Rift Propagation Research Articles

Variations in axial morphology, segmentation, and seafloor roughness along the Pacific‐Antarctic Ridge between 56°S and 66°S

The spreading rate at the Pacific‐Antarctic Ridge (PAR) increases rapidly from 54 mm/yr near Pitman Fracture Zone (FZ) up to 76 mm/yr near Udintsev FZ, resulting in three domains of axial morphology: an axial valley south of Pitman FZ, an axial high north of Saint Exupéry FZ, and in between, the transitional domain extends over 650 km. It comprises sections of ridge with an axial valley or an axial high and generally displays a very low cross‐sectional relief. It is also characterized by two propagating rifts. Two domains of different seafloor roughness appear south of Udintsev FZ: east of 157°W these two domains are separated by a 1000‐km V‐shaped boundary. West of 157°W, the boundary approximately coincides with Chron 3a or Chron 4. The southward migration of the transitional area during the last 35 Myr explains the V‐shaped boundary: (1) increases in spreading rate above a threshold value produced changes in axial morphology; and (2) in the transition zone, rotations of the spreading direction were accommodated by the plate boundary, either by rift propagation or by transitions from fracture zones to non transform discontinuities, leaving trails on the seafloor that presently delineate the V. Seafloor roughness variations are not controlled by exactly the same spreading rate dependence as changes in axial morphology. The transition from rough to smooth seems to have occurred everywhere for spreading rates greater than 50 mm/yr, except in a domain presently centered on Saint‐Exupéry FZ, where it occurred for spreading rates >60 to 65 mm/yr. Independent results from melting model calculations of major elements [Vlastelic et al., 2000] indicate that the upper mantle temperature is likely to be cooler between Antipodes and La Rose FZs. The combination of these two results reveals the existence of a 700‐km‐long segmentation of the upper mantle, with a “cool” area centered on Saint‐Exupéry FZ.

Continental rifting parallel to ancient collisional belts: an effect of the mechanical anisotropy of the lithospheric mantle

Abstract Analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process. Rift propagation is not random, but tends to follow the trend of the orogenic fabric of the plates, systematically reactivating ancient lithospheric structures. Continental rifts often display a clear component of strike–slip deformation, in particular in the early rifting stage. Moreover, although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process, there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts. These observations suggest that the deformation of the lithosphere, especially during rifting, is controlled by its preexisting structure. On the other hand, (1) the plasticity anisotropy of olivine single crystal and aggregates, (2) the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs, and (3) seismic anisotropy data, which require a tectonic fabric in the upper mantle coherent over large areas, suggest that preservation within the lithospheric mantle of a lattice preferred orientation (LPO) of olivine crystals may induce a large-scale mechanical anisotropy of the lithospheric mantle. We use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process. We assess the deformation of an anisotropic continental lithosphere in response to an axi-symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head. Model results show that a LPO-induced mechanical anisotropy of the lithospheric mantle may result in directional softening, leading to heterogeneous deformation. During continental rifting, this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique (30–55°) to the preexisting mantle fabric. This directional softening associated with olivine LPO frozen in the lithospheric mantle may also guide the propagation of the initial instability, that will follow the preexisting structural trend. The preexisting mantle fabric also controls the deformation regime, imposing a strong strike–slip shear component. A LPO-induced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting (structural inheritance), the plume–rift paradox, and the onset of transtension within continental rifts.

The role of the Spitsbergen shear zone in determining morphology, segmentation and evolution of the Knipovich Ridge

In 1989–1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km2 of the seafloor in the Norwegian-Greenland Sea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from its oblique (115° ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determine whether or not the ancient Spitsbergen Shear Zone continued to play any involvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Ridge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V-shaped-oriented flank faults which have their apex at the present day Molloy Transform, and rift related faults that extend north of the present Molloy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial highs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi stationary, focused magma centers creating (axial highs) located between long oblique rift basins. Present day segment discontinuities on the Knipovich Ridge are aligned along highly oblique, probably strike-slip faults, which could have been created in response to rotating shear couples within zones of transtension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formation of en-echelon pull apart basins. The curved stress trajectories create arcuate faults and subsiding elongate basins while focusing most of the volcanism through the boundary faults. As a result, the Knipovich Ridge is characterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple shear detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbergen Shear Zone on the evolution of the Knipovich Ridge is the primary reason that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along the Knipovich Ridge and Mohns Ridge (280° vs. 330°, respectively) cause the crust on the western side of the intersection of these two ridges to buckle and uplift via compression as is evidenced by the uplifted western wall province and the large 60 mGal free air gravity anomalies in this area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading regime has not evolved along this ridge.

The tectonic and geomagnetic significance of paleomagnetic observations from volcanic rocks from central Afar, Africa

Deformation throughout Afar over the past 2 myr has been characterized by widespread and intense crustal fragmentation that results from inhomogeneous extension across the region. In eastern Afar, this situation has evolved to localized extension associated with the westward propagation of the Gulf of Aden/Gulf of Tadjurah seafloor spreading system into the Asal–Ghoubbet Rift. During the gradual process of rift propagation and localization, crustal blocks in eastern Afar sustained clockwise rotations of ∼11°. To better understand the processes of rift propagation and localization and how they affect the rest of Afar, we have collected and analyzed over 400 oriented paleomagnetic samples from 67 lava flows from central and southern Afar. Unlike eastern Afar, the mean paleomagnetic direction from central Afar indicates that vertical-axis rotations are statistically insignificant (3.6°±4.4°), though small clockwise rotations (<8°) are permitted. Thus, propagation and localization in central Afar have not had the same influence in causing crustal block rotations or, perhaps more likely, have not reached the same stage of evolution as seen in eastern Afar. In addition, several of the lava flows record intriguing geomagnetic field behavior associated with polarity transitions, excursions, or large secular variation events. Interestingly, the transitional or anomalous virtual geomagnetic poles (VGPs) tend to cluster in two nearly antipodal regions, one in the northern Pacific Ocean and the other in the southwest Indian Ocean. One lava flow has recorded both of the antipodal transitional components, with the two components residing in magnetic minerals with unblocking temperatures above and below ∼500°C, respectively. Reheating and partial remagnetization by the overlying flow cannot explain either of the transitional directions because both differ significantly from that of the reversely magnetized overlying flow. The high-temperature component gives a VGP in the northern Pacific, whereas the lower-temperature component gives a nearly antipodal VGP south of Cape Town, South Africa. Hence, the configuration of the geomagnetic field appears to have jumped nearly instantaneously from a northern-hemisphere transitional state to a southern-hemisphere one during this normal-to-reverse polarity transition.

Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa

Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa

Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa

Research Article| February 01, 2000 Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa C. J. Ebinger; C. J. Ebinger 1Department of Geology, Royal Holloway, University of London, Egham TW20 0EX, United Kingdom Search for other works by this author on: GSW Google Scholar T. Yemane; T. Yemane 2Ethiopian Institute of Geological Surveys, Addis Ababa, Ethiopia3Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar D. J. Harding; D. J. Harding 4NASA Goddard Space Flight Center, Greenbelt, Maryland 20771 Search for other works by this author on: GSW Google Scholar S. Tesfaye; S. Tesfaye 2Ethiopian Institute of Geological Surveys, Addis Ababa, Ethiopia5Department of Geological Sciences, University of Colorado, Boulder, Colorado 80305 Search for other works by this author on: GSW Google Scholar S. Kelley; S. Kelley 3Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar D.C. Rex D.C. Rex 6Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (2): 163–176. https://doi.org/10.1130/0016-7606(2000)112<163:RDMAPL>2.0.CO;2 Article history received: 22 Oct 1997 rev-recd: 28 Feb 1998 accepted: 01 Apr 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. J. Ebinger, T. Yemane, D. J. Harding, S. Tesfaye, S. Kelley, D.C. Rex; Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa. GSA Bulletin 2000;; 112 (2): 163–176. doi: https://doi.org/10.1130/0016-7606(2000)112<163:RDMAPL>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Main Ethiopian and Eastern (Gregory) rifts, sectors of the East African rift system, overlap in a 300-km-wide system of extensional basins that is more than three times the breadth of either rift away from the zone of overlap. The oldest volcanic rocks (Eocene) and possibly the oldest rift basins (Oligocene) of the East African rift system occur in this zone of overlap. The objectives of field, remote sensing, and geochronology (K-Ar and 40Ar/39Ar) studies in southwestern Ethiopia were to establish a chronology of rifting and volcanism in the zone of overlap, and to correlate stratigraphic sequences with those in the Kenya rift to the south and in the Main Ethiopian rift to the north. Field observations and cross sections show that basins are bounded by steeply dipping faults, stratal dips are <30°, and that extension accommodated by the intrusion of dikes is volumetrically insignificant. Thus, the style of faulting is similar to that elsewhere in East Africa south of the Afar rift. Initial volcanism between ca. 45 and 33 Ma preceded faulting and uplift, except for reactivation of some Mesozoic rift structures near the Sudan-Ethiopia border and in northern Kenya. Extensional basins began to form in late Oligocene time in the Eastern rift, and in early Miocene time in the Main Ethiopian rift. Small degrees of extension and associated volcanism in the broadly rifted zone may have been triggered by extension in the Red Sea, as well as by lithospheric heating above a mantle plume. The anomalous breadth of the zone is a consequence of rift propagation and migration, rather than basin-and-range–style extension; both the Main Ethiopian rift and Eastern rifts have propagated along north-south lines, and the Eastern rift has migrated ∼200 km eastward since late Oligocene time. The distribution of seismicity and Quaternary volcanism suggest that the Eastern and Main Ethiopian rifts are currently linked across a 200-km-wide zone between the Omo and Segen basins. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Analogue experiments of propagation of oblique rifts

We present the results of analogue models of oblique rifting, which were carried out using a isostatically compensated two-layered small-scale models. The experiments show that structures formed in the brittle layer under oblique extension comprise a series of lenticular, rifted basins, set en échelon along the axial zone of the experiment box. In time, the basins propagate lengthways to merge and form a unified composite rift. However, traces of the initial en échelon rifts, expressed in deeper axial basins separated by shallower threshold zones, are clearly discernible in the mature rifts. Both the basins and the threshold zones are located within the composite rift, so that the rift margins are abrupt and steep along the basins, and more diffuse along the threshold zones. The thinning in the brittle layer along the rift is enhanced by diapirs that ascend from the ductile layer and penetrate the rift floor. Model experiments of nearly orthogonal extension showed a single, unified rift, which differed conspicuously from the composite oblique rifts. The results of the experiments suggest that oblique extension forms en échelon rift segments, the axes of which are perpendicular to the extension direction, separated by threshold zones. Therefore, there is ground to presume that occurrence of a series of alternating basins and threshold zones inside the axial section of a major rift could suggest a regime of oblique tectonic extension.

Rift kinematics during the incipient stages of continental extension: Evidence from the nascent Okavango rift basin, northwest Botswana

High-resolution aeromagnetic data from the nascent Okavango rift in northwest Botswana provide an unprecedented view of rift kinematics during the incipient stages of continental extension. Crosscutting relationships between west-northwest–trending 180 Ma Karoo dikes and reactivated northeast-trending Proterozoic basement faults are used to document the kinematics of Cenozoic faulting during the initial stages of rifting. Depth estimates to the top of the dikes using three-dimensional Euler deconvolution solutions have produced the following interpretations. (1) The Okavango rift is a half graben with a downthrow of ∼200–300 m. (2) The width of the Okavango rift (100 ± 20 km) is similar to that of more mature continental rifts such as the Tanganyika and Baikal rifts. This suggests that the width of continental rifts is acquired early in their evolution and reflects neither the age and maturity of the rift basin, nor the amount of extension. It is suggested that the cumulative downthrow (sediment infill included) and subsidence may be a better indicator of the relative maturity of rift basins. (3) Preexisting basement faults exert a major control during rifting, and reactivation processes do not occur synchronously along the entire length of preexisting faults. (4) The Okavango rift is defined by normal faults; there is no evidence of major strike-slip faults, thus excluding a pull-apart tectonic model for this nascent continental rift stage. (5) The preexisting Sekaka shear zone terminates the Okavango rift to the south, suggesting that such shear zones represent major barriers during longitudinal propagation of rifts. This probably explains why such shear zones commonly evolve into accommodation or transfer zones during further evolution of continental rifts.

Forces driving ridge propagation

Recent multibeam bathymetric and geophysical data from the Southeast Indian Ridge (SEIR) reveal eight propagating ridges (PRs) moving down the regional bathymetry gradient toward the Australian‐Antarctic Discordance (AAD). These PRs are propagating at a nearly uniform rate of 40‐mm yr−1 despite dramatic variations in axial morphology and segment length, and local reversals of the regional bathymetric gradient. Because existing dynamic rift propagation models do not explain the continued, uniform propagation of the diverse SEIR PRs, we have developed a new model that includes variable crustal thickness and thermally‐driven, along‐axis asthenospheric flow, in addition to topographic gradients, as positive contributions to the forces driving rift propagation. Along‐axis asthenospheric flow and variations in crustal thickness along the SEIR appear to provide the first‐order driving force required for ridge propagation, even in the highly segmented, magmatically starved region in and near the AAD where previous models for ridge propagation fail. Further, our model development requires that the rift valley at the propagating tip is a consequence of rifting in the cooler thermal regime of preexisting lithospherc, rather than a consequence of viscous resistance to flow. This explanation of PR tip topography is consistent with gravity data from the SEIR that suggest that the rift tip and associated pseudofaults are at least partially compensated by crustal thinning.

Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia

Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet–spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser–ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field.

Paleomagnetic and geochronological identification of the Réunion subchron in Ethiopian Afar

This paper reports the paleomagnetic and geochronological analysis of a suite of samples from a basaltic fault‐related scarp of Pliocene age in the central part of the Afar depression (Ethiopia). Paleomagnetic work uncovers a clear characteristic remanent magnetization carried by (titano‐) magnetites, with high unblocking temperatures, close to the Curie temperature of pure magnetite. The characteristic direction, based on 27 non transitional flows, is D=8.1°, I=10.1° (α95=4.1°). This is consistent with earlier determinations of the clockwise tectonic rotation, due to rift propagation and overlap, of the block to which the section belongs. Magnetic stratigraphy consists of a succession from bottom to top of 16 reversed, 7 normal, and 4 reversed flows. One flow at the upper normal to reverse transition demonstrates complex behavior upon thermal and alternating field (AF) demagnetization, which is due to remagnetization by the overlying flow. K/Ar dating of five samples provides consistent determinations, averaging 2.07 ± 0.05 Ma. All ages agree with this average, given their rather large individual uncertainties (from 0.04 to 0.08 Ma at the 1σ level). Comparison with recent reference geomagnetic polarity timescales (GPTS) indicates that the normal subchron recorded in the Gamarri section must be linked with (one of) the Réunion event(s). The normal subchron and an earlier episode of large secular variation could provide the best volcanic record of a double Réunion event. Although uncertainties in ages are too large to allow unequivocal inferences, our age determination for the Réunion event is in better agreement with earlier determinations of McDougall and Watkins [1973] than with a more recent value proposed by Baksi et al. [1993] and used in the GPTS.

On causal links between flood basalts and continental breakup

Temporal coincidence between continental flood basalts and breakup has been noted for almost three decades. Eight major continental flood basalts have been produced over the last 300 Ma. The most recent, the Ethiopian traps, erupted in about 1 Myr at 30 Ma. Rifting in the Red Sea and Gulf of Aden, and possibly East African rift started at about the same time. A second trap-like episode occurred around 2 Ma and formation of true oceanic crust is due in the next few Myr. We find similar relationships for the 60 Ma Greenland traps and opening of the North Atlantic, 65 Ma Deccan traps and opening of the NW Indian Ocean, 132 Ma Parana traps and South Atlantic, 184 Ma Karoo traps and SW Indian Ocean, and 200 Ma Central Atlantic Margin flood basalts and opening of the Central Atlantic Ocean. The 250 Ma Siberian and 258 Ma Emeishan traps seem to correlate with major, if aborted, phases of rifting. Rifting asymmetry, apparent triple junctions and rift propagation (towards the flood basalt area) are common features that may, together with the relative timings of flood basalt, seaward dipping reflector and oceanic crust production, depend on a number of plume- and lithosphere- related factors. We propose a mixed scenario of `active/passive' rifting to account for these observations. In all cases, an active component (a plume and resulting flood basalt) is a pre-requisite for the breakup of a major oceanic basin. But rifting must be allowed by plate-boundary forces and is influenced by pre-existing heterogeneities in lithospheric structure. The best example is the Atlantic Ocean, whose large-scale geometry with three large basins was imposed by the impact points of three mantle plumes.

Episodic triple-junction migration by rift propagation and microplates

Research Article| October 01, 1999 Episodic triple-junction migration by rift propagation and microplates Robert T. Bird; Robert T. Bird 1Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907, Australia Search for other works by this author on: GSW Google Scholar Sarah F. Tebbens; Sarah F. Tebbens 2Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701, USA Search for other works by this author on: GSW Google Scholar Martin C. Kleinrock; Martin C. Kleinrock 3Department of Geology, Vanderbilt University, Nashville, Tennessee 37235, USA Search for other works by this author on: GSW Google Scholar David F. Naar David F. Naar 2Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Robert T. Bird 1Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907, Australia Sarah F. Tebbens 2Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701, USA Martin C. Kleinrock 3Department of Geology, Vanderbilt University, Nashville, Tennessee 37235, USA David F. Naar 2Department of Marine Science, University of South Florida, St. Petersburg, Florida 33701, USA Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1999) 27 (10): 911–914. https://doi.org/10.1130/0091-7613(1999)027<0911:ETJMBR>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert T. Bird, Sarah F. Tebbens, Martin C. Kleinrock, David F. Naar; Episodic triple-junction migration by rift propagation and microplates. Geology 1999;; 27 (10): 911–914. doi: https://doi.org/10.1130/0091-7613(1999)027<0911:ETJMBR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We describe a model for episodic, open-ocean triple-junction migration based on observations of actual triple-junction evolutions. Migration progresses by repeated episodes of rift propagation, microplate formation, and microplate accretion to an adjacent, larger plate. These episodes may be highly variable in space and time depending on triple-junction geometry, velocity triangle, and other factors affecting local thermal and rheological conditions. Resulting tectonic features may include an abandoned transform fault, straight and potentially curving pseudofaults, sheared and potentially rotated abyssal-hill fabric, and a paleomicroplate with no associated failed rift. This model, developed mainly from the evolution of the Pacific-Antarctic-Nazca triple junction, may be relevant for other types of triple junctions such as the Bouvet and Azores triple junctions and ridge-ridge-ridge triple junctions in the Indian and Pacific Oceans. Episodic migration is found to occur even when the triple junction is kinematically stable. This model highlights the difference between predicted kinematic stability of triple junctions and observations of their true tectonic history. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

History of the northeast Brazilian rift system: kinematic implications for the break-up between Brazil and West Africa

Abstract The record of diachronic rift stages and widespread crustal deformation within the Brazilian-West African rift basins indicates changes in plate tectonics regimes and associated stress fields during the opening of the South Atlantic. Similarly to the presently active East African Rift System, the intracontinental rift basins of northeast Brazil provide a good example of an ancient intracontinental rift zone. They illustrate the three-dimensional geometries of rift systems and the control exercised by the basement structural framework, as well as important chronostratigraphic data that constrain the break-up of Gondwanaland between Brazil and West Africa. The stepwise opening of the South Atlantic led to considerable deformation, recorded in northeast Brazil as magmatic and sedimentary events, and by at least three tectonic stages, Syn-rift I, II and III. The tectonic stages are defined in northeast Brazil by discrete episodes of half-graben formation distributed over three main rift axes: (1) the Gabon-Sergipe-Alagoas trend; (2) the Recôncavo-Tucano-Jatobá trend; (3) the Cariri-Potiguar trend. Extensional deformation commenced in the Late Jurassic-early Berriasian and culminated in the late Aptian, when oceanic crust began to be emplaced between northeast Brazil and West Africa. Rift propagation is a matter of debate in the Equatorial Atlantic. It is suggested that the break-up progressed from south to north in the southern South Atlantic (Southern Branch) and concurrently from east and west in the Equatorial Branch. The rifting has been dated as Early Cretaceous (Neocomian-Barremian time) in the Southern Branch, and younger (Aptian-Albian) in the Equatorial Branch Palaeontological data indicate that these two branches intersected between northeast Brazil and Cameroon, Africa. Numerous reconstructions of the opening history of the South Atlantic have been proposed. However, most of these models are based on partial observations or an incomplete understanding of the sedimentary and tectonic record, and have resulted in derivation of improbable regional stress fields. This paper provides a regional view of a series of intracontinental rift basins in northeast Brazil, emphasizing their differences and similarities, as well as the diachronism within the equatorial and the southern basins. These basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases, especially during the Neocomian. Palaeostress directions for the northeast Brazilian rift basins are, even with the best available data sets, difficult to determine because different tectonic regimes can result in similarly oriented rift basins, especially in the Borborema Province, northeast Brazil, where large Proterozoic shear zones determine the pre-rift tectonic framework. Examination of this region demonstrates the results of changes in the regional kinematic framework through time and the control exercised by the extensively deformed Proterozoic continental crust of rift geometries.

Palaeozoic to Tertiary rift and basin dynamics: mid-Norway to the Bay of Biscay – a new context for hydrocarbon prospectivity in the deep water frontier

Since the advent of North Sea exploration, a variety of new advances have been made in understanding basin dynamics and regional variations in structural style that are well-founded on the stratigraphy of the better known basins. New insights have also been made into the influence of Palaeozoic compressional and inversion tectonics on the structure of Mesozoic successor basins. This paper, based in part on these advances, offers a new, post plate tectonics, model for the structural and stratigraphic development of Mesozoic and Tertiary basins between mid-Norway and the Bay of Biscay. The tectono-stratigraphic evolution of this region can now be simply described in terms of a southward propagating ‘Arctic’ rift and a northward propagating ‘Atlantic’ rift. These rifts, formed by polyphase extension, remained largely separate entities until the Late Jurassic or Early Cretaceous. Thereafter, the rifts and their successor basins became a single entity represented by the Rockall Trough, Faeroe–Shetland Basin and Møre Basins only modified by later Cretaceous events and the Early Tertiary break-up of the North Atlantic and Norwegian–Greenland Sea. The ‘Arctic’ rift was initiated possibly as early as the Late Palaeozoic, nucleating on Caledonian and Late Devonian structures in the strike-parallel Caledonian terranes. Polyphase extensional events between the Permo-Triassic and Late Jurassic with intervening phases of passive subsidence resulted in rifts and basins of opposing polarity, sometimes superposed or offset, trending from the Barents Sea into the greater North Sea area. In contrast, the ‘Atlantic’ rift was probably initiated by rifting between North America and Africa in Late Triassic to Early Jurassic times. Its northward propagation was at first limited by the E-W and NE-SW Hercynian and Caledonian terranes of the British Isles and northeast Canada. The main subsequent major phase of extension initiated in the Late Jurassic and continuing through the Early Cretaceous progressively opened the North Atlantic from south to north through into the Bay of Biscay. The northward propagation of the Atlantic rift from Biscay into the Rockall Trough and Faeroe–Shetland Basin–Møre Basin completed the linkage to the Arctic rift possibly as early as the Late Jurassic and certainly by the Albo–Aptian. The ensuing rift, underlain by highly stretched continental crust, extended from the Barents Sea via the Vøring Plateau, Møre Basin into the Rockall Trough. The subsequent Late Cretaceous history of the linked system of rifts is characterized by both extension and inversion of unknown origin. In the Early Tertiary, rifting associated with voluminous volcanism along the entire margin and on the scale of the Deccan Traps heralded the break-up by spreading of the Norwegian–Greenland Sea and the northern North Atlantic. A subsequent phase of rifting elevated the Jameson Land Basin of East Greenland exhuming major oil accumulations. In contrast, co-eval phases of inversion in the Vøring Basin, Møre Basin and the Faeroe–Shetland Basin have had a positive impact on prospectivity. Inversion in the Celtic Sea is more probably related to the Pyrennean orogeny. The model of ‘Arctic’ and ‘Atlantic’ rifts offers a new and simpler predictive model of source and reservoir distribution. In the ‘Arctic’ rift, i.e. the North Sea, West of Shetland and offshore Norway–Greenland, volumetrically significant source-bed systems are confined to the Permo–Triassic, Toarcian, Middle and Late Jurassic. In the ‘Atlantic’ rift, between the Celtic Sea, Iberia and the Grand Banks, the major sources seem to be of Toarcian and Late Jurassic age. Cretaceous source beds are minor in the linked rift. However, the multiple phases of rifting and later burial or inversion history have severely affected the maturity of the Jurassic, although appeal to remigration is clearly necessary in the Faeroe–Shetland Basin. Distribution of the major reservoirs in both rifts can be best understood in relation to phases of rift propagation and subsequent subsidence. Thus, the early rifts and their immediate successor passive basins are characterized by shallow marine sedimentation. In contrast, the Late Jurassic-Early Cretaceous rifts were subject to stretching that resulted in initially deep marine environments and very thick infill of later Cretaceous sediments. Significant sand input to the linked rifts only began in the Early Tertiary as a result of the uplift and unroofing of Scotland that heralded the opening to the North Atlantic.

Microplate rotation in northeast Brazil during South Atlantic rifting: Analogies with the Sinai microplate

The Early Cretaceous northeast Brazilian Sergipe microplate, formed at the northern end of the South Atlantic rift between South America and Africa, closely resembles the modern Sinai microplate at the northern end of the Red Sea in size, shape, and relative motion. Both formed where east-northeast–trending transverse shear zones arrested northward rift propagation, causing the Tucano-Reconcavo and Gulf of Suez rifts to fail and be replaced by northeast-trending leaky transforms (Sergipe-Alagoas and Dead Sea transforms) as the new paths of continental breakup. Bordered by the failed rift, the leaky transform, and the transverse shear zone, both microplates were rotated counterclockwise by drag along their eastern transform margins. Rotation thrust the edge of the Sergipe microplate over part of its northern border, creating the Arcoverde thrust wedge. The northwest-trending Vaza-Barris fault sheared the microplate, transferring the rift and evaporite sequence from the Sergipe-Alagoas to the Gabon continental margin. In Albian time, heating of the lithosphere in the Cabo igneous province near Recife permitted the South Atlantic rift to propagate across the Arcoverde thrust wedge, completing continental breakup.

Volcanic margin formation and Mesozoic rift propagators in the Cuvier Abyssal Plain off Western Australia

The western margin of Australia is characterized by synrift and postrift magmatism which is not well understood. A joint interpretation of magnetic anomaly, satellite gravity anomaly and seismic data from the Cuvier Abyssal Plain and margin shows that the breakup between India and Australia started circa 136 Ma (M14) and was followed by two rift propagation events which transferred portions of the Indian Plate to the Australian Plate. Post breakup magmatism continued with the emplacement of the Wallaby and Zenith plateaus (∼17–18 km thick at their centers) along a transform margin. Two narrow magmatic edifices adjacent to the Wallaby Plateau (Sonne and Sonja ridges) represent an extinct ridge and a pseudofault, respectively. They formed by excess volcanism, probably by lateral migration of buoyant melt along upside‐down crustal drainage channels from the melt source underneath the Wallaby Plateau. In a mantle plume scenario a small plume (∼400 km diameter) located underneath the rift could have locally uplifted the Bernier Platform and Exmouth Sub‐basin in the Early Cretaceous and left a track consistent with the azimuth of the Wallaby and Zenith plateaus. In this case, ridge‐plume interaction would have caused two consecutive ridge propagation events towards the plume while the ridge moved away from the hotspot. The abrupt end of the hotspot track west of the Zenith Plateau would be a consequence of the accelerating south‐eastward motion of the spreading ridge relative to the mantle after 120 Ma, leaving the mantle plume underneath the Indian Plate. An alternative nonmantle‐plume scenario is based on the observation that between breakup and chron M0 (∼120 Ma) the ocean crust in the southern Cuvier Abyssal Plain was formed while the spreading ridge abutted Indian continental crust. Small‐scale convection may have been initiated during rifting in the Early Cretaceous and maintained until the Wallaby‐Zenith ridge‐transform intersection passed by the eastern edge of Indian continental crust at chron M0.

Rift propagation, detachment faulting, and associated magmatism in Bransfield Strait, Antarctic Peninsula

Multichannel seismic data from Bransfield Strait, a young (&lt;4 Ma?) marginal basin located at the NW tip of the Antarctic Peninsula, reveal a complex array of rifting styles short of clearly defined seafloor spreading. The strait is undergoing basinwide extension. While enriched mid‐ocean ridge basalts have been dredged from topographic highs in the NE part of the basin, seismic evidence does not support the formation of oceanic crust there. Acoustic signatures of volcanic crust at known axial deep highs extend laterally subseafloor; their distribution and style support propagation of volcanism and associated extension from NE to SW. Active extensional structures along both margins control a strong physiographic asymmetry. The South Shetland Islands margin is composed of a few, closely spaced, large‐offset normal faults; the gentler Antarctic Peninsula margin exhibits broader, distributed extension. To the NE, normal fault polarity reversal structures (NFPRs) dominate; listric faults sole into NW dipping, low‐angle reflectors interpreted as detachments. To the SW, larger NFPRs and convex‐up geometries up to ∼20 km across suggest broad uplift rather than detachment‐based extension. Such uplift may precedes seafloor volcanism and rapid associated deflation along detachments. NW‐SE trending accommodation zones also segment the basin along‐strike. Bransfield Strait extension may result both from plate boundary tectonics at the South Shetland Trench and at the evolving Antarctic‐Scotia plate boundary. Extension was probably initiated by onset of subduction rollback at the trench, as a slab window opened from the SW. Augmented extension in the NE part of the basin may be a response to left‐lateral transtension associated with reorganization of the triple junction at the Shackleton Fracture Zone‐South Scotia Ridge intersection.

Rheological heterogeneity, mechanical anisotropy and deformation of the continental lithosphere

This paper aims to present an overview on the influence of rheological heterogeneity and mechanical anisotropy on the deformation of continents. After briefly recapping the concept of rheological stratification of the lithosphere, we discuss two specific issues: (1) as supported by a growing body of geophysical and geological observations, crust/mantle mechanical coupling is usually efficient, especially beneath major transcurrent faults which probably crosscut the lithosphere and root within the sublithospheric mantle; and (2) in most geodynamic environments, mechanical properties of the mantle govern the tectonic behaviour of the lithosphere. Lateral rheological heterogeneity of the continental lithosphere may result from various sources, with variations in geothermal gradient being the principal one. The oldest domains of continents, the cratonic nuclei, are characterized by a relatively cold, thick, and consequently stiff lithosphere. On the other hand, rifting may also modify the thermal structure of the lithosphere. Depending on the relative stretching of the crust and upper mantle, a stiff or a weak heterogeneity may develop. Observations from rift domains suggest that rifting usually results in a larger thinning of the lithospheric mantle than of the crust, and therefore tends to generate a weak heterogeneity. Numerical models show that during continental collision, the presence of both stiff and weak rheological heterogeneities significantly influences the large-scale deformation of the continental lithosphere. They especially favour the development of lithospheric-scale strike-slip faults, which allow strain to be transferred between the heterogeneities. An heterogeneous strain partition occurs: cratons largely escape deformation, and strain tends to localize within or at the boundary of the rift basins provided compressional deformation starts before the thermal heterogeneity induced by rifting are compensated. Seismic and electrical conductivity anisotropies consistently point towards the existence of a coherent fabric in the lithospheric mantle beneath continental domains. Analysis of naturally deformed peridotites, experimental deformations and numerical simulations suggest that this fabric is developed during orogenic events and subsequently frozen in the lithospheric mantle. Because the mechanical properties of single-crystal olivine are anisotropic, i.e. dependent on the orientation of the applied forces relative to the dominant slip systems, a pervasive fabric frozen in the mantle may induce a significant mechanical anisotropy of the whole lithospheric mantle. It is suggested that this mechanical anisotropy is the source of the so-called tectonic inheritance, i.e. the systematic reactivation of ancient tectonic directions; it may especially explain preferential rift propagation and continental break-up along pre-existing orogenic belts. Thus, the deformation of continents during orogenic events results from a trade-off between tectonic forces applied at plate boundaries, plate geometry, and the intrinsic properties (rheological heterogeneity and mechanical anisotropy) of the continental plates.

Plate tectonic reconstructions of the Juan Fernandez microplate: Transformation from internal shear to rigid rotation

Side‐scan sonar, swath bathymetry and magnetic anomaly date define a detailed, three‐phase history of the Juan Fernandez microplate. The ∼6 m.y. history is presented in a series of discrete time steps to document the growth and reorganization of propagating spreading centers and structural feature, and microplate kinematic evolution. Prior to the microplate, the East Pacific Rise at the Pacific‐Antarctic‐Nazca triple junction was offset by a long transform fault zone, likely the fastest slipping transform on Earth at anomaly 3A time. The microplate originated from an intratransform setting between anomaly 3A (5.95 Ma) and anomaly 3 (5.24 Ma) time. Its early development resembled a large propagating rift system, and microplate core structures suggest the entire offset zone may have experienced deformation. Fast propagation of the East Ridge dominated microplate growth until ∼2.6–1.9 Ma when seafloor spreading became the dominant growth process. The microplate rotation rate increased threefold (from 9 to 29° m.y.−1 average) from phase 1 (4.2–2.6 Ma) to phase 2 (2.6–1.1 Ma) of the microplate's history, then reduced fourfold (29 to 7° m.y. −1 average; phase 3, 1.1 Ma to Present). Phases 2 and 3 of the microplate's rotational history support the edge‐driven model for microplate kinematics of Schouten and others to a good approximation. The Pacific‐Nazca shear couple drove microplate rotation during phase 2, but development of the southeastern boundary enabled a transfer to the Nazca‐Antarctic plate pair (phase 3). West Ridge propagation and reorganization of the south‐western boundary may have decoupled the Pacific plate from the microplate, thus facilitating the shear couple transfer. The recent continued deceleration in microplate rotation rate and westward migration of the Pacific‐Antarctic ridge axis relative to the microplate may indicate that the process of microplate “death” has begun. We speculate that the Juan Fernandez microplate will accrete to the Antarctic plate, perhaps within the next million years, like the extinct Friday microplate has done, thereby accomplishing another northward migration of the Pacific‐Antarctic‐Nazca triple junction. Our reconstructions illustrate that the Easter and Juan Fernandez microplates are more similar than previously thought in terms of their origin, growth, rift propagation, ridge segmentation and overall tectonic evolution.