http://www.solid-earth.net/Solid Earth Latest Articlesenhttp://www.solid-earth.net/3/29/2012/<b>Phanerozoic environments of black shale deposition and the Wilson Cycle</b><br /><br />Solid Earth, 3, 29-42, 2012<br /><br />Author(s): J. Trabucho-Alexandre, W. W. Hay, and P. L. de Boer<br /><br />The spatial and temporal distribution of black shales is related to the development of environments in which they accumulate and to a propitious combination of environmental variables. In recent years, much has been done to improve our understanding of the mechanisms behind the temporal distribution of black shales in the Phanerozoic and of the environmental variables that result in their deposition. However, the interpretation of ancient black shale depositional environments is dominated by an oversimplistic set of three depositional models that do not capture their complexity and dynamics. These three models, the restricted circulation, the (open) ocean oxygen minimum and the continental shelf models, are an oversimplification of the variety of black shale depositional environments that arise and coexist throughout the course of a basin's Wilson Cycle, i.e. the dynamic sequence of events and stages that characterise the evolution of an ocean basin, from the opening continental rift to the closing orogeny. We examine the spatial distribution of black shales in the context of the Wilson Cycle using examples from the Phanerozoic. It is shown that the geographical distribution of environments of black shale deposition and the position of black shales in the basin infill sequence strongly depend on basin evolution, which controls the development of sedimentary environments where black shales may be deposited. The nature of the black shales that are deposited, i.e. lithology and type of organic matter, also depends on basin evolution and palaeogeography. We propose that in studies of black shales more attention should be given to the sedimentary processes that have led to their formation and to the interpretation of their sedimentary environments.Thu, 02 Feb 2012 00:00:00 +0100http://www.solid-earth.net/3/1/2012/<b>Bio-chemostratigraphy of the Barremian-Aptian shallow-water carbonates of the southern Apennines (Italy): pinpointing the OAE1a in a Tethyan carbonate platform</b><br /><br />Solid Earth, 3, 1-28, 2012<br /><br />Author(s): M. Di Lucia, A. Trecalli, M. Mutti, and M. Parente<br /><br />Low biostratigraphic resolution and lack of chronostratigraphic calibration hinder precise correlations between platform carbonates and coeval deep-water successions. These are the main obstacle when studying the record of Mesozoic oceanic anoxic events in carbonate platforms. In this paper carbon and strontium isotope stratigraphy are used to produce the first chronostratigraphic calibration of the Barremian-Aptian biostratigraphy of the Apenninic carbonate platform of southern Italy. According to this calibration, the segment of decreasing &delta;¹³C values, leading to the negative peak that is generally taken as the onset of the Selli event, starts a few metres above the last occurrence of <i>Palorbitolina lenticularis</i> and <i>Voloshinoides murgensis</i>. The following rise of &delta;¹³C values, corresponding to the interval of enhanced accumulation of organic matter in deep-water sections, ends just below the first acme of <i>Salpingoporella dinarica</i>, which roughly corresponds to the segment of peak &delta;¹³C values. The whole carbon isotope excursion associated with the oceanic anoxic event 1a is bracketed in the Apenninic carbonate platform between the last occurrence of <i>Voloshinoides murgensis</i> and the "Orbitolina level", characterized by the association of <i>Mesorbitolina parva</i> and <i>Mesorbitolina texana</i>. Since these bioevents have been widely recognized beyond the Apenninic platform, the calibration presented in this paper can be used to pinpoint the interval corresponding to the Early Aptian oceanic anoxic event in other carbonate platforms of central and southern Tethys. This calibration will be particularly useful to interpret the record of the Selli event in carbonate platform sections for which a reliable carbon isotope stratigraphy is not available.Fri, 27 Jan 2012 00:00:00 +0100http://www.solid-earth.net/2/315/2011/<b>Influence of the Ringwoodite-Perovskite transition on mantle convection in spherical geometry as a function of Clapeyron slope and Rayleigh number</b><br /><br />Solid Earth, 2, 315-326, 2011<br /><br />Author(s): M. Wolstencroft and J. H. Davies<br /><br />We investigate the influence on mantle convection of the negative Clapeyron slope ringwoodite to perovskite and ferro-periclase mantle phase transition, which is correlated with the seismic discontinuity at 660 km depth. In particular, we focus on understanding the influence of the magnitude of the Clapeyron slope (as measured by the Phase Buoyancy parameter, <i>P</i>) and the vigour of convection (as measured by the Rayleigh number, <i>Ra</i>) on mantle convection. We have undertaken 76 simulations of isoviscous mantle convection in spherical geometry, varying <i>Ra</i> and <i>P</i>. Three domains of behaviour were found: layered convection for high <i>Ra</i> and more negative <i>P</i>, whole mantle convection for low <i>Ra</i> and less negative <i>P</i>, and transitional behaviour in an intervening domain. The boundary between the layered and transitional domain was fit by a curve <i>P = &alpha; Ra^&beta;</i> where <i>α</i> = −1.05, and <i>β</i> = −0.1, and the fit for the boundary between the transitional and whole mantle convection domain was <i>α</i> = −4.8, and <i>β</i> = −0.25. These two curves converge at <i>Ra</i> ≈ 2.5 × 10⁴ (well below Earth mantle vigour) and <i>P</i> &approx; &minus;0.38. Extrapolating to high <i>Ra</i>, which is likely earlier in Earth history, this work suggests a large transitional domain. It is therefore likely that convection in the Archean would have been influenced by this phase change, with Earth being at least in the transitional domain, if not the layered domain.Mon, 19 Dec 2011 00:00:00 +0100http://www.solid-earth.net/2/303/2011/<b>Domains of Archean mantle lithosphere deciphered by seismic anisotropy &ndash; inferences from the LAPNET array in northern Fennoscandia</b><br /><br />Solid Earth, 2, 303-313, 2011<br /><br />Author(s): J. Plomerová, L. Vecsey, V. Babuška, and LAPNET Working Group<br /><br />An international LAPNET array (2007–2009, <a href="http://www.oulu.fi/sgo-oty/lapnet" target="_blank">http://www.oulu.fi/sgo-oty/lapnet</a>) of the POLENET/LAPNET sub-project of the POLENET-IPY consortium, related to seismic and geodetic studies in the Arctic regions, consisted of about 60 broadband seismic stations located on the territory of northern Finland and adjacent parts of Sweden, Norway and Russia. We analyze relative P-wave travel-time deviations evaluated for a subset of 90 teleseismic events recorded by the LAPNET array and show examples of lateral variations of shear-wave splitting to demonstrate variability of fabrics of the Archean mantle lithosphere. The initial results clearly demonstrate the Archean mantle lithosphere consists of domains with consistent fabrics reflecting fossil anisotropic structures. 3-D self-consistent anisotropic models with inclined symmetry axes accommodate two independent sets of body-wave anisotropic observations. Individual domains are delimited by boundaries (sutures), where the anisotropic parameters change. The results obtained from the LAPNET array fill a gap in structural studies of the upper mantle beneath northern Fennoscandia.Thu, 15 Dec 2011 00:00:00 +0100http://www.solid-earth.net/2/283/2011/<b>Erosion rates deduced from seasonal mass balance along the upper Urumqi River in Tianshan</b><br /><br />Solid Earth, 2, 283-301, 2011<br /><br />Author(s): Y. Liu, F. Métivier, J. Gaillardet, B. Ye, P. Meunier, C. Narteau, E. Lajeunesse, T. Han, and L. Malverti<br /><br />We report measurements performed during two complete flow seasons on the Urumqi River, a proglacial mountain stream in the northeastern flank of the Tianshan, an active mountain range in Central Asia. This survey of flow dynamics and sediment transport (dissolved, suspended and bed loads), together with a 25-year record of daily discharge, enables the assessment of secular denudation rates on this high mountain catchment of Central Asia. Our results show that chemical weathering accounts for more than one-third of the total denudation rate. Sediment transported as bed load cannot be neglected in the balance, given that sand and gravel transport accounts for one third of the solid load of the river. Overall, the mean denudation rates are low, averaging 46 t × km⁻² × yr⁻¹(17–18 m Myr⁻¹). We furthermore analyse the hydrologic record to show that the long-term sediment budget is not dominated by extreme and rare events but by the total amount of rainfall or annual runoff. The rates we obtain are in agreement with rates obtained from the mass balance reconstruction of the Plio-Quaternary gravely deposits of the foreland but signicantly lower than the rates recently obtained from cosmogenic dating of the Kuitun River sands, west of the Urumqi River. We show that the resolution of this incompatibility may have an important consquence for our understanding of the interplay between erosion and tectonics in the semi-humid ranges of Central Asia.Tue, 13 Dec 2011 00:00:00 +0100http://www.solid-earth.net/2/271/2011/<b>Remobilization of silicic intrusion by mafic magmas during the 2010 Eyjafjallajökull eruption</b><br /><br />Solid Earth, 2, 271-281, 2011<br /><br />Author(s): O. Sigmarsson, I. Vlastelic, R. Andreasen, I. Bindeman, J.-L. Devidal, S. Moune, J. K. Keiding, G. Larsen, A. Höskuldsson, and Th. Thordarson<br /><br />Injection of basaltic magmas into silicic crustal holding chambers and subsequent magma mingling or mixing is a process that has been recognised since the late seventies as resulting in explosive eruptions. Detailed reconstruction and assessment of the mixing process caused by such intrusion is now possible because of the exceptional time-sequence sample suite available from the tephra fallout of the 2010 summit eruption at Eyjafjallajökull volcano in South Iceland. Fallout from 14 to 19 April contains three glass types of basaltic, intermediate, and silicic compositions recording rapid magma mingling without homogenisation, involving evolved FeTi-basalt and silicic melt with composition identical to that produced by the 1821–1823 AD Eyjafjallajökull summit eruption. The time-dependent change in the magma composition suggests a binary mixing process with changing end-member compositions and proportions. Beginning of May, a new injection of primitive basalt was recorded by deep seismicity, appearance of Mg-rich olivine phenocrysts together with high sulphur dioxide output and presence of sulphide crystals. Thus, the composition of the basaltic injection became more magnesian and hotter with time provoking changes in the silicic mixing end-member from pre-existing melt to the solid carapace of the magma chamber. Finally, decreasing proportions of the mafic end-member with time in the erupted mixed-magma demonstrate that injections of Mg-rich basalt was the motor of the 2010 Eyjafjallajökull explosive eruption, and that its decreasing inflow terminated the eruption. Significant quantity of silicic magma is thus still present in the interior of the volcano. Our results show that detailed sampling during the entire eruption was essential for deciphering the complex magmatic processes at play, i.e. the dynamics of the magma mingling and mixing. Finally, the rapid compositional changes in the eruptive products suggest that magma mingling occurs on a timescale of a few hours to days whereas the interval between the first detected magma injection and eruption was several months.Fri, 02 Dec 2011 00:00:00 +0100http://www.solid-earth.net/2/259/2011/<b>Paleointensities on 8 ka obsidian from Mayor Island, New Zealand</b><br /><br />Solid Earth, 2, 259-270, 2011<br /><br />Author(s): A. Ferk, R. Leonhardt, K.-U. Hess, and D. B. Dingwell<br /><br />The 8 ka BP (6050 BCE) pantelleritic obsidian flow on Mayor Island, Bay of Plenty, New Zealand, has been investigated using 30 samples from two sites. Due to a very high paramagnetic/ferromagnetic ratio, it was not possible to determine the remanence carriers. This is despite the fact that the samples were studied intensively at low, room, and high temperatures. We infer that a stable remanence within the samples is carried by single- or close to single-domain particles. Experiments to determine the anisotropy of thermoremanence tensor and the dependency on cooling rate were hampered due to alteration resulting from the repeated heating of the samples to temperatures just below the glass transition. Nonetheless, a well-defined mean paleointensity of 57.0 ± 1.0 μT, based on individual high quality paleointensity determinations, was obtained. This field value compares very well to a paleointensity of 58.1 ± 2.9 μT, which Tanaka et al. (2009) obtained for 5500 BCE at a site 100 km distant. Agreement with geomagnetic field models, however, is poor. Thus, gathering more high-quality paleointensity data for the Pacific region and for the southern hemisphere in general to better constrain global field models is very important.Mon, 21 Nov 2011 00:00:00 +0100http://www.solid-earth.net/2/245/2011/<b>An open ocean record of the Toarcian oceanic anoxic event</b><br /><br />Solid Earth, 2, 245-257, 2011<br /><br />Author(s): D. R. Gröcke, R. S. Hori, J. Trabucho-Alexandre, D. B. Kemp, and L. Schwark<br /><br />Oceanic anoxic events were time intervals in the Mesozoic characterized by widespread distribution of marine organic matter-rich sediments (black shales) and significant perturbations in the global carbon cycle. These perturbations are globally recorded in sediments as carbon isotope excursions irrespective of lithology and depositional environment. During the early Toarcian, black shales were deposited on the epi- and pericontinental shelves of Pangaea, and these sedimentary rocks are associated with a pronounced (ca. 7 &permil;) negative (organic) carbon isotope excursion (CIE) which is thought to be the result of a major perturbation in the global carbon cycle. For this reason, the lower Toarcian is thought to represent an oceanic anoxic event (the T-OAE). If the T-OAE was indeed a global event, an isotopic expression of this event should be found beyond the epi- and pericontinental Pangaean localities. To address this issue, the carbon isotope composition of organic matter (&delta;¹³C_org of lower Toarcian organic matter-rich cherts from Japan, deposited in the open Panthalassa Ocean, was analysed. The results show the presence of a major (>6 &permil;) negative excursion in &delta;¹³C_org that, based on radiolarian biostratigraphy, is a correlative of the lower Toarcian negative CIE known from Pangaean epi- and pericontinental strata. A smaller negative excursion in &delta;¹³C_org (ca. 2 &permil;) is recognized lower in the studied succession. This excursion may, within the current biostratigraphic resolution, represent the excursion recorded in European epicontinental successions close to the Pliensbachian/Toarcian boundary. These results from the open ocean realm suggest, in conjunction with other previously published datasets, that these Early Jurassic carbon cycle perturbations affected the active global reservoirs of the exchangeable carbon cycle (deep marine, shallow marine, atmospheric).Fri, 11 Nov 2011 00:00:00 +0100http://www.solid-earth.net/2/219/2011/<b>Metamorphic history and geodynamic significance of the Early Cretaceous Sabzevar granulites (Sabzevar structural zone, NE Iran)</b><br /><br />Solid Earth, 2, 219-243, 2011<br /><br />Author(s): M. Nasrabady, F. Rossetti, T. Theye, and G. Vignaroli<br /><br />The Iranian ophiolites are part of the vast orogenic suture zones that mark the Alpine-Himalayan convergence zone. Few petrological and geochronological data are available from these ophiolitic domains, hampering a full assessment of the timing and regimes of subduction zone metamorphism and orogenic construction in the region. This paper describes texture, geochemistry, and the pressure-temperature path of the Early Cretaceous mafic granulites that occur within the Tertiary Sabzevar ophiolitic suture zone of NE Iran. Whole rock geochemistry indicates that the Sabzevar granulites are likely derived from a MORB-type precursor. They are thus considered as remnants of a dismembered dynamo-thermal sole formed during subduction of a back-arc basin (proto-Sabzevar Ocean) formed in the upper-plate of the Neotethyan slab. The metamorphic history of the granulites suggests an anticlockwise pressure-temperature loop compatible with burial in a hot subduction zone, followed by cooling during exhumation. Transition from a nascent to a mature stage of oceanic subduction is the geodynamic scenario proposed to accomplish for the reconstructed thermobaric evolution. When framed with the regional scenario, results of this study point to diachronous and independent tectonic evolutions of the different ophiolitic domains of central Iran, for which a growing disparity in the timing of metamorphic equilibration and of pressure-temperature paths can be expected to emerge with further investigations.Fri, 11 Nov 2011 00:00:00 +0100http://www.solid-earth.net/2/205/2011/<b>New zircon data supporting models of short-lived igneous activity at 1.89 Ga in the western Skellefte District, central Fennoscandian Shield</b><br /><br />Solid Earth, 2, 205-217, 2011<br /><br />Author(s): P. Skyttä, T. Hermansson, J. Andersson, M. Whitehouse, and P. Weihed<br /><br />New U-Th-Pb zircon data (SIMS) from three intrusive phases of the Palaeoproterozoic Viterliden intrusion in the western Skellefte District, central Fennoscandian Shield, dates igneous emplacement in a narrow time interval at about 1.89 Ga. A locally occurring quartz-plagioclase porphyritic tonalite, here dated at 1889 ± 3 Ma, is considered the youngest of the intrusive units, based on the new age data and field evidence. This supports an existing interpretation of its fault-controlled emplacement after intrusion of the dominating hornblende-tonalite units, in this study dated at 1892 ± 3 Ma. The Viterliden magmatism was synchronous with the oldest units of the Jörn type early-orogenic intrusions in the eastern part of the district (1.89–1.88 Ga; cf. Gonzàles Roldán, 2010). A U-Pb zircon age for a felsic metavolcanic rock from the hanging-wall to the Kristineberg VMS deposit, immediately south of the Viterliden intrusion, is constrained at 1883 ± 6 Ma in this study. It provides a minimum age for the Kristineberg ore deposit and suggests contemporaneous igneous/volcanic activity throughout the Skellefte District. Furthermore, it supports the view that the Skellefte Group defines a laterally continuous belt throughout this "ore district". Tentative correlation of the 1889 ± 3 Ma quartz-plagioclase porphyritic tonalite with the Kristineberg "mine porphyry" suggests that these units are coeval at about 1.89 Ga. Based on the new age determinations, the Viterliden intrusion may equally well have intruded into or locally acted as a basement for the ore-hosting Skellefte Group volcanic rocks.Wed, 26 Oct 2011 00:00:00 +0200http://www.solid-earth.net/2/199/2011/<b>The 11 May 2011 earthquake at Lorca (SE Spain) viewed in a structural-tectonic context</b><br /><br />Solid Earth, 2, 199-204, 2011<br /><br />Author(s): R. L. M. Vissers and B. M. L. Meijninger<br /><br />The Lorca earthquake of 11 May 2011 in the Betic Cordillera of SE Spain occurred almost exactly on the Alhama de Murcia fault, a marked fault that forms part of a NE-SW trending belt of faults and thrusts. The fault belt is reminiscent of a strike-slip corridor, but recent structural studies have provided clear evidence for reverse motions on these faults. Focal mechanisms of the main earthquake, but also of a foreshock, are strikingly consistent with structural observations on the Alhama de Murcia fault. This strengthens the conclusion that, rather than a strike-slip fault, the fault is at present a contractional fault with an oblique reverse sense of motion, presumably in response to the NW-directed motion of Africa with respect to Europe.Wed, 05 Oct 2011 00:00:00 +0200http://www.solid-earth.net/2/191/2011/<b>Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka</b><br /><br />Solid Earth, 2, 191-198, 2011<br /><br />Author(s): G. Delisle<br /><br />Heat flow measurements were carried out in 2009 offshore Kamchatka during the German-Russian joint-expedition KALMAR. An area with elevated heat flow in oceanic crust of Cretaceous age – detected ~30 yr ago in the course of several Russian heat flow surveys – was revisited. One previous interpretation postulated anomalous lithospheric conditions or a connection between a postulated mantle plume at great depth (>200 km) as the source for the observed high heat flow. However, the positive heat flow anomaly – as our bathymetric data show – is closely associated with the fragmentation of the western flank of the Meiji Seamount into a horst and graben structure initiated during descent of the oceanic crust into the subduction zone offshore Kamchatka. This paper offers an alternative interpretation, which connects high heat flow primarily with natural convection of fluids in the fragmented rock mass and, as a potential additional factor, high rates of erosion, for which evidence is available from our collected bathymetric image. Given high erosion rates, warm rock material at depth rises to nearer the sea floor, where it cools and causes temporary elevated heat flow.Mon, 26 Sep 2011 00:00:00 +0200http://www.solid-earth.net/2/159/2011/<b>Dynamical geochemistry of the mantle</b><br /><br />Solid Earth, 2, 159-189, 2011<br /><br />Author(s): G. F. Davies<br /><br />The reconciliation of mantle chemistry with the structure of the mantle inferred from geophysics and dynamical modelling has been a long-standing problem. This paper reviews three main aspects. First, extensions and refinements of dynamical modelling and theory of mantle processing over the past decade. Second, a recent reconsideration of the implications of mantle heterogeneity for melting, melt migration, mantle differentiation and mantle segregation. Third, a recent proposed shift in the primitive chemical baseline of the mantle inferred from observations of non-chondritic ¹⁴²Nd in the Earth. It seems most issues can now be resolved, except the level of heating required to maintain the mantle's thermal evolution. <br><br> A reconciliation of refractory trace elements and their isotopes with the dynamical mantle, proposed and given preliminary quantification by Hofmann, White and Christensen, has been strengthened by work over the past decade. The apparent age of lead isotopes and the broad refractory-element differences among and between ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) can now be quantitatively accounted for with some assurance. <br><br> The association of the least radiogenic helium with relatively depleted sources and their location in the mantle have been enigmatic. The least radiogenic helium samples have recently been recognised as matching the proposed non-chondritic primitive mantle. It has also been proposed recently that noble gases reside in a so-called hybrid pyroxenite assemblage that is the result of melt from fusible pods reacting with surrounding refractory peridotite and refreezing. Hybrid pyroxenite that is off-axis may not remelt and erupt at MORs, so its volatile constituents would recirculate within the mantle. Hybrid pyroxenite is likely to be denser than average mantle, and thus some would tend to settle in the D" zone at the base of the mantle, along with some old subducted oceanic crust. Residence times in D" are longer, so the hybrid pyroxenite there would be less degassed. Plumes would sample both the degassed, enriched old oceanic crust and the gassy, less enriched hybrid pyroxenite and deliver them to OIBs. These findings can account quantitatively for the main He, Ne and Ar isotopic observations. <br><br> It has been commonly inferred that the MORB source is strongly depleted of incompatible elements. However it has recently been argued that conventional estimates of the MORB source composition fail to take full account of mantle heterogeneity, and in particular focus on an ill-defined "depleted" mantle component while neglecting less common enriched components. Previous estimates have also been tied to the composition of peridotites, but these probably do not reflect the full complement of incompatible elements in the heterogeneous mantle. New estimates that account for enriched mantle components suggest the MORB source complement of incompatibles could be as much as 50–100 % larger than previous estimates. <br><br> A major difficulty has been the inference that mass balances of incompatible trace elements could only be satisfied if there is a deep enriched layer in the mantle, but the Earth's topography precludes such a layer. The difficulty might be resolved if either the Earth is depleted relative to chondritic or the MORB source is less depleted than previous estimates. Together these factors can certainly resolve the mass balance difficulties.Tue, 13 Sep 2011 00:00:00 +0200http://www.solid-earth.net/2/155/2011/<b>The role of karst in engineering and environmental geosciences</b><br /><br />Solid Earth, 2, 155-158, 2011<br /><br />Author(s): H. C. Ho<br /><br />Karst is a unique landform developed by soluble rock. It usually relates to the groundwater drainage system, and provides important water resources. Current researches indicate that karst is closely related to the Earth system and environmental protection, and it can also create potential natural hazards such as sinkhole flooding and land subsidence in urban area. Its relationship with hydrogeology has also been an important factor for studying water pollution and nutrient cycles in engineering geosciences and agricultural geology.Tue, 30 Aug 2011 00:00:00 +0200http://www.solid-earth.net/2/143/2011/<b>Optical method for measuring bed topography and flow depth in an experimental flume</b><br /><br />Solid Earth, 2, 143-154, 2011<br /><br />Author(s): A. Limare, M. Tal, M. D. Reitz, E. Lajeunesse, and F. Métivier<br /><br />We describe an optical method known as moiré for acquiring quasi-simultaneous measurements of bed topography and flow depth in laboratory experiments. The moiré method is based on projecting a fringe pattern (grating) on the bed and analyzing the deformation of the pattern caused by the topography with respect to a reference plane. The height of the object is encoded in the phase of the pattern and can be retrieved either through Fourier transform or phase shifting algorithms. The methodology enables image-based non-contact measurements over a continuous surface at very high spatial and temporal resolutions. We use a commercial software package of a moiré method called Light3D to map bed topography and flow depth in an experimental braided channel and demonstrate how the method can be used to characterize a full range of statistics not previously possible.Thu, 28 Jul 2011 00:00:00 +0200http://www.solid-earth.net/2/135/2011/<b>First observational evidence for the CO₂-driven origin of Stromboli's major explosions</b><br /><br />Solid Earth, 2, 135-142, 2011<br /><br />Author(s): A. Aiuppa, M. Burton, P. Allard, T. Caltabiano, G. Giudice, S. Gurrieri, M. Liuzzo, and G. Salerno<br /><br />We report on the first detection of CO₂ flux precursors of the till now unforecastable "major" explosions that intermittently occur at Stromboli volcano (Italy). An automated survey of the crater plume emissions in the period 2006–2010, during which 12 such explosions happened, demonstrated that these events are systematically preceded by a brief phase of increasing CO₂/SO₂ weight ratio (up to >40) and CO₂ flux (>1300 t d⁻¹) with respect to the time-averaged values of 3.7 and ~500 t d⁻¹ typical for standard Stromboli's activity. These signals are best explained by the accumulation of CO₂-rich gas at a discontinuity of the plumbing system (decreasing CO₂ emission at the surface), followed by increasing gas leakage prior to the explosion. Our observations thus supports the recent model of Allard (2010) for a CO₂-rich gas trigger of recurrent major explosions at Stromboli, and demonstrates the possibility to forecast these events in advance from geochemical precursors. These observations and conclusions have clear implications for monitoring strategies at other open-vent basaltic volcanoes worldwide.Tue, 19 Jul 2011 00:00:00 +0200http://www.solid-earth.net/2/125/2011/<b>Geophysical characterisation of two segments of the Møre-Trøndelag Fault Complex, Mid Norway</b><br /><br />Solid Earth, 2, 125-134, 2011<br /><br />Author(s): A. Nasuti, C. Pascal, J. Ebbing, and J. F. Tønnesen<br /><br />The Møre-Trøndelag Fault Complex (MTFC) has controlled the tectonic evolution of Mid Norway and its shelf for the past 400 Myr through repeated reactivations during Palaeozoic, Mesozoic and perhaps Cenozoic times, the very last phase of reactivation involving normal to oblique-slip faulting. Despite its pronounced signature in the landscape, its deep structure has largely remained unresolved until now. We focused on two specific segments of the MTFC (i.e. the Tjellefonna and Bæverdalen faults) and acquired multiple geophysical datasets (i.e. gravity, magnetic, resistivity and shallow refraction profiles). <br><br> A 100–200 m-wide zone of gouge and/or brecciated bedrock steeply dipping to the south is interpreted as being the Tjellefonna fault sensu stricto. The fault appears to be flanked by two additional but minor damage zones. A secondary normal fault also steeply dipping to the south but involving indurated breccias was detected ~1 km farther north. The Bæverdalen fault, ~12 km farther north, is interpreted as a ~700 m-wide and highly deformed zone involving fault gouge, breccias and lenses of intact bedrock. As such, it is probably the most important fault segment in the studied area and accommodated most of the strain during presumably Late Jurassic normal faulting. Our geophysical data are indicative of a Bæverdalen fault dipping steeply towards the south, in agreement with the average orientation of the local tectonic grain. Our findings suggest that the influence of Mesozoic normal faulting along the MTFC on landscape development is more complex than previously thought.Mon, 11 Jul 2011 00:00:00 +0200http://www.solid-earth.net/2/107/2011/<b>Re-evaluation of the Mentelle Basin, a polyphase rifted margin basin, offshore southwest Australia: new insights from integrated regional seismic datasets</b><br /><br />Solid Earth, 2, 107-123, 2011<br /><br />Author(s): D. Maloney, C. Sargent, N. G. Direen, R. W. Hobbs, and D. R. Gröcke<br /><br />Vintage 2-D (two-dimensional) seismic reflection surveys from the sparsely explored Mentelle Basin (western Australian margin) have been reprocessed and integrated with a recent high-quality 2-D seismic survey and stratigraphic borehole data. Interpretation of these data sets allows the internal geometry of the Mentelle Basin fill and depositional history to be reanalysed and new insights into its formation revealed. Basin stratigraphy can be subdivided into several seismically defined megasequences separated by major unconformities related to both breakup between India-Madagascar and Australia-Antarctica in the Valanginian-Late Hauterivian and tectonically-driven switches in deposition through the Albian. <br><br> Resting on the Valanginian-Late Hauterivian breakup unconformity are several kilometre-scale mounded structures that formed during Late Jurassic to Early Cretaceous extension. These have previously been interpreted as volcanic edifices although direct evidence of volcanic feeder systems is lacking. An alternative interpretation is that these features may be carbonate build-ups. The latter interpretation carries significant climatic ramifications since carbonate build-ups would have formed at high palaeolatitude, ~60° S. <br><br> Soon after breakup, initial subsidence resulted in a shallow marine environment and deposition of Barremian-Aptian silty-sandy mudstones. As subsidence continued, thick successions of Albian ferruginous black clays were deposited. Internally, seismic megasequences composed of successions of black clays show previously unresolved unconformities, onlapping and downlapping packages, which reflect a complex depositional, rifting and subsidence history at odds with their previous interpretation as open marine sediments. <br><br> Southwestwards migration of the Kerguelen hotspot led to thermal contraction and subsidence to the present day water depth (~3000 m). This was accompanied by Turonian-Santonian deposition of massive chalk beds, which are unconformably overlain by pelagic Palaeocene-Holocene sediments. This substantial unconformity is related to the diachronous breakup and onset of slow spreading between Australia and Antarctica, which may have led to the reactivation and inversion of basement faults and was followed by rapid seafloor spreading from the Middle Eocene to the present.Thu, 07 Jul 2011 00:00:00 +0200http://www.solid-earth.net/2/95/2011/<b>Exploring the potentials and limitations of the time-reversal imaging of finite seismic sources</b><br /><br />Solid Earth, 2, 95-105, 2011<br /><br />Author(s): S. Kremers, A. Fichtner, G. B. Brietzke, H. Igel, C. Larmat, L. Huang, and M. Käser<br /><br />The characterisation of seismic sources with time-reversed wave fields is developing into a standard technique that has already been successful in numerous applications. While the time-reversal imaging of effective point sources is now well-understood, little work has been done to extend this technique to the study of finite rupture processes. This is despite the pronounced non-uniqueness in classic finite source inversions. <br><br> The need to better constrain the details of finite rupture processes motivates the series of synthetic and real-data time reversal experiments described in this paper. We address questions concerning the quality of focussing in the source area, the localisation of the fault plane, the estimation of the slip distribution and the source complexity up to which time-reversal imaging can be applied successfully. The frequency band for the synthetic experiments is chosen such that it is comparable to the band usually employed for finite source inversion. <br><br> Contrary to our expectations, we find that time-reversal imaging is useful only for effective point sources, where it yields good estimates of both the source location and the origin time. In the case of finite sources, however, the time-reversed field does not provide meaningful characterisations of the fault location and the rupture process. This result cannot be improved sufficiently with the help of different imaging fields, realistic modifications of the receiver geometry or weights applied to the time-reversed sources. <br><br> The reasons for this failure are manifold. They include the choice of the frequency band, the incomplete recording of wave field information at the surface, the excitation of large-amplitude surface waves that deteriorate the depth resolution, the absence of a sink that should absorb energy radiated during the later stages of the rupture process, the invisibility of small slip and the neglect of prior information concerning the fault geometry and the inherent smoothness of seismologically inferred Earth models that prevents the beneficial occurrence of strong multiple-scattering. <br><br> The condensed conclusion of our study is that the limitations of time-reversal imaging – at least in the frequency band considered here – start where the seismic source stops being effectively point-localised.Tue, 21 Jun 2011 00:00:00 +0200http://www.solid-earth.net/2/75/2011/<b>Native American lithic procurement along the international border in the boot heel region of southwestern New Mexico</b><br /><br />Solid Earth, 2, 75-93, 2011<br /><br />Author(s): K. E. Zeigler, P. Hogan, C. Hughes, and A. Kurota<br /><br />Multidisciplinary field projects can be very useful to a more fundamental understanding of the world around us, though these projects are not as common as they should be. In particular, the combination of archeology and geology combines our understanding of human behavior and human use of the landscape with an intimate knowledge of geologic processes and the materials available for human use in order to gain a broader understanding of human-Earth interaction. Here we present data from a cross-disciplinary project that uses a common dataset, archeological artifacts, to explore the anthropological and geologic implications of useage patterns. Archeological excavations and surveys conducted by the Office of Contract Archeology in 2007 along the route of the proposed international border fence reveal patterns of use of geologic materials by Archaic, Formative and Protohistoric Native Americans in the Boot Heel of southwestern New Mexico. Thousands of artifacts were recorded in multiple sites from Guadalupe Pass in the southern Peloncillo Mountains to the Carrizalillo Hills west of Columbus. We identified the lithologies of artifacts, ranging from projectile points to groundstones, and then constructed material movement maps based on either known procurement sites ("quarries") or outcrops identified as the closest source to a given site for each lithology. Not unexpectedly, the majority of the rock types utilized by native peoples are local siliceous volcanic materials. However, several artifacts constructed from obsidian were transported into the region from northern Mexico and eastern Arizona, indicating long-distance travel and/or trade routes. We also examine useage pattern difference between Archaic, Formative and Protohistoric sites. Additionally, a dramatic change in distribution of sources for geologic materials occurs between one pre-Spanish site and one post-Spanish site that are adjacent to one another.Tue, 14 Jun 2011 00:00:00 +0200