Crustal Spreading Research Articles

Readjustment of fluvial systems to Quaternary tectonic disruption in the Central Afar (Ethiopia)

The Afar region constitutes the diffuse triple junction adjoining the Red Sea, Gulf of Aden, and Main Ethiopian Rifts. Extension in the Afar is distributed between on-axis, magmatic crustal spreading, and off-axis, amagmatic extension, such as in Dobe graben. The timing of initiation and development of these amagmatic grabens is key to understanding the final stages of continental extension/breakup in central Afar. All major graben-bounding faults in central Afar cut through the Quaternary Afar Stratoid Series, constraining the ages of initiation to younger than approximately 1 million years. The Afar Stratoid Series presents a widespread thick lava unit that smoothed and infilled the topography created by the prior 30 Ma of deformation. The ∼1 Ma top surface of the Afar Stratoid Series serves as a regional marker predating most Quaternary deformation. The initiation of a particular fault (or location along a graben-bounding fault) begins a process of local lowering of base level – this is recorded in the landscape as a knickpoint that incises the paleo drainage in the footwalls of the grabens, connecting it to the basin floor with a deep canyon. The rate at which knickpoints retreat upstream permits a space-for-time approach to assessing initiation. This study develops and applies a landscape evolution model to estimate relative maturity and age of drainage basins by measuring the long profile response to tectonic forcings over intermediate timescales in central Afar. This is applied under the assumption that faulting postdates Afar Stratoid Series emplacement. The model is calibrated by analysis of stream longitudinal profiles and terrestrial cosmogenic nuclide (TCN) analysis of samples collected in the field. Longitudinal stream valley profiles are mapped through remote sensing and photogrammetry, and TCN concentrations are assessed to calculate the uplift rate along the graben-bounding faults. Results indicate that faulting of Dobe Graben in the central Afar initiated in the southeast and propagated to the northwest. Along the southern margin of Dobe Graben, ages of 503 ± 116 ka were calculated for fault initiation at Hann Canyon in the southeast, and 339 ± 80 ka at Ishyilu Canyon to the northwest. These timing constraints have implications for competing tectonics models for central Afar. The general approach developed for modeling knickpoint retreat has broader utility beyond Afar, allowing for application to extensional tectonic regimes elsewhere where similar geological and surface constraints exist.

Magmatism at an ultra-slow spreading rift: high-resolution geomorphological studies of a Red Sea Rift segment in Hadarba Deep

The mid-ocean rift in the Red Sea is one of the youngest rifting systems on Earth. Only recently, state-of-the-art methods and modern deep-sea instruments have been used to explore this young and unique volcanic system. During the first autonomous underwater vehicle surveys of the Red Sea Rift in Spring 2022, we collected multibeam bathymetry, backscatter, sub-bottom profiler data, and water column data over a 9 km long ridge segment in the Hadarba Deep between 22.49°N and 22.56°N to investigate the volcano-tectonic processes of this ultra-slow spreading segment (12 mm/year spreading rate). The high-resolution hydroacoustic data was used to (1) delineate and quantify the geometry of tectonic structures and individual lava flows, (2) define lava flow morphology and eruption style, (3) estimate relative ages of flows and features, and (4) retrace the evolution of the volcanic activity. In addition, the geochemistry of several young lava flows provides information on the relation between the different magma that supply these eruptions. About 90 eruptive units with variable sedimentary cover have been identified within the 43 km2mapped region. The oldest lava flows are buried under 3 to 4.2 m of sediment, indicating ages of up to ~30 ka based on average sedimentation rate estimates (~14 cm/ka), while the youngest eruptions are covered by<10 cm of sediment, and are thus younger than 700 years. Three volcanic phases have been identified based on changes in flow morphology and distribution, and tectonic pattern. All three axial phases have an average eruptive frequency of ~100-250 years. The segment displays an overall low tectonic extension (<10% of the total extension) and low vertical offset. Our geomorphological maps, analyses, and statistics reveal a moderately faulted, ultra-slow spreading MOR segment in the Red Sea with a surprisingly large amount of magmatic extension, implying that the segment has been underlined by a large magma supply for at least 15 ka. All these observations provide valuable implications for the formation history of the Red Sea Rift and the formation of ultra-slow spreading crust.

Geological and topographical analysis of Anshar Sulcus, Ganymede: Implications for grooved terrain formation

The formation of grooved terrains on the icy surface of Ganymede is still debated and it could involve extensive rifting or spreading, strike-slip tectonics, and minor cryovolcanic resurfacing. Anshar Sulcus is a terminal portion of a groove network imaged at high resolution by the Galileo Solid State Imaging subsystem (SSI). To investigate the origin and evolution of the grooved terrains, we performed a geomorphological and structural analysis of Anshar Sulcus producing a geomorphological map of the study area in 1:500,000 scale, and we conducted a topographical analysis producing a Digital Elevation Model (DEM) of this region. We found several indicators that the formation of Anshar Sulcus' grooved terrain is the result of two tectonic events, the first being a right lateral strike-slip type and the second a crustal spreading. We found that the right lateral strike-slip movement that divided the dark terrain of this area into two distinct regions represents the precursor of the formation of the sulcus, and that the crustal spreading subsequently occurred through transtensional movement of the regions. We also found that the grooves were formed within the solid-state raised material by brittle fracturing and tilting of the newly formed crust.

Application of the Electromagnetic Geothermometer in Geothermics and Geothermal Exploration

Abstract —The latest results on electromagnetic geothermometry and its use in geothermics and geothermal exploration have been covered. Temperature models for the well-known geothermal areas, Larderello-Travale (Italy), Soultz-sous-Forêts (France) and Hengill (Iceland), constructed from electromagnetic sounding data and temperature well logs have been analyzed. 2D temperature model built along the magnetotelluric profile crossing the Travale (Italy) geothermal system allowed to locate a potential supercritical reservoir at the depth between two isotherms, TSCF ~ 400 °C and TBDT ~ 600 °C, coinciding with the seismic reflectors, detected earlier. 3D temperature model of the Hengill (Iceland) geothermal area built up to the depth 20 km, using electromagnetic sounding data, allowed us to locate high-temperature magma pockets at shallow depth (2–5 km); these magma pockets can be regarded as targets for geothermal exploration. Joint analysis of the specific resistivity and temperature models, taking into account gravity anomalies, allowed us to locate the zones of different thermal regimes within the crust; this, in turn, explained the observed seismicity pattern by thermo-mechanical properties, rather than by crust spreading. Deep temperature model, built for the Soultz-sous-Forêts (France) geothermal area from magnetotelluric data and temperature well logs, used to locate deep heat sources and predict heat transfer mechanism at depth up to 8 km.

Isotopes track Tethyan seamount subduction beneath the Troodos spreading centre, Cyprus

Supra-subduction zone volcanoes record the chemical transfer of sedimentary and igneous material from the subducting plate to the overlying mantle and lithosphere. This transfer is thought to preferentially involve fluid-mobile elements, which swamp the signature of similar trace elements in the supra-subduction mantle. Using high-resolution isotope measurements of fluid-mobile Pb, we have traced the evolution and inputs beneath the supra-subduction spreading ridge that is now the Troodos Ophiolite, Cyprus. It is apparent that variable but significant amounts of Pb are actually derived from the mantle wedge, and substantial amounts of the fluid-immobile elements like Nb are introduced from the slab. Our study identifies a chemical transformation of the spreading crust, which reflects a spatial change in the Neo-Tethyan slab. The west was influenced by the subduction of pelagic/carbonate sediments overlying MORB, whereas the east and south also include a high 206Pb/204Pb component, similar to high-μ mantle plumes distributed around northern Africa. This chemo-tectonic switch is interpreted to represent the location of supra-subduction spreading relative to the path of a seamount chain colliding with and subducting beneath the Troodos trench. The Hecataeus Rise, located to the southeast of Cyprus, is a candidate to be a foundered relic of this seamount chain, locked in the subduction zone. We also demonstrate that the supra-subduction mantle wedge was of an Atlantic/Pacific-type composition, but the subducting Neo-Tethyan oceanic crust had an Indian Ocean signature. As such, this destructive plate boundary can be defined as the Cretaceous interface between the Atlantic and Indian mantle domains.

Post‐50 Ma Evolution of India‐Asia Collision Zone From Paleomagnetic and GPS Data: Greater India Indentation to Eastward Tibet Flow

AbstractWe re‐evaluate 357 Jurassic‐Holocene paleomagnetic datasets from Tibet‐Indochina and compare them with present‐day Global Position System velocity field. SE Tibet NW of the East Himalaya Syntaxis (EHS) underwent 20°–30° counterclockwise rotations around 50 Ma, and mostly clockwise rotations after 40 Ma. NE of the EHS, post‐50 Ma clockwise rotation occurred, whereas highly scattered clockwise rotations took place on northern Indochina at 25–15 Ma, after a remagnetization episode. We suggest that the indentation of Greater India NE corner at ∼50 Ma resulted in a wide orogenic reentrant characterized by opposite rotations at orocline limbs. Rotations East of EHS after 40 Ma were likely due to local strike‐slip fault activity. After 30 Ma, the ongoing India collision fragmented Indochina into km‐scale blocks that experienced independent rotations. The present‐day clockwise rotation pattern around the EHS started at 15–10 Ma along with eastward Tibet crustal spreading, and has not produced yet a detectable paleomagnetic rotation.

Magma evolution of the South China Sea basin from continental-margin rifting to oceanic crustal spreading: Constraints from In-situ trace elements and Sr isotope of minerals

The South China Sea (SCS) has undergone a complete Wilson cycle. We studied the volcanic rocks from IODP Sites U1502, U1500, U1431, U1433, and U1434 that record the exact mantle dynamics of the SCS basin from continental rifting to oceanic crustal spreading. We present in-situ major and trace element data for minerals (olivine, clinopyroxene, and plagioclase) and melt inclusions; and in-situ Sr isotopic ratios for plagioclases from the volcanic samples. All SCS volcanic rocks belong to the tholeiite series. Most Site U1431 basalts are slightly depleted in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) and similar to that of normal mid-oceanic ridge basalts (N-MORB). All clinopyroxenes and olivines, and the calculated equilibrium melts of clinopyroxene cores display depletion of LREEs and enrichment of heavy rare earth elements (HREEs) similar to N-MORB-type primary magma. Thus, these volcanic rocks are likely formed by the extension decompression partial melting of shallow depleted magma. The Sites U1502, U1500, U1433, and U1434 bulk-rocks and melt inclusions show flat LREE and HREE patterns similar to that of enriched MORB (E-MORB), likely attributed to the partial melting of a depleted mantle source with different degrees of lower continental crustal contamination. The increase in 87Sr/86Sr ratios from core to rim in Site U1500 plagioclases further prove the crustal contamination during magmatism. Site U1431 clinopyroxene and Site U1500 plagioclase show normal zoning textures and slight enrichment of incompatible elements in their rims but depletion in their cores, likely indicating significant crystallization of late-stage differentiated melts. Although some clinopyroxene and plagioclase were likely crystallized in a relatively closed environment, the existence of melt inclusions and the large Sr isotopic variations between the plagioclase rim and groundmass suggest a dynamic and open magmatic system that likely experienced melt–rock interactions and magma mixing processes. Melt supply rate changes from low to high during the SCS evolution process from initial rifting (Site U1500) to later spreading (Sites U1431 and U1433) indicating that: (1) the number of high-An plagioclase-hosted melt inclusions has gradually decreased; and (2) the zoning textures of clinopyroxene and plagioclase have gradually changed from simple to complex. The influence of mantle plumes and the upwelling of fertile subduction-related asthenospheric magma were negligible during the continental-margin breakup and initial SCS spreading, but played an important role during the late SCS spreading period, as revealed by the variation in Sr isotopic compositions of high-An plagioclase cores, the different mantle potential temperatures and source lithologies.

Imaging Early Oceanic Crust spreading in the Equatorial Atlantic Ocean: Insights from the MAGIC wide-angle experiment

During the MAGIC (Margins of brAzil, Ghana, and Ivory Coast) experiment, five combined wide-angle, and reflection seismic profiles were acquired in the Pará-Maranhão/Barreirinhas/Ceará basins northern Brazil. This is a pull-apart passive margin, with two strike-slip borders. The equipment deployed includes 143 sea-bottom seismometers (OBS), a 4.5-km seismic streamer, and a 7587-in3 airgun array. In this paper, we focus on the distal parts of three profiles, and one entire transverse NW-SE profile, located on the presumed Cretaceous oceanic crust.Forward modelling of these wide-angle data sets reveals an E-W lateral evolution of the oceanic crust spreading initiation with: 1) just after the so-called intermediate domain, 60 km-wide domain that consists of a 5-km-thick crystalline crust. The basement presents two layers characterized by high acoustic velocity. This domain is bounded to the NW by a NW-SE volcanic line (Volcano Alignment), and 2) a 5-km-thick oceanic crust consisting of two layers characterized by “normal velocities” spanning between the two main fracture zones that fringe the Pará-Maranhão-Barreirinhas-Ceará segment. Despite a similar thickness, these two sub-domains present different velocity distribution in their two layers. They are both overlain by 5.5 km of sedimentary deposits. Forward wide-angle modelling confirms that the seafloor spreading process was progressive, with firstly the emplacement of a proto-oceanic crust, and then a thin oceanic crust. The “proto-oceanic” crust presents a similar seismic velocity with the intermediate domain interpreted as exhumed lower continental crust except for the lower part where the intruded lower crust gives place to a very sharp Moho at the base of the proto-oceanic domain. By contrast, the thin oceanic crust domain has a lower velocity structure in its upper layer, that is interpreted as basalt and is absent in the proto-oceanic crust. This eastward evolution, as also observed in the Provençal Basin, and the Santos Basin, suggests the involvement of the lower continental crust in the first steps of seafloor spreading.

Influence of Precambrian shear zones on the formation of oceanic fracture zones along the continental margin of Brazil

Abstract Structural inheritance control of oceanic fracture zone formation by a large basement shear zone during continental breakup is still a matter of debate. In this research, we investigated the reactivation of Precambrian shear zones and their role in the formation of the Ascension and Fernando Poo fracture zones during the evolution of the South Atlantic Ocean. We used aeromagnetic and multichannel 2D seismic reflection data associated with previous geological and geophysical data to analyze how the Precambrian shear zones may have influenced the formation of oceanic fracture zones. Our results suggest that the fracture zones are extensions of the shear zones in the transition between continental and oceanic crust. The Sao Miguel do Aleixo and Pernambuco shear zones were reactivated during the Pangea breakup and acted as zones of weakness controlling the location of the Ascension and Fernando Poo fracture zones during the process of oceanic crust spreading. Our model suggests that the formation of these fracture zones influenced by the shear zones occurred in the early stages of ocean opening. It indicates that the fracture zones that formed in the late opening stages were related to the thermal subsidence stage, as suggested in previous studies.

Analysis strategies for combining continuous and episodic GNSS for studies of neo-tectonics in Northern-Norway

Crustal deformation in the seismically active Nordland area in Northern Norway is estimated based on a combination of data from local episodic epGNSS campaigns (three 5-day campaigns in 1999, 2008 and 2015) and continuously operating cGNSS stations in the area that were mainly established in 2008 and in 2009. To establish a local long-term stable reference frame, which is consistent both with the epGNSS network and the network of newer cGNSS, a three-step procedure for reference frame realization is used to get consistent results from all the stations in the area. Analysis of the main error sources shows that uncertainties for the episodic epGNSS stations are around 0.2mm/yr in the horizontal components and 0.5mm/yr in the vertical component. The results support earlier findings that Ranafjord area of the Nordland is undergoing crustal spreading with horizontal displacement velocities of ca. 1.0±0.2mm/yr, predominantly in the east-west direction. The results also show a gradient in the uplift along the coast of Nordland that is larger than predicted by existing glacial isostatic adjustment models.

Ancient hydrothermal seafloor deposits in Eridania basin on Mars

The Eridania region in the southern highlands of Mars once contained a vast inland sea with a volume of water greater than that of all other Martian lakes combined. Here we show that the most ancient materials within Eridania are thick (>400 m), massive (not bedded), mottled deposits containing saponite, talc-saponite, Fe-rich mica (for example, glauconite-nontronite), Fe- and Mg-serpentine, Mg-Fe-Ca-carbonate and probable Fe-sulphide that likely formed in a deep water (500–1,500 m) hydrothermal setting. The Eridania basin occurs within some of the most ancient terrain on Mars where striking evidence for remnant magnetism might suggest an early phase of crustal spreading. The relatively well-preserved seafloor hydrothermal deposits in Eridania are contemporaneous with the earliest evidence for life on Earth in potentially similar environments 3.8 billion years ago, and might provide an invaluable window into the environmental conditions of early Earth.

Fissure swarms and fracture systems within the Western Volcanic Zone, Iceland – Effects of spreading rates

The Western Volcanic Zone (WVZ) in Iceland is ∼120 km long and 40 km wide. It offers an opportunity to study rift zones in a local ultra-slow spreading area close to a hotspot. Fractures were mapped from aerial photographs and digital elevation models. Most surface fractures are located in the southern part of the WVZ. The majority of the fractures have a north-northeasterly orientation, some deviations occur from this, especially in the north part of the WVZ. Fracture orientations are therefore quite uniform in the southern, faster spreading part of the WVZ, but more irregular in the slower-spreading northern part. This suggests different stress fields in the north part, which could be due to the influence of the Hreppar microplate and possibly also due to stress fields induced by crustal deformation because of changes in glacial load in the area. Such glacially-induced stress fields may have similar or even more influence than crustal spreading in the slower spreading northern part of the WVZ. Lower fracture density towards the north within the WVZ suggests lower frequency of rifting events in the north part, in accordance with less spreading in the north as measured by GPS geodetic measurements.

Fault-related biogenic mounds in the Ceduna Sub-basin, Australia. Implications for hydrocarbon migration

The Early Cenozoic in the Bight Basin represents a transitional period from the slow to fast oceanic crust spreading of the Southern Ocean between Australia and Antarctica. This period is characterised by widespread igneous activity and the occurrence of biogenic mound complexes. In the proximal part of the Ceduna Sub-basin these mound complexes thrive on local topographic highs, exploiting the shut-down in siliciclastic sediment supply during the Eocene. New interpretation of 2D and 3D seismic data from the central Ceduna Sub-basin shows additional, undrilled, Paleocene-Eocene biogenic mound complexes. They display rugose morphology, progradational and aggradational internal organisation and high impedance contrast with the surrounding sediments. These mounds typically show a clear relationship with underlying reactivated fault segments. These faults intersect potential oil mature Cretaceous source rock intervals and therefore one plausible mechanism for the development of fault-related mound complexes in the central Ceduna Sub-basin relies on natural hydrocarbon seeps along faults supplying nutrients for a benthic chemosynthetic organisms and localised biogenic mound growth. The presence of a sub-bottom profile anomaly overlaid by a vertical water column acoustic contact in close vicinity of a mound complex support this model. The occurrence of seep related biogenic mounds would validate the presence of mature source rock(s) and hydrocarbon charge in the central Ceduna Sub-basin. If confirmed the biogenic mounds would imply fluid migration occurrence until at least the Eocene.

The geological nature and geodynamics of the Okinawa Trough, Western Pacific

AbstractThere is a continuing hot debate on whether the geodynamics in the West Pacific indicate either a marginal basin or back‐arc basin, since the nature, evolution and formation mechanism are not well known. Research on the Okinawa Trough (OT), which is an active back‐arc basin, has important scientific significance for discussing the opening and development of a marginal basin or back‐arc basin. This paper synthesizes data on the crustal thickness, heat flow, hydrothermal activity and fault patterns to determine the geological nature of the OT. By comparing the thicknesses of initial oceanic crusts of global passive continental margins and marginal basins in the West Pacific, the central and southern OT are interpreted to be at the early stage of seafloor spreading, generating new oceanic crust at the spreading centre, while the northern OT is at the mature stage of continental rifting, having the character of transitional crust. The NE‐ and NNE‐trending faults are dominant in the OT. The NE‐trending strike‐slip faults control the stratigraphic succession and structural framework of the OT. The NNE‐trending strike‐slip faults triggered the back‐arc rifting or extension and evolved into transform faults with the emergence of oceanic crust. The Cenozoic tectonic jumping in the East Asia Continental Margin, including the OT, indicates that the eastward movement of the mantle flow under the Eurasian Plate should be the main dynamic source. Because the Philippine Sea Plate is obliquely and passively subducted, the evolution of the OT can be divided into three stages. The first stage is the initial rifting, triggered by the back‐arc normal faulting. The second stage is the passive spreading, triggered by the strike‐slip pull‐apart process or transtensional faulting of NNE‐trending faults. The third stage is the initial divergence of the relic and Ryukyu island arcs, triggered by new‐born oceanic crustal spreading. Copyright © 2016 John Wiley & Sons, Ltd.

Autonomous geodynamics of the Pamir–Tien Shan junction zone from seismology data

The geodynamics of the Tajik Depression, the junction zone of the Pamirs and Tien Shan, is typically considered in the context of plate tectonic concept, which implies intense subhorizontal compression of the zone resulting from the subduction of the Indian and Eurasian lithospheric plates. This convergence has been reliably confirmed by the GPS measurements. However, the joint analysis of the geological structure, seismicity, and geodimeter measurements conducted during a few years at the Garm geodynamical testing site of the Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, demonstrates a widening of the Tajik Depression instead of its shortening, as should be expected from the subhorizontal compression predominant in the present-day stress-state of this region. This conclusion, together with the data from the other regions, suggests that, along with the plate tectonic mechanisms, there are also other, local, autonomous drivers that contribute to the tectogenesis of this region. Besides, the probable existence of these autonomous sources within the Tajik Depression directly follows from the seismology data. Among them is the crustal spreading within the depression suggested by the seismotectonic displacements in the focal mechanisms of the earthquakes. These displacements are directed in different azimuths off the axial’s most subsided part of the depression at a depth of 20–30 km. Above this region the distribution of seismotectonic deformations (STD) is chaotic. This pattern of deformation is barely accounted for by a simple model of subhorizontal compression of the Earth’s crust in the region. In our opinion, these features of the seismotectonic deformation in the crust within the studied part of the Tajik Depression is probably associated with the gain in the volume of the rocks due to the inflow of the additional material, which is supplied from the bottom crust or upper mantle by the deep fluids. This increase in the rock volume produces the state of horizontal compression (or, more exactly, outward all-round pressure). We suggest considering the subvertical pillar-like object, identified as a cluster of the earthquake hypocenters in the northern part of the region with chaotic STD, as one of the probable channels of fluid supply. This structure’ plane is isometric and has a diameter of about 20 km. It stretches downwards to a depth of 40 km, where the seismicity ceases. The suggested scenario of generation of the outward all-round pressure (autonomous geodynamics) can be considered as an independent driver of tectogenesis along with the traditional mechanisms relating the emergence of the stressstate of subhorizontal compression to the plate-tectonic convergence of the Eurasian and Indian lithospheric plates.

Magmatic accretion and thermal convection at the sheeted dike complex–gabbro boundary in superfast spreading crust, ODP Hole 1256D

The magmatic processes responsible for accretion of new oceanic plutonic crust at fast-spreading centers remain unclear. Expedition 312, at Ocean Drilling Program Hole 1256D, on the eastern flank of the East Pacific Rise, barely drilled through the uppermost gabbros but offers unparalleled opportunities to constrain deep magmatic processes and the mode of construction of the gabbroic layer.Gabbro 1 and gabbro 2, respectively 52 and 24m-thick, were intruded in an altered and baked sheeted dike complex. Both gabbros lack a macroscopic fabric and were until now considered isotropic. Here, we use digital 3-D fabric analysis of plagioclase phenocrysts in 33 non-azimuthally oriented samples. In contrast with previous observations, the two gabbros display a distinct and consistent shape-preferred orientation. Both gabbros exhibit magmatic, equilibrated microstructures, with minor submagmatic microstructures. The calculated Rayleigh numbers range between 1015 and 1018, proving that the gabbroic magma was initially undergoing turbulent magmatic flow. Magmatic foliations dip moderately, with a mean of 48° in gabbro 1 and 61° in gabbro 2. Magmatic lineations plunge in gabbro 1 between 1 and 44° (mean 28°), while in Gabbro 2, plunges range from 6 to 69° (mean 41°). These inclined magmatic structures, combined with the inclined magmatic contacts between the gabbros and their host-rock, show that these two gabbro bodies, instead of being horizontal sills, more likely are inclined intrusions that cut obliquely through the sheeted dike complex.Seismic reflection data around Hole 1256D shows inclined reflectors similar to those produced by gabbroic saucer-shaped intrusions in passive margins. On the basis of the inclined fabrics, inclined contacts and the regional seismic data, we propose that the uppermost gabbroic intrusions at fast-spreading centers form with a saucer-shape. This new shape of intrusion, never reported before in the oceanic environment, may constitute a significant component of oceanic accretion.

Anatomy of an extinct magmatic system along a divergent plate boundary: Alftafjordur, Iceland

AbstractRecent rifting episodes highlight the role of magmatic systems with propagating dikes on crustal spreading. However, our knowledge of magmatic systems is usually limited to surface observations and geophysical data. Eastern Iceland allows direct access to extinct and eroded deeper magmatic systems. Here we collected field structural and AMS (anisotropy of magnetic susceptibility) data on 187 and 19 dikes, respectively, in the 10–12 Ma old Alftafjordur magmatic system. At a paleodepth of ~1.5 km, the extension due to diking is at least 1–2 orders of magnitude larger than that induced by regional tectonics, confirming magmatism as the key mechanism for crustal spreading. This magma‐induced extension, inferred from the aspect ratio of the magmatic system, was of ~8 mm/yr, lower than the present one. AMS data suggest that most of dikes have geometrically normal fabric, at least at the margins, consistent with prevalent subvertical magma flow and propagation.

Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data

Multi-channel seismic (MCS) reflection profiles across the Pacific Plate south of the Alaska Peninsula reveal the internal structure of mature oceanic crust (48–56 Ma) formed at fast to intermediate spreading rates during and after a major plate re-organization. Oceanic crust formed at fast spreading rates (half spreading rate ∼74 mm/yr) has smoother basement topography, thinner sediment cover with less faulting, and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates (half spreading rate ∼28–34 mm/yr). MCS data across fast-spreading oceanic crust formed during plate re-organization contain abundant bright reflections, mostly confined to the lower crust above a highly reflective Moho transition zone, which has a reflection coefficient (RC) of ∼0.1. The lower crustal events dip predominantly toward the paleo-ridge axis at ∼10–30°. Reflections are also imaged in the uppermost mantle, which primarily dip away from the ridge at ∼10–25°, the opposite direction to those observed in the lower crust. Dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re-organization, where a Moho reflection is weak or absent. Our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt. The abundance and reflection amplitude strength of these events (RC∼0.15) can be explained by a combination of solidified melt that was segregated within the shear structures, mylonitization of the shear zones, and crystal alignment, all of which can result in anisotropy and constructive signal interference. Formation of shear zones with this geometry requires differential motion between the crust and upper mantle, where the upper mantle moves away from the ridge faster than the crust. Active asthenospheric upwelling is one possible explanation for these conditions. The other possible interpretation is that lower crustal reflections are caused by magmatic (mafic/ultramafic) layering associated with accretion from a central mid-crustal magma chamber. Considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re-organizations associated with ridge jumps or rift propagation, we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle, and therefore to shearing in the ductile lower crust or, alternatively, that plate reorganization could produce magmatic pulses which may lead to mafic/ultramafic banding.

Assessing long‐term postseismic deformation following the M7.2 4 April 2010, El Mayor‐Cucapah earthquake with implications for lithospheric rheology in the Salton Trough

AbstractThe 4 April 2010 Mw 7.2 El Mayor‐Cucapah (EMC) earthquake provides the best opportunity to date to study the lithospheric response to a large (>M6) magnitude earthquake in the Salton Trough region through analysis of Global Positioning System (GPS) data. In conjunction with the EarthScope Plate Boundary Observatory (PBO), we installed six new continuous GPS stations in the months following the EMC earthquake to increase station coverage in the epicentral region of northern Baja California, Mexico. We modeled the pre‐EMC deformation field using available campaign and continuous GPS data for southern California and northern Baja California and inferred a pre‐EMC secular rate at each new station location. Through direct comparison of the pre‐ and post‐EMC secular rates, we calculate long‐term changes associated with viscoelastic relaxation in the Salton Trough region. We fit these velocity changes using numerical models employing an elastic upper crustal layer underlain by a viscoelastic lower crustal layer and a mantle half‐space. Forward models that produce the smallest weighted sum of squared residuals have an upper mantle viscosity in the range 4–6 × 1018 Pa s and a less well‐resolved lower crustal viscosity in the range 2 × 1019 to 1 × 1022 Pa s. A high‐viscosity lower crust, despite high heat flow in the Salton Trough region, is inconsistent with felsic composition and might suggest accretion of mafic lower crust associated with crustal spreading obscured by thick sedimentary cover.

Microbial iron mats at the Mid-Atlantic Ridge and evidence that Zetaproteobacteria may be restricted to iron-oxidizing marine systems.

Chemolithoautotrophic iron-oxidizing bacteria play an essential role in the global iron cycle. Thus far, the majority of marine iron-oxidizing bacteria have been identified as Zetaproteobacteria, a novel class within the phylum Proteobacteria. Marine iron-oxidizing microbial communities have been found associated with volcanically active seamounts, crustal spreading centers, and coastal waters. However, little is known about the presence and diversity of iron-oxidizing communities at hydrothermal systems along the slow crustal spreading center of the Mid-Atlantic Ridge. From October to November 2012, samples were collected from rust-colored mats at three well-known hydrothermal vent systems on the Mid-Atlantic Ridge (Rainbow, Trans-Atlantic Geotraverse, and Snake Pit) using the ROV Jason II. The goal of these efforts was to determine if iron-oxidizing Zetaproteobacteria were present at sites proximal to black smoker vent fields. Small, diffuse flow venting areas with high iron(II) concentrations and rust-colored microbial mats were observed at all three sites proximal to black smoker chimneys. A novel, syringe-based precision sampler was used to collect discrete microbial iron mat samples at the three sites. The presence of Zetaproteobacteria was confirmed using a combination of 16S rRNA pyrosequencing and single-cell sorting, while light micros-copy revealed a variety of iron-oxyhydroxide structures, indicating that active iron-oxidizing communities exist along the Mid-Atlantic Ridge. Sequencing analysis suggests that these iron mats contain cosmopolitan representatives of Zetaproteobacteria, but also exhibit diversity that may be uncommon at other iron-rich marine sites studied to date. A meta-analysis of publically available data encompassing a variety of aquatic habitats indicates that Zetaproteobacteria are rare if an iron source is not readily available. This work adds to the growing understanding of Zetaproteobacteria ecology and suggests that this organism is likely locally restricted to iron-rich marine environments but may exhibit wide-scale geographic distribution, further underscoring the importance of Zetaproteobacteria in global iron cycling.