whole Alboran Domain during the Late Oligocene–Early Miocene. Such thermal structure and metamorphic evolution can be explained by generalized extension in the whole Alboran Domain crustal sequence. At a larger scale, the present thermal structure of the Alboran Domain is roughly spatially consistent around the Beni Bousera peridotites in the Rif, but much more affected by late brittle tectonics around the Ronda peridotites in the Western Betics. Therefore, on the basis of the observed thermal structure, the metamorphic evolution of the Alboran Domain can be interpreted as the result of the ascent of hot mantle units contemporaneous with thinning of the whole lithosphere during an Oligo‐Miocene extensional event. The resulting structure has however been dismembered by late brittle tectonics in the Western Betics.

A model of episodic melt extraction for plumes

A model of melt segregation and extraction within rising plumes is proposed. It is based on two‐phase porous flow within the partially molten region, combined with only three extraction parameters. The conservation equations of mass, momentum, and energy are solved for a two‐phase melt matrix system. As a rising hot mantle region (plume) reaches the asthenosphere, decompression melting occurs, and the melt begins to percolate with respect to the matrix. Accumulation layers form, which might be the locus for the formation of buoyancy‐driven propagating dikes. As dike propagation requires a minimum dike length, melt extraction is parameterized by dex, ϕ1, and ϕ2. Here dex is the critical thickness of the partially molten layer in which a critical melt fraction ϕ2 is exceeded. If this condition is met within a certain region of the melt source region, melt might be extracted from that region in the form of one or several propagating dikes, leaving behind a region of residual melt fraction ϕ1. This simple extraction model is tested in one dimension for rising hot mantle flow. Depending on the chosen extraction parameters, multiple extraction events may be observed with a characteristic episodicity and a saw‐tooth‐like depth distribution. Exploring the parameter space shows that for values of dex and ϕ2 of a few kilometers and a few melt percent, respectively, typical extraction cycles have the order of 103–104 years, and they extract melt volumes per surface area of 50 to several hundred meters each. Tentatively assuming that eruptions are tied to mantle ascent at depth, the model is applied to observed eruption frequencies and multiple extraction depths, and values for ϕ2 of about 2% and dex of 3–5 km are derived.

Differences between Archean and Proterozoic lithospheres: Assessment of the possible major role of thermal conductivity

We study heat transfer through the conductive lithosphere and convective mantle on the basis of a 2‐D convection model in order to better understand the differences between Archean and Proterozoic lithospheres. The original improvement in the modeling consists of a precise track of the cutoff temperature between conduction and convection. The conductive lithosphere is undeformable, and the convective mantle has a constant viscosity. The conductive lithosphere in Archean cratons is assumed to have a lower radiogenic heat production in the crust and/or the conductive mantle, and/or a higher cutoff temperature (attributed to a stiffening of the more depleted mantle) relative to Proterozoic terrains. We also investigate the effect of a fourth factor never considered before: an enhancement of the vertical thermal conductivity in the conductive Archean mantle due to a vertical lineation. Our model successfully predicts the observations on the Canadian and South African shields. A high vertical thermal conductivity in the Archean lithospheric mantle possibly associated to a radiogenic crust depletion explains many observations: i.e., (1) the development of a thick Archean cold root, (2) a dipping cold convective plume beneath Archean cratons, (3) a uniform mantle heat flux along Archean and Proterozoic terrains, and (4) strong high seismic velocity anomalies over Archean cratons. In addition, a lower radiogenic heat production of the lithospheric mantle or a larger cutoff temperature in Archean cratons is not favored by our models. Finally, we investigate how a cold root and a sub‐Archean cold mantle plume react to the shear induced by large‐scale mantle flow and to the collision with a hot mantle plume. Models show that shear removes the cold plume from below the Archean craton, but cold dipping instabilities are periodically generated inside the convective boundary layer below the Archean cratonic lid. However, the thick Archean lithospheric root may survive several orogeneses. In addition, the models show that it takes at least 200 Myr for a hot mantle plume to erode the thick lithospheric root. This suggests that an Archean root may only disappear if the plate remains above a hot plume for a very long time.

Na depletion in modern adakites via melt / rock reaction within the sub-arc mantle

Interaction between slab-derived melt and mantle peridotite and the role of slab melt as a metasomatizing agent in the sub-arc mantle is being increasingly recognized. Adakite, the slab melt erupted on the surface, usually exhibits anomalously high MgO, CaO, Cr and Ni contents that indicate interaction with mantle peridotitite. Here we note that Cenozoic adakites have Na 2O contents below 5.8 wt.% with ∼95% samples lower than 5.0 wt.%, and are generally depleted in this component relative to experimental basalt partial melts (mostly beyond 5.0 wt.% and up to 9.0 wt.% Na 2O) produced under 1.5–3.0 GPa conditions that are most relevant to adakite production. We interpret the adakite Na depletion to be also a consequence of the melt / rock reaction that takes place within the hot mantle wedge. During ascent and reaction with mantle peridotite, primary adakite melts gain mantle components MgO, CaO, Cr and Ni but lose Na 2O, SiO 2 and perhaps K 2O to the mantle, leading to Na-rich mantle metasomatism. Selective assimilation of predominately mantle clinopyroxene, some spinel and minor olivine at high T/P has been considered to be an important process in producing high-Mg adakites from primary low-Mg slab melts [Killian, R., Stern, C. R., 2002. Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths. Eur. J. Mineral. 14, 25–36]. In such a process, Na depletion in the assimilated melt is the result of dilution due to the increase in melt mass. Phase relationships in the reaction system siliceous melt + peridotite and quantitative calculation suggest that assimilation of mantle clinopyroxene, olivine and spinel and fractional crystallization of sodic amphibole and orthopyroxene, under conditions of moderate T/P and increasing melt mass, is also an important process that modifies the composition of adakites and causes the Na depletion.

Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range

Research Article| September 01, 2006 Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range Ian S.E. Carmichael; Ian S.E. Carmichael 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Search for other works by this author on: GSW Google Scholar Rebecca A. Lange; Rebecca A. Lange 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Search for other works by this author on: GSW Google Scholar Chris M. Hall; Chris M. Hall 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Search for other works by this author on: GSW Google Scholar Paul R. Renne Paul R. Renne 3Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA, and Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Ian S.E. Carmichael 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Rebecca A. Lange 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Chris M. Hall 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA Paul R. Renne 3Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA, and Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Publisher: Geological Society of America Received: 18 Oct 2005 Revision Received: 25 Mar 2006 Accepted: 04 Apr 2006 First Online: 08 Mar 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 Geological Society of America GSA Bulletin (2006) 118 (9-10): 1196–1211. https://doi.org/10.1130/B25918.1 Article history Received: 18 Oct 2005 Revision Received: 25 Mar 2006 Accepted: 04 Apr 2006 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 Ian S.E. Carmichael, Rebecca A. Lange, Chris M. Hall, Paul R. Renne; Faulted and tilted Pliocene olivine-tholeiite lavas near Alturas, NE California, and their bearing on the uplift of the Warner Range. GSA Bulletin 2006;; 118 (9-10): 1196–1211. doi: https://doi.org/10.1130/B25918.1 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 Late Miocene–Pliocene (8–3 Ma) olivine basalt lavas, dated in this study by the 40Ar/39Ar method, have been faulted and tilted on both the east and west sides of the Warner Range of NE California, which is itself a tilted block rising to 2960 m at its crest that is composed of Miocene-Oligocene lavas and volcaniclastic rocks. The late Miocene–Pliocene lavas, distinctively poor in K2O and rich in MgO, are called low-K olivine tholeiites and have a different mantle source region than that of the older subduction-related lavas of the main Warner Range. Hays Canyon Range (max. elev. 2400 m) lies to the east of the Warner Range, and the broad Surprise Valley separates the two fault-bounded ranges. Middle Miocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap the Hays Canyon Range and overlie Oligocene silicic ash-flow deposits and a basaltic andesite spatter volcano. Middle Miocene basic lavas also form the crest of the Warner Range and its westerly dip slope (∼15°). Nearly horizontal basic lavas of the same age are also found on both sides of the Warner Range, and it is a plausible conclusion that these middle Miocene basalts were a contiguous group before faulting and uplift of the Warner Range.Derived estimates of uplift rates (∼1 mm/yr) of the Warner Range indicate that uplift could have been initiated at ca. 4 Ma, a period of the most voluminous eruption of low-K olivine tholeiite lavas. If the slower Cretaceous exhumation rate of the Sierra Nevada (0.5–1.0 mm/yr) is applied to the total offset of the Warner Range (4270 m), and it did not vary with time, then the uplift of the Warner Range was initiated at ca. 8 Ma, which coincides with the age of the oldest low-K olivine tholeiite lava (8 Ma). Low-K olivine tholeiites require a hot shallow asthenospheric source, and it is the rise of this hot mantle that is presumed to have caused the uplift of the Warner Range. Whether or not the widespread eruption of small volumes of Pliocene low-K olivine tholeiites in central and eastern Oregon is associated with crustal uplift is unknown. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Are splash plumes the origin of minor hotspots?

It has been claimed that focused hot cylindrical upwelling plumes cause many of the surface volcanic hotspots on Earth. It has also been argued that they must originate from thermal boundary layers. In this paper, we present spherical simulations of mantle circulation at close to Earth-like vigor with significant internal heating. These show, in addition to thermal boundary layer plumes, a new class of plumes that are not rooted in thermal boundary layers. These plumes develop as instabilities from the edge of bowls of hot mantle, which are produced by cold downwelling material deforming hot sheets of mantle. The resulting bowl and plume structure can look a bit like the “splash� of a water droplet. These splash plumes might provide an explanation for some hotspots that are not underlain by thermal boundary layer–sourced plumes and not initiated by large igneous provinces. We suggest that in Earth's mantle, lithospheric instabilities or small pieces of subducting slab could play the role of the model downwelling material in initiating splash plumes. Splash plumes would have implications for interpreting ocean-island basalt geochemistry, plume fixity, excess plume temperature, and estimating core heat flux. Improved seismic imaging will ultimately test this hypothesis.

Suboceanic earthquake location and seismic structure in the Kanto district, central Japan

Abstract We present a combined method, using sP depth-phase data and double-difference arrival times, to determine the precise hypocenter locations of earthquakes that occur under the Pacific Ocean outside of the area covered by the land-based seismic network. We assess the effectiveness of the combined method using a data set of P- and S-wave arrival times and sP depth phase from suboceanic earthquakes recorded by both land-based seismic stations and offshore seismic stations (OFS). The hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and OFS data. The differences in the hypocenters relocated by the two methods are less than 4 km. We applied the method to the subduction region that underlies the Kanto district, central Japan, and located a large number of earthquakes that occurred beneath the Pacific Ocean. We then determined the detailed 3D seismic velocity structure by inverting a large number of arrival times of P- and S-waves and sP depth phase from the relocated earthquakes in the study region. High-velocity anomalies related to the cold subducting Pacific slab and low-velocity anomalies related to the hot mantle wedge are clearly imaged. Beneath active volcanoes, low-velocity zones are visible from the surface to a depth of 100 km, reflecting fluids released by dehydration of the subducting Pacific slab. Strong lateral heterogeneities are revealed on the upper boundary of the Pacific slab beneath the forearc region. The low-velocity areas under the offshore region are associated with low seismicity and weak interplate coupling. A low-velocity layer is imaged along the upper boundary of the Philippine Sea slab in the northern part of Kanto district, which may reflect dehydration of the slab. Our tomographic images indicate that the overlaying Philippine Sea plate has effects on the spatial distribution of active volcanoes related to the subducting Pacific slab in the study region.

Volcanic passive margins

Abstract Compared to non-volcanic ones, volcanic passive margins mark continental break-up over a hotter mantle, probably subject to small-scale convection. They present distinctive genetic and structural features. High-rate extension of the lithosphere is associated with catastrophic mantle melting responsible for the accretion of a thick igneous crust. Distinctive structural features of volcanic margins are syn-magmatic and continentward-dipping crustal faults accommodating the seaward flexure of the igneous crust. Volcanic margins present along-axis a magmatic and tectonic segmentation with wavelength similar to adjacent slow-spreading ridges. Their 3D organisation suggests a connection between loci of mantle melting at depths and zones of strain concentration within the lithosphere. Break-up would start and propagate from localized thermally-softened lithospheric zones. These ‘soft points’ could be localized over small-scale convection cells found at the bottom of the lithosphere, where adiabatic mantle melting would specifically occur. The particular structure of the brittle crust at volcanic passive margins could be interpreted by active and sudden oceanward flow of both the unstable hot mantle and the ductile part of the lithosphere during the break-up stage. To cite this article: L. Geoffroy, C. R. Geoscience 337 (2005).

Seismic imaging of the entire arc of Tohoku and Hokkaido in Japan using P-wave, S-wave and sP depth-phase data

In order to better understand seismic structure and seismotectonics of the entire arc of Tohoku and Hokkaido in Japan, we combined arrival time data from earthquakes beneath Tohoku and Hokkaido land areas, and beneath the Pacific Ocean to determine the three-dimensional (3D) velocity structures (Vp and Vs) under the entire Northeast (NE) Japan–Kuril arc. We adopted 176,431 P-wave and 110,953 S-wave arrival times, from 5123 local earthquakes, and 2843 sP depth-phase data from 385 events that occurred beneath the Pacific Ocean. The 385 suboceanic events were accurately relocated by using P-wave, S-wave and sP depth-phase arrival time data jointly. The obtained results confirmed the major features delineated by previous studies and revealed some new features of the structural heterogeneity beneath NE Japan and the Kuril forearcs. High-velocity anomalies of the cold subducting Pacific slab and low-velocity anomalies in the hot mantle wedge were imaged clearly. Strong lateral heterogeneities were revealed on the upper boundary of the Pacific slab under the forearc region, which showed a good correlation with the spatial distribution of large interplate earthquakes. These results indicated that strong coupling sections (or asperities) and weak-coupled or decoupled patches might exist along the upper boundary of the Pacific slab. Widespread low-velocity anomalies were visible in the forearc mantle above the subducting Pacific slab, which might reflect serpentinization of the forearc mantle associated with the dehydration process of the subducting slab. Our results also showed a general tendency for seismic coupling in the asperities to be located around low-velocity areas on the slab boundary under the suboceanic region.

The chemical structure of the Hawaiian mantle plume

The Hawaiian-Emperor volcanic island and seamount chain is usually attributed to a hot mantle plume, located beneath the Pacific lithosphere, that delivers material sourced from deep in the mantle to the surface. The shield volcanoes of the Hawaiian islands are distributed in two curvilinear, parallel trends (termed 'Kea' and 'Loa'), whose rocks are characterized by general geochemical differences. This has led to the proposition that Hawaiian volcanoes sample compositionally distinct, concentrically zoned, regions of the underlying mantle plume. Melt inclusions, or samples of local magma 'frozen' in olivine phenocrysts during crystallization, may record complexities of mantle sources, thereby providing better insight into the chemical structure of plumes. Here we report the discovery of both Kea- and Loa-like major and trace element compositions in olivine-hosted melt inclusions in individual, shield-stage Hawaiian volcanoes--even within single rock samples. We infer from these data that one mantle source component may dominate a single lava flow, but that the two mantle source components are consistently represented to some extent in all lavas, regardless of the specific geographic location of the volcano. We therefore suggest that the Hawaiian mantle plume is unlikely to be compositionally concentrically zoned. Instead, the observed chemical variation is probably controlled by the thermal structure of the plume.

Crustal velocity structure across the Main Ethiopian Rift: results from two-dimensional wide-angle seismic modelling

SUMMARY We present the results of velocity modelling of a recently acquired wide-angle seismic reflection/refraction profile across the Main Ethiopian Rift. The models show a continental type of crust with significant asymmetry between the two sides of the rift. A 2- to 5-km-thick layer of sedimentary and volcanic sequences is modelled across the entire region. This is underlain b ya 40- to 45-km-thick crust with a c. 15-km-thick high-velocity lowest crustal layer beneath the western plateau. This layer is absent from the eastern side, where the crust is 35 km thick beneath the sediments. We interpret this layer as underplated material associated with the Oligocene flood basalts of the region with possible subsequent addition by recent magmatic events. Slight crustal thinning is observed beneath the rift, where Pn velocities indicate the presence of hot mantle rocks containing partial melt. Beneath the rift axis, the velocities of the upper crustal layers are 5‐10 per cent higher than outside the rift, which we interpret as resulting from mafic intrusions that can be associated with magmatic centres observed in the rift valley. Variations in seismic reflectivity suggest the presence of layering in the lower crust beneath the rift, possibly indicating the presence of sills, as well as some layering in the proposed underplated body.

Geophysical evaluation of the enigmatic Bedout basement high, offshore northwestern Australia

The Bedout High in the Roebuck Basin (formerly offshore Canning Basin) on the northwest shelf of Australia is an unusual structure, which has been controversially interpreted as an end-Permian impact structure similar in size to the K–T boundary Chicxulub Crater. We present a geophysical perspective of the associated debate, based on deep seismic reflection, refraction and well data. The basement and crust in the Roebuck Basin display a number of features that distinguish them from other basins along the northwest Australian margin, including major crustal thinning and the presence of a thick layer of interpreted magmatic underplating. The Bedout High consists of two separate highs separated by a Paleozoic fault, and is associated with a Moho uplift of 7–8 km, and is about 40–50 km wide. The normal fault separating the two highs trends NNW–SSE, roughly paralleling a Paleozoic fault system associated with rifting in the Canning Basin and terminating below the interpreted top-Permian reflection. There are no circular, symmetric fault zones bounding the proposed annular trough, and the distinct difference in seismic character normally associated with impact breccias versus layered sediments above are not expressed in deep multichannel seismic data. The end-Permian horizon exhibits little topography, with well-layered units both below and above. The area around the Bedout High stands out as an area of low velocity basement: 5400–5600 m/s compared to 5800–6000 m/s for other nearby basement areas located in a similar depth range, but known complex impact sites are not characterized by a unique seismic basement velocity signature. Both seismic velocity analysis, revealing a thick underplated layer in the lower crust, and thermal modelling based on data from well La Grange-1 and basalts drilled on top of the Bedout High, are consistent with rifting above anomalously hot mantle. The available geophysical and geological data are compatible with an interpretation of the Bedout structure as a basement high formed by two consecutive Paleozoic and Mesozoic episodes of rifting roughly orthogonal to each other, associated with basin formation east and west of the Bedout High, but fail nearly all unequivocal criteria for impact crater recognition.