Passive Margin Formation Research Articles

The Neotethyan Sanandaj‐Sirjan zone of Iran as an archetype for passive margin‐arc transitions

AbstractThe Sanandaj‐Sirjan zone of Iran is a northwest trending orogenic belt immediately north of the Zagros suture, which represents the former position of the Neotethys Ocean. The zone contains the most extensive, best preserved record of key events in the formation and evolution of the Neotethys, from its birth in Late Paleozoic time through its demise during the mid‐Tertiary collision of Arabia with Eurasia. The record includes rifting of continental fragments off of the northern margin of Gondwanaland, formation of facing passive continental margins, initiation of subduction along the northern margin, and progressive development of a continental magmatic arc. The latter two of these events are critical phases of the Wilson Cycle that, elsewhere in the world, are poorly preserved in the geologic record because of superimposed events. Our new synthesis reaffirms the similarity between this zone and various terranes to the north in Central Iran. Late Paleozoic rifting, preserved as A‐type granites and accelerated subsidence, was followed by a phase of pronounced subsidence and shallow marine sedimentation in Permian through Triassic time, marking the formation and evolution of passive margins on both sides of the suture. Subduction and arc magmatism began in latest Triassic/Early Jurassic time, culminating at ~170 Ma. The extinction of arc magmatism in this zone, and its shift northeastward to form the subparallel Urumieh‐Dokhtar arc, occurred diachronously along strike, in Late Cretaceous or Paleogene time. Post‐Cretaceous uplift transformed the zone from a primarily marine borderland into a marine archipelago that persisted until mid‐Tertiary time.

Basin architecture and evolution in the Mount Isa mineral province, northern Australia: Constraints from deep seismic reflection profiling and implications for ore genesis

Deep seismic reflection profiling confirms that the Paleo- to Mesoproterozoic Mount Isa mineral province comprises three vertically stacked and partially inverted sedimentary basins preserving a record of intracontinental rifting followed by passive margin formation. Passive margin conditions were established no later than 1655Ma before being interrupted by plate convergence, crustal shortening and basin-wide inversion at 1640Ma in both the 1730–1640Ma Calvert and 1790–1740Ma Leichhardt superbasins. Crustal extension and thinning resumed after 1640 Ma with formation of the 1635–1575Ma Isa Superbasin and continued up to ca. 1615Ma when extensional faulting ceased and a further episode of basin inversion commenced. The 1575Ma Century Pb–Zn ore-body is hosted by syn-inversion sediments deposited during the initial stages of the Isan Orogeny with basin inversion accommodated on east- or northeast-dipping reactivated intrabasinal extensional faults and footwall shortcut thrusts. These structures extend to considerable depths and served as fluid conduits during basin inversion, tapping thick syn-rift sequences of immature siliciclastic sediments floored by bimodal volcanic sequences from which the bulk of metals and mineralising fluids are thought to have been sourced. Basin inversion and fluid expulsion at this stage were entirely submarine consistent with a syn-sedimentary to early diagenetic origin for Pb–Zn mineralisation at, or close to, the seafloor. Farther east, a change from platform carbonates to deeper water continental slope deposits (Kuridala and Soldiers Cap groups) marks the position of the original shelf break along which the north–south-striking Selwyn-Mount Dore structural corridor developed. This corridor served as a locus for strain partitioning, fluid flow and iron oxide–copper–gold mineralisation during and subsequent to the onset of basin inversion and peak metamorphism in the Isan Orogeny at 1585Ma. An episode of post-orogenic strike-slip faulting and hydrothermal alteration associated with the subvertical Cloncurry Fault Zone overprints west- to southwest-dipping shear zones that extend beneath the Cannington Pb–Zn deposit and are antithetic to inverted extensional faults farther west in the same sub-basin. Successive episodes of basin inversion and mineralisation were driven by changes in the external stress field and related plate tectonic environment as evidenced by a corresponding match to bends in the polar wander path for northern Australia. An analogous passive margin setting has been described for Pb–Zn mineralisation in the Paleozoic Selwyn Basin of western Canada.

Succession of Permian and Mesozoic metasomatic events in the eastern Pyrenees with emphasis on the Trimouns talc–chlorite deposit

Recent studies proposing pre-orogenic mantle exhumation models have helped renew the interest of the geosciences community in the Pyrenees, which should be now interpreted as a hyper-extended passive margin before the convergence between Iberia and Eurasia occurred. Unresolved questions of the Pyrenean geology, as well as the understanding of the formation of hyper-extended passive margins, are how the crust was thinned, and when, where and how the crustal breakoff occurred. The study of the Variscan and pre-Variscan Pyrenean basement is thus critical to document and understand this Cretaceous crustal thinning. In order to specify the timing of Mesozoic metasomatism and the associated deformation in the pre-Mesozoic basement of the Pyrenees, we carried out a U–Th–Pb laser ablation ICP–MS study on a large panel of REE and titanium-rich minerals (titanite and rutile) from talc–chlorite ores from the eastern Pyrenees, with a special emphasis on the Trimouns deposit, the world’s largest talc quarry. Our results suggest that the Trimouns talc formation was restricted to the upper Aptian–Cenomanian time, while the talc and chlorite formation in the eastern Pyrenees occurred during several distinct Permian, Jurassic and Cretaceous episodes. These results give strong constraints on the tectonic setting of the Pyrenean domain during the transition between the Variscan and Alpine orogenic cycles, and particularly on when and how the upper crust was thinned before the crustal breakoff and the final mantle exhumation.

High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the North Pyrenean passive paleomargin

Abstract. An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40Ar–39Ar (amphibole, micas) and U–Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos–Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos–Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.

Oblique rifting of the Equatorial Atlantic: Why there is no Saharan Atlantic Ocean

Rifting between large continental plates results in either continental breakup and the formation of conjugate passive margins, or rift abandonment and a set of aborted rift basins. The nonlinear interaction between key parameters such as plate boundary configuration, lithospheric architecture, and extension geometry determines the dynamics of rift evolution and ultimately selects between successful or failed rifts. In an attempt to evaluate and quantify the contribution of the rift geometry, we analyze the Early Cretaceous extension between Africa and South America that was preceded by ∼20–30 m.y. of extensive intracontinental rifting prior to the final separation between the two plates. While the South Atlantic and Equatorial Atlantic conjugate passive margins continued into seafloor-spreading mode, forming the South Atlantic Ocean basin, Cretaceous African intraplate rifts eventually failed soon after South America broke away from Africa. We investigate the spatiotemporal dynamics of rifting in these domains through a joint plate kinematic and three-dimensional forward numerical modeling approach, addressing (1) the dynamic competition of Atlantic and African extensional systems, (2) two-stage kinematics of the South Atlantic Rift System, and (3) the acceleration of the South America plate prior to final breakup. Oblique rifts are mechanically favored because they require both less strain and less force in order to reach the plastic yield limit. This implies that rift obliquity can act as selector between successful ocean basin formation and failed rifts, explaining the success of the highly oblique Equatorial Atlantic rift and ultimately inhibiting the formation of a Saharan Atlantic Ocean. We suggest that thinning of the last continental connection between Africa and South America produced a severe strength-velocity feedback responsible for the observed increase in South America plate velocity.

The African Plate: A history of oceanic crust accretion and subduction since the Jurassic

We present a model for the Jurassic to Present evolution of plate boundaries and oceanic crust of the African plate based on updated interpretation of magnetic, gravity and other geological and geophysical data sets. Location of continent ocean boundaries and age and geometry of old oceanic crust (Jurassic and Cretaceous) are updated in the light of new data and models of passive margin formation. A new set of oceanic palaeo-age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the African plate palaeo-stresses and may have influenced the evolution of intracontinental sedimentary basins. As a case study, we compute a simple model of palaeo-stress for the Late Cretaceous time in order to assess how ridge push, slab pull and horizontal mantle drag might have influenced the continental African plate. We note that the changes in length of various plate boundaries (especially trenches) do not correlate well with absolute plate motion, but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity.

Development peculiarities of the magmatism synchronous to the formation of the North Atlantic passive margins

Magmatism synchronous to the formation of passive margins of the North Atlantic is discussed. The main features and causes of the geochemical enrichment of the primary magmas at the margins have been established. This paper is based on the published data on the Norwegian-Greenland tectonotype of volcanic margins and the West Iberia-Newfoundland tectonotype of nonvolcanic margins. In the first tectonotype the hot rifting and active magmatism gave rise to the formation of a thick crust at the margin and the adjacent oceanic zone. The second tectonotype is characterized by cold amagmatic rifting and slow initial spreading, which led to the widespread occurrence of ancient continental complexes and serpentinized mantle rocks at the margin, as well as the thin and disturbed oceanic crust nearby. In order to characterize the magmatism and initial oceanic opening, the geological and geochemical data pertaining to the reference sections chosen for each margin were compared in detail. In particular, the geochemical and isotopic data on the flood basalts and suites of parallel dikes related to the pre- and synbreakup magmatic phases were involved for the Norwegian-Greenland region. The predominance of tholeiites enriched in lithophile elements and radiogenic isotopes, as well as a significant contribution of continental material to them, are typical of the volcanic margins. No less than two enriched magma sources for the lower part of the volcanic complex are suggested, whereas a depleted or slightly enriched source is established for the upper part. A more enriched source as compared with the volcanic margins of the Norwegian-Greenland region is suggested for the low-volume magmatic manifestations at the nonvolcanic Iberian margin. The tectonic settings of margins development and their relationships with the effect of deep plumes and the propagation of the extension zone toward the cold Atlantic lithosphere are discussed.

Neoproterozoic Blaini glacial diamictite and Ediacaran Krol carbonate sedimentation in the Lesser Himalaya, India

Abstract The breakup of Rodinia resulted in the formation of rift basins and passive margins c. 650 Ma. Major palaeoclimatic vents such as Neoproterozoic global glaciation (known as ‘snowball Earth’; Hoffman et al. 1998 ) followed by global warming have been recorded on different continents, including the Indian Lesser Himalaya (Blaini–Krol Cryogenian–Ediacaran Period). The reconstruction of the Rodinia supercontinent ( Powell et al. 1993 ; Li et al. 2003 ) and the palaeoposition of India (including Lesser Himalaya–Southern China shelf facies, Tewari 2010 ) strongly suggest that a connection of the Lesser Himalayan Neoproterozoic sedimentary basins with Rodinia must have existed. Early Earth possibly witnessed its most extreme climatic fluctuations during the mid late Neoproterozoic between 750 and 550 Ma. Palaeoglaciers even reached the equator c. 635 Ma, covering the whole Earth. Evidence from Australia, Africa, Antarctica, South America, South China and the Indian Lesser Himalaya suggest that there may have been three or more palaeoglacial events during this 200 million year interval. The global decline of Meso-Neoproterozoic stromatolites, biotic evolution, diversification, extinctions and the discovery of Ediacaran life following the cold climate are of great significance. Carbon isotopic excursions from all pink cap carbonates from the Lesser Himalaya capping the Blaini glacial diamictites have shown strong negative excursions, whereas the overlying Ediacaran Krol carbonates are characterized by a positive shift in carbon isotope ratios ( Tewari 2010 ). In the Lesser Himalayan Krol belt, extending from the Solan in Himachal to Nainital in Uttarakhand Lesser Himalaya, the Upper Krol Formation (Krol D Member, Auden, 1934 , 1937 ) is a typical Ediacaran (terminal Neoproterozoic) microbial carbonate sedimentation facies. Krol D contains Ediacaran metazoan fossils and abundant microbial structures in the form of columnar, domal and stratified stromatolites. Peloids, oncoids and microphytolites are also microbially formed structures. Carbonate grainstones and packstones are common in Krol D, and the allochemical constituents are intraclasts, ooids, coated grains, peloids, microbial grains and catagraphs. A diagenesis and cathodoluminescence study of the Krol ooids has been done in detail for the first time. The sedimentary structures present in the non-stromatolitic Krol carbonates include bird's eye, cross beddings, symmetrical ripples and shallow channel structures. An intertidal to supratidal carbonate ramp depositional environment with some moderate currents and intermittent periods of exposure has been suggested for the Upper Krol carbonates of the Lesser Himalaya. A moderate to high content of organic matter (kerogen) in the Neoproterozoic microbial fossils indicates the presence of probable hydrocarbon source rocks.

Dextral shear, terrane accretion and basin formation in the Northern Andes: best explained by interaction with a Pacific-derived Caribbean Plate?

Abstract The structure, stratigraphy and magmatic history of northern Peru, Ecuador and Colombia are only adequately explained by Pacific-origin models for the Caribbean Plate. Inter-American models for the origin of the Caribbean Plate cannot explain the contrasts between the Northern Andes and the Central Andes. Persistent large magnitude subduction, arc magmatism and compressional deformation typify the Central Andes, while the Northern Andes shows back-arc basin and passive margin formation followed by dextral oblique accretion of oceanic plateau basalt and island arc terranes with Caribbean affinity. Cretaceous separation between the Americas resulted in the development of a NNE-trending dextral–transpressive boundary between the Caribbean and northwestern South America, becoming more compressional when spreading in the Proto-Caribbean Seaway slowed towards the end of the Cretaceous. Dextral transpression started at 120–100 Ma, when the Caribbean Arc formed at the leading edge of the Caribbean Plate as a result of subduction zone polarity reversal at the site of the pre-existing Trans-American Arc, which had linked to Central America to South America in the vicinity of the present-day Peru–Ecuador border. Subsequent closure of the Andean Back-Arc Basin resulted in accretion of Caribbean terranes to western Colombia. Initiation of flat-slab subduction of the Caribbean Plate beneath Colombia at about 100 Ma is associated with limited magmatism, with no subsequent development of a magmatic arc. This was followed by northward-younging Maastrichtian to Eocene collision of the trailing edge Panama Arc. The triple junction where the Panama Arc joined the Peru–Chile trench was located west of present-day Ecuador as late as Eocene time, and the Talara, Tumbes and Manabi pull-apart basins directly relate to its northward migration. Features associated with the subduction of the Nazca Plate, such as active calc-alkaline volcanic arcs built on South American crust, only became established in Ecuador, and then Colombia, as the triple junction migrated to the north. Our model provides a comprehensive, regional and testable framework for analysing the as yet poorly understood collage of arc remnants, basement blocks and basins in the Northern Andes.

Structural style of formation of passive margins, insights from dynamical modelling

We use thermo-mechanical finite element model experiments to investigate factors that are potentially important controls during volcanic and non-volcanic passive margin formation which may explain these characteristic differences. Our focus is on processes that create shear zones, on the rheological stratification of the lithosphere, and on processes that lead to differential thinning of upper and lower lithosphere during rifting. Dynamic modelling cases are compared where the crust is strong, weak, or very weak, and the mantle lithosphere is either strong or weak. Strain softening takes the form of a reduction in the internal angle of friction with increasing strain. Predicted rift modes belong to three fundamental types: 1) narrow, asymmetric rifting in which the geometry of both the upper and lower lithosphere is approximately asymmetric; 2) narrow, asymmetric, upper lithosphere rifting concomitant with narrow, symmetric, lower lithosphere extension; and 3) wide, symmetric, crustal rifting concomitant with narrow, mantle lithosphere extension.

The East European Craton (Baltica) before and during the assembly of Rodinia

Prior to participating in Rodinia, the East European Craton (Baltica) had undergone a complex process of development. Many of the structures, which were important during the amalgamation of Rodinia, were formed between ca. 1.7 and 1.4 Ga. From ca. 1.6 Ga onwards, the evolution of the western and eastern parts of this Craton followed very different path. While accretion of juvenile continental crust, and eventually, continental collision took place in the west, rifting and extension consistently dominated in the east. Between 1.14 and 0.90 Ga, the Sveconorwegian orogeny marked the incorporation of the East European Craton (Baltica) into Rodinia. This process involved four distinct phases related to Baltica's movements. During the 1.14–1.10 Ga Arendal phase there was accretion and early collision, during the 1.05–0.98 Ga Agder phase continent–continent collision took place, while the 0.98–0.96 Ga Falkenberg phase and the 0.96–0.90 Ga Dalane phase involved final convergence and post-collisional relaxation, respectively. The differences of tectonic regime in the East European Craton during the late Mesoproterozoic were determined by the movement and rotation of this megaterrane concomitantly with the Rodinia assembly. This led to collisional tectonics in the present west while break-up and the formation of passive margins occurred in the east.

Late Mesozoic–Cenozoic structural and stratigraphic correlations between the conjugate mid-Norway and NE Greenland continental margins

Late Mesozoic–Cenozoic tectonostratigraphic correlations between the conjugate mid-Norway and NE Greenland continental margins are provided, based on the recently refined structural and stratigraphic framework off mid-Norway, new early opening plate reconstructions of Eurasia versus Greenland between the Jan Mayen and Senja fracture zones, and a sparse regional grid of seismic reflection profiles off NE Greenland. The Norwegian margin exhibits a distinct along-strike margin segmentation governed by across-margin transfer systems. In particular, the Bivrost Fracture Zone and its landward transfer zone prolongation are well-defined features. Corresponding features are here interpreted on the conjugate NE Greenland margin. Together, these conjugate transfer/fracture zones represent a first-order, across-margin tectonomagmatic boundary of prolonged structural inheritance. Regional transects across both margins reveal important vertical and lateral variations in crustal configuration and composition resulting from a complex history of rifting prior to and during the last rift episode in Late Cretaceous–Early Tertiary time, leading to break-up and volcanic passive margin formation. Although the composite Late Jurassic–earliest Cretaceous rifting was the dominant tectonic episode, one also observes structural and stratigraphic relations that indicate an Aptian–?Albian rift phase, probably co-eval with similar events elsewhere on the NE Atlantic margins. Late Cretaceous rifting, with onset in middle Campanian time, was characterized by low-angle detachment faulting, culminating in regional uplift, intrusive igneous activity and subsequent erosion towards the end of the Paleocene. Thick seaward-dipping reflector sequences indicate massive eruptions of lavas during break-up at the Paleocene–Eocene transition. The post-break-up passive margin development was characterized by the transport and deposition of large amounts of sediment in response to margin subsidence and continental uplift, particularly during two distinct phases of outbuilding in Oligocene?/Miocene and Plio-Pleistocene times. Of special interest are a number of mid-Cenozoic intra-basin inversion features recognized both off Norway and off Greenland, revealing a regional compressive regime.

A wide ocean-continent transition along the south-west Australian margin: first results of the MARGAU/MD110 cruise

Syn-rift exhumation of mantle rocks in a continental breakup zone was highlighted along the present-day west Iberian passive margin [e.g. Boillot et al., 1988, 1995; Whitmarsh et al., 1995, 2001; Beslier et al., 1996; Brun and Beslier, 1996; Boillot and Coulon, 1998; Krawczyk et al., 1996; Girardeau et al., 1998] and along the fossil Tethyan margins [e.g. Froitzheim and Manatschal, 1996; Manatschal and Bernoulli, 1996; Marroni et al., 1998; Muntener et al., 2000; Desmurs et al., 2001]. Along the west Iberian margin, serpentinized peridotite and scarce gabbro and basalt lay directly under the sediments, over a 30 to 130 km-wide transition between the thinned continental crust and the first oceanic crust [Girardeau et al., 1988, 1998; Kornprobst and Tabit, 1988; Boillot et al., 1989; Beslier et al., 1990, 1996; Cornen et al., 1999]. The formation of a wide ocean-continent transition (OCT), mostly controlled by tectonics and associated with an exhumation of deep lithospheric levels, would be an essential stage of continental breakup and a characteristic of magma-poor passive margins. The southwest Australian margin provides an opportunity to test and to generalize the models proposed for the west Iberian margin, as both margins present many analogies. The south Australian margin formed during the Gondwana breakup in the Mesozoic, along a NW-SE oblique extension direction [Willcox and Stagg, 1990]. From north to south, the continental slope is bounded by (1) a magnetic quiet zone (MQZ) where the nature of the basement is ambiguous [Talwani et al., 1979; Tikku and Cande, 1999; Sayers et al., 2001], (2) a zone where the basement shows a rough topography associated with poorly expressed magnetic anomalies [Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999; Sayers et al., 2001], and which is the eastward prolongation of the Diamantina Zone, and (3) an Eocene oceanic domain. The continental breakup zone is believed to be located near or at the southern edge of the MQZ [Cande and Mutter, 1982; Veevers et al., 1990; Sayers et al., 2001]. Breakup is dated at 125 Ma [Stagg and Willcox, 1992], 95 ± 5 Ma [Veevers, 1986] or at 83 Ma [Sayers et al., 2001], and followed by ultra-slow seafloor spreading until the Eocene (43 Ma), and fast spreading afterwards [Weissel and Hayes, 1972; Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999]. The western end of the margin (fig. 1) is starved and bounded in the OCT by basement ridges where peridotite, gabbro and basalt were previously dredged [Nicholls et al., 1981]. Altimetry data [Sandwell and Smith, 1997] show that some of these ridges are continuous over 1500 km along the OCT of the south Australian margin and of the conjugate Antarctic margin. The objectives of the MARGAU/MD110 cruise (May-June 1998; [Royer et al., 1998]; fig. 2) were to define the morpho-structure and the nature and evolution of the basement in the SW Australian OCT. An area of 180 000 km2 was explored with swath bathymetry. Gravimetric data (11382 km) were simultaneously recorded whereas few single channel seismic (1353 km) and magnetic (5387 km) data were obtained due to technical difficulties. Crystalline basement rocks, made of varied and locally well-preserved lithologies, were dredged at 11 sites located on structural highs.

Mafic volcanic rocks on King Island, Tasmania: evidence for 579 Ma break-up in east Gondwana

The eastern coast of King Island in southeastern Australia exposes a thick, well-preserved sequence of latest Neoproterozoic volcanic, and related shallow intrusive rocks. These rocks are associated with shallow marine carbonates and siltstones and pass up into massive conglomerates representing a marine flooding event and unconformity, during continental break-up and subsequent volcanic passive margin formation. Unusual differentiated sills (Grimes Intrusive suite) with extreme internal variation (wehrlite to andesite compositions) intrude deformed Proterozoic metasediments of the Rodinian basement. A thin, basal tholeiitic basaltic volcanic unit (City of Melbourne Volcanics) is less contaminated than the underlying sills, and preceded eruption of a thick sequence of highly depleted picritic pillows, sub-aerial flows and hyaloclastites (Shower Droplet Volcanics). The picrite sequence is overlain by thick tholeiitic basalts and reworked volcanogenic conglomerates (Bold Head Formation) that show a strong compositional similarity to enriched mid ocean ridge basalts. Both the picrites and the upper tholeiitic basalts are not crustally contaminated and have an Nd–Sm isochron age of 579 ± 16 Ma with initial ɛNd of +4.2. The lithostratigraphy and range of compositions represented are analogous to early magmatism associated with continental break-up and volcanic passive margin formation, including voluminous Seaward Dipping Reflector Sequences, in the Mesozoic North Atlantic volcanic margins.

Unravelling a long-term multi-event thermal record in the cratonic interior of southern Finland through apatite fission track thermochronology

Apatite fission track thermochronology (AFTT) has been applied to the Precambrian basement rocks of southern Finland in an attempt to detect within the long-term thermal history, thermal manifestations in the cratonic interior of tectonic events at the craton margin. The likely subtle magnitude of these manifestations means that AFTT is a useful technique for such a study due to its low temperature sensitivity. A total of 10 samples have been analysed, generating AFTT ages, length statistics and thermal models. Ages range from 313 ± 22 to 848 ± 60 Ma and mean track lengths range from 11.0 ± 1.6 to 13.3 ± 1.8 μm. The data suggests the presence of thermal overprinting of an earlier cooling event. Thermal modelling produces similar results for all samples and typically contains the following major events: (1) two phases of Late-Proterozoic cooling, (2) Late-Silurian re-heating, (3) Cenozoic cooling. The first phase of Late-Proterozoic cooling is interpreted to be due to aulacogen inversion as a result of stress propagation from the collisional tectonics of the Sveconorwegian orogeny. The second phase is discussed in relation to passive margin formation and possible asthenospheric diaper induced relief and exhumation. The Late-Silurian re-heating coincides in time with a proposed Caledonian foreland basin. The Cenozoic cooling is interpreted to represent the latest exposure resulting from North Atlantic Margin formation induced uplift and associated denudation.

Geophysical models for the tectonic framework of the Lake Vostok region, East Antarctica

Aerogeophysical and seismological data from a geophysical survey in the interior of East Antarctica were used to develop a conceptual tectonic model for the Lake Vostok region. The model is constrained using three independent data sets: magnetic, seismic, and gravimetric. A distinct change in the aeromagnetic anomaly character across Lake Vostok defines a crustal boundary. Depth to magnetic basement estimates image a 400-km-wide and more than 10-km-deep sedimentary basin west of the lake. Analysis of teleseismic earthquakes suggests a relatively thin crust beneath Lake Vostok consistent with predictions from kinematic and flexural gravity modelling. Magnetic, gravity, and subglacial topography data reveal a tectonic boundary within East Antarctica. Based on our kinematic and flexural gravity modelling, this tectonic boundary appears to be the result of thrust sheet emplacement onto an earlier passive continental margin. No data presently exist to date directly either the timing of passive margin formation or the subsequent shortening phase. The preserved thrust sheet thickness is related to the thickness of the passive margin crust. Because a significant amount of time is required to erode the thrust sheet topography, we suggest that these tectonic events are Proterozoic in age. Minor normal reactivation of the thrust sheet offers a simple mechanism to explain the formation of the Lake Vostok Basin. A low level of seismicity exists in the vicinity of this tectonic boundary. The existence of a crustal boundary in the Antarctic interior provides new constraints on the Proterozoic architecture of the East Antarctic craton.

The role of lower crust and continental upper mantle during formation of non-volcanic passive margins: evidence from the Alps

Abstract The remnants of a Mesozoic passive continental margin and of the Tethyan ocean floor are preserved in the Austroalpine and Upper Penninic nappes in eastern Switzerland and northern Italy. Reconstructions of the continent-ocean transition indicate that large areas of subcontinental mantle rocks, but only limited areas of lower-crustal rocks were exposed on the Tethyan sea floor. Microstructures, large shear zones, and the retrograde metamorphic evolution of peridotite and gabbro from Malenco (northern Italy) are investigated to evaluate the role of lower crust and upper mantle during formation of non-volcanic passive continental margins. The combination of petrological constraints and microstructures suggests two contrasting stages: (1) high-temperature (> 650 °C) shearing and annealing of microstructures are attributed to pre-rift tectonics; (2) localized mylonitic shear zones cut the high-temperature structures and developed during nearly isothermal decompression ( T <600°C), followed by cooling and hydration of the rocks. These shear zones formed during exhumation of the lower crust and upper mantle and are related to early rifting of the Adriatic passive continental margin. The microstructures of the hydrous mylonites display drastic grain-size reduction, which results from a combination of dynamic recrystallization and metamorphic hydration reactions at temperatures <650°C. Strain softening facilitated the formation of crustal-scale shear zones along which the lower crust and upper mantle were exhumed to shallow crustal levels of c. 10–15 km. Such large shear zones excised 10–20 km of mostly intermediate and lower crust, and are linked to and contemporaneous with the formation of rift-related basins in the upper crust. Boudinage of the lower crust during early rifting is proposed as a major process to explain the scarcity of exposed lower crust along non-volcanic passive margins. The compilation of pressure-temperature data and rift-related structures in the deep crust and upper continental mantle from the Alps suggests that most peridotites preserve a high-temperature evolution that is not related to Mesozoic rifting. Granulite-facies rocks occur in pre-rift lower and middle continental crust. Exhumed granulites along passive continental margins preserve much of a history that is not related to the exhumation itself, but to tectonic processes predating rifting.

Formation and evolution of the Solander Basin, southwestern South Island, New Zealand, controlled by a major fault in continental crust and upper mantle

Seismic reflection and refraction data from the Solander Basin, southern New Zealand, show that its structural evolution has been controlled by a major fault, named here the Tauru Fault, that cuts the entire crust and splays into a diffuse zone in the upper mantle. The tectonic setting of the Solander Basin has evolved from Eocene‐Oligocene extension and transtension to Miocene‐Quaternary transpression and subduction. The Tauru Fault is 100 km east of the active Puysegur subduction zone thrust and is part of the overriding plate. On the basis of lower crustal reflectivity, the base of the crust beneath the adjacent Stewart Island shelf is at ∼30 km depth (∼9 s two‐way time (TWT)), and rises to ∼20 km (∼8 s TWT) beneath the Solander Basin. This is consistent with gravity data. Prominent dipping reflections show that the Tauru Fault can be traced to ∼30 km depth (∼12 s TWT), where it merges with a zone of subhorizontal reflectors in the upper mantle. The Tauru Fault dips ∼30° northeast and appears to offset the Moho in a reverse sense. Stratigraphic relationships show that the Tauru Fault was active as a normal fault during Eocene extension, when Solander Basin crust was thinned and ocean crust was generated farther south in the Solander Trough. It has been reactivated as a reverse fault during at least two phases of Miocene‐Quaternary compression and is still active. The strike of the Tauru Fault, which is parallel to Paleozoic‐Mesozoic structures and was poorly oriented for the known Eocene extension direction, strongly suggests that it formed prior to Eocene time. The Tauru Fault significantly influenced the geometry of Eocene basin formation, producing a strongly asymmetric basin dominated by east dipping normal faults, with a single eastern boundary fault. Our data demonstrate that Miocene‐Quaternary simple shear associated with the Tauru Fault cuts the whole crust and continues into the upper mantle. We conclude that variations in strength of the lithosphere, particularly associated with inherited structures in the crust and upper mantle, may control many aspects of basin development, passive margin formation, and the kinematics of continental deformation zones.

Tectonic, magmatic and depositional processes at passive continental margins

The papers presented in this thematic collection all derive from a meeting on geological processes at passive margins, held at Burlington House in February 1996. The meeting was co-hosted by the Geological Society, The Norwegian Geological Society and The Norwegian Petroleum Society. We would like to thank our colleagues Bjørn Larsen and Olav Eldholmfor ensuring the strong Norwegian participation at the meeting and Mike Daly for organizing contributions from the UK hydrocarbon industry. The formation of passive continental margins during continental rifting to form new oceans is a fundamental component of the plate tectonic process yet many aspects of the formation of passive margins are still poorly understood. Passive margins are amongst the more difficult areas of the earth's surface to study, because of their great water depths and the difficulty of acquiring data. Much of the current upsurge of attention to passive continental margins, which is particularly strong for the Irish, UK and Norwegian Atlantic margins, arises from the increased interest in their hydrocarbon potential resulting from advances in deep water exploration and production technology. This renewed interest has led to the collection of new data and the increased exchange of ideas between industry and academia, some of the results of which are presented here. The successful geological analysis of passive margins calls for the integration of many different disciplines. The papers presented here show how we need to understand the link between tectonics, depositional systems and magmatic processes in order to understand margin evolution. In addition much of

Gravity anomalies and spatial variations of flexural rigidity at mountain ranges

New continent‐wide compilations of gravity and topography data have been used to determine spatial variations in the long‐term mechanical properties of the foreland lithosphere at mountain ranges. In the southern Appalachians, a large elastic thickness Te (in the range 40–70 km) is required with variation only along the length of the mountain range. To explain the Appalachian Bouguer gravity anomaly “high”, a subsurface load and crustal thinning are necessary. For the Andes, Te varies in the range 5–85 km perpendicular and parallel to the length of the mountain range. In the Alps, Te is high in the French Alps and then decreases in the western Swiss Alps and increases toward the eastern Alps. In the Romanian Carpathians, Te variations in the range 5–20 km are found. Although the thermal state, curvature, and inelastic yielding may play some role, these variations in Te are interpreted as the result of inheritance of the thermal and mechanical properties of foreland lithosphere. Inheritance plays a role in influencing the development of the next plate tectonic event. At mountain ranges, preexisting strength variations offer some control on tectonic style, the geometry of fold and thrust belts, and the stratigraphic patterns that develop in the adjacent foreland basin. Foreland lithosphere is involved in a cycle of events that began with continental rifting and passive margin formation and ended in continental collision. Compilations of Te from continental rifts, passive margins, and foreland lithosphere show large variations. The lowest values are associated with rifts, margins, and foreland lithosphere which has recently been been involved in continental breakup. The highest values are associated with foreland lithosphere for which a long time has elapsed since the previous rifting event. We speculate that continental lithosphere undergoes some form of strength recovery.