Mantle Exhumation Research Articles

Geological and geophysical interpretation of the Rio Grande Rise, south-eastern Brazilian margin: extensional tectonics and rifting of continental and oceanic crusts

ABSTRACT This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North–south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east–west direction, and locally it is marked by rift structures aligned along a NW–SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east–west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporite sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.

A short electromagnetic profile across the Kane Oceanic Core Complex

A short seafloor electromagnetic profile across part of the Kane Oceanic Core Complex shows a large variation in seafloor electrical properties (uppermost 30m) with a sharp transition from conductive to resistive seafloor over a distance of less than 350m. The transition is in a location consistent with a sharp, but deeper, lateral gradient in seismic velocity, inferred to mark a transition from serpentinized peridotite to either gabbro or pristine mantle rocks, and is close to a mapped outcrop of serpentinized peridotites. We relate this variability in shallow structure primarily to changes in porosity related to composition.

Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies

[1] The Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian-Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Bearn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian-early Albian. They are unconformably overlain by the Albian-Cenomanian flysch formations and have experienced high temperature-low pressure mid-Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust-mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low-angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm.

Seismic structure of an oceanic core complex at the Mid‐Atlantic Ridge, 22°19′N

We present results from a seismic refraction and wide‐angle experiment surveying an oceanic core complex on the Mid‐Atlantic Ridge at 22°19′N. Oceanic core complexes are settings where petrological sampling found exposed lower crustal and upper mantle rocks, exhumed by asymmetric crustal accretion involving detachment faulting at magmatically starved ridge sections. Tomographic inversion of our seismic data yielded lateral variations of P wave velocity within the upper 3 to 4 km of the lithosphere across the median valley. A joint modeling procedure of seismic P wave travel times and marine gravity field data was used to constrain crustal thickness variations and the structure of the uppermost mantle. A gradual increase of seismic velocities from the median valley to the east is connected to aging of the oceanic crust, while a rapid change of seismic velocities at the western ridge flank indicates profound differences in lithology between conjugated ridge flanks, caused by un‐roofing lower crust rocks. Under the core complex crust is approximately 40% thinner than in the median valley and under the conjugated eastern flank. Clear PmP reflections turning under the western ridge flank suggest the creation of a Moho boundary and hence continuous magmatic accretion during core complex formation.

Tectonic versus magmatic extension in the presence of core complexes at slow-spreading ridges from a visualization of faulted seafloor topography

We develop a forward model of the generation of faulted seafloor topography (visualization) to estimate the relative roles of tectonic and magmatic extension in the presence of core complexes at slow-spreading ridges. The visualization assumes flexural rotation of 60° normal faults, a constant effective elastic thickness, Te , of young lithosphere, and a continuous infill of the depressed hanging wall by lava flowing from the spreading axis. We obtain a new estimate of Te = 0.5–1 km from the shapes of the toes of 6 well-documented oceanic core complexes. We model an 80-km-long bathymetric profile in the equatorial Atlantic across a core complex and the ridge axis at 13°20′N and estimate the variation in tectonic extension, which yields the variation in the fraction of upper crust extension, M , by magmatic diking at the ridge axis. Core complex formation appears to be stable for all values of M < 0.5. The visualization shows how gabbro emplaced at the base of the lithosphere during extension by magmatic diking is partitioned to each side of the spreading axis, and predicts a high probability of finding gabbros in the domes of core complexes.

Oceanic plagiogranites as a result of interaction between magmatic and hydrothermal systems in the slow-spreading mid-ocean ridges

The paper considers some petrological and geochemical aspects of the formation of oceanic plagiogranites (OPG)—felsic intrusive rocks, which were found in the plutonic complexes of modern mid-ocean ridges (MOR) and ophiolites of paleo-collisional zones. Based on the multi-equilibrium clinopyroxene-orthopyroxene-amphibole-plagioclase geothermobarometry, typical OPG found in gabbros and peridotites were formed at temperatures of 820–850°C and pressure of 2–2.5 kbar. Close temperature estimates (825 ± 50°C) were obtained from literature data on Ti content in zircon, with allowance for lowered TiO2 activity in the rock. Under these P-T parameters, OPG can be generated only in the presence of fluid of water activity \( \left( {a_{H_2 O} } \right) \) close to 0.9. OPG and associated recrystallized gabbroids contain high-temperature hornblende with significant Cl content (0.5–2 wt %). In addition, the plagiogranites are characterized by particular geochemical features such as extremely high Na2O/K2O (up to 135), sharp LREE enrichment ((Ce/Yb)cn and (La/Sm)cn up to 10 and 4, respectively), and elevated 87Sr/86Sr ratio relative to DMM. All these facts point to the key role of hydrothermal fluid, the seawater derivative, in the OPG formation. The fluid with \( a_{H_2 O} = 0.9 \) (approximately 28 wt % NaCl) could be produced from seawater due to hydration reactions at the higher lower temperature horizons of oceanic crust in the course of its percolation to the OPG generation areas. The formation of plagiogranites in the MOR oceanic core complexes possibly reflects the fundamental feature of oceanic accretion: practically simultaneous (at the geological time scale) proceeding of exogenic (neptunic) and endogenous (plutonic) processes.

The structure of oceanic core complexes controlled by the depth distribution of magma emplacement

Extension at mid-ocean ridges can be accommodated by detachment faults, forming oceanic core complexes that develop under low rates of magma intrusion. Modelling reveals that oceanic core complexes can also form under high rates of magma intrusion, if the magma is injected into the lower ductile layer of the crust. At mid-ocean spreading centres, extension can be accommodated by slip on large, long-lived (1–2 Myr) detachment faults that expose large tracts of lower crustal gabbroic rocks and mantle peridotite. These structures are known as oceanic core complexes. The development of detachment faults is controlled by the rate at which magma is injected into the brittle lithosphere as intrusive dykes. Recent modelling studies suggested that oceanic core complexes form under low magma injection rates, when only 30–50% of total plate separation is accommodated by the injection of magma into the lithosphere1,2,3. Yet, paradoxically, field observations4,5,6,7,8,9,10,11 document oceanic core complexes that have formed under a spectrum of magma injection rates, from amagmatic to fully magmatic conditions. Here we present a numerical model of oceanic core complex formation that explicitly considers magma intrusion not only in the brittle lithospheric layer, as in earlier simulations1,2,3, but also in the underlying ductile asthenosphere. We find that the rate of magma intrusion into the brittle layer controls fault evolution, whereas the rate of intrusion below the brittle–ductile transition has no influence on fault development, but controls the volume of gabbro exhumed. Our findings suggest that oceanic core complexes can form under high magma intrusion rates if intrusion is accommodated mainly by the ductile asthenosphere, thus reconciling the disparity between prevailing models and field observations.

Strain localization along the Atlantis Bank oceanic detachment fault system, Southwest Indian Ridge

Microstructural observations and mineral thermometry from in situ samples collected from the Atlantis Bank oceanic core complex (SW Indian Ridge) indicate that detachment faulting was initiated under hypersolidus conditions in the ductile regime and continued through subgreenschist temperatures through the ductile, semibrittle, and brittle regimes as strain localized along the exposed, now subhorizontal fault surface. Ductile, semibrittle, and brittle fabrics are developed within dominantly gabbroic rocks. Footwall rocks exhibit crystal plastic fabrics distributed over a structural thickness up to 400 m below the denuded fault surface exposed at the seafloor, whereas semibrittle and brittle fabrics are concentrated in the 80 and 30 m immediately below the principal slip surface of the detachment fault, respectively. Sample fabrics suggest that strain localization was achieved by dynamic recrystallization of plagioclase at temperatures between 910°C and 650°C, by amphibole‐accommodated dissolution‐precipitation creep at temperatures ∼750°C–450°C, by chlorite‐accommodated reaction softening at temperatures ∼450°C–300°C, and by brittle fracturing and cataclasis at temperatures &lt;300°C.

Alpine peridotites from the Ligurian Tethys: an updated critical review

We present an updated overview of the petrogenetic evolution of upper mantle peridotites exposed in the Alpine–Apennine ophiolites; the latter represent fragments of Jurassic oceanic lithosphere derived from the Ligurian–Piedmontese basin. This synthesis is based on the results of recent multidisciplinary studies of these peridotite massifs in the Eastern, Central and Western Alps, Ligurian Alps, Northern Apennines and Alpine Corsica, and has been developed within the framework of the geodynamic evolution of the Ligurian–Piedmontese basin between Europe and Adria. The Ligurian–Piedmontese basin began forming through continental rifting during the Triassic. As the continental lithosphere was progressively stretched and thinned, the underlying asthenosphere underwent adiabatic upwelling and decompressional partial melting. The subsequent crustal break-up during the Jurassic led to the development of the non-volcanic passive margins of Europe and Adria, and to the exhumation and exposure of the sub-continental lithospheric mantle on the seafloor. The rifting stage of the basin was characterized by percolation through diffuse porous flow of asthenospheric mid-ocean ridge basalt (MORB) melts via the continental lithospheric mantle. Melt–rock interaction caused depletion/re-fertilization of the lithospheric mantle and generated the extreme compositional heterogeneity of the upper mantle peridotites, which were later exposed on the seafloor at distal settings of the basin. Depleted spinel peridotites, which were previously interpreted as refractory residua after asthenosphere partial melting, are now recognized as reactive peridotites formed by melt–rock interaction. MORB melts migrating through the sub-continental lithospheric mantle along the axial zone of the extending system modified the upper mantle peridotites significantly and were entrapped by interstitial crystallization.

Crustal structure of a rifted oceanic core complex and its conjugate side at the MAR at 5°S: implications for melt extraction during detachment faulting and core complex formation

We present results of a seismic refraction experiment which determines the crustal and upper-mantle structure of an oceanic core complex (OCC) and its conjugate side located south of the 5°S ridge–transform intersection at the Mid-Atlantic Ridge. The core complex with a corrugated surface has been split by a change in location of active seafloor spreading, resulting in two massifs on either side of the current spreading axis. We applied a joint tomographic inversion of wide-angle reflected and refracted phases for five intersecting seismic profiles. The obtained velocity models are used to constrain the magmatic evolution of the core complex from the analysis of seismic layer 3 and crustal thickness. An abrupt increase of crustal velocities at shallow depth coincides with the onset of the seafloor corrugations at the exposed footwall. The observed velocity structure is consistent with the presence of gabbros directly beneath the corrugated fault surface. The thickness of the high-velocity body is constrained by PmP reflections to vary along and across axis between <3 and 5 km. The thickest crust is associated with the central phase of detachment faulting at the higher-elevated northern portion of the massif. Beneath the breakaway of the OCC the crust is 2.5 km thick and reveals significantly lower velocities. This implies that the fault initially exhumed low-velocity material overlying the gabbro plutons. In contrast, crust formed at the conjugate side during OCC formation is characterized by an up to 2-km-thick seismic layer 2 overlying a 1.7-km-thick seismic layer 3. Obtained upper-mantle velocities range from 7.3 to 7.9 km s−1 and seem to increase with distance from the median valley. However, velocities of 7.3–7.5 km s−1 beneath the older portions of the OCC may derive from deep fluid circulation and related hydrothermal alteration, which may likely be facilitated by the subsequent rifting. Our velocity models reveal a strongly asymmetric velocity structure across the ridge axis, associated with the accretion of gabbros into the footwall of the detachment fault and upper-crustal portions concentrated at the conjugate side. Our results do not support a substantial increase in the axial ridge's melt supply related to the final phase of detachment faulting. Hence, the footwall rifting at 5°S may be a generic mechanism of detachment termination under very low melt conditions, as predicted by recent numerical models of Tucholke et al.

Mantle Melting, Melt Transport, and Delivery Beneath a Slow-Spreading Ridge: The Paleo-MAR from 23 15'N to 23 45'N

Kane Megamullion, an oceanic core complex near the Mid-Atlantic Ridge (MAR) abutting the Kane Transform, exposes nearly the full plutonic foundation of the MARK paleo-ridge segment. This provides the first opportunity for a detailed look at the patterns of mantle melting, melt transport and delivery at a slow-spreading ridge. The Kane lower crust and mantle section is heterogeneous, as a result of focused mantle melt flow to different points beneath the ridge segment in time and space, over an ∼300–400 kyr time scale. The association of residual mantle peridotite, dunite and troctolite with a large ∼1 km+ thick gabbro section at the Adam Dome Magmatic Center in the southern third of the complex probably represents the crust–mantle transition. This provides direct evidence for local melt accumulation in the shallow mantle near the base of the crust as a result of dilation accompanying corner flow beneath the ridge. Dunite and troctolite with high-Mg Cpx represent melt–rock reaction with the mantle, and suggest that this should be taken into account in modeling the evolution of mid-ocean ridge basalt (MORB). Despite early precipitation of high-Mg Cpx, wehrlites similar to those in many ophiolites were not found. Peridotite modes from the main core complex and transform wall define a depletion trend coincident with that for the SW Indian Ridge projecting toward East Pacific Rise mantle exposed at Hess Deep. The average Kane transform peridotite is a lherzolite with 5·2% Cpx, whereas that from the main core complex is a harzburgite with only 3·5% Cpx. As the area corresponds to a regional bathymetric low, and the crust is apparently thin, it is likely that most residual mantle along the MAR is significantly more depleted. Thus, harzburgitic and lherzolitic ophiolite subtypes cannot be simply interpreted as slow- and fast-spreading ridges respectively. The mantle peridotites are consistent with a transform edge effect caused by juxtaposition of old cold lithosphere against upwelling mantle at the ridge–transform intersection. This effect is far more local, confined to within 10 km of the transform slip zone, and far smaller than previously thought, corresponding to ∼8% as opposed to 12·5% melting of a pyrolitic mantle away from the transform. Excluding the transform, the overall degree of melting over 3 Myr indicated by the peridotites is uniform, ranging from ∼11·3 to 13·8%. Large variations in composition for a single dredge or ROV dive, however, reflect local melt transport through the shallow mantle. This produced variable extents of melt–rock reaction, dunite formation, and melt impregnation. At least three styles of late mantle metasomatism are present. Small amounts of plagioclase with elevated sodium and titanium and alumina-depletion in pyroxene relative to residual spinel peridotites represent impregnation by a MORB-like melt. Highly variable alumina depletion in pyroxene rims in spinel peridotite probably represents cryptic metasomatism by small volumes of late transient silica-rich melts meandering through the shallow mantle. Direct evidence for such melts is seen in orthopyroxenite veins. Finally, a late hydrous fluid may be required to explain anomalous pyroxene sodium enrichment in spinel peridotites. The discontinuous thin lower crust exposed at Kane Megamullion contrasts with the >700 km2 1·5 km+ thick Atlantis Bank gabbro massif at the SW Indian Ridge (SWIR), clearly showing more robust magmatism at the latter. However, the SWIR spreading rate is 54% of the MAR rate, the offset of the Atlantis II Fracture Zone is 46% greater and Na8 of the spatially associated basalts 16% greater—all of which predict precisely the opposite. At the same time, the average compositions of Kane and Atlantis II transform peridotites are nearly identical. This is best explained by a more fertile parent mantle beneath the SWIR and demonstrates that crustal thickness predicted by simply inverting MORB compositions can be significantly in error.

Oceanic Core Complex Development in Modern and Ancient Oceanic Lithosphere: Gabbro‐Localized versus Peridotite‐Localized Detachment Models

Abstract Drilling and submersible studies of the Atlantis Bank (Southwest Indian Ridge) and the Atlantis Massif (Mid‐Atlantic Ridge) oceanic core complexes reveal “gabbro‐localized” and “peridotite‐localized” end‐member models of strain localization and deformation during core complex development, in which the gabbroic fault rocks exhibit extensive and rare high‐temperature ductile‐deformation fabrics, respectively. Both models emphasize a footwall cored by gabbroic intrusions, therefore precluding an amagmatic origin of core complex formation. We test these models by using relict oceanic core complexes preserved in the western Mirdita Ophiolite in Albania. The Puka and Krabbi massifs display traits of the peridotite‐localized detachment model, whereby amagmatic tectonic extension is not required for the formation of this category of core complexes.

Structural architecture and nature of the continent-ocean transitional domain at the Camamu and Almada Basins (NE Brazil) within a conjugate margin setting

Abstract Regional seismic reflection profiles and potential field data across the conjugate magma-poor Camamu/Almada–Gabon margins, complemented by crustal-scale gravity modelling and plate reconstructions, are used to reveal and illustrate the relationship of crustal structure to along-margin variation of potential field anomalies, to refine and constrain the continent–ocean boundary, as well as to study the structural architecture and nature of the continent–ocean transitional domain. The analysis reveals that the prominent conjugate Salvador–N'Komi transfer system appears to be a first-order structural element, governing the margin segmentation and evolution, and may have acted as an intraplate decoupling zone. The continent–ocean transitional domain, offshore northeastern Brazil, is characterized by rotated fault-blocks and wedge-shaped syn-rift sedimentary sequences overlying a prominent and undulated reflector (‘M-reflector’), which in turn characterizes the boundary between an extremely thinned, possibly magmatically intruded, continental crust and normal lithospheric mantle. The ‘M-reflector’ in the northeastern Brazilian margin shows remarkable similarities to the S-reflector at the West Iberia margin. In the same way, the ‘M-reflector’ is interpreted as a detachment surface that was active during rifting. Unlike the well studied central and northern segments of the West Iberia margin, however, the present study of the northeastern Brazilian margin does not clearly reveal evidence of an exhumation phase. The latter predicts exhumation of middle and lower crust followed by mantle exhumation. Increase in volcanic activity during the late stages of rifting may have ‘interrupted’ the extensional system, implying a failed exhumation phase. In this setting, the break-up and drift phase may have replaced the exhumation phase. Nevertheless, the available observations cannot discount the possibility that the ‘M-reflector’ is underlain by partially serpentinized mantle. Our study further leads to the development of a detailed conceptual model, accounting for the complex tectonomagmatic evolution of the conjugate northeastern Brazilian–Gabon margins. This model substantiates a polyphase rifting evolution mode, which is associated with a complex time-dependent thermal structure of the lithosphere. In the conjugate margin setting, asymmetrical lithospheric extension resulted in the formation of the thinned continental crust domain prior to the formation of the approximately symmetrical transitional domain.

Properties and distribution of lower crustal bodies on the mid-Norwegian margin

Abstract Anomalously high velocity and high density bodies have been detected in the lower crust on the mid-Norwegian margin. The lower crustal bodies (LCB) are pronounced on the Møre and Vøring margins segments and have mainly been interpreted as either magmatic or high-grade metamorphic in origin. Evolutionary models of the whole margin are heavily affected by the interpretation of the LCB and so are estimates of vertical movements and thermal structure in the area. A 3D gravity and magnetic model of the mid-Norwegian margin was constructed to map the main geological features of the margin and acquire the distribution of the LCB. The model utilizes the most recent potential field compilations on the margin and is constrained by extensive reflection seismic data and published refraction profiles. Further constraints on the model were attained from studying the isostatic state of the lithosphere. We present a map showing the distribution of the different LCB and discuss the implications for the structural and thermal evolution of the margin. The properties of the LCB vary across the margin and at least three different processes may be responsible for their existence. The LCB is commonly interpreted as igneous rock either intruded into the lower crust or underplated beneath it. The distribution of the LCB along the Vøring margin has an apparent correlation with the offshore prolongations of major onshore detachments stemming from Late Caledonian orogenic collapse. This may point towards some relation between the LCB and these old zones of weakness and that the LCB represents high-grade metamorphic rocks. Detailed modelling on the Møre margin shows a spatial link between parts of the LCB and extremely thin crustal thickness, suggesting a serpentinized exhumed mantle origin.

Interactions between continental breakup dynamics and large‐scale delta system evolution: Insights from the Cretaceous Ceduna delta system, Bight Basin, Southern Australian margin

The interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern Australian margin (Bight Basin) are designed to analyze the influence of the Australia‐Antarctica continental breakup process on the kinematic evolution of the Cretaceous Ceduna delta system. The data show that the structural architecture of this delta system consists of two stacked delta systems. The lower White Pointer delta system (Late Albian‐Santonian) is an unstable tectonic wedge, regionally detached seaward above Late Albian ductile shales. Sequential restoration suggests that the overall gravitational sliding behavior of the White Pointer delta wedge (∼45 km of seaward extension, i.e., ∼27%) is partially balanced by the tectonic denudation of the subcontinental mantle. We are able to estimate the horizontal stretching rate of the mantle exhumation between ∼2 and 5 km Ma−1. The associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the White Pointer delta system. Lithospheric failure occurs at ∼84 Ma through the rapid exhumation of the mantle. The upper Hammerhead delta system (Late Santonian‐Maastrichtian) forms a stable tectonic wedge developed during initial, slow seafloor spreading and sag basin evolution of the Australian side margin. Lateral variation of basin slope (related to the geometry of the underlying White Pointer delta wedge) is associated with distal raft tectonic structures sustained by high sedimentation rates. Finally, we propose a conceptual low‐angle detachment fault model for the evolution of the Australian‐Antarctic conjugate margins, in which the Antarctic margin corresponds to the upper plate and the Australian margin to the lower plate.

Seismogenic Shear Zones in the Lithospheric Mantle: Ultramafic Pseudotachylytes in the Lanzo Peridotite (Western Alps, NW Italy)

At Mt. Moncuni (Lanzo Massif, Western Alps) plagioclase peridotites and early mid-ocean ridge basalt (MORB) gabbroic dykes are deformed by shear zones containing cataclastic bands and both fault-vein and injection-vein pseudotachylytes, which are crosscut by late MORB porphyritic dykes. Fault-vein pseudotachylytes have thicknesses of the order of 1 mm; injection-vein pseudotachylytes have a typical thickness of 1–10 cm and contain spinifex textures. Structural, petrological and geochemical data show that the pseudotachylytes formed by near-complete melting of the host peridotite, at ambient temperature–pressure conditions (T = 600 ± 100°C, P < 0·5 GPa) close to the brittle–ductile transition of ultramafic rocks, during exhumation of the lithospheric mantle in the early stages of formation of the Ligurian Tethys oceanic basin. Estimates of the average volume fraction of unmelted clasts and of the ambient and liquidus temperature, together with thermophysical parameters, allow the determination of the melting energy per unit volume. Coseismic displacement is not observable at Mt. Moncuni, and consequently the dynamic shear resistance cannot be inferred. We show that commonly proposed relations between fault-vein thickness and displacement are of limited value, given the difficulty in identifying ‘single-event’ pseudotachylytes and the mobility of the melt. However, we also show that dynamic shear resistance can be predicted to decrease sharply if the melt coats the whole fault plane, partly as a consequence of the nonlinear viscosity of silicate melts at high strain rates. The Mt. Moncuni pseudotachylytes are the result of upper mantle seismicity at shallow depth (z < 20 km) over a time period of at most 5 Myr. Estimation of the total seismic energy release and moment (caused by an unspecified number of small to moderate earthquakes) requires an assessment of the total pseudotachylyte volume. This is highly uncertain, with a probable qualitative error margin of ±1 order of magnitude. The inferred values of cumulative seismic energy release and moment are of the order of 1015 ± 1 J and 1019 ± 1 N m, respectively, resulting in a seismic energy release rate of approximately 108 ± 1 J/a. This value is compatible with present-day seismic rates at extensional plate margins.

Numerical Modeling of the Formation and Evolution of Rifted Continental Margins

AbstractOn the basis of the isoviscous corner flow model, we establish an upwelling divergent mantle flow field which acts on the base of the continental lithosphere. This model can explain the deformation and thinning of the continental lithosphere leading to breakup and the initiation of sea‐floor spreading. The numerical modeling results indicate that the lithosphere experiences depth‐dependent stretch which depends on varied factors at different depths, and the surface heat flow significantly increases in response to an upwelling divergent mantle flow. The isostatic adjustment due to the mantle flow‐induced thermal disturbance leads to surface subsidence; meanwhile the uplift of hot material reduces the rheological strength of the lithosphere, especially at the deformation center. The critical depth of brittle deformation reduces and the range of ductile deformation extends greatly. This deformation finally evolves into continental breakup, mantle exhumation, and the formation of sea‐floor spreading and the rifted continental margin under the everlasting effect of the upwelling divergent mantle flow.

Heterogeneous seismic velocity structure of the upper lithosphere at Kane oceanic core complex, Mid‐Atlantic Ridge

The Kane oceanic core complex (OCC) is a large, corrugated megamullion that was formed by a long‐lived detachment fault at the axis of the Mid‐Atlantic Ridge adjacent to Kane Fracture Zone between 2.1 and 3.3 Ma. We use refracted arrivals recorded along a 6‐km‐long hydrophone streamer during a multichannel seismic survey to constrain the shallow seismic velocity structure of the OCC. Results are presented in high‐resolution traveltime seismic tomographic models along six lines that cover all of the main morphological features of the megamullion. The models show large lateral variability in P wave velocity within the upper ∼0.5–2.0 km of the lithosphere, and these variations correlate to first order with observed variations in lithology, documented by in situ basement samples and seafloor morphology. Lithological interpretation of the velocity models indicates that there is marked lateral variability in distribution of gabbroic intrusions, serpentinized peridotites, and basalts at scales of a few kilometers to ∼10 km. Serpentinized peridotites appear to dominate the central and older parts of the OCC. High‐velocity gabbros are consistently (but not exclusively) present closer to the termination of the Kane detachment fault and toward the ends of the OCC. The structure of the lithosphere exhumed by the Kane detachment fault is far from the standard ophiolite‐based Penrose model, and it does not show segment‐centered magmatism that is commonly interpreted at slow spreading ridges. If the gabbros exhumed toward the termination of the OCC were emplaced deep (∼10 km) beneath the spreading axis, they may have constituted a weak zone that focused initiation of the Kane detachment fault. Alternately, as the OCC footwall was being exhumed the gabbros may have been emplaced because of dynamic changes in melt supply, changes in mantle fertility, or decompression melting. Late stage volcanism is clearly associated with a major high‐angle normal fault that cuts the detachment surface; this volcanism may have been stimulated or enhanced by bending stresses in the bending footwall. The shape of the large‐scale corrugated morphology of the OCC is nearly invariant in the dip direction across major changes in basement lithology, indicating that once established, the form of the Kane detachment fault was highly resistant to modification.

Life cycle of oceanic core complexes

Oceanic core complexes are the uplifted footwalls of very-large-offset low-angle normal faults that exhume lower crust and mantle rocks onto the seafloor at slow-spreading ridges. Although it is suggested on the basis of numerical modelling that they form during periods of relatively reduced magma supply, little is known about how they initiate and become inactive, nor why only certain normal fault systems develop into core complexes. In this paper we present results from a near-bottom sidescan sonar/bathymetric profiler survey and sampling study of the Mid-Atlantic Ridge near 13°N that identify the critical controls on oceanic core complex development and evolution. We show that core complex detachment faults initiate as high-angle (65° ± 10°) normal faults no different from surrounding valley-wall faults and, like them, rapidly flatten to dips of ∼ 30° in response to flexural unloading; however, on certain structures slip continues rather than being relayed inward onto a new normal fault. Runaway displacement appears to be triggered primarily by local waning of magma supply below a critical threshold, then aided by strain localisation resulting from seawater penetration and talc formation along the fault zones. Spreading becomes markedly asymmetric when the core complexes are active, and volcanism is suppressed or absent. When the asymmetry is such that the detachments accommodate more than half the total plate separation the active faults migrate across the axial valley. As a consequence magma is emplaced into and captured by the footwall of the detachment fault rather than being injected into the hanging wall, explaining the frequent presence of gabbro bodies and other melt relicts at oceanic core complexes. Core complexes are ultimately terminated when sufficient magma is emplaced to overwhelm the detachment fault; in the 13°N area by neovolcanic ridges propagating laterally across them from magmatically robust segments along strike.

Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the “rolling-hinge” model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.