Onset Of Rifting Research Articles

STRUCTURAL ARCHITECTURE OF TARBOUL BASIN, SOUTHERN GULF OF SUEZ, EGYPT

The surface structural architecture basin configuration and petroleum system are important steps in studying and evaluating frontier sedimentary basins. The Tarboul basin (TH) is a typical marginal half garben at the onshore part of the southern Gulf of Suez rift. It is bounded from the west by the NW-SE striking Oligo-Miocene rift boundary fault (RBF) and from the East by the NW-SE striking Gemsa basin major bounding fault. The TH has a tripartite geometry bordered by the RBF, rift onset unconformity and the post rift unconformity. The Pre-rift strata include undifferentiated Nubia sandstones of Paleozoic-Lower Cretaceous age, Raha Formation of Cenomanian age, Wata Formation of Turonian age, Matulla Formation of Coniacian - Santonian age, Duwi Formation of Campanian age, Sudr Formation of Masstrachtian age and Thebes Formation of lower Eocene age. The synrift strata are represented by Lower Miocene Nukhul Formation and Lower-Middle Miocene Rudeis Formation. The Syn-rift packages roll into and stratigraphically thicken toward the RBF. The up dip northeastern part of the TH is characterized by a flexural faulted margin where there is an exposed monoclinal structure. Integrating surface and subsurface mapping of the monocline facilitated the prediction of the upward propagated fault that formed this structure. The TH has a SE plunging NW-SE oriented synclinal structure representing a hanging wall syncline related to the upward propagation of the RBF. The exposed Pre-Miocene rock units at the northwest part of this structure indicate the decrease in the throw of the RBF toward the onshore part of the adjoining Morgan Transfer Zone.

Mesozoic and older rift basins on the SE Greenland Shelf offshore Ammassalik

Abstract Seismic reflection data and shallow cores from the SE Greenland margin show that rift basins formed by the mid- to Late Cretaceous in the offshore area near Ammassalik. Here termed the Ammassalik Basin, this contribution documents the area using reprocessed older shallow seismic reflection data together with a more recent, commercial deep seismic reflection profile. The data show that the basin is at least 4 km deep and may be regionally quite extensive. Interpretation of gravity anomaly data indicate that the basin potentially covers an area of nearly 100 000 km 2 . The sediments in the basins are at least of Cretaceous age, as indicated by a sample from just below the basalt cover that was dated as Albian. Dipping sediment layers in the basins indicate that older sediments are present. Comparison of the data to the conjugate Hatton margin where older basins are exposed beneath the volcanic cover shows similar stratigraphy of similar ages. Reconstructions of the position of the basin during the Permian–Triassic and Jurassic suggest that older sedimentary strata could also be possible. In contrast to the conjugate Hatton margin, possible older strata subcrop out below the seafloor along the shallow margin, providing a future opportunity to sample some of the oldest sediments to determine the onset of rifting between SE Greenland and the Hatton margin.

Thermochronometric evidence for diffuse extension and two-phase rifting within the Central Arabian Margin of the Red Sea Rift

Numerical time-temperature models derived from a 2D network of apatite and zircon (U-Th)/He ages reveal a three-stage thermotectonic history for the central Arabian rift flank (CARF) of the Red Sea Rift (RSR) system. The pre-rift Arabian-Nubian Shield existed as part of a passive Paleo-Tethyan margin until a widespread tectonic event ca. 350 Ma exhumed the proto-CARF to mid-to-upper crustal structural levels. After remaining thermally stable through the Mesozoic, the first phase of RSR extension began with a distinct rift pulse ca. 23 Ma when fault blocks across a 150 km-wide area were exhumed along a diffuse set of rift-parallel faults from an average pre-rift flank depth of 1.7 ± 0.8 km. This rift onset age is mirrored in thermochronometric and sequence stratigraphic analyses elsewhere along the Red Sea Nubian and Arabian margins, confirming that rifting occurred concomitantly along the full Red Sea-Gulf of Suez rift system. Diffuse lithospheric extension lasted for 8 million years before a second rift pulse ca. 15 Ma, coincident with regional stress realignment, transferred active faulting basinward towards the modern RSR axial trough. CARF time-temperature models indicate that the prevalent rift style during both RSR extensional phases was one of localized, structurally controlled block faulting and contemporaneous dike injection, not wholesale rift flank uplift.

Jurassic failed rift system below the Filchner-Ronne-Shelf, Antarctica: New evidence from geophysical data

During the austral summer of 1994/95, reasonable ice conditions in the Weddell Sea allowed the acquisition of new high quality seismic refraction data parallel to the Filchner-Ronne Ice Shelf (FRS), Antarctica. Although pack ice conditions resulted in some data gaps, the final velocity-depth/2D-density models cover the entire FRS in E-W direction using all available deep seismic data/picks from this remote area. The velocity-depth model shows a sedimentary basin with a thickness up to 12km and a large velocity inversion in the lowermost sedimentary unit. The crustal thickness reaches a maximum of 40km along the basin's margins in the Antarctic Peninsula and East Antarctica. In the central shelf area, numerous interfering seismic phases occur from the crust-mantle boundary at decreasing distances indicating a thinning of the crust. Here, the modelled velocities and densities reveal a thickness of 20km for the igneous crust. This corridor of overthickened oceanic or close to oceanic crust is 160km wide. The corridor is characterized by weak, but in general continuous magnetic anomalies, which we interpret as isochrons developed during the rifting or the initial formation of oceanic crust. If the crustal composition represents an old stripe of oceanic crust, a minimum estimate for the early formation of the oceanic crust is 145/148Ma (Late Jurassic). However, based on the velocity of rift propagation during the initial opening of the adjacent Weddell Sea the oceanic crust is likely to have formed around 160Ma. The onset of rifting and development of a thick igneous crust can be related to stresses developed between the interior and the southwestern paleo-Pacific subduction margin of the fragmenting Gondwana supercontinent in combination with additional melt supply from a deeper mantle source that arrived and spread in the period 183–155Ma.

Large igneous province or long-lived magmatic arc along the eastern margin of Australia during the Cretaceous? Insights from the sedimentary record

U-Pb geochronology and Lu-Hf isotope analysis of detrital zircon from the mid-Cretaceous Winton and Mackunda Formations in the Eromanga Basin were employed to investigate regional provenance patterns in order to better understand the tectonic setting and paleogeography of eastern Australia during the late Mesozoic. A suite of Mesozoic-aged zircon populations recovered from these formations suggests that volcanism along the eastern margin of Australia was relatively continuous from the Triassic (252 Ma) to at least the mid-Cretaceous (ca. 92 Ma). Cretaceous-age zircon populations dominate the provenance record, and a distinct upsection younging trend in Cretaceous grain ages indicates that deposition was largely synchronous with ongoing volcanism to the east. Lu-Hf isotopic data suggest that these zircon populations were sourced from igneous rocks of a mixed juvenile and crustal source, similar to Lu-Hf isotopic systematics for eastern Australian zircons from Pennsylvanian–Permian igneous assemblages (307–252 Ma), for which an active convergent margin association is well established. An extensive Cretaceous volcanic terrain, now limited to the Whitsunday Igneous Association, was once located along the northeastern margin of Australia. Results from this study support the hypothesis that the Whitsunday igneous association was the main source of Cretaceous sediment to the Eromanga Basin, and likely for sediment transported across the continent southward and into the Ceduna Delta system offshore South Australia. The Whitsunday igneous association has been interpreted as a siliceous large igneous province associated with the onset of rifting in the region and linked to opening of the Tasman and Coral Seas. Yet, in this study, we document a relatively continuous Late Triassic to Late Cretaceous (240–92 Ma) age range for detrital zircons from the Mackunda and Winton Formations, consistent with relatively uninterrupted magmatic activity along the continental margin until ca. 92 Ma (earliest Turonian). Furthermore, zircon grains across this age spectrum exhibit dominantly positive to strongly positive eHf(t) values, between +4 and +12, consistent with values known for zircon suites from older magmatic arc rocks of eastern Australia. Although these data do not support a conclusive interpretation, they are consistent with an east Australian magmatic arc related to westward subduction of paleo-Pacific oceanic crust beneath eastern Australia enduring into the Cretaceous, as distinct from extensional siliceous large igneous province magmatism unrelated to subduction and generated by rupture of continental crust.

Mesozoic rift to post-rift tectonostratigraphy of the Sverdrup Basin, Canadian Arctic

Jurassic-Cretaceous rift successions and basin geometries of the Sverdrup Basin are reconstructed from a review and integration of stratigraphy, igneous records, outcrop maps, and subsurface data. The rift onset unconformity is in the Lower Jurassic portion of the Heiberg Group (approximately 200–190 Ma). Facies transgress from early syn-rift sandstones of the King Christian Formation to marine mudstones of the Jameson Bay Formation. The syn-rift succession of marine mudstones in the basin centre, Jameson Bay to Deer Bay formations, ranges from Early Jurassic (Pleinsbachian) to Early Cretaceous (Valanginian). Early post-rift deposits of the lower Isachsen Formation are truncated by the sub-Hauterivian unconformity, which is interpreted as a break up unconformity at approximately 135–130 Ma. Cessation of rift subsidence allowed for late post-rift sandstone deposits of the Isachsen Formation to be distributed across the entire basin. Marine deposition to form mudstone of the Christopher Formation throughout the Canadian Arctic Islands and outside of the rift basin records establishment of a broad marine shelf during post-rift thermal subsidence at the start of a passive margin stage. The onset of the High Arctic Large Igneous Province at approximately 130 Ma appears to coincide with the breakup unconformity, and it is quite typical that magma-poor rifted margins have mainly post-rift igneous rocks. We extend the magma-poor characterization where rifting is driven by lithospheric extension, to speculatively consider that the records from Sverdrup Basin are consistent with tectonic models of retro-arc extension and intra-continental rifting that have previously been proposed for the Amerasia Basin under the Arctic Ocean.

Evolution of the East African rift: Drip magmatism, lithospheric thinning and mafic volcanism

The origin of the Ethiopian-Yemeni Oligocene flood basalt province is widely interpreted as representing mafic volcanism associated with the Afar mantle plume head, with minor contributions from the lithospheric mantle. We reinterpret the geochemical compositions of primitive Oligocene basalts and picrites as requiring a far more significant contribution from the metasomatized subcontinental lithospheric mantle than has been recognized previously. This region displays the fingerprints of mantle plume and lithospheric drip magmatism as predicted from numerical models. Metasomatized mantle lithosphere is not dynamically stable, and heating above the upwelling Afar plume caused metasomatized lithosphere with a significant pyroxenite component to drip into the asthenosphere and melt. This process generated the HT2 lavas observed today in restricted portions of Ethiopia and Yemen now separated by the Red Sea, suggesting a fundamental link between drip magmatism and the onset of rifting. Coeval HT1 and LT lavas, in contrast, were not generated by drip melting but instead originated from shallower, dominantly anhydrous peridotite. Looking more broadly across the East African Rift System in time and space, geochemical data support small volume volcanic events in Turkana (N. Kenya), Chyulu Hills (S. Kenya) and the Virunga province (Western Rift) to be derived ultimately from drip melting. The removal of the gravitationally unstable, metasomatized portion of the subcontinental lithospheric mantle via dripping is correlated in each case with periods of rapid uplift. The combined influence of thermo-mechanically thinned lithosphere and the Afar plume together thus controlled the locus of continental rift initiation between Africa and Arabia and provide dynamic support for the Ethiopian plateau.

La apertura del segmento central del Atlántico Sur: simetría y discrepancia en la extensión

The rifted margins of the central segment of the South Atlantic are considered in terms of magmatism, symmetry and the amount and timing of extension, in comparison to the margins of the North Atlantic. It is suggested that the South Atlantic margins are probably moderately magmatic, with most of the magmatism occurring in the Hauterivian–Barremian, after the onset of rifting in the Berriasian. Most of the crustal extension took place between the Berriasian and early Barremian, before focusing along a narrow line of breakup in the Aptian. The changing locus of the rifting with time led to the development of considerable late asymmetry, and the stranding of the large part of the salt basins on variously the African and South American margin. The amount of Berriasian– Barremian extension may have been underestimated and may be sufficient to explain most of the observed crustal thinning. The apparent limited subsidence by the time of deposition of the Aptian evaporites may result from either delayed thermal subsidence due to the influx of hot asthenosphere or indicate that Early Cretaceous global sea level was well above the local water level within this isolated, partly empty basin.

Facies architecture of the K10 supersequence in the Browse Basin: when sequence stratigraphy meets lithostratigraphy

Regional seismic sequence stratigraphic mapping of the K10 supersequence (Berriasian–Valanginian) has brought into focus the relationship between sequence stratigraphic and lithostratigraphic units. In this extended abstract, the relationship between seismically-defined sandstone bodies and the Brewster Member, an important exploration target and reservoir in the Browse Basin, is investigated. Deposition of the K10 supersequence started at the onset of rifting between Greater India and the Northern Carnarvon Basin. Sediment sourced from uplifted areas resulted in deposition of the Barrow Delta in the Exmouth and Barrow sub-basins. In the Browse Basin, K10 is also a sand-rich progradational sequence, albeit diminutive compared to the Barrow Delta. Seismic mapping of K10 in the Caswell Sub-basin has resolved stratal geometries and, with the integration of well data, recognition gross depositional facies. Sandstone-prone siliciclastic platform facies (clinoform topsets) include the Brewster Member. The first reference to the Brewster Member in the Australian Stratigraphic Units Database applied the name to the upper part of the K10 sequence between 3,284 m and 3,405 m in Heywood–1. Usage of this name, however, has been inconsistently applied to sandstone-prone units within K10. Given the significance of the Brewster Member as an exploration target, and as a reservoir in the Ichthys-Prelude and Burnside gas fields, attention to integration of sequence stratigraphic and lithostratigraphic frameworks for this interval is timely.

Mesozoic rift onset and its impact on the sequence stratigraphic architecture of the Northern Carnarvon Basin

Ages varying from Late Triassic to Early Jurassic have been proposed by different authors for the onset of rifting in the Northern Carnarvon Basin. Seismic sections from the Exmouth Sub-basin and outer Exmouth Plateau demonstrate significant growth strata associated with displacement on normal faults starting at least at the base of the R. rhaetica zone (Rhaetian). This tectonic event coincides with a marked change in sequence architecture and a large landward shift (~300 km) of the paleo-shoreline to the vicinity of the Rankin and Alpha Arch trends. Rapid creation of accommodation in the inboard narrow rift basins (Exmouth, Barrow and Dampier sub-basins) is suggested to be the most likely cause of this major transgression. The preferential development of associated carbonate build-ups during the Rhaetian on the footwall side of active tilted fault blocks provides additional evidence for the onset of significant extensional faulting occurring during this time. An earlier more subtle initiation phase of rifting, however, is interpreted during the Norian, from around the middle part of the H. balmei biozone time, above which a change in stratigraphic architecture from aggrading to retrograding occurs. The observed structural and stratigraphic transitions can be related to typical phases of evolution described in many rift basins around the world. The work highlights the importance of integrating regional structural geology, sequence stratigraphy and depositional systems observations to provide robust constraints for basin evolutions.

Continental rifting and sediment infill in the northwestern Gulf of Mexico

The opening of the Gulf of Mexico was an important Mesozoic tectonic event that provides new insight in the role of magmatism and lithospheric stresses in the initiation of continental rifting. A new seismic velocity profile based on seismic refraction data in the northwestern Gulf of Mexico offshore Texas, where the basin started opening in the Early Jurassic, shows a rifted margin with strong lateral heterogeneity beneath the shelf and slope. The structure of the thinned crust is consistent with large-scale extensional faulting and moderate amounts of synrift magmatism before continental breakup. These new seismic constraints do not indicate the presence of a volcanic margin along the Texas coast, as has sometimes been proposed based on magnetic data. The Laurentian continental lithosphere of central Texas may have been too thick at the onset of rifting (>100 km) to let magmatic diking control the extension. In contrast, the continental lithosphere of the northeastern Gulf of Mexico may have been thinner, such that magma-assisted rifting formed a volcanic margin there later in the Jurassic.

Tectonic evolution of the Sicilian Maghrebian Chain inferred from stratigraphic and petrographic evidences of Lower Cretaceous and Oligocene flysch

Abstract The occurrence of a Lower Cretaceous flysch group, cropping out from the Gibraltar Arc to the Balkans with a very similar structural setting and sedimentary provenance always linked to the dismantling of internal areas, suggests the existence of only one sedimentary basin (Alpine Tethys s.s.), subdivided into many other minor oceanic areas. The Maghrebian Basin, mainly developed on thinned continental crust, was probably located in the westernmost sector of the Alpine Tethys. Cretaceous re-organization of the plates triggered one (or more) tectonic phases, well recorded in almost all the sectors of the Alpine Tethys. However, the Maghrebian Basin seems to have been deformed by Late- or post-Cretaceous tectonics, connected with a “meso-Alpine” phase (pre-Oligocene), already hypothesized since the beginning of the nineties. Field geological evidence and recent biostratigraphic data also support this important meso- Alpine tectonic phase in the Sicilian segment of the Maghrebian Chain, indicated by the deformations of a Lower Cretaceous flysch sealed by Lower Oligocene turbidite deposits. This tectonic development is emphasized here because it was probably connected with the onset of rifting in the southern paleomargin of the European plate, the detaching of the so-called AlKaPeCa block (Auct.; i.e. Alboran + Kabylian + Calabria and Peloritani terranes) and its fragmentation into several microplates. The subsequent early Oligocene drifting of these microplates led to the progressive closure of the Maghrebian Basin and the opening of new back-arc oceanic basins, strongly controlled by extensional processes, in the western Mediterranean (i.e. Gulf of Lion, Valencia Trough, Provençal Basin and Alboran Sea).

Subcontinental rift initiation and ocean-continent transitional setting of the Dinarides and Vardar zone: Evidence from the Krivaja–Konjuh Massif, Bosnia and Herzegovina

The Dinaride and Vardar zone ophiolite belts extend from the south-eastern margins of the Alps to the Albanian and Greek ophiolites. Detailed sampling of the Krivaja–Konjuh massif, one of the largest massifs in the Dinaride belt, reveals fertile compositions and an extensive record of deformation at spinel peridotite facies conditions. High Na2O clinopyroxene and spinel–orthopyroxene symplectites after garnet indicate a relatively high pressure, subcontinental origin of the southern and western part of Krivaja, similar to orogenic massifs such as Lherz, Ronda and the Eastern Central Alpine peridotites. Clinopyroxene and spinel compositions from Konjuh show similarities with fertile abyssal peridotite. In the central parts of the massif the spinel lherzolites contain locally abundant patches of plagioclase, indicating impregnation by melt. The migrating melt was orthopyroxene undersaturated, locally converting the peridotites to massive olivine-rich troctolites. Massive gabbros and more evolved gabbro veins cross-cutting peridotites indicate continued melt production at depth. Overall we infer that the massif represents the onset of rifting and early stages of formation of a new ocean basin. In the south of Krivaja very localized chromitite occurrences indicate that much more depleted melts with supra-subduction affinity traversed the massif that have no genetic relationship with the peridotites. This indicates that volcanics with supra-subduction affinity at the margins of the Krivaja–Konjuh massif record separate processes during closure of the ocean basin. Comparison with published compositional data from other Balkan massifs shows that the range of compositions within the Krivaja–Konjuh massif is similar to the compositional range of the western massifs of the Dinarides. The compositions of the Balkan massifs show a west to east gradient, ranging from subcontinental on the western side of the Dinarides to depleted mid-ocean ridge/arc compositions in the Vardar zone in the east. This is consistent with the hypothesis that both ophiolite belts originate in a single ocean, rather than from two separate basins. A distinct decrease in fertility occurs in the south of the Dinarides towards the Albanian ophiolites with supra-subduction affinity.

Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia

Research Article| May 01, 2014 Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia Robert C. Mahon; Robert C. Mahon 1Idaho State University Department of Geosciences, 921 S. 8th Ave., Stop 8072, Pocatello, Idaho 83209-8072, USA †Current address: University of Wyoming Department of Geology and Geophysics, Department 3006, 1000 University Ave., Laramie, Wyoming 82071, USA; e-mail: mahorobe@isu.edu; rmahon1@uwyo.edu. Search for other works by this author on: GSW Google Scholar Carol M. Dehler; Carol M. Dehler 2Utah State University Department of Geology, 4505 Old Main Hill, Logan, Utah 84322-4505, USA Search for other works by this author on: GSW Google Scholar Paul K. Link; Paul K. Link 1Idaho State University Department of Geosciences, 921 S. 8th Ave., Stop 8072, Pocatello, Idaho 83209-8072, USA Search for other works by this author on: GSW Google Scholar Karl E. Karlstrom; Karl E. Karlstrom 3University of New Mexico Department of Earth and Planetary Sciences, MSCO3-2040, Albuquerque, New Mexico 87131, USA Search for other works by this author on: GSW Google Scholar George E. Gehrels George E. Gehrels 4University of Arizona Department of Geosciences, 1040 E. 4th St., Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2014) 126 (5-6): 652–664. https://doi.org/10.1130/B30956.1 Article history received: 07 Jul 2013 rev-recd: 03 Dec 2013 accepted: 08 Jan 2014 first online: 08 Mar 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 C. Mahon, Carol M. Dehler, Paul K. Link, Karl E. Karlstrom, George E. Gehrels; Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia. GSA Bulletin 2014;; 126 (5-6): 652–664. doi: https://doi.org/10.1130/B30956.1 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 Pahrump Group in the Death Valley region of eastern California records a rich history of Mesoproterozoic to Neoproterozoic tectonic, climatic, and biotic events. These include the formation, stability, and onset of rifting of the Rodinia supercontinent, two potentially low-latitude glaciations correlative with global “snowball Earth” glacial intervals, and the onset of complex microbiota (e.g., testate amoebae). Poor direct age control, however, has significantly hindered the progress of understanding of these important stratigraphic units. New LA-ICPMS (laser ablation–inductively coupled plasma mass spectrometry) detrital zircon data from clastic units directly overlying a major unconformity within the Mesoproterozoic Crystal Spring Formation provide a maximum depositional age of 787 ± 11 Ma for the upper member of the Crystal Spring Formation. This unconformity, representing a duration of ≥300 Ma, is now recognized in sedimentary successions across southwestern Laurentia. These new age data, in addition to the distinct stratigraphic style above and below the unconformity, result in the proposed formal stratigraphic revision to elevate the upper member of the Crystal Spring Formation to the Neoproterozoic Horse Thief Springs Formation and separate it from the remainder of the underlying Mesoproterozoic Crystal Spring Formation (ca. 1100 Ma). New age relations and revised stratigraphic nomenclature significantly clarify stratigraphic and tectonic correlations and imply ca. 1250–1070 Ma assembly, 1070–780 Ma stability, and 780–600 Ma breakup of the supercontinent Rodinia along the southwestern Laurentian margin. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Stratigraphic, geochronologic, and geochemical record of the Cryogenian Perry Canyon Formation, northern Utah: Implications for Rodinia rifting and snowball Earth glaciation

The Cryogenian Perry Canyon Formation, formally named herein with a type section located in northern Utah, consists of a 0.3- to 1.5-km-thick succession of diamictite- and volcanic-bearing strata that record glacial events and early rifting along western Laurentia. The formation is divided into seven informal members, from bottom to top: (1) arkosic grit deposited as proximal turbidites during onset of rifting; (2) quartzite and grit deposited as medial turbidites; (3) pebbly slate with dropstones deposited during an older glacial episode; (4) slate with interbedded quartzite deposited mostly as distal turbidites; (5) mafic volcanic and intrusive rocks; (6) diamictite with basement and volcanic clasts deposited during younger glaciation; and (7) graywacke with rare carbonate layers. The formation has a basal nonconformity on Archean to Paleoproterozoic basement rocks of the Facer Formation and is overlain by arkosic quartzite of the Maple Canyon Formation and argillite of the Kelley Canyon Formation. Changes in member thicknesses reflect syndepositional faulting. Vertical and lateral changes in provenance and maximum depositional ages were evaluated from detrital zircon (DZ) U-Pb geochronology of five well-exposed stratigraphic sections across the sedimentary basin. DZ patterns for the arkosic grit record local sources during onset of rifting, patterns for the pebbly slate and slate members record distal Laurentian sources and possible sediment recycling, and patterns for diamictite and graywacke record basement sources along the rift margin and reworking of mafic to felsic volcanic rocks. Young zircon grains derived from volcanic rocks give maximum depositional ages of 703 ± 6 Ma and 667 ± 5 Ma for the lower part of the diamictite and graywacke members, respectively, suggesting a late Sturtian age for the younger glaciation. Associated cap carbonates, however, have Marinoan-style features, indicating that global correlations based only on lithology may not be valid. DZ data for the Kelley Canyon Formation record a change to distal Laurentian sources during final stages of early rifting. Mafic volcanic and intrusive rocks are subalkaline to alkaline and have trace-element ratios indicative of a rift setting. Volcanic clasts in diamictite include basalt, trachyte, and rhyolite. Volcanic zircon grains have e Hf = –1 to –21, which record crustal contamination during igneous activity. Rare mafic to ultramafic dikes in the upper part of the section were intruded during waning stages of igneous activity. Early rifting spanned ca. 720 Ma to 660 Ma but was incomplete and followed by deposition of mature quartzite of the overlying lower Brigham Group across a broad basin, indicating that Rodinia rift models showing pre–720 Ma separation of western Laurentia may need revision. Final rifting and transition to drift occurred ca. 550 Ma during deposition of the upper Brigham Group.

Crustal structure of the rifted volcanic margins and uplifted plateau of Western Yemen from receiver function analysis

We analyse P-wave receiver functions across the western Gulf of Aden and southern Red Sea continental margins in Western Yemen to constrain crustal thickness, internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism, faulting and mechanical crustal thinning during continental breakup. We analyse teleseismic data from 21 stations forming the temporary Young Conjugate Margins Laboratory (YOCMAL) network together with GFZ and Yemeni permanent stations. Analysis of computed receiver functions shows that (1) the thickness of unextended crust on the Yemen plateau is ?35?km; (2) this thins to ?22?km in coastal areas and reaches less than 14?km on the Red Sea coast, where presence of a high-velocity lower crust is evident. The average Vp/Vs ratio for the western Yemen Plateau is 1.79, increasing to ?1.92 near the Red Sea coast and decreasing to 1.68 for those stations located on or near the granitic rocks. Thinning of the crust, and by inference extension, occurs over a ?130-km-wide transition zone from the Red Sea and Gulf of Aden coasts to the edges of the Yemen plateau. Thinning of continental crust is particularly localized in a <30-km-wide zone near the coastline, spatially co-incident with addition of magmatic underplate to the lower crust, above which on the surface we observe the presence of seaward dipping reflectors (SDRs) and thickened Oligo-Miocene syn-rift basaltic flows. Our results strongly suggest the presence of high-velocity mafic intrusions in the lower crust, which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the SDRs. Our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western Gulf of Aden and southern Red Sea volcanic margins followed by a second phase of extension along the Red Sea margin.

Late Oligocene to Middle Miocene rifting and synextensional magmatism in the southwestern Sierra Madre Occidental, Mexico: The beginning of the Gulf of California rift

Although Basin and Range–style extension affected large areas of western Mexico after the Late Eocene, most consider that extension in the Gulf of California region began as subduction waned and ended ca. 14–12.5 Ma. A general consensus also exists in considering Early and Middle Miocene volcanism of the Sierra Madre Occidental and Comondú Group as subduction related, whereas volcanism after ca. 12.5 Ma is extension related. Here we present a new regional geologic study of the eastern Gulf of California margin in the states of Nayarit and Sinaloa, Mexico, backed by 43 new Ar-Ar and U-Pb mineral ages, and geochemical data that document an earlier widespread phase of extension. This extension across the southern and central Gulf Extensional Province began between Late Oligocene and Early Miocene time, but was focused in the region of the future Gulf of California in the Middle Miocene. Late Oligocene to Early Miocene rocks across northern Nayarit and southern Sinaloa were affected by major approximately north-south– to north-northwest–striking normal faults prior to ca. 21 Ma. Between ca. 21 and 11 Ma, a system of north-northwest–south-southeast high-angle extensional faults continued extending the southwestern side of the Sierra Madre Occidental. Rhyolitic domes, shallow intrusive bodies, and lesser basalts were emplaced along this extensional belt at 20–17 Ma. Rhyolitic rocks, in particular the domes and lavas, often show strong antecrystic inheritance but only a few Mesozoic or older xenocrysts, suggesting silicic magma generation in the mid-upper crust triggered by an extension-induced basaltic influx. In northern Sinaloa, large grabens were occupied by huge volcanic dome complexes ca. 21–17 Ma and filled by continental sediments with interlayered basalts dated as 15–14 Ma, a stratigraphy and timing very similar to those found in central Sonora (northeastern Gulf of California margin). Early to Middle Miocene volcanism occurred thus in rift basins, and was likely associated with decompression melting of upper mantle (inducing crustal partial melting) rather than with fluxing by fluids from the young and slow subducting microplates. Along the eastern side of the Gulf of California coast, from Farallón de San Ignacio island offshore Los Mochis, Sinaloa, to San Blas, Nayarit, a strike distance of >700 km, flat-lying basaltic lavas dated as ca. 11.5–10 Ma are exposed just above the present sea level. Here crustal thickness is almost half that in the unextended core of the adjacent Sierra Madre Occidental, implying significant lithosphere stretching before ca. 11 Ma. This mafic pulse, with subdued Nb-Ta negative spikes, may be related to the detachment of the lower part of the subducted slab, allowing an upward asthenospheric flow into an upper mantle previously modified by fluid fluxes related to past subduction. Widespread eruption of very uniform oceanic island basalt–like lavas occurred by the late Pliocene and Pleistocene, only 20 m.y. after the onset of rifting and ∼9 m.y. after the end of subduction, implying that preexisting subduction-modified mantle had now become isolated from melt source regions. Our study shows that rifting across the southern-central Gulf Extensional Province began much earlier than the Late Miocene and provided a fundamental control on the style and composition of volcanism from at least 30 Ma. We envision a sustained period of lithospheric stretching and magmatism during which the pace and breadth of extension changed ca. 20–18 Ma to be narrower, and again after ca. 12.5 Ma, when the kinematics of rifting became more oblique.

The sedimentary fill of the Baikal Basin: Implications for rifting age and geodynamics

Synthesis of the available stratigraphic data on the Baikal basin sediments exposed around the lake and their correlation with offshore lake sediments and with onshore sections in the Baikal Foredeep allows a new perspective of the Baikal rift history. The basin sediments on the Baikal shore comprise three tectonic–lithological–stratigraphic complexes (TLSC) which correspond to three seismic stratigraphic sequences (SSS) in the lake sediments and to three complexes in the Baikal Foredeep.The oldest unit, TLSC-1, has a particular lithology being deposited in an environment which never repeated in the later history of the area. This proves the validity of its lithostratigraphic correlation with Masstraichtian–Early Oligocene sediments of the Baikal Foredeep constrained by biostratigraphy. Further support comes from isotope dating, paleontology, and other evidence.Unlike seismostratigraphy-derived models, SSS-1 is correlated in the new model with TLSC-1 rather than with the Tankhoi Formation (which actually represents TLSC-2), and the onset of rifting is placed at the Late Cretaceous–Paleogene rather than at the Oligocene (or Miocene). Thus, the Baikal rifting began prior to the India–Eurasia collision, and the first rifting pulse originally had other causes. This inference agrees with fission-track apatite thermochronology indicating Cretaceous ages of samples from the Barguzin rift basin in the northeastern flank of the rift system.Rifting developed successively in three different tectonic settings and was driven by different geodynamic mechanisms at each stage. First it was a passive response to Late Cretaceous–Eocene distributed continent-wide extension in Asia (purely passive rifting). At the second stage spanning Late Oligocene–Early/Late Pliocene time, the area was subject to compressive impact from the India–Eurasia collision, which propagated from the southwest since the Eocene and reached the region about 30 Ma ago to take control over its geodynamics (conventionally passive “impactogenic” rifting). Finally, the Pliocene–Quaternary evolution has been driven by extension from a local source associated with hot mantle material rising to the base of the rifted crust (active rifting).The major rifting stages are further subdivided into substages: two substages in the second stage with the boundary at ~ 10 Ma (Middle–Late Miocene) and three substages in the third stage, with boundaries at 1.0–0.8 and 0.15–0.12 Ma. The stages and substages of rifting are separated by events of tectonic activity and stress reversal when additional compression produced folds and shear structures. The events that mark the stage boundaries show up as gaps, unconformities, and deformation features in the deposition patterns.Thus, the three units of synrift sediments composed of eluvium, early molasse, and late molasse, respectively, were deposited during preorogenic, early orogenic, and postorogenic stages of rifting driven by passive (first purely passive and then “impactogenic”) and active mechanisms.The new model of the Baikal rift history agrees with data obtained by different other methods.

Geomechanical modelling of fault reactivation in the Ceduna Sub-basin, Bight Basin, Australia

Abstract The Ceduna Sub-basin is located within the Bight Basin on the Australian southern margin. Recent structural analysis using newly acquired two-dimensional (2D) and three-dimensional (3D) seismic data demonstrates two Late Cretaceous delta–deepwater fold–thrust belts (DDWFTBs), which are overlain by Cenozoic sediments. The present-day normal fault stress regime identified in the Bight Basin indicates that the maximum horizontal stress ( S Hmax ) is margin parallel; Andersonain faulting theory therefore suggests the delta-top extensional faults are oriented favourably for reactivation. A breached hydrocarbon trap encountered in the Jerboa-1 well demonstrates this fault reactivation. Faults interpreted from 3D seismic data were modelled using the Poly3D © geomechanical code to determine the risk of reactivation. Results indicate delta-top extensional faults that dip 40–70° are at moderate–high risk of reactivation, while variations in the orientation of the fault planes results in an increased risk of reactivation. Two pulses of inversion are identified in the Ceduna Sub-basin and correlate with the onset of rifting and fault reactivation in the Santonian. We propose a ridge-push mechanism for this stress which selectively reactivates extensional faults on the delta-top, forming inversion anticlines that are prospective for hydrocarbon exploration.

From rifting to oceanic spreading in the Gulf of Aden: a synthesis

We present here a synthesis of the evolution of rifted continental margin systems in the Gulf of Aden. These margins are volcanic to the west of the Gulf of Aden, where they are influenced by the Afar hotspot, and non-volcanic east of longitude 46° E. The combined use of magnetics, gravity, seismic reflection, field observations (tectonic, stratigraphic and sedimentological) and oil well data allowed us to obtain better constraints on the timing of continental rifting and seafloor spreading. From the Permo-Triassic to the Oligocene, the Arabian-African plate was subject to distributed extension, probably due, at least from the Cretaceous, to tensile stresses related to the subduction of the Tethysian slab in the north. In Late Eocene-Early Oligocene, 34-33 Ma ago, rifting started to localise along the future area of continental breakup. Initially guided by the inherited basins, continental rifting then occurred synchronously over the entire gulf before becoming localised on the northern and southern borders of the inherited grabens, in the direction of the Afar hotspot. In the areas with non-volcanic margins (in the east), the faults marking the end of rifting trend parallel to the inherited grabens. Only the transfer faults cross-cut the inherited grabens, and some of these faults later developed into transform faults. The most important of these transform faults follow a Precambrian trend. Volcanic margins were formed in the west of the Gulf, up to the Guban graben in the southeast and as far as the southern boundary of the Bahlaf graben in the northeast. Seaward dipping reflectors can be observed on many oil industry seismic profiles. The influence of the hotspot during rifting was concentrated on the western part of the gulf. Therefore, it seems that the western domain was uplifted and eroded at the onset of rifting, while the eastern domain was characterised by more continuous sedimentation. The phase of distributed deformation was followed by a phase of strain localisation during the final rifting stage, just before formation of the Ocean-Continent Transition (OCT), in the most distal graben (DIM graben). About 20 Ma ago, at the time of the continental break-up, the emplacement of the OCT started in the east with exhumation of the subcontinental mantle. Farther west, the system was heated up by the strong influence of the Afar hotspot, which led to breakup with much less extension. In the Gulf of Aden (s.str), up to the Shukra El Sheik fracture zone, oceanic spreading started 17.6 Ma ago. West of this fracture zone, oceanic accretion started 10 Ma ago, and 2 Ma ago in the Gulf of Tadjoura. Post-rift deformation of the eastern margins of the Gulf of Aden can be seen in the distal and proximal domains. Indeed, the substantial post-rift uplift of these margins could be associated with either the continental break-up, or activity of the Afar hotspot and related volcanic/magmatic activity. Uplift of the northern proximal margin was still active (e.g. stepped beach rocks exposed at 60 m of 2 Ma; 30 m of 35,200 years; 10 and 2 m) and active volcanoes can be inferred at depths of between 70 and 200 km beneath the margin (at 5-10 km distance from the coast). On the distal margin, heat flow measurements show a high value that is associated with post-rift volcanic activity and the development of a volcano (with flows and sills) shortly after the formation of the OCT. The Afar hotspot is therefore important for several reasons. It allows the localisation of deformation along the Red Sea/Aden system and the rapid opening of the Gulf after the continental break-up; its influence also seems to persist during the post-rift period.