Description: Publication date: August 2018 Source: Earth-Science Reviews, Volume 183

Description: Publication date: Available online 19 June 2018 Source: Earth-Science Reviews Author(s): Yaoling Niu, David H. Green The plate tectonics theory established ~ 50 years ago has formed a solid framework for understanding how the earth works on all scales. In this theory, movement of the tectonic plates relative to the subjacent asthenosphere is one of the fundamental tenets. However, the nature of the boundary between the lithosphere and asthenosphere (LAB) beneath ocean basins remains under debate. The current consensus is that the oceanic lithosphere thickens with age by accreting asthenosphere material from below, and reaches its full thickness ( L ) of ~ 90 km at the age ( t ) of ~ 70 Ma. This lithospheric thickening fits the relation L ∝ t 1/2 , consistent with conductive cooling to the seafloor. A puzzling observation is that although conductive cooling continues, the oceanic lithosphere ceases to grow any thicker than ~ 90 km when t  > 70 Ma. Small scale convection close beneath the LAB has been generally invoked to explain this puzzle, but why such convection does not occur until L ~ 90 km at t  > 70 Ma has been a matter of conjecture. In this paper, we summarize the results of many years of experimental petrology and petrological studies of oceanic basalts, which indicate consistently that the LAB is a petrological phase boundary marking the intersection of the geotherm with the solidus of amphibole (pargasite)-bearing lherzolite. That is, petrologically, the LAB is an isotherm of ~ 1100 °C with L ∝ t 1/2 for t  〈 70 Ma and an isobar of ~ 3 GPa (~ 90 km) for t  > 70 Ma. This unifying concept explains why the LAB depth increases with age for t  〈 70 Ma and maintains constant (~ 90 km) for t  > 70 Ma. The LAB, that is intrinsically determined by petrological phase equilibria, does not require small-scale convection. However, because the mantle above the LAB is the conductive lithosphere (pargasite-bearing lherzolite/harzburgite) and below the LAB is the viscosity-reduced convective asthenosphere (lherzolite/harzburgite + incipient melt), the small-scale convection in the asthenosphere close beneath the LAB under older seafloors becomes possible, whose convective heat supply balances the conductive heat loss, maintaining the constant heat flow, seafloor depth and lithosphere thickness.

Description: Publication date: Available online 18 June 2018 Source: Earth-Science Reviews Author(s): Nester M. Korolev, Aleksey E. Melnik, Xian-Hua Li, Sergey G. Skublov In the past decade the oxygen isotope composition in rock-forming minerals of mantle eclogites (δ 18 O = 2–12‰) has been actively discussed as a most reliable proxy on their origin and evolution. In the present study, the possibility of using the oxygen isotope composition as a proxy for the origin of mantle eclogites is carefully tested, and conclusions regarding its application limits are drawn. Contributions of all processes leading to variations in the isotope composition from protolith (oceanic crust) formation to ascent of eclogites by kimberlite to the surface are discussed, including hydrothermal alteration by sea water; fractionation of oxygen isotopes upon metamorphism, dehydration and the removal of volatile components in subduction zone; metasomatism in subduction zone; partial melting; mantle metasomatism; and variation in the oxygen isotope composition upon the exhumation of mantle xenoliths by kimberlitic magma. The total contribution of all processes typically does not exceed a deviation of ±3.5‰ from the initial δ 18 O value established in a protolith. Variations of δ 18 O in mantle eclogites (2–12‰; and 〈3% out of all samples display δ 18 O over 8‰), which have inferred basaltic and cumulate (gabbroic) protoliths, do not fully match those in the oceanic crust (0–15‰). This limited overlap could be attributed primarily to the small initial volume of protoliths with δ 18 O > 8‰, which subducts into the mantle (up to 5%), and the preferential erosion and partial melting of the uppermost layers of the oceanic crust. The statistical data show that eclogite garnet mainly preserves more ancient oxygen isotope signatures (including the initial δ 18 O of protolith) than clinopyroxene. Garnets with δ 18 O of 〈4.5‰ typical of the cumulate portion of oceanic crust, occur mainly in mantle eclogites from the Kaapvaal Craton (22.3% of finds, a total of 157 samples). For all other cratons, the percentage of garnet with δ 18 O 〈 4.5‰ is not >1.5% (a total of 451 samples). Oxygen isotope composition in a considerable portion of mantle eclogites that originated from the oceanic gabbro based on their chemical composition (δ 18 O 〈 6‰) was re-equilibrated, and they acquired “basalt” isotopic signatures (δ 18 O > 6‰).

Description: Publication date: Available online 15 June 2018 Source: Earth-Science Reviews Author(s): G. Pennacchioni, N.S. Mancktelow The length, thickness and strain gradients of small-scale (10 −3 –10 −1  m thick) ductile shear zones are pre-determined by the presence of surface precursors (e.g. a fracture or a compositional layer) and associated fluid-rock interaction. Fractures, with their surrounding host-rock damage zone and concurrent tectonic under-pressure during dilation, provide efficient fluid pathways and networks with diffusive fluid-rock interaction at their margins. The fluid-induced, gradational to zoned compositional haloes that symmetrically surround a fracture control the strain gradients of developing shear zones and result in a diversity of geometric types, including single homogeneous-to-heterogeneous shear zones and paired shear zones. As a consequence, geochemical differences between shear zone and host rock may reflect fluid-rock interaction during the precursor brittle history rather than during slip, especially considering that shear zones with even a small component of stretch may be over-pressured and therefore unable to drain fluids from the surrounding rocks. Support from field observation for this interpretation is given by (1) the lack of correlation between shear zone thickness and accumulated displacement; and (2) the similar thickness of shear zones and locally unexploited alteration haloes surrounding fracture precursors. Most small-scale shear zones in massive rocks (granitoids) are initially neither thickening nor narrowing with increasing strain. This concept may also apply to more foliated rocks, but does not necessarily hold for larger-scale shear zones.

Description: Publication date: Available online 15 June 2018 Source: Earth-Science Reviews Author(s): Sergio G. Longhitano The present paper reviews a number of geophysical and geological datasets acquired in recent and less recent times on the modern Messina Strait, aiming at reconstructing the sedimentary dynamics of this complex system. In a second paper, the results achieved with the present study are used as proxy for interpreting the analogue lower Pleistocene succession cropping out along the margins of the modern strait. The tide-dominated Messina Strait separates the Italian peninsula and Sicily in the central Mediterranean Sea. This 3-km-wide marine passageway is governed by bi-directional tidal currents flowing along the strait axis, because of semi-diurnal tidal phase opposition, so that high tide in the Ionian Sea corresponds with low tide in the Tyrrhenian Sea and vice versa. Tidal currents cyclically accelerate passing through the strait central restriction and decelerate towards the strait exits, following reverse directions during each tidal phase. This hydrodynamics generates specific by-pass areas, sediment routes and accumulation patterns in the modern strait, based upon the varying bed-shear stress exerted onto the strait bottom by the converging/diverging M2 tidal waves. Recent multibeam-based investigations on the modern Messina Strait bottom reveal subaqueous morpho-bathymetric features, which allow a number of areas with specific morphologic characters and sedimentary processes to be identified: (i) the ‘strait-centre zone’ is the narrowest and shallowest by-pass sector, where the tidal currents reach their maximum strength eroding and scouring the substrate. (ii) Towards NE and S, two larger ‘dune-bedded zones’ occur. Here, clastic sediments transported as bed load and in suspension are accumulated and continuously reworked forming characteristic bedform fields, whose features reflect complex current patterns due to a strong tidal asymmetry. (iii) The ‘strait-end zones’ represent distal depositional areas, where decelerating currents rework fine-grained smaller moribund dunes. (iv) The ‘strait-margin zones’ are diffusely characterised by mass-wasting processes along the steepest sublittoral sectors, where sediment instability are caused by earthquakes or sea storms. Fan-deltas prograde from the gentler-sloping coastal segments, represent the main sediment entry points with episodic, high-magnitude discharge and are influenced by the tidal circulation only along their delta-front parts. However, a dogmatic zone partition, based on a ‘mutually-evasive’ mechanism of sediment distribution and consequent decrease in the transport capacity towards the strait ends is not everywhere applicable to the Messina Strait sedimentary dynamics. The examination of measured tidal ellipses in the Messina Strait reveals that although flood and ebb tidal currents are approximately equal in speed, they are collinear only across the narrowest zone, being separated as the strait enlarges. This flow separation has possibly generated two main populations of flood and ebb bedforms, which migrate at high angle and with reverse direction in the two dune-bedded zones of the strait. Moreover, because of a strong water-mass stratification due to a marked difference in salinity, the denser flood constituent dominates over the lighter ebb current. As a consequence, the largest dunes observed in both the strait opposite depositional areas have northward-oriented lee faces, whereas superimposed smaller dunes have southward-oriented fronts, reflecting flood and ebb tidal constituents, respectively. Erosional features in the central sill, uncharacteristic bedforms having orientations not explained by modern currents and coarser grains in some places implying thresholds of motion larger than the modern currents all suggest that different, most likely, larger currents, probably occurred at some point in the past. It is suggested here that these features reflect an early stage of strait connection, during the initial sea-level rise after the Last Glacial Maximum. The data reviewed in the present work summarise the complex sedimentary dynamics of the modern Messina Strait and allow a comparative analysis of the ancient strait exposed along the modern margins, also suggesting insights for other examples of tidal straits developed under very similar oceanographic conditions.

Description: Publication date: October 2018 Source: Earth-Science Reviews, Volume 185 Author(s): Michał Gradziński, Pavel Bella, Peter Holúbek Constructional caves in freshwater limestone appear to be the least described of the most commonly occurring types of cave. They are also known under the names “primary”, “syngenetic” or “framework” caves. Such caves develop in tufa and travertine during the growth of their host rock. Two main genetic categories of constructional caves have been recognized, described herein as progradational and aggradational . Progradational caves develop chiefly in tufa, less commonly in travertine, as the result of progradation occurring on a steep depositional surface. An empty void is often left between an older surface and a new carbonate mass growing outwards from it as a vertical or inclined curtain. The size and shape of a progradational cave depend on: (i) energy of water over the steep section, (ii) height of the steep section, (iii) shape of the depositional surface, (iv) water chemistry. The category of progradational caves includes also caves roofed by bridges of travertine or tufa. Progradational caves are richly decorated with speleothems like those seen in other limestone caves. The growth of the speleothems is facilitated by efficient percolation of water through the cave ceilings of carbonate with high effective porosity. Clastic sediments may accumulate in relict progradational caves, and often contain palaeontological, archaeological and palaeoanthropological finds. Aggradational caves may develop within freshwater carbonate (predominantly travertine) buildups at the outlets of artesian springs where the orifice lips constantly migrate upward. An active aggradational cave acts as a feeder channel for such a spring, so it is filled with water. The sizes and shapes of aggradational caves depend upon: (i) the nature of bedrock beneath travertine, (ii) local hydrological situation which controls the position of the piezometric surface, (iii) topography around the spring orifice, (iv) chemistry of water, and (v) evolution of the above conditions during the buildup growth. In addition to progradational and aggradational caves, tree-mould caves and cavities created by the subrosion of active carbonate buildups develop during their host carbonate growth. The majority of known constructional caves are of Quaternary, predominantly Holocene, age. This article is a review of constructional caves based on the available literature.

Description: Publication date: Available online 14 June 2018 Source: Earth-Science Reviews Author(s): Jin Lai, Guiwen Wang, Song Wang, Juntao Cao, Mei Li, Xiaojiao Pang, Zhenglong Zhou, Xuqiang Fan, Quanqi Dai, Liu Yang, Zhibo He, Ziqiang Qin The tight sandstones are characterized by low porosity, low permeability, complex pore structure and strong heterogeneity due to the extensive diagenetic modifications they experienced. Understanding of the impact of diagenetic alterations on reservoir quality is crucial to the hydrocarbon exploration and production in tight sandstones. Diagenetic facies, which is the comprehensive description of the diagenesis and diagenetic minerals, determines the formation and distribution of sweet spot. By correlating the diagenetic facies to well log responses, the subsurface distribution of porosity and permeability can be predicted. However, the prediction of diagenetic facies and reservoir quality via well logs in tight sandstones remains a challenging task. This paper critically reviews the impact of diagenesis and diagenetic minerals on reservoir quality in tight sandstones, and establishes a model for prediction of diagenetic facies via well logs, as assessed from peer reviewed papers in the literature as well as from the authors' personal experiences. This review begins with reviewing the impacts of compaction, cementation, dissolution and various types of diagenetic minerals on reservoir quality evolution. The definition and classification schemes of diagenetic facies are then discussed, and the reservoir quality as well as diagenetic evolution sequence of various diagenetic facies is summarized. The same diagenetic facies commonly display similar compositional and textural attributes, matrix and cement, as well as porosity systems. The well log responses (GR, AC, DEN, CNL, and RT) of various diagenetic facies are summarized by the calibration of log values with cores and related thin sections. By translating the diagenetic facies to conventional well logs, a predictable model, which can be used for subsurface reservoir quality prediction, is established. Then the theory of ECS logs is reviewed, and the application of ECS logs in diagenetic facies evaluation is discussed. At last, the quantitative characterization for various type and degree of diagenesis is reviewed, and the subsurface diagenetic facies is predicted by quantitative calculation of the compactional porosity loss, cementational porosity loss and dissolution porosity content via well logs. Correlating the diagenetic facies to well logs provides a powerful tool to predict the distribution of high quality reservoirs in tight sandstones. This review will provide insights into the reservoir quality evaluation and sweet spot prediction via well logs in tight sandstones.

Description: Publication date: Available online 12 June 2018 Source: Earth-Science Reviews Author(s): Marissa J. Betts, John R. Paterson, Sarah M. Jacquet, Anita S. Andrew, Philip A. Hall, James B. Jago, Elizabeth A. Jagodzinski, Wolfgang V. Preiss, James L. Crowley, Tom Brougham, Ciaran P. Mathewson, Diego C. García-Bellido, Timothy P. Topper, Christian B. Skovsted, Glenn A. Brock The most successful chronostratigraphic correlation methods enlist multiple proxies such as biostratigraphy and chemostratigraphy to constrain the timing of globally important bio- and geo-events. Here we present the first regional, high-resolution shelly fossil biostratigraphy integrated with δ 13 C chemostratigraphy (and corresponding δ 18 O data) from the traditional lower Cambrian (Terreneuvian and provisional Cambrian Series 2) of South Australia. The global ZHUCE, SHICE, positive excursions II and III and the CARE are captured in lower Cambrian successions from the Arrowie and Stansbury basins. The South Australian shelly fossil biostratigraphy has a consistent relationship with the δ 13 C results, bolstering interpretation, identification and correlation of the excursions. Positive excursion II straddles the boundary between the Kulparina rostrata and Micrina etheridgei zones, and the CARE straddles the boundary between the M. etheridgei and Dailyatia odyssei zones, peaking in the lower parts of the latter zone. New CA-TIMS zircon dates from the upper Hawker Group and Billy Creek Formation provide geochronologic calibration points for the upper D. odyssei Zone and corresponding chemostratigraphic curve, embedding the lower Cambrian successions from South Australia into a global chronostratigraphic context. This multi-proxy investigation demonstrates the power of integrated methods for developing regional biostratigraphic schemes and facilitating robust global correlation of lower Cambrian successions from South Australia (part of East Gondwana) with coeval terranes on other Cambrian palaeocontinents, including South and North China, Siberia, Laurentia, Avalonia and West Gondwana.

Description: Publication date: Available online 12 June 2018 Source: Earth-Science Reviews Author(s): D.C.P. Peacock, D.J. Sanderson Seven distinct types of structural analysis can be defined, each with their own data and uses: (1) basic geological descriptions; (2) geometries and topology; (3) age relationships; (4) kinematics; (5) tectonics; (6) mechanics and (7) fluid flow. We illustrate these types of analysis using the example of faults and fractures, which typically form networks of interacting and connected segments. A framework for describing and characterising fault and fracture networks is presented for each of the structural analysis types. We suggest that any structural study be tailored to suit the desired outcome and that this scheme of analysis types should be used as a basis for the development of workflows, for the design of research projects and for testing hypotheses. For example, prediction of fluid flow through a fracture network must begin with the basic geological description of fracture types. Basic geological descriptions should be followed by measuring their geometries and topologies, understanding their age relationships, kinematic and mechanics, and developing a realistic, data-led model for related fluid flow. Missing steps can lead to fundamentally flawed interpretations.

Description: Publication date: Available online 7 June 2018 Source: Earth-Science Reviews Author(s): Rosario Esposito, Kimberly Badescu, Matthew Steele-MacInnis, Claudia Cannatelli, Benedetto De Vivo, Annamaria Lima, Robert J. Bodnar, Craig E. Manning One of the main goals of studying melt inclusions (MI) is to constrain the pre-eruptive physical and chemical processes that have occurred in a magma reservoir at the micro-scale. Recently, several studies that focused on magmatic differentiation of volcanic systems produced detailed interpretations based on data from MI trapped at different times and locations in the plumbing system. Ideally, MI data should be collected and tested following the melt inclusion assemblage (MIA) protocol that consists of studying and analyzing groups of MI that were trapped at the same time, and, thus, at the same chemical and physical conditions. However, the rarity of MIA in juvenile volcanic phenocrysts precludes this methodology in many cases, leading to uncertainty concerning the validity of the MI as recorders of pre-eruptive conditions. In this study, we focused on MI from the Campi Flegrei (CF) and Procida Island (PI) volcanic systems in southern Italy, including data from this study and data from the literature. The database included MI hosted in sanidine, clinopyroxene, plagioclase, biotite and olivine, and, thus, represents melts trapped at various stages in the overall differentiation process. We developed a protocol to select the most reliable MI from a dataset associated with a single magmatic system. As a first step we compare MI data with bulk rock data for the same magmatic system. This comparison reveals that most MI show major element compositions that fall within or close to the range for bulk rocks – these MI are considered to be “normal”. Some MI show anomalous compositions and are not representative of the melt in equilibrium with the phenocryst host and were excluded from the data set. In the second step we selected only bubble-free MI from the previously identified “normal” MI to interpret the volatile evolution. In the third step we compare compositions of the “normal” bubble-free MI to compositions predicted by rhyolite-MELTS simulations, assuming a variety of initial conditions. Comparison of data obtained from basaltic-trachybasaltic MI with rhyolite-MELTS predictions indicates that one group of MI records the geochemical evolution of a volatile-saturated magma differentiating by polybaric fractional crystallization from ≥200 MPa (≥7.5 km) to 30 MPa (~1 km). Another group of MI records recharge of the magma chamber by a primitive basaltic magma that mixes with the preexisting primitive trachybasaltic magma before eruption. Extensive isobaric crystallization of the trachybasaltic magmas at ~7.5 km beneath CF may have generated trachytic-phonolitic magmas, such as those associated with the Neapolitan Yellow Tuff (NYT) that is characterized by a relatively high H 2 O content. These volatile-saturated trachytic-phonolitic magmas likely trigger high-magnitude eruptions during their ascent to the surface.