Magma-poor Rifted Margins Research Articles

Exhumation of ultra-high pressure (UHP) rocks modulated by rifted margin-subduction feedback: Implications for their preservation in old collisional orogens

In continental subduction, rifted margins can be carried to mantle depths (> 90 km) where ultra-high pressure (UHP) metamorphism above coesite stability is attained. Although different exhumation mechanisms for UHP rocks have been discussed, none of them integrate the recent understanding of rifted continental margins. Here, we perform high-resolution thermomechanical numerical experiments to demonstrate that segments of magma-poor rifted margins that reach UHP conditions can efficiently exhume back to shallower levels, while segments of magma-rich rifted margins cannot. This is because the thick layer of rocks with a basaltic composition in magma-rich margins becomes negatively buoyant during metamorphism, preventing their exhumation. This new concept might be pivotal for explaining why exhumed UHP rocks, a key feature of modern-style continental orogens, only appeared and became common late in Earth's history. We suggest that higher mantle potential temperatures and fertility in Earth's early history favored magma-rich rifted margins, making exhumation of UHP crust inefficient. Conditions for magma-poor rifted margins may have arisen during Earth's middle age (1.5–0.8 Ga) due to a colder, more refractory mantle that limited melting and magmatism. We argue that at the end of Neoproterozoic, these colder and positively buoyant magma-poor rifted margins were then subducted and efficiently exhumed to form the large collisional orogens of Gondwana where Earth's oldest coesite-bearing UHP rocks have been unequivocally found. Since then, UHP rocks have become a key and ubiquitous feature of continental orogens.

Serpentinization as a Tape Recorder of (Dis)Continuous Mantle Exhumation along the Alpine Tethys Ocean-Continent-Transition

Abstract Serpentinization has been widely documented and investigated at mid-ocean ridges (MOR) and subduction zones. In contrast, at magma-poor rifted margins serpentinization has received much less attention, despite its importance in controlling rheology and mass fluxes during breakup and establishing of a steady-state MOR. In this study, we present new petrological and geochemical data on subcontinental exhumed serpentinized peridotites from the spectacularly exposed Platta, Tasna and Totalp nappes in the Eastern Central Alps in SE Switzerland, belonging to the Alpine Tethys Ocean Continent Transition (OCT). The results testify of a complex history of fluid–rock interactions recorded by several serpentinization events starting with lizardite mesh and bastite textures (S1), subsequently followed by a succession of serpentine-filling veins with distinct textures and serpentine polysomes that include spherical polyhedral serpentine (S2); chrysotile ± polygonal ± lizardite banded veins (S3); lamellar antigorite veins and patches (S4) and chrysotile crack-seal (S5). The serpentinization sequence differs at proximal (i.e. continentwards) and distal (i.e. oceanwards) domains of the OCT. At proximal domains of the OCT (Upper Platta, Tasna) serpentinites record the complete serpentinization sequence (S1 to S5), whereas at distal domains (Lower Platta) serpentinization is restricted to pseudomorphic mesh and bastite (S1) and chrysotile crack-seal (S5). We attribute this discrepancy to contrasted mechanisms of mantle exhumation along the OCT. While at proximal domains mantle is unroofed along continuous and single large offset detachment faults allowing for the formation of all serpentine generations, mantle exhumation at distal domains is a more discontinuous process, controlled by sequential out-of-sequence detachment and flip-flop faults preventing the full development of all serpentine generations. In this frame, the nature and order of formation of the serpentine polysomes are directly controlled by the conditions of serpentinization (i.e. temperature, mantle composition and fluid/rock ratio). We propose that this new conceptual model can be extrapolated to serpentinization at slow to ultra-slow MORs, where close similarities in the serpentinization sequences have been recently reported.

Extensional fault geometry and evolution within rifted margin hyper-extended continental crust leading to mantle exhumation and allochthon formation

Abstract. Seismic reflection interpretation at magma-poor rifted margins shows that crustal thinning within the hyper-extended domain occurs by in-sequence oceanward extensional faulting which terminates in a sub-horizontal reflector in the topmost mantle immediately beneath tilted crustal fault blocks. This sub-horizontal reflector is interpreted to be a detachment surface that develops sequentially with oceanward in-sequence crustal faulting. We investigate the geometry and evolution of active and inactive extensional faulting due to flexural isostatic rotation during magma-poor margin hyper-extension using a recursive adaptation of the rolling-hinge model of Buck (1988) and compare modelling results with published seismic interpretation. In the case of progressive in-sequence faulting, we show that sub-horizontal reflectors imaged on published seismic reflection profiles can be generated by the flexural isostatic rotation of faults with initially high-angle geometry. Our modelling supports the hypothesis of Lymer et al. (2019) that the S reflector on the Galician margin is a sub-horizontal detachment generated by the in-sequence incremental addition of the isostatically rotated soles of block-bounding extensional faults. Flexural isostatic rotation produces shallowing of emergent fault angles, fault locking, and the development of new high-angle shortcut fault segments within the hanging wall. This results in the transfer and isostatic rotation of triangular pieces of hanging wall onto exhumed fault footwall, forming extensional allochthons which our modelling predicts are typically limited to a few kilometres in lateral extent and thickness. The initial geometry of basement extensional faults is a long-standing question. Our modelling results show that a sequence of extensional listric or planar faults with otherwise identical tectonic parameters produce very similar seabed bathymetric relief but distinct Moho and allochthon shapes. Our preferred interpretation of our modelling results and seismic observations is that faults are initially planar in geometry but are isostatically rotated and coalesce at depth to form the seismically observed sub-horizontal detachment in the topmost mantle. In-sequence extensional faulting of hyper-extended continental crust results in a smooth bathymetric transition from thinned continental crust to exhumed mantle. In contrast, out-of-sequence faulting results in a transition to exhumed mantle with bathymetric relief.

Inverted Magma-rich Versus Magma-poor Rifted Margins: Implications for Early Orogenic Systems

Mountain belts are often the result of former inverted rifts or rifted margins. Up to date, relics of magma-poor rifted margins have been found in orogens allowing the investigation of how these margins reactivate and control the formation of mountain belts. In contrast, magma-rich rifted margins have barely been recognized within orogenic systems, and consequently how these margins reactivate and control subsequent orogenesis is poorly understood. We use a thermo-mechanical model to investigate the mechanical behaviour of reactivated magma-rich versus magma-poor rifted margins during early orogenesis (i.e. margin inversion). Input data for our modelling experiments are obtained from two natural laboratories. One is the Demerara Plateau characterized by a mildly shortened magma-rich rifted margin. Its décollement level is observed at the top of the syn-kinematic volcanics, which propagates downwards into a frozen incipient subduction. The other study-case is the Basque-Cantabrian Belt consisting of a reactivated magma-poor hyper-extended rift system with the décollement level at the bottom of the syn-rift sediments. Our modelling results, for inverted magma-rich rifted margin, show that the syn-kinematic volcanics undergo subduction as they are mechanically coupled with the underlying rifted lithosphere. Hence, only post-rift sediments are accreted within the early orogenic accretionary wedge. In contrast, both the syn- and post-rift sediments from a reactivated magma-poor rifted margin are expected to be preserved within the accretionary wedge. Therefore, we conclude that the presence or absence of syn-rift sediments within an accretionary prism may represent a robust indicator to determine the nature of reactivated rifted margins within orogens. We believe that our results may strongly contribute to recognize, the so far poorly identified, magma-rich rifted margins within present-day orogenic systems.

Mantle serpentinization and associated hydrogen flux at North Atlantic magma-poor rifted margins

AbstractMantle serpentinization influences the rheology of altered peridotites and the global fluxes of energy and volatiles, the generation of seafloor and sub-seafloor chemolithotrophic life, and the carbon cycle. As a by-product of serpentinization, molecular hydrogen (H2) is generated, which supports chemosynthetic communities, and this mechanism may have driven the origin of life on early Earth. At continent-ocean transition zones (COTs) of magma-poor rifted margins, the mantle is exposed and hydrated over hundreds of kilometers across the rift, but the H2 fluxes associated with this process are poorly known. Here, we coupled a thermomechanical model with serpentinization reaction equations to estimate associated H2 release during mantle exhumation at COTs. This reproduced a tectonic structure similar to that of the West Iberia margin, one of the best-studied magma-poor margins. We estimated the rate of H2 production from mantle hydration at (7.5 ± 2.5) × 107 mol/(yr × km). By estimating the area of exhumed mantle from wide-angle seismic profiles at North Atlantic magma-poor margins, we calculated that the accumulated H2 production could have been as high as ~4.3 × 1018 mol (~8.6 × 1012 metric tons) prior to opening of the North Atlantic Ocean, at a rate of ~1.4 × 1017 mol/m.y. This is one quarter of the total predicted flux produced by the global system of mid-ocean ridges, thus highlighting the significance of H2 generation at magma-poor margins in global H2 fluxes, to hydrogenothropic microbial life, and, perhaps, as a potential energy source.

Geophysical evidence for lithospheric scale asymmetry and inherited mantle in the SE Brazilian-Angola and Newfoundland-Iberia rifted margins

Rift systems in the central South Atlantic and the southern Northern Atlantic margins exhibit anomalous wide ocean-continent transitions, extreme thinning of the continental crust, and scarce volcanism indicative of low magmatic budgets during rifting and breakup, characteristic of magma-poor rifted margins. While numerous studies investigated the crustal architecture at these magma-poor rifted margins, the nature of the underlying mantle lithosphere remains virtually unknown. In this work, we have interpreted seismic tomography and electromagnetic data with the aim of comparing large-scale aspects and mantle geophysical anomalies at these conjugate rifted margins. The data exhibits low electrical conductivity and high P-wave velocity in the mantle underlying the Iberia and the Angolan margins, while the Newfoundland and SE-Brazil margins show a lack of high-velocity mantle and exhibit higher electrical conductivity. The P-wave tomography values at middle lithospheric depths are suggestive of an oceanward lateral continuity of rigid lithospheric mantle at both the Iberian and Angola margins that may correspond to an anomalous mantle. Rifting-induced delamination at the onset of Atlantic opening is supported by previous geochemical studies. In addition, low electrical conductivity areas may indicate dry mantle conditions, which can be caused by inherited subcontinental mantle, and/or pre-to syn-rift magmatic extraction. A lack of high-velocity and high electrical conductivity in the mantle under the Newfoundland and Santos-Campos margins suggest a relatively weaker and newly formed mantle, in agreement with the removing and/or initial absence of a strong middle-to-lower inherited lithosphere beneath those margins. Therefore, the geophysical observations support different types of the nature of the middle mantle lithospheres flooring the two conjugate, magma-poor rifted margins.

Mantle exhumation at magma-poor rifted margins controlled by frictional shear zones

The transition zone from continental crust to the mature mid-ocean ridge spreading center of the Iberia-Newfoundland magma-poor rifted margins is mostly composed of exhumed mantle characterized by highs and domes with varying elevation, spacing and shape. The mechanism controlling strain localization and fault migration explaining the geometry of these peridotite ridges is poorly understood. Here we show using forward geodynamic models that multiple out-of-sequence detachments with recurring dip reversal form during magma-poor rifting and mantle exhumation as a consequence of the strength competition between weak frictional-plastic shear zones and the thermally weakened necking domain beneath the exhuming footwall explaining geometry of these peridotite ridges. Model behaviour also shows that fault types and detachment styles vary with spreading rate and fault strength and confirm that these results can be compared to other magma poor passive margins such as along Antarctica-Australia and to ultra-slow mid-ocean spreading systems as the South-West Indian Ridge.

Serpentinization-Driven H2 Production From Continental Break-Up to Mid-Ocean Ridge Spreading: Unexpected High Rates at the West Iberia Margin

Molecular hydrogen (H2) released during serpentinization of mantle rocks is one of the main fuels for chemosynthetic life. Processes of H2 production at slow-spreading mid-ocean ridges (MORs) have received much attention in the past. Less well understood is serpentinization at passive continental margins where different rock types are involved (lherzolite instead of harzburgite/dunite at MORs) and the alteration temperatures tend to be lower (<200°C vs. >200°C). To help closing this knowledge gap we investigated drill core samples from the West Iberia margin. Lherzolitic compositions and spinel geochemistry indicate that the exhumed peridotites resemble sub-continental lithospheric mantle. The rocks are strongly serpentinized, mainly consist of serpentine with little magnetite, and are generally brucite-free. Serpentine can be uncommonly Fe-rich, with XMg = Mg/(Mg + Fe) < 0.8, and shows distinct compositional trends toward a cronstedtite endmember. Bulk rock and silicate fraction Fe(III)/∑Fe ratios are 0.6–0.92 and 0.58–0.8, respectively; our data show that 2/3 of the ferric Fe is accounted for by Fe(III)-serpentine. Mass balance and thermodynamic calculations suggest that the sample’s initial serpentinization produced ∼120 to >300 mmol H2 per kg rock. The cold, late-stage weathering of the serpentinites at the seafloor caused additional H2 formation. These results suggest that the H2 generation potential evolves during the transition from continental break-up to ultraslow and, eventually, slow MOR spreading. Metamorphic phase assemblages systematically vary between these settings, which has consequences for H2 yields during serpentinization. At magma-poor rifted margins and ultraslow-spreading MORs, serpentine hosts most Fe(III). Hydrogen yields of 120 to >300 mmol and 50–150 mmol H2 per kg rock, respectively, may be expected at temperatures of <200°C. At slow-spreading MORs, in contrast, serpentinization may produce 200–350 mmol H2, most of which is related to magnetite formation at >200°C. Since, in comparison to slow-spreading MORs, geothermal gradients at magma-poor margins and ultraslow-spreading MORs are lower, larger volumes of low-temperature serpentinite should form in these settings. Serpentinization of lherzolitic rocks at magma-poor margins should produce particularly high amounts of H2 under conditions within the habitable zone. Magma-poor margins may hence be more relevant environments for hydrogenotrophic microbial life than previously thought.

Widespread hydrothermal vents and associated volcanism record prolonged Cenozoic magmatism in the South China Sea

Abstract The continental margin of the northern South China Sea is considered to be a magma-poor rifted margin. This work uses new seismic, bathymetric, gravity, and magnetic data to reveal how extensively magmatic processes have reshaped the latter continental margin. Widespread hydrothermal vent complexes and magmatic edifices such as volcanoes, igneous sills, lava flows, and associated domes are confirmed in the broader area of the northern South China Sea. Newly identified hydrothermal vents have crater- and mound-shaped surface expressions, and occur chiefly above igneous sills and volcanic edifices. Detailed stratigraphic analyses of volcanoes and hydrothermal vents suggest that magmatic activity took place in discrete phases between the early Miocene and the Quaternary. Importantly, the occurrence of hydrothermal vents close to the present seafloor, when accompanied by shallow igneous sills, suggest that fluid seepage is still active, well after main phases of volcanism previously documented in the literature. After combining geophysical and geochemical data, this study postulates that the extensive post-rift magmatism in the northern South China Sea is linked to the effect of a mantle plume over a long time interval. We propose that prolonged magmatism resulted in contact metamorphism in carbon-rich sediments, producing large amounts of hydrothermal fluid along the northern South China Sea. Similar processes are expected in parts of magma-poor margins in association with CO2/CH4 and heat flow release into sea water and underlying strata.

Organic matter in an Ocean Continent Transitions of present-day magma-poor rifted margins: the examples of Iberia and Newfoundland

Organic matter in an Ocean Continent Transitions of present-day magma-poor rifted margins: the examples of Iberia and Newfoundland

The nature of the interface between basalts and serpentinized mantle in oceanic domains: Insights from a geological section in the Alps

The association of mafic extrusive rocks and serpentinized mantle rocks is a common feature encountered in numerous ultra-distal magma-poor rifted margins as well as in (ultra)slow-spreading oceanic settings where detachment faulting and mantle exhumation occur. Although seismic imaging and high-resolution TOBI radar allow imaging this association, the nature of this interface is poorly understood (e.g. nonconformity vs. tectonic channelized vs. diffuse fluid flow along the interface). The timing between magmatism and active detachment faulting is often poorly constrained and it remains difficult to decipher if mafic extrusives are pre-, syn- or post-exhumation of mantle rocks at the seafloor. In this study, we characterize the nature of this interface from a section of a Jurassic ultra-distal margin preserved in the Platta nappe, SE Swiss Alps. We show that the basalt-serpentinized mantle contact does neither correspond to an Alpine thrust system, nor to a nonconformity or a detachment plane exhuming mantle rocks from underneath already emplaced basalts. Rather, the basalt-serpentinite interface corresponds to a weak decoupling level formed by conjugate low-angle normal faults onto which conjugate high-angle normal faults branch. These structures are diagnostic of co-axial extensional tectonics occurring during and after emplacement of basalts over a formerly exhumed serpentinized mantle. Carbonate extensional and shear veins and foliated ophicalcites formed during this Jurassic extensional event. Their distribution across and along the basalt-serpentinite interface demonstrates that this decoupling interface was a high-permeability layer that channelized high fluid flow responsible for the widespread hydrothermal alteration. A positive feedback between fluid flow and extensional deformation is proposed.

Unravelling the architecture and evolution of the inverted multi-stage North Iberian-Bay of Biscay rift

Current rift models propose polyphase rift evolutions that develop with the same kinematic framework and accommodate continuous and sequential extension, through the stacking of deformation modes. Several rifted margins show, however, evidence for multiple and out of sequence rift events, which developed within different kinematic frameworks, with a complex spatial and temporal evolution. In this work, we address the problem of multi-stage rifting based on the study of the central North Iberian margin, located at the southern Bay of Biscay triangular oceanic domain. This magma-poor rifted margin recorded three major Mesozoic rift events and a subsequent Alpine compressional reactivation, representing a unique setting to study the architecture of multi-stage rift systems and the exerted control on subsequent reactivation. Relying on a dense set of 2D seismic reflection profiles, boreholes, and published velocity models, we map and describe structural domains and major extensional and compressional structures, and we construct depth and thickness maps of syn-rift units. These new maps display the geometry and spatial distribution of major rift basins and bounding structures. Distinctive scenarios resulting from this tectono-stratigraphic approach led us to define three rift systems. A first, diffuse and widespread Triassic system, with classical fault-bounded half-graben basins; a second, narrow, deep and localised Late Jurassic to Barremian transtensional system, including laterally confined pull-apart basins; and a third, widely distributed, Aptian to Cenomanian hyperextended system. Our results show that each rift system controlled the successive extensional events and the subsequent contractional deformation, resulting in a complex 3D template. Deciphering the tectono-stratigraphic architecture of such, multi-stage systems provide key insights on the spatial, temporal, and thermal evolution of divergent plate boundaries. It is likewise indispensable to propose and test kinematic plate deformable models on conjugate rifted margins and to comprehend the implications for their reactivation.

Reply to Discussion on ‘Breakup continents at magma poor rifted margins: a seismic v. outcrop perspective’. Journal of the Geological Society, London , 175, 875-882

Dr. Giancarlo Molli constructively replied to our paper ‘Breakup continents at magma poor rifted margins: a seismic v. outcrop perspective’ (Molli 2020) with a twofold argument (i) criticizing the collection of data, and, more importantly, (ii) questioning our discussion on the syn-tectonic behavior of basalts in the study area. 1. Regarding the first point, G. Molli produced a new, high quality interpretative map and a set of retro-deformed sections of the Levanto-Bonassola area to support the idea that Alpine deformation was underestimated by our work. At least three high-quality, detailed maps of the area were published (Decandia and Elter 1969; Barrett 1982; Cortesogno et al. 1987) and, in addition, there are a number of maps produced in the frame of PhD thesis (among which Molli 1995) that are, at present, not yet published. The fact that none of these maps are perfectly identical one to each other does not surprise any field geologist, as each geological map mainly derives from the balance of missing data (cover) and outcrops, especially in highly-anthropized areas, as is the case for the Levanto-Bonassola area. For example, if the maps of Cortesogno et al. (1987) and Decandia and Elter (1969) are compared for the area north of the Bonassola village, it can be seen that different solutions were proposed introducing or not prominent Alpine structures. This example is striking, in particular if one considers the period in which these distinguished geologists worked, because it shows how different maps gave priority to specific primary or late features that were often functional to the proposed framework. For what concerns our work, we wish to stress the concept that the map in Decarlis et al. (2018) was functional to a broader discussion and we therefore highlighted primary structures. The choice of prioritizing some observations (nature of contacts …

Discussion on ‘Breaking up continents at magma-poor rifted margins: a seismic v. outcrop perspective’ Journal of the Geological Society, London , 175, 875–882

The contribution of Decarlis et al. (2018) focuses on the breakup of continents at magma-poor rifted margins, a topic which, as claimed by the authors, ‘is a complex yet little understood process accounting for intricate interactions between tectonic and magmatic processes’. The authors use field observations of a classical Ligurian ophiolites exposure, the Bracco–Levanto, to compare it with the setting of the East Antarctica margin analysed by seismic survey and interpretations (Geoscience Australia Survey 228). The authors state that the overall aim is to combine detailed structural mapping and petrological data from the fossil exhumed example (Bracco–Levanto) with architectural features observed in seismic sections from present-day ocean–continent transitions (OCTs). Moreover, as explicitly mentioned in the title, the authors aim at discussing the implications of their analysis for the interpretation of the nature of seismic interfaces and of the character of rocks at ultra-distal margins. It also helps to constrain the timing and type of processes controlling lithospheric breakup and onset of steady-state, localized seafloor spreading. Actually, the 3D complexity of the tectonic architecture of the Ligurian ophiolites, if not properly taken into account, can cause uncertainty in the analyses and projected goals; therefore, in the following pages I will complement and refine the geological information provided by Decarlis et al. (2018), with the purpose of helping future works on the subject. Moreover, my contribution aims to complete the references on the study area, providing further structural information and details and amending some misinterpretations included in the paper by Decarlis et al. (2018). As recognized by Decarlis et al. (2018), the Bracco–Levanto area represents a key region for the studies of the Ligurian Tethys ophiolites, having a special value in the history of modern geological knowledge. In the early 1960s Bailey & McCallien (1960) analysed the local geology …

Polyphase tectono-magmatic evolution during mantle exhumation in an ultra-distal, magma-poor rift domain: example of the fossil Platta ophiolite, SE Switzerland

Despite the fact that many studies have investigated mantle exhumation at ultra-slow-spreading ridges and magma-poor rifted margins, there are still numerous questions concerning the 3D architecture, magmatic, fluid, and thermal evolution of these domains that remain unexplained. Indeed, it has been observed in seismic data from ultra-distal magma-poor rifted margins that top basement is heavily structured and complex; however, the associated morpho-tectonic and magmatic processes remain ill constrained. The aim of this study is to describe the 3D top basement morphology, timing, and processes controlling the formation of an exhumed mantle domain preserved over about 200 km2 in the Platta nappe in SE Switzerland. Detailed mapping of parts of the Platta nappe enabled to document the top basement architecture of an exhumed mantle domain, and to investigate its link to later, rift/oceanic structures, magmatic additions, and hydrothermal fluid systems. Our observations show: (1) a polyphase deformation history associated with mantle exhumation along exhumation faults overprinted by later high-angle normal faults, (2) a structured top basement morphology capped by magmato-sedimentary sequences, (3) a tectono-magmatic evolution that includes gabbros, emplaced at deeper levels and subsequently exhumed and overlain by younger extrusive magmatic additions, and (4) fluid systems related to serpentinization, calcification, hydrothermal vents, rodingitization, and spilitization affecting exhumed mantle and associated magmatic rocks. The overall observations provide new information on the temporal and spatial evolution of the tectonic and magmatic processes and their link to hydrothermal and sedimentary systems controlling the formation of ultra-distal, magma-poor rifted margins, and lithospheric breakup.

Seismic characteristics and evolution of post-rift igneous complexes and hydrothermal vents in the Lingshui sag (Qiongdongnan basin), northwestern South China Sea

The study of morphology, distribution, and characteristics of igneous complexes has great significance to the understanding of magma plumbing processes, geodynamics, and tectonic evolution of continental margins. Previous studies concentrated partly on the magma-rich rifted basins, where the lateral magma transport mainly affects the igneous complexes' connection and distribution. However, due to seismic wave shielding effects of the large shallow magmatic bodies, the underlying igneous complexes and their corresponding magma plumbing systems in the magma-poor rifted margins are still in debate. In this study, 2D/3D seismic data and well data are utilized to describe the morphology, architecture, and spatial-temporal distribution of igneous complexes in the Lingshui sag of the Qiongdongnan basin, northwestern South China Sea margin. The identified igneous complexes include 98 intrusive sills and feeder dykes beneath some of the isolated sills. Twenty-six cone-shaped mounds that overlie intruded sills through internal disturbed conduits were also described. Drilled well samples and seismic expressions suggest that these mounds are hydrothermal vents. A uniform Bottom Mounds Horizon of these vents suggests that they probably formed at the same time. Constrained by biostratigraphic data and sedimentation rate of underlying and overlying sedimentary layers, the magma emplacement was dated to the middle Miocene (ca. 14.6 Ma). Most of the hydrothermal vents are distributed along the F2 fault zone and have direct linkage with the underlying sills, while the large sill complexes that are connected with limited vents are mainly present above the hyperextended continental crust, where the crust thins to 6–10 km. The sills intruded into different layers, from the lower Oligocene to the lower Miocene and the emplaced depth of sills is 1.2–6.3 km, whether or not they feed any vents above. Unlike most of the large volume and laterally linked sills found in the magma-rich rifted margins, the scattered distribution of sills at different levels indicates that dykes probably play an important role in magma transport, which might coexist with numerous polygonal or small faults and interference reflections. This work highlights the critical role of basin structures in controlling the distribution of post-rift igneous complexes in magma-poor margins, including thinned continental crust, sedimentary thickness, and faults.

Nature of the crust in the northern Gulf of California and Salton Trough

AbstractIn the southern Gulf of California, the generation of new oceanic crust has resulted in linear magnetic anomalies and seafloor bathymetry that are characteristic of active seafloor-spreading systems. In the northern Gulf of California and the onshore (southeastern California, USA) Salton Trough region, a thick sedimentary package overlies the crystalline crust, masking its nature, and linear magnetic anomalies are absent. We use potential-field data and a geotherm analysis to constrain the composition of the crust (oceanic or continental) and develop a conceptual model for rifting. Gravity anomalies in the northern Gulf of California and Salton Trough are best fit with crustal densities that correspond to continental crust, and the fit is not as good if densities representative of mafic rocks, i.e., oceanic crust or mafic underplating, are assumed. Because extensive mafic underplated bodies would produce gravity anomalies that are not in agreement with observed gravity data, we propose, following earlier work, that the anomalies might be due to serpentinized peridotite bodies such as found at magma-poor rifted margins. The density and seismic velocities of such serpentinized peridotite bodies are in agreement with observed gravity and seismic velocities. Our conceptual model for the Salton Trough and northern Gulf of California shows that net crustal thinning here is limited because new crust is formed rapidly from sediment deposition. As a result, continental breakup may be delayed.

The syn-rift stratigraphic record across a fossil hyper-extended rifted margin: the example of the northwestern Adriatic margin exposed in the Central Alps

To understand the overall spatial and temporal evolution and resulting sedimentary stacking patterns within an evolving hyper-extended rift system, we investigated the Austroalpine and Upper Penninic nappes in the Central Alps. These nappes preserve pristine remnants of the northwestern Adriatic rifted margin showing the transition from stretched and hyper-extended continental crust to exhumed mantle of a sediment-starved, magma-poor rifted margin. Our paper reviews sedimentological and structural data that enable us to determine a general chrono-tectono-stratigraphic framework for the syn-rift succession of the margin. The detailed study of the facies distribution within the different rift domains, including proximal, necking, hyper-extended, and exhumed mantle domains, allow us to identify syn- and post-tectonic sedimentary packages. The correlation of these sedimentary packages between the rift domains is based on the identification of basin-wide correlative stratigraphic intervals linked to global or basin-wide Jurassic events: (1) the Early Toarcian Oceanic Anoxic Event; (2) an Early Bajocian bio-siliceous event; and (3) the onset of Tithonian carbonate-dominated sedimentation. Using these timelines, we could recognize the oceanward propagation of the syn-tectonic package from the proximal to the exhumed mantle domains as a function of time. A key observation is that syn-tectonic packages can be compartmentalized into four tectonic system tracts across the margin. Each system tract and their associated bounding surfaces record the stratigraphic evolution and change in deformation mode related to distinct and successive syn-rift phases that correspond to the stretching (~ 10My), necking (~ 6My), hyper-extension (~ 15My), and mantle exhumation (~ 18My) phases.

Long-lived mega fault-scarps and related breccias at distal rifted margins: insights from present-day and fossil analogues

Thinning of the continental crust during rifting is accommodated by a number of major faults, only a few of which produce long-lived mega fault-scarps. In this paper, we investigate mega fault-scarps and the sedimentary system located at their toe across magma-poor rifted margins. Our approach combines observations from subsurface examples along present-day margins and field analysis of fossil examples exposed in the Alpine Tethys margins. Whereas present-day examples of rifted margins imaged by seismic techniques provide details about the architecture of the top basement and the relationship between faults and sediments, outcrops give access to the sedimentary features of rocks related to mega fault-scarps. Our study shows that (1) mega fault-scarps are preferentially located at rift domain boundaries, implying a topographic escarpment that juxtaposes rift domains of different crustal thickness, and (2) mega fault-scarps are long-lived local sources for syn- and post-tectonic breccia, respectively produced during and after fault activity.

Evaluating magmatic additions at a magma-poor rifted margin: An East Indian case study

Evaluating magmatic additions at a magma-poor rifted margin: An East Indian case study