Passive Margin Formation Research Articles

Neoproterozoic passive margin formation and evolution during the Rodinia–Gondwana supercontinent cycle at the eastern margin of the West African Craton

Abstract Petrographical and geochemical data from the Togo structural unit (TSU), also referred to as the Atacora structural unit, are presented together with the existing dataset; geochemical and age data from the sedimentary and metasedimentary rocks from the passive margin sequences of the Dahomeyide belt in Ghana to infer their provenance and depositional setting and expand the discussion on the Rodina–Gondwana supercontinent assembly during the Pan-African orogeny. The metasedimentary rocks of the TSU are quartzites and phyllites. The framework grains of the quartzites consisting dominantly of quartz and small amounts of feldspar grains and relict lithic fragments classify them as quartz arenite, subarkose and sublitharenite. Generally, the studied rocks show similar rare-earth element and multi-element patterns, which imply derivation from similar sources. Elemental ratios, including (La/Lu)N, Th/Sc and La/Sc, suggest sediments sourced from intermediate to felsic rocks. Provenance and depositional setting indicators of the TSU suggest deposition in a passive margin setting, with the West African and Amazonian cratons’ granitoids and granitic gneisses as possible provenance, akin to siliciclastic rocks of the Buem structural unit and the Voltaian Supergroup of the Volta Basin. The deformational history of the TSU is similar to those of the Buem structural unit and the eastern margin of the Voltaian Supergroup, indicating the effect of the Pan-African orogeny on the passive margin of the Dahomeyide belt. We, therefore, propose the formation and evolution of a Neoproterozoic passive margin unit, which was tectonically deformed during the Rodinia–Gondwana supercontinent cycle.

Seismically constrained gravity inversions reveal magmatic and metamorphic processes at a major lithospheric boundary in northwestern Australia

Magmatic and metamorphic processes modifying the lower crust and uppermost mantle are key to understanding continental growth and evolution. However, our ability to identify and quantify such processes is challenged by inaccessibility, data sparsity, and uncertainties in defining properties at depth. As a result, knowledge of composition, metamorphic grade, and the nature of the continental crust-mantle transition is often limited. Here, we model the structure and properties of the crust and uppermost mantle between the West and North Australian Cratons to infer tectonic processes at this major lithospheric boundary. Seismically constrained 3D gravity inversions were used to estimate crustal and mantle densities and Moho depths. The application of isostatic principles and the joint interpretation of gravity, active and passive seismic models, well data, and outcrop data are used to identify and characterize anomalous masses that are otherwise unresolvable using supracrustal datasets. The results suggest a thick, dense crust with a mafic bulk composition, a high-density, reflective lower crust, and a low-density uppermost mantle beneath the southern Canning Basin. We interpret these crustal and mantle features as reflecting mafic mantle-derived magmatism and crustal anatexis related to Proterozoic-Cambrian Large Igneous Provinces. In contrast, the Proterozoic Paterson Orogen exhibits a thick, dense, high-grade metamorphic bulk crust with a felsic to intermediate composition, a high-density, low-reflectivity lower crust, and low-density uppermost mantle, inferred to result from convergent tectonics. The northern Canning Basin displays a thinner, less dense crust with an intermediate to mafic composition, and mafic, high-density, reflective lower crust, associated with mafic magma emplacement during rifting and passive margin formation. These variations in crustal structure reveal a complex interplay of Proterozoic terrane amalgamation to the cratonic margin, repeated intraplate high-volume mafic magmatic events, and rifting of the offshore basin, which have collectively shaped the deep crust and uppermost mantle in this region.

The Middle Jurassic Cheshmeh-Ghasaban High-Mg gabbro (Hamedan, NW Iran): A plume-influenced intracontinental rifting event?

The Jurassic mafic to felsic magmatism affecting the older Ediacaran-to-Cambrian basement of the Sanandaj-Sirjan Zone of Iran has been traditionally interpreted as the product of arc and/or back-arc magmatism related to the early stages of Neo-Tethys subduction beneath Iran in the early Jurassic. Recent works and new compositional and geochronological data have started challenging this commonly accepted model in favor of scenarios involving continental rifting, mantle plume activity, and/or passive margin formation. In the Hamedan area of the central sector of the Sanandaj-Sirjan Zone, the Jurassic Cheshmeh-Ghasaban gabbro (ca. 165 Ma) is a key formation to better understand the tectono-magmatic framework of the whole area. Our new data, combined with the existing literature, suggest a transitional to alkaline OIB-like compositional character for this gabbro similar to the nearby but slightly younger (ca. 145 Ma) Panjeh and Ghalaylan basaltic complexes (in the Songhor-Ghorveh area). When integrated with the existing geochemical data of Jurassic mafic rocks from the central Sanandaj-Sirjan Zone, our results point to a scenario of intracontinental rifting, possibly involving the upwelling old metasomatized (by Proto-Tethys subduction?) mantle or mantle-plume activity.

Thermochronometry constraints on south West Greenland passive continental margin development

Passive continental margins (PCMs) represent the interface between the marine and terrestrial realms. However, topographic evolution of PCMs is often difficult to decipher due to paucity of the preserved geological record. Here, we report uranium-thorium-helium ((U-Th)/He) analysis of the Precambrian crystalline basement from southern West Greenland that help constrain the process of rifting between Greenland and North America and contributes to the debate about the West Greenland PCM development. The majority of zircon (U-Th)/He dates (220-580 Ma) imply several kilometres of burial of the basement by Paleozoic (and potentially Mesozoic) sediments. Apatite (U-Th)/He dates (80-230 Ma) record thermal processes associated with extensional tectonism starting in the Late Triassic and passive margin formation in the Early Cretaceous. Our data provide no evidence of thermal activity during Cenozoic times, suggesting that the thermal effects of Paleogene rifting and break-up were negligible and the magnitude of Cenozoic erosion was <3.5 km in the study area.

Kinematic Boundary Conditions Favouring Subduction Initiation at Passive Margins Over Subduction at Mid-oceanic Ridges

We perform numerical modelling to simulate the shortening of an oceanic basin and the adjacent continental margins in order to discuss the relationship between compressional stresses acting on the lithosphere and the time dependent strength of the mid-oceanic ridges within the frame of subduction initiation. We focus on the role of stress regulating mechanisms by testing the stress–strain-rate response to convergence rate, and the thermo-tectonic age of oceanic and continental lithospheres. We find that, upon compression, subduction initiation at passive margin is favoured for thermally thin (Palaeozoic or younger) continental lithospheres (&amp;lt;160 km) over cratons (&amp;gt;180 km), and for oceanic basins younger than 60 Myr (after rifting). The results also highlight the importance of convergence rate that controls stress distribution and magnitudes in the oceanic lithosphere. Slow convergence (&amp;lt;0.9 cm/yr) favours strengthening of the ridge and build-up of stress at the ocean-continent transition allowing for subduction initiation at passive margins over subduction at mid-oceanic ridges. The results allow for identifying geodynamic processes that fit conditions for subduction nucleation at passive margins, which is relevant for the unique case of the Alps. We speculate that the slow Africa–Europe convergence between 130 and 85 Ma contributes to the strengthening of the mid-oceanic ridge, leading to subduction initiation at passive margin 60–70 Myr after rifting and passive margin formation.

Analogue modelling of marginal flexure in Afar, East Africa: Implications for passive margin formation

The Afar region in East Africa is a key locality for studying continental break-up. Within Afar, passive margins are developing, of which the Southern Afar Margin (SAM) contains synthetic (basinward) faulting, whereas crustal flexure, antithetic faulting and marginal grabens occur along the Western Afar Margin (WAM). Numerous conflicting scenarios for the evolution of the WAM exist. In this analogue modelling study we test various factors that may affect the development of a WAM-style passive margin: brittle crustal thickness, (en echelon) rheological contrasts, sedimentation and oblique extension.Our experimental results illustrate how marginal flexure due to a weak lower crust below Afar can elegantly account for the structural features of the WAM. Brittle crustal thickness controls what structures occur: a thinner brittle crust accommodates flexure internally, whereas increasing brittle thicknesses lead to faulting. Large escarpment faults develop early on, followed by late-stage antithetic faulting and marginal grabens. A thicker brittle crust also causes enhanced subsidence, and increased strength contrasts between lower crustal domains leads to more localized deformation. Basin-wide sedimentation causes enhanced subsidence, as well as longer activity along large synthetic (escarpment) faults. Finally, oblique extension clearly prevents the development of marginal grabens, which only form in near-orthogonal extension.These results support a tectonic scenario involving initial oblique extension due to Arabian plate motion, creating echelon synthetic escarpment faults along the WAM. After the Danakil Block started its independent rotation, near-orthogonal extension conditions were established, allowing (enhanced) marginal flexure, antithetic faulting and marginal graben formation along the older en echelon escarpment. Differences in extension obliquity may also explain the differences in structural architectures between the WAM and SAM. The characteristics of the WAM are typical of magma-rich passive margins, and the margin has great potential for studying continental break-up and (magma-rich) passive margin formation.

Microcontinents and Continental Fragments Associated With Subduction Systems

AbstractMicrocontinents and continental fragments are small pieces of continental crust that are surrounded by oceanic lithosphere. Although classically associated with passive margin formation, here we present several preserved microcontinents and continental fragments associated with subduction systems. They are located in the Coral Sea, South China Sea, central Mediterranean and Scotia Sea regions, and a “proto‐microcontinent,” in the Gulf of California. Reviewing the tectonic history of each region and interpreting a variety of geophysical data allows us to identify parameters controlling the formation of microcontinents and continental fragments in subduction settings. All these tectonic blocks experienced long, complex tectonic histories with an important role for developing inherited structures. They tend to form in back‐arc locations and separate from their parent continent by oblique or rotational kinematics. The separated continental pieces and associated marginal basins are generally small and their formation is quick (&lt;50 Myr). Microcontinents and continental fragments formed close to large continental masses tend to form faster than those created in systems bordered by large oceanic plates. A common triggering mechanism for their formation is difficult to identify, but seems to be linked with rapid changes of complex subduction dynamics. The young ages of all contemporary pieces found in situ suggest that microcontinents and continental fragments in these settings are short lived. Although presently the amount of in‐situ subduction‐related microcontinents is meager (an area of 0.56% and 0.28% of global, non‐cratonic, continental crustal area and crustal volume, respectively), through time microcontinents contributed to terrane amalgamation and larger continent formation.

Structural Analysis of the Western Afar Margin, East Africa: Evidence for Multiphase Rotational Rifting

AbstractThe Afar region in East Africa represents a key location to study continental breakup. We present an integrated structural analysis of the Western Afar Margin (WAM) aiming to better understand rifted margin development and the role of plate rotation during rifting. New structural information from remote sensing, fieldwork, and earthquake data sets reveals that the N‐S striking WAM is still actively deforming and is characterized by NNW‐SSE normal faulting as well as a series of marginal grabens. Seismicity distribution analysis and the first‐ever borehole‐calibrated sections of this developing passive margin show recent slip concentrated along antithetic faults. Tectonic stress parameters derived from earthquake focal mechanisms reveal different extension directions along the WAM (82°N), in Afar (66°N) and in the Main Ethiopian Rift (108°N). Fault slip analysis along the WAM yields the same extension direction. Combined with GPS data, this shows that current tectonics in Afar is dominated by the local rotation of the Danakil Block, considered to have occurred since 11 Ma. Earlier stages of Afar development (since 31–25 Ma) were most likely related to the large‐scale rotation of the Arabian plate. Various authors have proposed scenarios for the evolution of the WAM. Any complete model should consider, among other factors, the multiphase tectonic history and antithetic fault activity of the margin. The findings of this study are not only relevant for a better understanding of the WAM but also provide insights into the role of multiphase rotational extension during rifting and passive margin formation in general.

Cretaceous intracontinental rifting at the southern Chatham Rise margin and initialisation of seafloor spreading between Zealandia and Antarctica

Passive continental margins are commonly classified as magma-poor and magma-rich types. Related breakup processes are often associated with far-field tectonic stresses or upwelling mantle plumes. The Chatham Rise east off New Zealand records a sequence of Late Cretaceous tectonic events, which include subduction and collision of the oceanic Hikurangi Plateau to subsequent continental rifting and breakup. The mechanisms triggering the change in tectonic forces are poorly understood but address open questions regarding the formation of passive margins. We acquired wide-angle seismic reflection/refraction, multi-channel seismic and potential field data along three profiles crossing the southern Chatham Rise margin and SE Chatham Terrace to the oceanic crust in order to image and understand the crustal structure and breakup mechanisms. Variations in crustal thickness along the highly faulted Chatham Rise are most likely related to the collision with the Hikurangi Plateau. Our data indicate that the SE Chatham Terrace represents a broad continent-ocean transition zone (COTZ), which we interpret to consist of very thin continental crust affected by magmatic activity. Along the southern Chatham Rise margin, features of both, magma-poor and magma-rich rifted margins are present. We suggest that passive rifting initiated at 105–100 Ma related to slab dynamics after the Hikurangi Plateau collision. We revise the onset of seafloor spreading south of the eastern Chatham Rise to ~88 Ma from the extent of our inferred COTZ. Geographically extensive, but low-volume intraplate magmatism affected the margin at 85–79 Ma. We suggest that this magmatism and the onset of seafloor spreading are a response to upwelling mantle through a slab window after 90 Ma. After 85 Ma, spreading segments became connected leading to the final separation of Zealandia from Antarctica. We interpret the southern Chatham Rise margin as a unique hybrid margin whose tectonic history was influenced by passive continental rifting and mantle upwelling.

Zircon U‐Pb Chronostratigraphy and Provenance of the Cycladic Blueschist Unit and the Nature of the Contact With the Cycladic Basement on Sikinos and Ios Islands, Greece

AbstractSikinos and Ios Islands, located in the Southern Cyclades, represent part of a Cenozoic metamorphic core complex system that exposes subduction‐related metamorphic rocks in the highly extended back‐arc region of the Hellenic subduction zone. These exhumed HP‐LT metamorphic units are composed of Mesozoic metasedimentary rocks of the Cycladic Blueschist Unit (CBU) and the Paleozoic Cycladic Basement (CB). The magmatic and stratigraphic evolution of these units, as well as the nature of the contact between the CBU and CB, have remained poorly understood. We used zircon U‐Pb dating to determine crystallization ages of the CB on Sikinos and the maximum deposition ages and detrital provenance of the metasedimentary units to reconstruct the Mesozoic to early Cenozoic stratigraphic and tectonic evolution of the CBU on both islands. The results reveal that the CB in Sikinos is composed of Cambrian‐Silurian metasedimentary rocks intruded by Carboniferous granites and is overlain by metasedimentary rocks of the CBU with depositional ages spanning from Permo‐Triassic to Late Cretaceous. The provenance data from the CBU records a long‐lived tectonic evolution from Paleo‐Tethys subduction and rifting, to passive margin formation, and to subduction of the Neo‐Tethyan Pindos basin. The continuous stratigraphic record and provenance evolution from the CB into the CBU imply a para‐autochthonous relationship. On NE Sikinos and Ios, stratigraphic constraints suggest older‐over‐younger relationships along cryptic‐thrusts, supporting premetamorphic or synmetamorphic structural repetition of the CBU by imbrication, likely during subduction underplating.

Cenozoic development of northern Svalbard based on thermochronological data

AbstractNorthern Svalbard represents a basement high surrounded by the Norwegian‐Greenland Sea/Fram Strait, Eurasian Basin, the Barents Shelf and the onshore Central Tertiary Basin (CTB). Published apatite fission track (AFT) data indicate Mesozoic differential, fault‐controlled uplift and exhumation of the region. Thermal history modelling of published and new AFT and (U–Th–Sm)/He ages of 51–153 Ma in the context of regional stratigraphy and geomorphology implies at least two, possibly three, uplift and exhumation stages since late Mesozoic, separated by episodes of subsidence and sediment deposition. Late Cretaceous/Palaeocene exhumation and subsequent burial appear to be related with the transition of compressional to transpressional collision of Svalbard and Greenland during the Eurekan Orogeny. Renewed exhumation since the Oligocene probably results from passive margin formation after the separation of Svalbard and Greenland, when a new offshore sedimentary basin opened west of Svalbard. Final uplift since the Miocene eventually re‐exposed the palaeosurface of northern Svalbard.

Introduction to the thematic set: Rifts III: catching the wave

Our understanding of the complex interplay of geodynamic processes that operate during lithospheric extension and results in the formation of magmatic and amagmatic passive margins remains equivocal. The challenge of the third rift conference in the series was, how to better constrain interpretation of those mechanisms, contributing to the surface expression at rifted plate margins. The fundamental observation that not all extensional basins and rifted continental margins involve the development of process-related magmatism continues to court significant debate and controversy (Rifts III: Catching the Wave, Geological Society, London – this conference ). Consensus remains elusive; however empirical data are providing more informed insights into the boundary conditions for input into more realistic and testable geodynamic numerical models (Lavier et al. 2016). Studied margins record their own particular complexities and causally reflect the influence of important crustal and mantle-scale heterogeneities in addition to plume-lithosphere interactions associated with core-mantle boundary hot-spot anomalies (Ryberg et al. 2015). The apparent paradoxical juxtaposition of magma-rich and magma-poor continental margin segments is now more widely recognized (e.g. Koopmann et al. 2013). By inference, we now conclude that rupture of the lithosphere is less likely to correspond to a continuous, single phase, uniform extension model ( sensu McKenzie 1978). This simplistic, elegant concept, in which crust and mantle thin by the same factor, is unable to explain the diversity of documented rift margin relationships. Instead, either the …

The Wilson Cycle and Effects of Tectonic Structural Inheritance on Rifted Passive Margin Formation

AbstractThe parallelism between older collisional belts and younger rift systems is widely known and particularly well portrayed along the Atlantic Ocean. How tectonic inherited and new‐formed shear zones control rift nucleation and the final architecture of rifted conjugate passive margins is still poorly understood. Here we present lithospheric‐scale thermo‐mechanical numerical models that self‐consistently create extensional and contractional tectonic inheritance, where prior extension and contraction are systematically varied. Our results show that (1) initial reactivation occurs along the former lithospheric suture zones; (2) upper crustal thick‐skinned basement thrusts are partially or fully reactivated depending on the amount of prior contraction and size of the orogen; (3) with a small amount of contraction, thick‐skinned thrusts are efficiently reactivated in extension and provide the template for rifted margin formation; (4) with larger amounts of contraction, thick‐skinned thrusts distal to the lithospheric suture zone do not reactivate in extension; and (5) reactivation of prior contractional shear zones dominates during the early stages of rifting, while during the final stage of margin formation new‐formed extensional shear zones dominate. Force balance analysis predicts an inverse relation between midcrustal viscosity and the maximum offset for reactivation of weak upper crustal structures. Force balance also predicts that the degree of weakening or healing of the weak suture and the thermal thinning of the necking area control at which stage suture reactivation is deactivated and extension proceeds by mantle lithosphere thermal necking. Two rifted conjugate margins with orogenic inheritance in the North and South Atlantic are used for comparison.

The Jan Mayen microplate complex and the Wilson cycle

Abstract The opening of the North Atlantic region was one of the most important geodynamic events that shaped the present day passive margins of Europe, Greenland and North America. Although well-studied, much remains to be understood about the evolution of the North Atlantic, including the role of the Jan Mayen microplate complex. Geophysical data provide an image of the crustal structure of this microplate and enable a detailed reconstruction of the rifting and spreading history. However, the mechanisms that cause the separation of microplates between conjugate margins are still poorly understood. We assemble recent models of rifting and passive margin formation in the North Atlantic and discuss possible scenarios that may have led to the formation of the Jan Mayen microplate complex. This event was probably triggered by regional plate tectonic reorganizations rejuvenating inherited structures. The axis of rifting and continental break-up and the width of the Jan Mayen microplate complex were controlled by old Caledonian fossil subduction/suture zones. Its length is related to east–west-oriented deformation and fracture zones, possibly linked to rheological heterogeneities inherited from the pre-existing Precambrian terrane boundaries.

The role of asthenospheric flow during rift propagation and breakup

Continental rifting precedes the breakup of continents, leading to the formation of passive margins and oceanic lithosphere. Although rifting dynamics is classically described in terms of either active rifting caused by active mantle upwelling, or passive rifting caused by far-field extensional stresses, it was proposed that a transition from passive to active rifting can result from changes in buoyancy forces due to localized thinning of the lithosphere. Three-dimensional numerical experiments of rifting near an Euler pole allow the quantification of these buoyancy forces and show that gravitational stresses are strong enough not only to sustain rifting and drive axis-parallel motion in the asthenosphere dome, but also to promote along-axis asthenospheric flow and to drive the propagation of the rift tip toward its rotation pole. We show that gradients of gravitational potential energy due to the presence of the dome of asthenosphere induce time-dependent phases of compressional and transcurrent stress regimes, despite an overall divergent plate setting. Our experiments predict overdeepened bathymetry at the tip of the propagating rift, as well as the variability of focal mechanisms of shallow seismic events similar to those observed in such a setting. We also explain the episodes of basin inversion documented in many rifted continental margins.

The influence of extension rate and crustal rheology on the evolution of passive margins from rifting to break-up

Extension rate and crustal strength are two important factors that influence the shape of initial rift basins and final passive margin architecture. The extension rate will also affect crustal strength as the passive margin evolves, therefore adding more complexity to the system. We used thermomechanical numerical experiments to systematically explore the relationship of extension rate and initial crustal strength on final passive margin structure. Varying the initial amount of ductile crust from 33% to 70% and extension rates from 2 to 50mmyr−1 results in passive margins of different widths, shapes, and evolution histories. We find four distinct margin types: Type I–narrow symmetric margins, Type II–wide symmetric margins, Type III–asymmetric margins, and Type IV–rift jumping. Experiments with strong initial crust (33% ductile crust) have crust-mantle coupling of strain and produce narrow, symmetric margins with high rift flank uplift (Type I margins). Ultraslow extension experiments (2mmyr−1) undergo thermal cooling and produce wide, symmetric margins with hyperextended crust (Type II margins). Experiments with intermediate to fast extension rates (10–50mmyr−1) and thick layers of ductile crust will produce initial rift basins that are symmetric, but later rifting stages yield asymmetric conjugate margins with hyperextended domains 100–250km wide (Type III margins). In special cases with intermediate extension rates and thick ductile crustal layers, we observe a jump in rift location over 50km and abandoned rifts with isolated crustal blocks (Type IV margins). These results highlight how ductile crustal layers and extension rates influence passive margin formation during different stages of rifting.

Kinematic constraints on the Rodinia to Gondwana transition

Earth’s plate tectonic history during the breakup of the supercontinent Pangea is well constrained from the seafloor spreading record, but evolving plate configurations during older supercontinent cycles are much less well understood. A relative paucity of available palaeomagnetic and geological data for deep time reconstructions necessitates innovative approaches to help discriminate between competing plate configurations. More difficult is tracing the journeys of individual continents during the amalgamation and breakup of supercontinents. Typically, deep-time reconstructions are built using absolute motions defined by palaeomagnetic data, and do not consider the kinematics of relative motions between plates, even for occasions where they are thought to be ‘plate-pairs’, either rifting apart leading to the formation of conjugate passive margins separated by a new ocean basin, or brought together by collision and orogenesis. Here, we use open-source software tools (GPlates/pyGPlates) to assess quantitative plate kinematics inherent within alternative reconstructions, such as rates of relative plate motion. We analyse the Rodinia-Gondwana transition during the Neoproterozoic, investigating the proposed Australia-Laurentia configurations during Rodinia, and the motion of India colliding with Gondwana. We find that earlier rifting times provide more optimal kinematic results. The AUSWUS and AUSMEX configurations with rifting at 800Ma are the most kinematically supported configurations for Australia and Laurentia (average rates of 57 and 64mm/a respectively), and angular rotation of ∼1.4°/Ma, compared to a SWEAT configuration (average spreading rate ∼76mm/a) and Missing-Link configuration (∼90mm/a). Later rifting, at, or after, 725Ma necessitates unreasonably high spreading rates of >130mm/a for AUSWUS and AUSMEX and ∼150mm/a for SWEAT and Missing-Link. Using motion paths and convergence rates, we create a kinematically reasonable (convergence below 70mm/a) tectonic model that is built upon a front-on collision of India with Gondwana, while also incorporating sinistral strike-slip motion against Australia and East Antarctica. We use this simple tectonic model to refine a global model for the breakup of western Rodinia and the transition to eastern Gondwana.

Basin inversion and supercontinent assembly as drivers of sediment-hosted Pb–Zn mineralization in the Mount Isa region, northern Australia

Deep seismic reflection imaging combined with geochronological, palaeomagnetic and stratigraphic data indicate that the world class sediment-hosted Pb–Zn deposits of northern Australia are preferentially concentrated in the post-extensional syn-inversion fraction of the Calvert and Isa superbasins. These fractions were deposited at 1650–1640 Ma and 1615–1575 Ma respectively, and overlap the age of known orogenic events in both Australia and western Laurentia, pointing to a common origin linked to crustal shortening and supercontinent assembly. Crustal shortening resulted in thrust faulting and reactivation of earlier-formed extensional faults leading to upward expulsion of mineralizing fluids at or close to the seafloor while basin inversion and sedimentation were still in progress. This is contrary to most existing models for Pb–Zn ore genesis in northern Australia where fluid flow and mineralization have been attributed to syn-extensional processes accompanying continental breakup or thermal sag. Instead, mineralization post-dates passive margin formation and more probably occurred in a contractional or foreland basin setting involving orogenic loading of the continental crust from the east during assembly or reassembly of the Nuna supercontinent. A strikingly similar history of basin inversion and Pb–Zn mineralisation is shared by the late Neoproterozoic-early Paleozoic Selwyn and older epicratonic basins of western North America.

Metamorphic inheritance of Rheic passive margin evolution and its early‐Variscan overprint in the Teplá‐Barrandian Unit, Bohemian Massif

AbstractRift‐related regional metamorphism of passive margins is usually difficult to observe on the surface, mainly due to its strong metamorphic overprint during the subsequent orogenic processes that cause its exposure. However, recognition of such a pre‐orogenic evolution is achievable by careful characterization of the polyphase tectono‐metamorphic record of the orogenic upper plate. A multidisciplinary approach, involving metamorphic petrology, P–T modelling, structural geology and in situ U‐Pb monazite geochronology using laser‐ablation split‐stream inductively coupled plasma mass spectrometry, was applied to unravel the polyphase tectono‐metamorphic record of metapelites at the western margin of the Teplá‐Barrandian domain in the Bohemian Massif. The study resulted in discovery of three tectono‐metamorphic events. The oldest event M1 is LP–HT regional metamorphism with a geothermal gradient between 30 and 50 °C km−1, peak temperatures up to 650 °C and of Cambro‐Ordovician age (c. 485 Ma). The M1 event was followed by M2‐D2, which is characterized by a Barrovian sequence of minerals from biotite to kyanite and a geothermal gradient of 20–25 °C km−1. D2‐M2 is associated with a vertical fabric S2 and was dated as Devonian (c. 375 Ma). Finally, the vertical fabric S2 was overprinted by a D3‐M3 event that formed sillimanite to chlorite bearing gently inclined fabric S3 also of Devonian age. The high geothermal gradient of the M1 event can be explained as the result of an extensional, rift‐related tectonic setting. In addition, restoration of the deep architecture and polarity of the extended domain before the Devonian history – together with the supracrustal sedimentary and magmatic record – lead us to propose a model for formation of an Ordovician passive continental margin. The subsequent Devonian evolution is interpreted as horizontal shortening of the passive margin at the beginning of Variscan convergence, followed by detachment‐accommodated exhumation of lower‐crustal rocks. Both Devonian shortening and detachment occurred in the upper plate of a Devonian subduction zone. The tectonic evolution presented in this article modifies previous models of the tectonic history of the western margin of the Teplá‐Barrandian domain, and also put constraints on the evolution of the southern margin of the Rheic ocean from the passive margin formation to the early phases of Variscan orogeny.

The importance of structural softening for the evolution and architecture of passive margins.

Lithospheric extension can generate passive margins that bound oceans worldwide. Detailed geological and geophysical studies in present and fossil passive margins have highlighted the complexity of their architecture and their multi-stage deformation history. Previous modeling studies have shown the significant impact of coarse mechanical layering of the lithosphere (2 to 4 layer crust and mantle) on passive margin formation. We built upon these studies and design high-resolution (~100–300 m) thermo-mechanical numerical models that incorporate finer mechanical layering (kilometer scale) mimicking tectonically inherited heterogeneities. During lithospheric extension a variety of extensional structures arises naturally due to (1) structural softening caused by necking of mechanically strong layers and (2) the establishment of a network of weak layers across the deforming multi-layered lithosphere. We argue that structural softening in a multi-layered lithosphere is the main cause for the observed multi-stage evolution and architecture of magma-poor passive margins.