Quaternary Volcanism Research Articles

Quaternary Volcanism and Origin of the Kurile Basin (Sea of Okhotsk)

Quaternary Volcanism and Origin of the Kurile Basin (Sea of Okhotsk)

Quaternary Tholeiitic to Alkaline Volcanism in the Karasu Valley, Dead Sea Rift Zone, Southeast Turkey: Sr-Nd-Pb-O Isotopic and Trace-Element Approaches to Crust-Mantle Interaction

Rare-earth-element, radiogenic and oxygen isotope, and mineral chemical data are presented for tholeiitic and alkaline Quaternary volcanism from Karasu Valley (Hatay, southeastern Turkey). Karasu Valley is the northern segment of the Dead Sea transform fault and is filled with flood-basalt type volcanics of Quaternary age. This valley is an active fault zone that is known as “Karasu fault,” extending in a NE-SW direction. The Karasu Valley basaltic volcanics (KVBV) are subaphyric to porphyritic, with variable amounts of olivine, clinopyroxene, and plagioclase phenocrysts. Alkali basalts are generally characterized by high contents of olivine, clinopyroxene, and plagioclase phenocrysts. Their groundmass contains olivine, clinopyroxene, plagioclase, and Fe-Ti oxides. Tholeiitic basalts are subaphyric to porphyritic (high contents of olivine, clinopyroxene, and plagioclase). Their groundmass is similar to that of alkali basalts. The range of olivine phenocryst and microlite compositions for all analyzed samples is Fo81 to Fo43. Plagioclase compositions in both tholeiitic and alkali basalts range from andesine, An38 to bytownite, An72. Clinopyroxene compositions range from diopside to calcic augite. Most of the olivine, plagioclase, and clinopyroxene phenocrysts are normally zoned and/or unzoned. Fe-Ti oxides in both series are titanomagnetite and ilmenite. Based on normative and geochemical data, the Karasu Valley basaltic volcanics are mostly olivine and quartz-tholeiites, and relatively lesser amount of alkali olivine-basalts. KVBV have low K2O/Na2O ratios, typically between 0.25 and 0.45. Olivine- and quartz-tholeiites are older than alkali olivine-basalts. Olivine tholeiites have Zr/Nb and Y/Nb ratios similar to alkaline rocks, but their Ba/Nb, Ba/La, and La/Nb ratios are slightly higher than alkali olivine-basalts. In contrast, quartz-tholeiites have the highest Ba/Nb, Ba/La, Zr/Nb, and Y/Nb and the lowest Nb/La ratios among the KVBV. Alkali basalts have 87Sr/86Sr and 143Nd/144Nd ratios ranging from 0.703353 to 0.704410 and 0.512860 to 0.512910, respectively. In contrast, quartz-tholeiites have higher 87Sr/86Sr and lower 143Nd/144Nd ratios, which vary from 0.704410 to 0.705490 and 0.512628 to 0.512640, respectively. Olivine tholeiites have intermediate isotopic compositions ranging from 0.703490 to 0.704780 and 0.512699 to 0.512780, respectively. 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb isotopic ratios of KVBV range from 18.817 to 19.325, 15.640 to 15.718, and 39.054 to 39.223, respectively. The range of O isotope values is between +5.84 and +7.97‰. The higher O and Sr isotopes in olivine- and quartz-tholeiites relative to alkali olivine-basalts can be explained by contamination of magmas by crustal materials. The KVBV have intraplate chemistry similar to that of other tholeiitic and alkaline basalts in other within-plate environments, and isotopes range from isotopically depleted mantle to enriched isotope compositions similar to some enriched ocean islands. Trace-element and isotope data indicate that the KVBV are derived from a common OIB-like asthenospheric mantle source, but they have experienced different degrees of crustal contamination during their ascent to the surface, contemporaneous with little fractional crystallization. Although quartz-tholeiites display significant effects of crustal contamination, alkali olivine-basalts appear to have negligible or no crustal contamination in their geesis.

Epithermal gold occurrences in the lakes district of the Main Ethiopian Rift and Tendaho area of the Afar Rift: discovery of a metallogenic province

Plio-Quaternary volcanic products cover a large part of the Main Ethiopian Rift (MER). Epithermal gold occurrences related to Quaternary volcanics are at present being closely studied for their precious metal potential. Low sulphidation (Adularia-sericite-type) occurrences have been found. Analyses of 579 core and cutting samples collected for the purpose of exploratory geothermal studies from a number of localities (Gedemsa, Aluto and Corbetti calderas MER, and the Tendaho graben; Afar) were analyzed for gold. The results showed concentration of gold ranging from 100 ppb to 440 ppb. Values ranging between 200 and 300 ppb are very common. The overall characteristics of the known ore occurrences and the evolution of the Quaternary central volcanoes within the MER, and related epithermal processes seem to delineate an individual, homogeneous metallogenic province. A new field of investigation on epithermal ore occurrences which are unusual for the present Ethiopian metallogenic scenery is probably emerging. A study of these phenomena, in light of recent acquisitions in metallogenic knowledge, could be of interest not only for further scientific studies but also for new possibilities in mining activity. This study presents the occurrences of epithermal gold mineralization in the MER, the only one of its kind so far reported in the East African Rift System. SINET: Ethiopian Journal of Science Vol. 24, No. 1 (June 2001), pp. 69-91 Key words/phrases: Caldera, epithermal gold, metallogeny, Plio-Quaternary, rift volcanics

Variations of the lithospheric seismic scattering strength below the Massif Central, France and the Frankonian Jura, SE Germany

Small-scale heterogeneities in the lithosphere below the Frankonian Jura (SE Germany) and the Massif Central (France) are studied using scattered teleseismic wavefields. To compare both regions wavefields of earthquakes are analysed which were simultaneously recorded at permanent local networks. Because the average wavelets of the first P-arrivals are nearly identical in the Frankonian Jura and the Massif Central, it is possible to compare directly the scattering characteristics of the lithospheric environments. Although the overall form of the mean wavefields and the P-codas are quite similar in both areas, the intensity of the scattered seismic arrivals is increased in the Massif Central compared to the Frankonian Jura. The same result is obtained using waveform data from mobile stations. The increased seismic scattering strength in the Massif Central is interpreted as interaction of the incoming wavefield with solidified magmatic intrusions. The lithospheric heterogeneities are most likely related to the known upwelling of the asthenosphere and the Tertiary and Quaternary volcanism. In contrast, the lithosphere beneath the Frankonian Jura seems to be less heterogeneous, perhaps due to less structural alterations since its formation.

Dynamic elevation of the Cordillera, western United States

We introduce a methodology that synthesizes topography, gravity, crustal‐scale seismic refraction velocity, and surface heat flow data sets to estimate dynamic elevation, i.e., the topography deriving from buoyancy variations beneath the lithosphere. The geophysical data independently constrain the topographic effects of surface processes, crustal buoyancy, and thermal boundary layer thickness. Each of these are subtracted from raw elevation of the western U.S. Cordillera to reveal dynamic elevation that can exceed 2 km and is significant at >95% confidence. The largest (∼1000 km diameter) of the dynamic elevation anomalies resembles a numerical model of a hypothetical Yellowstone hotspot swell, but the swell model does not account for all of the significant features seen in the dynamic elevation map. Other dynamic elevation anomalies are spatially correlative with Quaternary volcanism, but partial melt can contribute no more than a few hundred meters of elevation. Hence much of the dynamic elevation likely derives from other thermodynamic anomalies. Possible alternative mechanisms include both superadiabatic upwelling and adiabatic phase boundary deflections maintained by latent heat effects. Comparison of seismicity and volcanism to effective viscosity gradients, estimated from lithospheric flexural rigidity to facilitate the numerical swell model, suggests that tectonism focuses where lithosphere with negligible mantle viscosity abuts lithosphere with significant uppermost mantle viscosity.

Fold uplift and synkinematic stratal architectures in a region of active transtensional tectonics and volcanism, eastern Tyrrhenian Sea

The detailed stratigraphic analysis of synkinematic stratal architectures permits the calculation of fold uplift rates and the reconstruction of fold kinematics. This approach was applied to active folds in the Bay of Pozzuoli, eastern Tyrrhenian Sea. This area features late Quaternary volcanism, shallow magmatic intrusions, and transtensional tectonics. Seismostratigraphic analysis of synkinematic strata is constrained by a physical correlation to an already established stratigraphic succession in the neighboring Campi Flegrei and by tephra layers in a gravity core located in Pozzuoli Bay. A stratigraphic resolution in thousands of years was established. Structural and stratigraphic analyses yield quantitative data on the timing of fold inception, kinematics, and the amount and rates of uplift. The folds extend over ∼2 km, and display a curved axial pattern and wedge-shaped packages of synkinematic strata. The rate of fold uplift ranges from 1 to 20 mm/yr. The structures feature limb rotation kinematics and decreasing uplift rates. The uplift rates and geometry of the Pozzuoli Bay folds suggest that they are of tectonic origin and are a product of detachment folding. Their limited longitudinal extent and arcuate axial trends are compatible with an east-trending left transtensional shear zone that was active along the eastern Tyrrhenian Sea during the late Quaternary. A relative sea-level curve was calculated for an anticline culminating in the city of Pozzuoli. This relative sea-level curve highlights the relationship between the deformation timing and the genesis of related abrupt changes in sedimentary facies and erosional surfaces.

A53 東北地方第四紀火山活動の特徴 : 火山カタログから

A53 東北地方第四紀火山活動の特徴 : 火山カタログから

Distribution, nature, and origin of Neogene-Quaternary magmatism in the northern Cordilleran volcanic province, Canada

The northern Cordilleran volcanic province encompasses a broad area of Neogene to Quaternary volcanism in northwestern British Columbia, the Yukon Territory, and adjacent eastern Alaska. Volcanic rocks of the northern Cordilleran volcanic province range in age from 20 Ma to ca. 200 yr B.P. and are dominantly alkali olivine basalt and hawaiite. A variety of more strongly alkaline rock types not commonly found in the North American Cordillera are locally abundant in the northern Cordilleran volcanic province. These include nephelinite, basanite, and peralkaline phonolite, trachyte, and comendite. The most MgO-rich nephelinites, basanites, and alkaline basalts from throughout the northern Cordilleran volcanic province show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to that for average oceanic island basalt and for post-5 Ma alkaline basalts from the Basin and Range. Our petrologic observations help constrain the origin of northern Cordilleran volcanic province magmatism as well as lithosphere changes between the four major basement terranes that underlie the province. Results from phase equilibria calculations and the spatial distributions of volcanic rock types and magmatic inclusions are more consistent with the existence of thicker lithosphere beneath Stikinia, which underlies the southern part of the northern Cordilleran volcanic province, than beneath the Cache Creek and Yukon-Tanana terranes, which underlie the northern part of the northern Cordilleran volcanic province. Our results support a model for initiation of northern Cordilleran volcanic province magmatism due to incipient rifting of the northern Cordillera, driven by changes in relative plate motion between the Pacific and North American plates ca. 15–10 Ma.

Alpha-recoil track dating of Quaternary volcanics

Like fission-track dating, alpha-recoil track (ART) dating is based on the accumulation of nuclear particles that are released by natural radioactivity and produce etchable tracks in solids. ARTs are formed during the alpha-decay of uranium and thorium as well as of their daughter nuclei. When emitting an alpha-particle, the heavy remaining nucleus recoils 30–40 nm, leaving behind a trail of radiation damage. Through etching the ART tracks become visible with interference phase-contrast microscopy. Under the presupposition that all tracks are preserved since the formation of a sample their total number is a measure for the sample's age. The technique has already been proposed more than three decades ago [Huang, W.H., Walker, R.M., 1967. Fossil alpha-particle recoil tracks: a new method of age determination. Science 155, 1103–1106.], but due to inherent difficulties, it was hardly applied or made any substantial progress since then. The present paper describes a methodology by which reliable ART ages of dark mica can be obtained. The technique is applicable to tiny (∼0.5 mm) single mica flakes in the 10 2 to 10 6 years age-range. It is tested on Quaternary volcanics from the Eifel region, Germany. Alpha-recoil dating has a great potential for Quaternary chronometry and tephrochronology.

Palaeomagnetism of Late Miocene to Quaternary volcanics from the eastern segment of the Trans-Mexican Volcanic Belt

A systematic palaeomagnetic study in the eastern part of the Trans-Mexican Volcanic Belt includes 39 Miocene, Pliocene and Quaternary volcanic rocks in the southeastern Mexico Basin (Sierra Nevada and Sierra de Rı́o Frı́o), the Altiplano area, and the Palma Sola Massif. A total of 430 samples have been selectively demagnetized using mostly alternating field demagnetizing methods, supplemented by thermal analyses. Most characteristic remanences are carried by low-Ti titanomagnetites, with occasional titanohematites or slightly maghemitized low-Ti titanomagnetites, of similar direction. Seven sites were discarded because they presented intermediate directions, hydrothermal alteration or were remagnetized by lightning strikes. The mean directions of 32 sites, together with 24 sites from Sierra de las Cruces in the western Mexico basin, indicate rocks older than 2 Ma are rotated some 10° counterclockwise with respect to Quaternary rocks, whereas there is no rotational difference between Miocene and Pliocene rocks. Statistical analyses between different regrouped populations confirm that the rotational pattern is due to the age of the volcanics rocks but not to their spatial distribution. The Quaternary mean direction from the three Mexico Basin ranges is consistent with the geographical reference pole. In contrast, the Pliocene mean direction from volcanic rocks of the Altiplano area and the Sierra de Las Cruces is slightly rotated some 10° westwards with respect to the reference direction from North America. No significant rotations have been observed in the eastern TMVB (from the western Mexico Basin to the border of the Altiplano), between late Miocene and late Pliocene time. It suggests that a very small, counterclockwise vertical-axis rotation may have been taken place in this segment of the TMVB between late Pliocene and Quaternary times. Comparisons of these results with a summary of the available palaeomagnetic data in the area indicate that the previously reported Quaternary rotations are of questionable reliability, and that the large counterclockwise rotations, reported in Cretaceous to Miocene rocks, probably took place before the late Miocene. These new palaeomagnetic data support the idea that the eastern TMVB since the late Miocene, has been a zone of extension with a little, left-lateral shear component.

Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area, southern China

The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO 2 with increasing SiO 2 content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10 −13.4 to 10 −10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.

Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa

Research Article| February 01, 2000 Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa C. J. Ebinger; C. J. Ebinger 1Department of Geology, Royal Holloway, University of London, Egham TW20 0EX, United Kingdom Search for other works by this author on: GSW Google Scholar T. Yemane; T. Yemane 2Ethiopian Institute of Geological Surveys, Addis Ababa, Ethiopia3Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar D. J. Harding; D. J. Harding 4NASA Goddard Space Flight Center, Greenbelt, Maryland 20771 Search for other works by this author on: GSW Google Scholar S. Tesfaye; S. Tesfaye 2Ethiopian Institute of Geological Surveys, Addis Ababa, Ethiopia5Department of Geological Sciences, University of Colorado, Boulder, Colorado 80305 Search for other works by this author on: GSW Google Scholar S. Kelley; S. Kelley 3Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar D.C. Rex D.C. Rex 6Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (2): 163–176. https://doi.org/10.1130/0016-7606(2000)112<163:RDMAPL>2.0.CO;2 Article history received: 22 Oct 1997 rev-recd: 28 Feb 1998 accepted: 01 Apr 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. J. Ebinger, T. Yemane, D. J. Harding, S. Tesfaye, S. Kelley, D.C. Rex; Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa. GSA Bulletin 2000;; 112 (2): 163–176. doi: https://doi.org/10.1130/0016-7606(2000)112<163:RDMAPL>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Main Ethiopian and Eastern (Gregory) rifts, sectors of the East African rift system, overlap in a 300-km-wide system of extensional basins that is more than three times the breadth of either rift away from the zone of overlap. The oldest volcanic rocks (Eocene) and possibly the oldest rift basins (Oligocene) of the East African rift system occur in this zone of overlap. The objectives of field, remote sensing, and geochronology (K-Ar and 40Ar/39Ar) studies in southwestern Ethiopia were to establish a chronology of rifting and volcanism in the zone of overlap, and to correlate stratigraphic sequences with those in the Kenya rift to the south and in the Main Ethiopian rift to the north. Field observations and cross sections show that basins are bounded by steeply dipping faults, stratal dips are <30°, and that extension accommodated by the intrusion of dikes is volumetrically insignificant. Thus, the style of faulting is similar to that elsewhere in East Africa south of the Afar rift. Initial volcanism between ca. 45 and 33 Ma preceded faulting and uplift, except for reactivation of some Mesozoic rift structures near the Sudan-Ethiopia border and in northern Kenya. Extensional basins began to form in late Oligocene time in the Eastern rift, and in early Miocene time in the Main Ethiopian rift. Small degrees of extension and associated volcanism in the broadly rifted zone may have been triggered by extension in the Red Sea, as well as by lithospheric heating above a mantle plume. The anomalous breadth of the zone is a consequence of rift propagation and migration, rather than basin-and-range–style extension; both the Main Ethiopian rift and Eastern rifts have propagated along north-south lines, and the Eastern rift has migrated ∼200 km eastward since late Oligocene time. The distribution of seismicity and Quaternary volcanism suggest that the Eastern and Main Ethiopian rifts are currently linked across a 200-km-wide zone between the Omo and Segen basins. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Discovery of Minoan tsunami deposits

The Hellenic arc is a terrane of extensive Quaternary volcanism. One of the main centers of explosive eruptions is located on Thera (Santorini), and the eruption of the Thera volcano in late Minoan time (1600–1300 B.C.) is considered to have been the most significant Aegean explosive volcanism during the late Holocene. The last eruptive phase of Thera resulted in an enormous submarine caldera, which is believed to have produced tsunamis on a large scale. Evidence suggesting seawater inundation was found previously at some archaeological sites on the coast of Crete; however, the cause of the tsunami and its effects on the area have not been well understood. On the Aegean Sea coast of western Turkey (Didim and Fethye) and Crete (Gouves), we have found traces of tsunami deposits related to the Thera eruption. The sedimentological consequences and the hydraulics of a Thera-caused tsunami indicate that the eruption of Thera volcano was earlier than the previous estimates and the tsunami did not have disruptive influence on Minoan civilization.

Neogene and Quaternary Volcanism of Southeastern Turkey

Abstract Potassium-argon dating indicates two episodes of basaltic magmatism in south eastern Turkey at c. 19–15 and c. 2.3–0.6 Ma. Each produced olivine-titanaugite basalts, whose chemical compositions are difficult to classify using any conventional model in both the Anatolian continental fragment and the Arabian Platform. It is proposed here that both episodes of volcanism, and the associated crustal thickening and surface uplift, result from heating of the mantle lithosphere by crustal thickening caused by inflow of plastic lower crust from adjoining regions. Thus, although this study region has remained in a plate boundary zone for tens of millions of years, its volcanism has no direct relationship to local plate motions. It is suggested instead that both episodes of volcanism are the result of loading effects caused by glacial to interglacial sea-level variations, which will cause net flow of lower crust from beneath the offshore shelf to beneath the land: the moderate glaciations of Antarctica which began in the Early-Middle Miocene, and the more intense lowland glaciations of the northern hemisphere which began around c. 2.5 Ma.

Geochronology of Neogene alkaline volcanic rocks (Miles Canyon basalt), southern Yukon Territory, Canada: the relative effectiveness of laser 40Ar/39Ar and K-Ar geochronology

Miles Canyon basalt is an informal term used to describe numerous exposures of young alkaline olivine basalt flows in southern Yukon. The volcanic rocks are part of the Northern Cordilleran volcanic province. K-Ar and Ar-Ar whole-rock dates indicate that the Miles Canyon succession of flows at the Whitehorse Rapids are clearly Late Miocene in age (ca. 8.4 Ma). The largest exposure of the Miles Canyon basalt occurs at the Alligator Lake volcanic complex where two nearly concordant Pliocene Ar-Ar dates indicate eruption at ca. 3.2 Ma. K-Ar analyses from other sites yield dates of 2.4 and 7.1 Ma and indicate an episodic Neogene volcanic history for the region. There is no evidence of Quaternary or postglacial volcanism. The dates are older than assumed by previous workers, and in some cases the K-Ar dates are strongly discordant from Ar-Ar determinations. More accurate Ar-Ar determinations may result from the method's ability to select smaller amounts of better material for analysis. Excess 40Ar was not encountered. As a result, the accuracy of any single or several discordant K-Ar determinations for Neogene subaerial volcanic rocks, particularly low-K rocks such as basalts, should be questioned and resulting interpretations made with caution. Models accounting for the eruption of the Northern Cordilleran volcanic province lavas have typically relied upon extension along north-trending faults that were generated by stresses along the continental margin. However, we consider a slab window model which better accounts for the initiation and distribution of northern Cordilleran Neogene volcanic activity.

Crustal structure of central Japan and its petrological implications

The crustal structure of central Honshu, Japan, has been influenced by several tectonic events: a series of collision processes during the late Tertiary, extensive faulting, highly compressive deformation and Quaternary volcanism. The southern part of the study area is covered by various granitic rocks which intrude into old Palaeozoic formations. Quaternary volcanoes occupy the northern part and cover the basement, which is mainly composed of granitic rocks and Tertiary formations. A traveltime inversion for velocity and interface depth was applied to the seismic data, together with constraints from amplitude forward modelling to produce a seismic velocity model of the crust in central Honshu. In this experiment both refracted P and S waves are clearly recorded along a 110 km profile. These provide constraints on the 2-D P- and S-wave velocity structure down to a depth of about 10 km. Moreover, clear P-wave reflections from intracrustal interfaces provide constraints on deeper parts of the crust down to about 25 km. For the P-wave velocity model, the velocities of near-surface layers show large lateral variations from 3.7 to 4.4 km s−1, increasing towards the south. Underlying layers have velocities ranging from 4.85 to 5.6 km s−1. Velocities in the range 5.85–6.2 km s−1 characterize the upper crust that extends to a depth of about 15 km. For the S-wave velocity model, the near-surface layer velocities change from 1.7 to 2.55 km s−1, showing remarkable lateral heterogeneity, whilst the upper crust is characterized by velocities of 3.4–3.6 km s−1. The deeper parts are modelled by two prominent reflectors in the mid-crust. The shallower one, which delineates the bottom of the upper crust, dips southwards from 14 to 16 km. The deeper reflector lies between 20 and 24 km depth and dips northwards. The velocity in this mid-crustal layer is 6.35 km s−1 at the top and 6.7 km s−1 at the bottom. The crustal thickness under the profile could not be determined because of a lack of Pn and/or Pm P phases. This study also presents the Vp /Vs ratio and the Poisson’s ratio distribution and gives the probable composition of the upper crust. Poisson’s ratio varies from 0.29 to 0.34, which can be ascribed to a porous and water-saturated structure for the near-surface layers in the northern part of the region. Poisson’s ratios of about 0.24–0.25 in the southern part and 0.22–0.24 in the upper crust, together with the geological data, show that the upper crust has a felsic content and is formed by granite–granodioritic rocks with a high quartz content. The changes in the quartz content explain the lateral variations of Vp /Vs and Poisson’s ratio in the model. The higher velocities in the middle crust suggest a more mafic structure, probably composed of diorites.

Recent geodynamic characteristics in the Arabian–Eurasian and Indian–Eurasian collision region by active fault data

A part of the Alpine–Himalayan orogenic belt is studied, limited by 30–104°E and by 26–46°N to the west of 64°E and 26–56°N to the east of 64°E. A technique is proposed to calculate the field of the recent (Late Pleistocene and Holocene) deformation rate tensors in the upper crust (15 to 20 km) by active fault data. A hydrodynamic model of the medium is used for the calculation. Monotonous fault segments as long as 10–15 km are taken as the elementary cells of the medium. The cell square (the length multiplied by the depth) multiplied by the displacement vector magnitude, gives the geometric moment. Similar components of the local tensors of the geometric moments are summarized within the windows into which the region is divided. Finally, parameters of the axes of principal rates of deformation are calculated, and their directions and magnitudes are mapped. This tectonic deformation is compared with the seismotectonic one calculated by focal mechanisms of earthquakes. The compiled maps show: a concentration of high deformation rates in the plate boundary zones (especially in front of the southern plate syntaxes) and a smaller concentration in some microplate boundary zones; a predominance of the N–S-trending shortening and of strike-slip type of deformation. The rates of shortening and lengthening are usually almost equal to each other and differ essentially (the shortening rate is higher than the lengthening) only in specific sites. These are areas of intense recent detachment or late Quaternary volcanism. It seems that the double-axis lengthening (extension) is suitable for these processes.

Tephrochronology of the Toluca Basin, central Mexico

The major element glass geochemistry of 10 Late Pleistocene and Holocene tephras found in the Toluca Basin in the Trans-Mexican Volcanic Belt is reported for the first time, as a key step towards establishing a tephrochronology for the region. The context for this tephrochronology is provided by a review of Late Quaternary volcanism of the basin. New tephra samples were collected from reference profiles around the basin in order to establish major element glass geochemical data for tephras described in previous publications, but not previously analysed. In addition, cores were taken from the basin floor to extend the known spatial distribution of the tephra layers and establish correlations with the lacustrine record. Improved chronological control has been provided by seven new AMS and conventional radiocarbon dates on the major tephras. Geochemical data are presented for 10 tephras ranging in age from ca. 25,000 BP (Lower Toluca Pumice) to about 8500 BP (Tres Cruces Tephra). Based on geochemical data, it is possible to correlate between sites within the Toluca Basin and between the Toluca Basin and the Basin of Mexico. The basis for a tephrochronological framework has been established.

Fire or ice: anticorrelation of volcanism and glaciation in California over the past 800,000 years

Compilation of published ages of Quaternary volcanism in eastern California indicates that volcanism was episodic, with maxima occurring during interglacial periods. The smoothed age distribution shows peaks at 10, 100, 185, 320, and 690 kyr, corresponding to interglacial cycles 1, 5, 7, 9, and 17. This implies that volcanism was modulated by changes in climate, although the processes that link the two are not well understood. Several factors, including transient loads imposed across rugged topography by ice and water and changes in groundwater regime, may provide the link. Although climatic modulation of volcanism is generally tied to changes in sea level, these data indicate that even volcanoes far from the coast can be affected by climatic change.

Quaternary volcanism near the Valley of Mexico: implications for subduction zone magmatism and the effects of crustal thickness variations on primitive magma compositions

The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary eruptive vents in addition to the large, composite volcanoes of Popocatepetl, Iztaccihuatl, and Nevado de Toluca. The eruptive vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous in the Sierra Chichinautzin that forms the southern terminus of the Valley of Mexico. The Chichinautzin volcanic field (CVF) is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites, andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO, and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements. Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust. In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities that are related to previous extraction of alkaline to sub-alkaline basaltic melts.