Rifting History Research Articles

Crustal structure of Baffin Bay from constrained three-dimensional gravity inversion and deformable plate tectonic models

Mesozoic to Cenozoic continental rifting, breakup and spreading between North America and Greenland led to the opening, from south to north, of the Labrador Sea and eventually Baffin Bay between Baffin Island, northeast Canada and northwest Greenland. Baffin Bay lies at the northern limit of this extinct rift, transform and spreading system and remains largely underexplored. With the sparsity of existing crustal-scale geophysical investigations of Baffin Bay, regional potential field methods and quantitative deformation assessments based on plate reconstructions provide two means of examining Baffin Bay at the regional scale and drawing conclusions about its crustal structure, its rifting history and the role of pre-existing structures in its evolution. Despite the identification of extinct spreading axes and fracture zones based on gravity data, insights into the nature and structure of the underlying crust have only been gleaned from limited deep seismic experiments, mostly concentrated in the north and east where the continental shelf is shallower and wider. Baffin Bay is partially underlain by oceanic crust with zones of variable width of extended continental crust along its margins. 3-D gravity inversions, constrained by bathymetric and depth to basement constraints, have generated a range of 3-D crustal density models that collectively reveal an asymmetric distribution of extended continental crust, approximately 25–30 km thick, along the margins of Baffin Bay, with a wider zone on the Greenland margin. A zone of 5–13 km thick crust lies at the centre of Baffin Bay, with the thinnest crust (5 km thick) clearly aligning with Eocene spreading centres. The resolved crustal thicknesses are generally in agreement with available seismic constraints, with discrepancies mostly corresponding to zones of higher density lower crust along the Greenland margin and Nares Strait. Deformation modelling from independent plate reconstructions using GPlates of the rifted margins of Baffin Bay was performed to gauge the influence of original crustal thickness and the width of the deformation zone on the crustal thicknesses obtained from the gravity inversions. These results show the best match with the results from the gravity inversions for an original unstretched crustal thickness of 34–36 km, consistent with present-day crustal thicknesses derived from teleseismic studies beyond the likely continentward limits of rifting around the margins of Baffin Bay. The width of the deformation zone has only a minimal influence on the modelled crustal thicknesses if the zone is of sufficient width that edge effects do not interfere with the main modelled domain.

The pre-breakup stratigraphy and petroleum system of the Southern Jan Mayen Ridge revealed by seafloor sampling

The Jan Mayen Microplate Complex (JMMC) in the NE Atlantic is interpreted to mostly consist of continental fragments with possible interstitial embryonic oceanic crust. A complex Cenozoic rifting history accompanied by extensive extrusive and intrusive volcanism have made the geological characterization of the JMMC challenging especially due to poor seismic imaging beneath the breakup basalt succession. The presence of continental crust in the JMMC is inferred by seismic and magnetic data, but ground truthing evidence have yet to be provided. Here, we present the results from a seafloor sampling campaign undertaken in 2011 on the Southern Jan Mayen Ridge complex. Seabed samples were recovered using a gravity corer and a dredge along a 1000 m high escarpment with a 19° slope. Sampling locations were selected based on the interpretation of seismic profiles that suggest the presence of possible pre-breakup successions outcropping along this steep escarpment. Results include a sequence of samples with age diagnostic palynomorph assemblages ranging from Permian/Triassic to Eocene, and including igneous samples related to the Early Eocene breakup volcanism. Importantly, the samples were retrieved from hard substrate in an erosional gully lacking overburden sediments and have ages arranged in younging upward sequential order, supporting their near in-situ position. The sampling results were integrated into a lithostratigraphic pseudo-well that can be used to constrain the evolution and breakup of the JMMC. Additionally, evidence for active migration of Jurassic sourced hydrocarbons comprise the first indication of a working hydrocarbon system, with important implications for the petroleum prospectivity of the Dreki area. Finally, these results confirm that the Southern Jan Mayen Ridge is indeed a sliver of continental crust.

Cooling, exhumation, and deformation in the Hindu Kush, NW Pakistan: New constraints from preliminary 40Ar/39Ar and fission track analyses

Asian crust in the Hindu Kush region in northern Pakistan records a protracted history of rifting, subduction and collision not commonly preserved within the Himalaya. Because of this, it is key to understanding the development of the southern Eurasian margin both prior to and after collision with India. New mica 40Ar/39Ar and apatite fission track geochronologic data from this region provide constraints on the kinematics of the Hindu Kush. 40Ar/39Ar muscovite and biotite ages from the late Cambrian Kafiristan pluton are 379.7 ± 1.7 Ma and 47.2 ± 0.3 Ma, respectively. The muscovite age may record cooling or partial resetting, while the biotite age is interpreted to record a thermal disruption associated with the early stages of continental collision in the Himalayan system. A 111.0 ± 0.6 Ma muscovite age from the northern part of the Tirich Mir pluton (∼123 Ma old; U-Pb) is interpreted to indicate a recrystallization event ∼12 Myrs after its intrusion. In addition, a younger muscovite age of 47.5 ± 0.2 Ma was derived from the opposite side of the same pluton in the immediate hanging wall of the Tirich Mir fault. This Eocene age is interpreted to represent the time of recrystallization during fault (re)activation in the early stages of India-Asia continent-continent collision. 40Ar/39Ar biotite analysis from the Buni-Zom pluton yields an age of 61.6 ± 1.1 Ma and is interpreted to reflect cooling at mid-upper crustal levels subsequent to the pluton’s emplacement in the middle Cretaceous. Finally, 17.1–21.3 Ma 40Ar/39Ar ages from the Garam Chasma pluton and surrounding metapelites indicate cooling immediately following crystallization of the leucogranite body in the earliest Miocene/latest Oligocene.The younger cooling history is resolved by fission track dating of apatite (AFT). In the vicinity of the bounding Tirich Mir fault, the Tirich Mir pluton yields an AFT age of 1.4 ± 0.3 Ma, which is consistent with active exhumation associated with the surface uplift of the 7700+ m Tirich Mir peak. The Garam Chasma pluton has a young age of 3.5 ± 0.2 Ma, which also records rapid rock uplift and exhumation in the area. Finally, an AFT age of 9.1 ± 2.1 Ma was extracted from a metapelite in the footwall of an east verging thrust fault separating it from the Garam Chasma pluton to the west. The difference in ages, Pliocene vs. late Miocene, reflect differential cooling/exhumation paths across that structure.

The thermal structure of volcanic passive margins

Over the past ten years we have numerically modelled the properties of the magmatism generated at four of the key areas where the ‘mantle plume–volcanic margin hypothesis’ is expected to be valid: the North Atlantic, South Atlantic, India–Seychelles and Afar. Our model incorporates many of the original assumptions in the classic White and McKenzie model, with pure shear of the lithospheric mantle, passive upwelling and decompressional melting. Our model is however two- rather than one-dimensional, can capture the rift history (extension rate changes and axis jumps) and tracks mantle depletion during melting. In all four of our study areas we require the sub-lithospheric mantle to be 100 – 200°C hotter than ‘normal’, non-volcanic margins to explain the characteristics of the magmatism. In the three passive margin cases we find this excess temperature is limited to a 50 – 100 km thick layer. We require this layer temperature to drop along-strike away from the proposed sites of plume impact at the base of the lithosphere. However, we also find that lithospheric thickness and rift history are as important as temperature for controlling the magmatism. Our work therefore lends support to the hypothesis that the excess magmatism at volcanic margins is due to a thermal anomaly in the asthenosphere, albeit with consideration of extra parameters. Supplementary material: Numerical model description is available at https://doi.org/10.6084/m9.figshare.c.3924505

Complex fault interaction controls continental rifting

Rifted margins mark a transition from continents to oceans and contain in their architecture a record of their rift history. Recent investigations of rift architecture have suggested that multiphase deformation of the crust and mantle lithosphere leads to the formation of distinct margin domains. The processes that control transitions between these domains, however, are not fully understood. Here we use high-resolution numerical simulations to show how structural inheritance and variations in extension velocity control the architecture of rifted margins and their temporal evolution. Distinct domains form as extension velocities increase over time and deformation focuses along lithosphere-scale detachment faults, which migrate oceanwards through re-activation and complex linkages of prior fault networks. Our models demonstrate, in unprecedented detail, how faults formed in the earliest phases of continental extension control the subsequent structural evolution and complex architecture of rifted margins through fault interaction processes, hereby creating the widely observed distinct margin domains.

Structural characteristics of the ocean-continent transition along the rifted continental margin, offshore central Labrador

Studies of the structural deformation and the nature of the ocean-continent transition zone at the rifted continental margin, offshore central Labrador, and its conjugate off West Greenland have been conducted over several decades and helped to define our understanding of the controlling factors in the development of magma-poor margins globally. New two-dimensional seismic reflection data are now available across the Labrador margin which can help to modify and refine this understanding. In this paper, we describe some of these new seismic data and the corresponding potential field data in relation to earlier studies, including an existing deep seismic crustal velocity profile. These data cross the three major structural and compositional zones below the sedimentary wedge defined in earlier deep seismic experiments as: 1) extended zone of continental crust, including the hyper-extended region, partly underlain by serpentinized mantle; 2) exhumed and serpentinized continental mantle zone; and 3) zone of oceanic crust. We are able to extend these zones laterally over 200 km along the margin. The rifted continental crust is hyper-extended over tens of kilometers landward of the breakup location, where many varieties of normal faults and possible low-angle detachment surfaces are present, some of which are near or at the base of the crust. Further seaward, mantle was serpentinized and likely exhumed, and possesses a seismic character very different from the continental crust. Basement in this zone may locally form prominent ridges. Within the oceanic zone we define a further sub-division into a 'proto-oceanic' domain with thin, variable oceanic crust of about 70–65 Ma in age (magnetic chrons 27 to 31) and a steady-state seafloor spreading region further seaward. Thus, seafloor spreading starts at about chron 31 time. Potential field data and models are broadly consistent with these zones. A strongly reflective, post-rift volcanic event is present across much of the region, especially over the exhumed mantle zone and parts of the oceanic crust. These results are discussed and compared to earlier studies of the West Greenland conjugate margin and to other well-studied magma-poor margins. Given that this rift occurred in thick, cold cratonic lithosphere, the results are remarkably similar to those off Iberia and Newfoundland, suggesting that a long rift history and/or mantle metasomatism may have weakened the lithosphere prior to the main rifting event.

Discovery of Wanyuan-Dazhou Intracratonic Rift and its significance for gas exploration in Sichuan Basin, SW China

Abstract New understandings of the geology of Sichuan Basin were achieved in the progress of natural gas exploration in the Sinian-Cambrian strata in Sichuan Basin. An NE trending intracratonic rift was found in the Wanyuan-Dazhou area, northeastern Sichuan Basin. Based on seismic data interpretation, outcrop data and regional tectonic background analysis, we studied the boundary, distribution, formation and evolution history of Wanyuan-Dazhou rift. The following findings were obtained: (1) The seismic section indicates that a steep-slope belt indicating platform margin facies was developed during the deposition of the first and second Member (Z2dn1-Z2dn2) of the Sinian Dengying Formation. The rift generally strikes NE. (2) The thicknesses of the first and second Member of the Dengying Formation are thicker than the third and fourth Member (Z2dn3-Z2dn4) at the periphery of the rift and vice versa inside the rift. (3) The rift formed during the deposition of Z2dn1-Z2dn2 in the Sinian. Filling and subsidence occurred during the deposition of Z2dn3-Z2dn4 in the Sinian. The shrinkage of the rift happened during the deposition of the Lower Cambrian Maidiping Formation – Qiongzhusi Formation. Regional extension and uplift in the Nanhua controlled rift formation. The discovery of the Wanyuan-Dazhou rift changed the traditional understanding of the tectonic and sedimentary framework of Sichuan Basin in the Sinian and Early Cambrian. The periphery of the rift has significant potential for natural gas exploration due to its superior natural gas accumulation conditions in the Sinian.

Morphotectonic inversion in the Tunka rift basin (southwestern Baikal region)

The general basement subsidence trend in the Tunka rift is locally interrupted by uplift (basin inversion). The inversion uplift causes deformation to basin sediments and shows up in the surface topography as morphostructures of two types. Inversion in the area is either part of rifting, when the subsidence-to-uplift change is driven by the rifting mechanism, or perturbs the rifting trend as superposed Gobi-type mountain growth, but is never associated with change from continental rifting to other tectonic setting. The presence of buried erosion cutouts in the rift valley floor indicates that wave-like vertical motions, with erosion during uplift and deposition in the erosion cutouts during subsidence, superpose on differentiated (orogenic) motions. The latest phase of basin inversion acted in the Tunka rift in the second half of the Late Pleistocene-Holocene, and the amount of uplift varied from a few tens to a few hundreds of meters. The highest 300 m uplift was in the Tor rift basin, as estimated from relative elevation of its ~55 kyr sediments. In general, inversion uplift occurred over 40% of the Tunka basin area (872 km2 of 2240 km2), and about 450 km2 of this uplift (49% of the uplifted area or 20% of the rift valley floor) grew by the Gobi-type mechanism. Quaternary sediments lie with a hiatus upon the Neogene strata in almost all sedimentary sections of basin margins, thus indicating that the deposition area reduced for a long period in the rift history and reached the former extent only in the earliest Late Pleistocene.

Two-phase Cretaceous–Paleocene rifting in the Taranaki Basin region, New Zealand; implications for Gondwana break-up

The break-up of Gondwana resulted in extension of New Zealand continental crust during the Cretaceous–Paleocene. Offshore the geometry and rift history are well imaged by new regional mapping of a large seismic reflection dataset, tied to wells, used here to document the Cretaceous–Paleocene ( c . 105 – 55 Ma) evolution of the greater Taranaki Basin region. Two temporally distinct phases of rifting have been recognized in the region, and record Gondwana break-up. The first (Zealandia rift phase) produced half-grabens trending NW to WNW during the mid-Cretaceous ( c . 105 – 83 Ma). These rift basins predate, and are parallel to, Tasman Sea spreading centres. They record distributed stretching of northern Zealandia prior to the onset of seafloor spreading in the Tasman Sea. A short period ( c . 83 – 80 Ma) of uplift and erosion followed, possibly representing a break-up unconformity, with erosion in southern Taranaki Basin and deposition of the ‘Taranaki Delta’ sequence in Deepwater Taranaki. The second, West Coast–Taranaki rift phase produced north- to NE-trending extensional half-grabens in the shelfal Taranaki Basin during the latest Cretaceous–Paleocene ( c . 80 – 55 Ma). This rift was narrow (<150 km wide), orthogonal to Zealandia phase rifting, affected mainly western Zealandia and did not progress to full break-up. Supplementary material: A full set of eight palaeogeographical maps as well as expanded versions of the seismic figures, with both uninterpreted and interpreted versions, are available at https://doi.org/10.6084/m9.figshare.c.3772175

A shifting rift—Geophysical insights into the evolution of Rio Grande rift margins and the Embudo transfer zone near Taos, New Mexico

We present a detailed example of how a subbasin develops adjacent to a transfer zone in the Rio Grande rift. The Embudo transfer zone in the Rio Grande rift is considered one of the classic examples and has been used as the inspiration for several theoretical models. Despite this attention, the history of its development into a major rift structure is poorly known along its northern extent near Taos, New Mexico. Geologic evidence for all but its young rift history is concealed under Quaternary cover. We focus on under­standing the pre-Quaternary evidence that is in the subsurface by integrating diverse pieces of geologic and geophysical information. As a result, we present a substantively new understanding of the tectonic configuration and evolution of the northern extent of the Embudo fault and its adjacent subbasin. We integrate geophysical, borehole, and geologic information to interpret the subsurface configuration of the rift margins formed by the Embudo and Sangre de Cristo faults and the geometry of the subbasin within the Taos embayment. Key features interpreted include (1) an imperfect D-shaped subbasin that slopes to the east and southeast, with the deepest point ∼2 km below the valley floor located northwest of Taos at ∼36° 26′N latitude and 105° 37′W longitude; (2) a concealed Embudo fault system that extends as much as 7 km wider than is mapped at the surface, wherein fault strands disrupt or truncate flows of Pliocene Servilleta Basalt and step down into the subbasin with a minimum of 1.8 km of vertical displacement; and (3) a similar, wider than expected (5–7 km) zone of stepped, west-down normal faults associated with the Sangre de Cristo range front fault. From the geophysical interpretations and subsurface models, we infer relations between faulting and flows of Pliocene Servilleta Basalt and older, ­buried basaltic rocks that, combined with geologic mapping, suggest a revised rift history involving shifts in the locus of fault activity as the Taos subbasin developed. We speculate that faults related to north-striking grabens at the end of Laramide time formed the first west-down master faults. The Embudo fault may have initiated in early Miocene southwest of the Taos region. Normal-oblique slip on these early fault strands likely transitioned in space and time to dominantly left-lateral slip as the Embudo fault propagated to the northeast. During and shortly after eruption of Servilleta Basalt, proto-­Embudo fault strands were active along and parallel to the modern, NE-aligned Rio Pueblo de Taos, ∼4–7 km basinward of the modern, mapped Embudo fault zone. Faults along the northeastern subbasin margin had northwest strikes for most of the period of subbasin formation and were located ∼5–7 km basinward of the modern Sangre de Cristo fault. The locus of fault activity shifted to more northerly striking faults within 2 km of the modern range front sometime after Servilleta volcanism had ceased. The northerly faults may have linked with the northeasterly proto-Embudo faults at this time, concurrent with the development of N-striking Los Cordovas normal faults within the interior of the subbasin. By middle Pleistocene(?) time, the Los Cordovas faults had become inactive, and the linked Embudo–Sangre de Cristo fault system migrated to the south, to the modern range front.

Effective elastic thickness along the conjugate passive margins of India, Madagascar and Antarctica: A re-evaluation using the Hermite multitaper Bouguer coherence application

Gondwana correlation studies had rationally positioned the western continental margin of India (WCMI) against the eastern continental margin of Madagascar (ECMM), and the eastern continental margin of India (ECMI) against the eastern Antarctica continental margin (EACM). This contribution computes the effective elastic thickness (Te) of the lithospheres of these once-conjugated continental margins using the multitaper Bouguer coherence method. The results reveal significantly low strength values (Te∼2km) in the central segment of the WCMI that correlate with consistently low Te values (2–3km) obtained throughout the entire marginal length of the ECMM. This result is consistent with the previous Te estimates of these margins, and confirms the idea that the low-Te segments in the central part of the WCMI and along the ECMM represents paleo-rift inception points of the lithospheric margins that was thermally and mechanically weakened by the combined action of the Marion hotspot and lithospheric extension during the rifting. The uniformly low-Te value (∼2km) along the EACM indicates a mechanically weak lithospheric margin, probably due to considerable stretching of the lithosphere, considering the fact that this margin remained almost stationary throughout its rift history. In contrast, the ECMI has comparatively high-Te variations (5–11km) that lack any correlation with the regional tectonic setting. Using gravity forward and inversion applications, we find a leading order of influence of sediment load on the flexural properties of this marginal lithosphere. The study concludes that the thick pile of the Bengal Fan sediments in the ECMI masks and has erased the signal of the original load-induced topography, and its gravity effect has biased the long-wavelength part of the observed gravity signal. The hence uncorrelated flat topography and deep lithospheric flexure together contribute a bias in the flexure modeling, which likely accounts a relatively high Te estimate.

The role of long‐term rifting history on modes of continental lithosphere extension

AbstractContinental lithosphere extension results in complex basin types with differing structural styles, subsidence, thermal histories, and melt production. Many studies have examined the role of initial rheological layering, geothermal gradients, and extension rates during a single rifting event. This approach neglects the tectonic history of many basins that are marked by multiple rifting events. Here we address the role of repeated extension on long‐term lithospheric strain modes and the resulting basins, highlighting cases most affected by previous rifting events. We use numerical models of a lithosphere undergoing two rifting events of differing extension rates and separated by cooling, to show the effect of early events on subsequent evolution. The combination of boundary displacement velocity in both events leads to the formation of various rift basin types, ranging from narrow to wide to hyperextended and with variation of subsidence patterns, degrees of symmetry, and melt yield. We show that basin type, subsidence, and melt production might be strongly affected by previous rifting events, illustrating cases in which the previous rifting history cannot be neglected. Our models reproduce the first‐order features of Earth's sedimentary basins and propose a classification to guide the interpretation of extensional basins and their evolution.

Measuring plume-related exhumation of the British Isles in Early Cenozoic times

Mantle plumes have been proposed to exert a first-order control on the morphology of Earth's surface. However, there is little consensus on the lifespan of the convectively supported topography. Here, we focus on the Cenozoic uplift and exhumation history of the British Isles. While uplift in the absence of major regional tectonic activity has long been documented, the causative mechanism is highly controversial, and direct exhumation estimates are hindered by the near-complete absence of onshore post-Cretaceous sediments (outside Northern Ireland) and the truncated stratigraphic record of many offshore basins. Two main hypotheses have been developed by previous studies: epeirogenic exhumation driven by the proto-Iceland plume, or multiple phases of Cenozoic compression driven by far-field stresses. Here, we present a new thermochronological dataset comprising 43 apatite fission track (AFT) and 102 (U–Th–Sm)/He (AHe) dates from the onshore British Isles. Inverse modelling of vertical sample profiles allows us to define well-constrained regional cooling histories. Crucially, during the Paleocene, the thermal history models show that a rapid exhumation pulse (1–2.5 km) occurred, focused on the Irish Sea. Exhumation is greatest in the north of the Irish Sea region, and decreases in intensity to the south and west. The spatial pattern of Paleocene exhumation is in agreement with the extent of magmatic underplating inferred from geophysical studies, and the timing of uplift and exhumation is synchronous with emplacement of the plume-related British and Irish Paleogene Igneous Province (BIPIP). Prior to the Paleocene exhumation pulse, the Mesozoic onshore exhumation pulse is mainly linked to the uplift and erosion of the hinterland during the complex and long-lived rifting history of the neighbouring offshore basins. The extent of Neogene exhumation is difficult to constrain due to the poor sensitivity of the AHe and AFT systems at low temperatures. We conclude that the Cenozoic topographic evolution of the British Isles is the result of plume-driven uplift and exhumation, with inversion under compressive stress playing a secondary role.

The Cenozoic volcanism in the Kivu rift: Assessment of the tectonic setting, geochemistry, and geochronology of the volcanic activity in the South-Kivu and Virunga regions

The Kivu rift is part of the western branch of the East African Rift system. From Lake Tanganyika to Lake Albert, the Kivu rift is set in a succession of Precambrian zones of weakness trending NW-SE, NNE-SSW and NE-SW. At the NW to NNE turn of the rift direction in the Lake Kivu area, the inherited faults are crosscut by newly born N-S fractures which developed during the late Cenozoic rifting and controlled the volcanic activity. From Lake Kivu to Lake Edward, the N-S faults show a right-lateral en echelon pattern. Development of tension gashes in the Virunga area indicates a clockwise rotation of the constraint linked to dextral oblique motion of crustal blocks. The extensional direction was W-E in the Mio-Pliocene and ENE-WSW in the Pleistocene to present time.The volcanic rocks are assigned to three groups: (1) tholeiites and sodic alkali basalts in the South-Kivu, (2) sodic basalts and nephelinites in the northern Lake Kivu and western Virunga, and (3) potassic basanites and potassic nephelinites in the Virunga area. South-Kivu magmas were generated by melting of spinel + garnet lherzolite from two sources: an enriched lithospheric source and a less enriched mixed lithospheric and asthenospheric source. The latter source was implied in the genesis of the tholeiitic lavas at the beginning of the South-Kivu tectono-volcanic activity, in relationships with asthenosphere upwelling. The ensuing outpouring of alkaline basaltic lavas from the lithospheric source attests for the abortion of the asthenospheric contribution and a change of the rifting process. The sodic nephelinites of the northern Lake Kivu originated from low partial melting of garnet peridotite of the sub-continental mantle due to pressure release during swell initiation. The Virunga potassic magmas resulted from the melting of garnet peridotite with an increasing degree of melting from nephelinite to basanite. They originated from a lithospheric source enriched in both K and Rb, suggesting the presence of phlogopite and the local existence of a metasomatized mantle. A carbonatite contribution is evidenced in the Nyiragongo lavas.New K-Ar ages date around 21 Ma the earliest volcanic activity made of nephelinites. A sodic alkaline volcanism took place between 13 and 9 Ma at the western side of the Virunga during the doming stage of the rift and before the formation of the rift valley. In the South-Kivu area, the first lavas were tholeiitic and dated at 11 Ma. The rift valley subsidence began around 8–7 Ma. The tholeiitic lavas were progressively replaced by alkali basaltic lavas until to 2.6 Ma. Renewal of the basaltic volcanism happened at ca. 1.7 Ma on a western step of the rift. In the Virunga area, the potassic volcanism appeared ca. 2.6 Ma along a NE-SW fault zone and then migrated both to the east and west, in jumping to oblique tension gashes.The uncommon magmatic evolution and the high diversity of volcanic rocks of the Kivu rift are explained by varying transtensional constraints during the rift history.

Sedimentology and regional significance of the ‘argillite unit’, a probable Cryogenian map unit in southeast Yukon, Canada

AbstractAn unnamed succession of volcaniclastic argillite, sandstone, and conglomerate (the ‘argillite unit’) is exposed in an inlier in southeastern Yukon (NTS 95C/5). These strata were hornfelsed during emplacement of the Pool Creek syenite (ca. 640–650 Ma) and are correlated with the late Cryogenian Hay Creek Group of the Mackenzie Mountains based on lithostratigraphic evidence and published constraints from detrital zircons. The unit preserves three argillite lithofacies, two sandstone lithofacies, and two conglomerate lithofacies. The argillite and sandstone can be grouped into three facies associations, reflecting differing volumes of sandstone and the presence or absence of chaotic bedding. Deposition was mainly from sediment‐gravity flows, particularly turbidity currents and debris flows. Post‐depositional slumping was common, and the succession is interpreted to have been deposited on a slope that trended NNW and dipped to the ENE. This orientation was subparallel to but facing towards the contemporaneous slope of the Hay Creek Group in the Mackenzie Mountains. Conglomeratic facies are dominated by angular to poorly rounded clasts of subalkali basalt that probably were entrained in debris flows during or soon after eruption and otherwise saw little transport or weathering. Geochemistry of the clasts is permissive of a rift‐related setting. Following deposition, but prior to deposition of overlying Cambro‐Ordovician strata, the argillite unit underwent compression that produced broad, open folds, consistent with recent proposals for late Neoproterozoic transtension–transpression on the present‐day northwest margin of Laurentia. The argillite unit provides a snapshot of the geological evolution of southeast Yukon during the late Cryogenian, providing a new data point for reconstructing the protracted and complex rifting history of Rodinia in western Canada. © 2016 Her Majesty the Queen in Right of Canada Geological Journal © 2016 John Wiley & Sons, Ltd.

Post-Caledonian extension in the West Norway–northern North Sea region: the role of structural inheritance

Abstract The northern North Sea region has experienced repeated phases of post-Caledonian extension, starting with extensional reactivation of the low-angle basal Caledonian thrust zone, then the formation of Devonian extensional shear zones with 10–100 km-scale displacements, followed by brittle reactivation and the creation of a plethora of extensional faults. The North Sea Rift-related approximately east–west extension created a new set of rift-parallel faults that cut across less favourably orientated pre-rift structures. Nevertheless, fault rock dating shows that onshore faults and shear zones of different orientations were active throughout the history of rifting. Several of the reactivated major Devonian extensional structures can be extrapolated offshore into the rift, where they appear as bands of dipping reflectors. They coincide with large-scale boundaries separating 50–100 km-wide rift domains of internally uniform fault patterns. Major north–south-trending rift faults, such as the Øygarden Fault System, bend or terminate against these boundaries, clearly influenced by their presence during rifting. Hence, the North Sea is one of several examples where pre-rift basement structures oblique to the rift extension direction can significantly influence rift architecture, even if most of the rift faults are newly-formed structures.

Conjugate rifted margins width and asymmetry: The interplay between lithospheric strength and thermomechanical processes

AbstractNumerical experiments have been used to relate the range in the distribution and the style of deformation observed in rifted margins to localizing/delocalizing thermomechanical processes. The experiments give rise to four end‐members of margins for varying initial lithospheric strength and extension rates. The first two end‐members are narrow and asymmetric and narrow and near‐symmetric, conjugate margins. The third end‐member is asymmetric conjugate margins, wherein one side is <100 km wide and the other is >100–300 km wide. Lastly, we explore wide rift systems that may form very asymmetric conjugate margins with one narrow margin and a very wide conjugate, 200 km to > 350 km across. With initial and boundary conditions close to that inferred from the North and South Atlantic margins, we find that not all margins experience a polyphase rifting history of stretching‐thinning‐exhumation. Instead, the stretching mode can be very short or protracted, and the thinning or the exhumation modes can be incomplete or absent. The deformation localization of the thinning mode is in places associated with the formation of a keystone block or “block H.” A new mechanism for the formation of the unstable crustal root under block H is described, wherein the bounding border faults lead to differential thinning of the crust and mantle lithosphere. Nonuniform extension also occurs in both types of wide rift systems and is related to the sequential deformation migration outward of an initial graben, associated with effective lithospheric strengthening that occurs during crustal thinning and bending.

The enigma of the Jonah high in the middle of the Levant basin and its significance to the history of rifting

Recent giant gas discoveries within deeply buried structural highs in the middle of the Levant basin have attracted the attention of the industrial and academic communities striving to understand the origin of such structures, their relations to the tectonic history of the basin, and their evolution through time. Here we focus on the Jonah high, which is one of the largest structures in the basin and is particularly enigmatic in its geometry, dimensions and location compared to nearby structures. It is buried under more than 3km of Late Tertiary sediments, and is associated with one of the largest magnetic anomalies in the basin, though no significant gravity anomaly is observed. Previous studies raised several possibilities explaining its origin: an ancient horst related to the early stage of basin formation (Late Paleozoic or early Mesozoic); a Syrian Arc fold (Late Cretaceous to Neogene); a giant volcanic seamount; and an intrusive magmatic body.A reconstruction of the evolution of this structure is proposed here based on newly produced pre-stack depth migration of five selected seismic reflection lines crossing the Jonah high combined with a basin-wide interpretation of more than 500 2-D time-migrated lines. We suggest that the Jonah high is a horst bounded by grabens, most probably formed during continental breakup related to the Neo-Tethys formation. However, unlike other extensional structures that were reactivated and inverted during the Syrian Arc deformation, the Jonah high was never reactivated. Rather, it formed a prominent seamount that persisted for 120–140Ma until the Early Miocene, when it was finally buried. In a wider perspective the Jonah horst is similar to the Eratosthenes seamount, a fragment of continental crust between the Levant and Herodotus basins.

Unzipping the Patagonian Andes—Long-lived influence of rifting history on foreland basin evolution

The Andean Cordillera is widely considered to be one of the type examples of a convergent margin setting. In the southernmost Andes, however, rifting and volcanism predated mid-Cretaceous breakup of Gondwana and formation of the South Atlantic Ocean by up to 40 m.y. and culminated in the opening of the Rocas Verdes backarc basin east of the Mesozoic Patagonian Batholith. We present new U-Pb geochronology from the Austral sector (49°S–50°S) that indicates rift volcanism occurred between 154 and 147 Ma near the northern terminus of the basin. Available data and observations from the southern Rocas Verdes Basin indicate larger-magnitude and longer-duration extension compared to the northern basin region. The Rocas Verdes Basin underwent progressive northward propagation and opening and was later backfilled concomitantly with the opening of the southern Atlantic Ocean by north-to-south deposition within a retroarc foreland setting. The influence of the inherited tectonic fabric of the Rocas Verdes backarc basin on the subsequent foreland basin explains many unique characteristics of the Patagonian Andes, such as a protracted deep basin that formed atop the previously rifted and weakened crust. Moreover, the early rift history helps account for intraplate deformation of southernmost South America during the opening of the South Atlantic Ocean.

Supercontinental inheritance and its influence on supercontinental breakup: The Central Atlantic Magmatic Province and the breakup of Pangea

AbstractThe Central Atlantic Magmatic Province (CAMP) is the large igneous province (LIP) that coincides with the breakup of the supercontinent Pangea. Major and trace element data, Sr‐Nd‐Pb radiogenic isotopes, and high‐precision olivine chemistry were collected on primitive CAMP dikes from Virginia (VA). These new samples were used in conjunction with a global CAMP data set to elucidate different mechanisms for supercontinent breakup and LIP formation. On the Eastern North American Margin, CAMP flows are found primarily in rift basins that can be divided into northern or southern groups based on differences in tectonic evolution, rifting history, and supercontinental inheritance. Geochemical signatures of CAMP suggest an upper mantle source modified by subduction processes. We propose that the greater number of accretionary events, or metasomatism by sediment melts as opposed to fluids on the northern versus the southern Laurentian margin during the formation of Pangea led to different subduction‐related signatures in the mantle source of the northern versus southern CAMP lavas. CAMP samples have elevated Ni and low Ca in olivine phenocrysts indicating a significant pyroxenite component in the source, interpreted here as a result of subduction metasomatism. Different collisional styles during the Alleghanian orogeny in the North and South may have led to the diachroneity of the rifting of Pangea. Furthermore, due to a low angle of subduction, the Rheic Plate may have underplated the lithosphere then delaminated, triggering both the breakup of Pangea and the formation of CAMP.