Rift-related Basins Research Articles

Late Paleoproterozoic to Mesoproterozoic rift successions in SW China: Implication for the Yangtze Block–North Australia–Northwest Laurentia connection in the Columbia supercontinent

The late Paleoproterozoic to Mesoproterozoic Dongchuan Group in the southwestern Yangtze Block formed in a series of fault-controlled, rift-related basins associated with the fragmentation of the Columbia supercontinent. The Dongchuan Group is composed, from the base upward, of the Yinmin, Luoxue, Etouchang and Luzhijiang formations. The Yinmin Formation has a basal layer of conglomerate and coarse sandstone, up to 2m thick, marking initiation of rifting. It is a laterally discontinuous layer, indicative of lateral variations in the accommodation space created adjacent to juvenile normal faults in the earliest stages of rifting. Fluvial to intertidal facies sedimentary rocks of the Yinmin Formation are characterized by fining-upward patterns from sandstone to interbedded carbonate and mudstone, representing syn-rifting sedimentation. Enlargement of the basin and establishment of a carbonate platform during sedimentation of the Luoxue Formation suggest that faulting and volcanism were significantly reduced, marking a transition from syn-rifting subsidence to post-rifting subsidence. Black carbonaceous shale of the Etouchang Formation likely formed in a continental slope and deep ocean basin, generated by slow thermal subsidence of heated subcrustal materials during the post-rifting stage. Subsequent shoaling led to re-establishment of a rimmed carbonate shelf, in which massive argillaceous dolostone of the Luzhijiang Formation started to accumulate. Two major igneous events marked the ca. 1710-Ma rifting climax and 1685–1660-Ma waning phase of rifting. The rift sequence in the southwestern Yangtze Block correlates with the late Paleoproterozoic to early Mesoproterozoic sequence in the Mount Isa Basin, North Australia. These successions have identical zircon age patterns and similar igneous and sedimentary assemblages. In addition, the rifting-related 1735–1663Ma Hornby Bay and ~1640–1600Ma Wernecke successions in northwestern Laurentia show similarities with those synchronous successions in the Yangtze Block and North Australia. This correlation suggests that the Yangtze Block was likely connected to northern Australia and northwestern Laurentia during the assembly of Columbia supercontinent but drifted away from these continents before ca. 1590Ma and showed tectonic evolution different from them after that.

Geophysical Constraints on the Hydrogeologic and Structural Settings of the Gulf of Suez Rift-Related Basins: Case Study from the El Qaa Plain, Sinai, Egypt

Groundwater has been identified as one of the major freshwater sources that can potentially meet the growing demands of Egypt’s population. Gravity data (from 381 ground gravity stations) were collected, processed, and analyzed together with the available aeromagnetic (800 line-km) data to investigate the hydrogeologic and structural settings, areal distribution, geometry, and water storage of the aquifers in El Qaa coastal plain in the southwest Sinai Peninsula, and to assess their longevity given projected extraction rates. Findings include (1) complete Bouguer anomaly and total magnetic intensity maps show two connected sub-basins separated by a narrow saddle with an average basin length of 43 km and an average width of 12 km; (2) two-dimensional modeling of both gravity and magnetic data indicates basin fill with a maximum thickness of 3.5 km; (3) using anomalous residual gravity, the volume of water in storage was estimated at 40–56 km3; and (4) progressive increases in extraction rates over time will deplete up to 40 % of the aquifers’ volume in 200–230 years and will cause the water quality to deteriorate due to seawater intrusion in 45 years. Similar geophysical exploration campaigns, if conducted over the entire coastal plains of the Red Sea and the Gulfs of Suez and Aqaba, could assist in the development of sound and sustainable management schemes for the freshwater resources in these areas. The adopted techniques could pave the way toward the establishment of sustainable utilization schemes for a much larger suite of similar aquifers worldwide.

Origin and tectonic significance of the Huangling massif within the Yangtze craton, South China

As the oldest exposed basement and the typical sedimentary cover of the Yangtze craton, the Huangling massif is a suitable place to decipher the tectonics of South China block. Structural analysis shows that the Huangling massif has an elliptic domal shape with N–S striking long axis, an asymmetric antiform with a steep western flank and a gentle eastern flank. There, three litho-tectonic units are recognized, from inner to outer parts: (1) Archean–Paleoproterozoic metamorphic rocks intruded by Neoproterozoic granitoids; (2) Neoproterozoic to Jurassic sedimentary envelope around the dome core; (3) Cretaceous terrigeneous alluvial–fluvial deposits, unconformably overlying the dome flanks. Coeval with the uplifting of the massif, the pre-Cretaceous strata on the western and eastern flanks of the Huangling massif were involved in a series of folds with nearly N–S axes and layer-parallel slip structures with top-to-the-W and top-to-the-E motion, respectively. The subsequent brittle normal faulting controlled the deposition of the graben or half-graben basins on both flanks. Cooling history reveals that the Huangling massif underwent uplifting between 160Ma and 110Ma with an average cooling rate of 2–3°C/Ma. Moreover, the Huangling area was not significantly affected by the Early Paleozoic and Triassic orogenies of South China. Comparable with the contemporaneous extensional structures, such as metamorphic core complexs, syntectonic plutons bounded by ductile normal faults, and rift-related basins in eastern China, it is proposed that the Huangling massif, might be an extensional structure controlled by a weak crustal extension. In this case, it will represent the western front of the Late Mesozoic lithospheric thinning in entire eastern China. However the compressional model cannot be ruled out.

Mesozoic fault reactivation along the St. Lawrence rift system, eastern Canada: Thermochronologic evidence from apatite fission-track dating

The St. Lawrence rift system is an active fault zone where reactivation of Late Proterozoic, Iapetus-related faults is believed to occur. It forms both a NE-SW–trending half graben and NW-SE–trending grabens that link and include the Ottawa-Bonnechere and Saguenay River grabens, both interpreted as aulacogens. The rift faults locally preserve kinematic evidence for normal-sense displacement. The lack of isotopic chronological constraints and the absence of strata younger than the Ordovician make it difficult to determine the timing of fault increments. Field relations in the Quebec City, Charlevoix, and Saguenay River regions suggest faulting event(s) younger than the Devonian-age Charlevoix crater. We determined apatite fission-track (AFT) ages for Grenvillian basement rocks from the hanging wall and footwall of typical rift faults at different locations of the St. Lawrence and Saguenay River fault systems along the St. Lawrence rift system. The footwall rocks of major faults yielded Late Triassic–Early Jurassic AFT ages ranging from 235 ± 28 Ma to 184 ± 22 Ma. The hanging-wall samples yielded older ages, between 443 ± 44 Ma and 279 ± 28 Ma along the NW-trending Saguenay River fault system that are consistent with Triassic–Early Jurassic normal faulting at ca. 250–200 Ma based on HeFTy inverse models of fission-track length distributions. Younger Late Jurassic AFT ages, ranging from 152 ± 17 Ma to 149 ± 14 Ma in the hanging wall of the NE-SW–trending St. Lawrence fault system, suggest the occurrence of localized faulting increments during the Late Jurassic. The latter AFT age discontinuities are interpreted as the result of normal faulting at ca. 250–200 Ma, followed by tectonic inversion. Evidence for the reactivation and inversion of preexisting normal faults during or after Jurassic time is also found in the Mesozoic rift-related basins of the Atlantic coast. The NW-SE–trending compressive stress field currently documented along the St. Lawrence River is consistent with tectonic inversion along the St. Lawrence rift system. This study provides geochronological support for Atlantic rift-related extensional deformation followed by compressional deformation in the interior of the Canadian Shield, more than 500 km west of the axis of the Mesozoic Atlantic rift basins.

Timing, styles, and kinematics of Cambro–Ordovician extension in the Teplá–Barrandian Unit, Bohemian Massif, and its bearing on the opening of the Rheic Ocean

This paper describes late Cambrian dikes and Early Ordovician volcano-sedimentary successions of the Prague Basin, Bohemian Massif, to discuss the timing and kinematics of breakup of the northern margin of Gondwana. Andesitic dikes indicate minor E–W crustal extension in the late Cambrian, whereas the Tremadocian to Dapingian lithofacies distribution and linear array of depocenters suggest opening of this Rheic Ocean rift-related basin during NW–SE pure shear-dominated extension. This kinematic change was associated with the onset of basic submarine volcanism, presumably resulting from decompression mantle melting as the amount of extension increased. We conclude from these inferences and from a comparison with other Avalonian–Cadomian terranes that the rifting along the northern Gondwana margin was a two-stage process involving one major pulse of terrane detachment in the early Cambrian and one in the Early Ordovician. While the geodynamic cause for the former phase remains unclear, but still may include effects of Cadomian subduction (roll-back, slab break-off), isostatic rebound, or mantle plume, the incipient stage of the latter phase may have been triggered by the onset of subduction of the Iapetus Ocean at around 510 Ma, followed by advanced extension broadly coeval (Tremadocian to Darriwilian) in large portions of the Avalonian–Cadomian belt. Unequal amounts of extension resulted in the separation and drift of some terranes, while other portions of the belt remained adjacent to Gondwana.

Structural evolution of the Texas Orocline, eastern Australia

The Texas Orocline, with a half wavelength of ~120km, is the largest and most obvious orocline in the southern New England Orogen and is clearly recognised in geological maps and geophysical images. In the area of the orocline, there is a major unconformity between Devonian–Carboniferous metasedimentary rocks (Texas beds) and the overlying Early Permian rift-related basins. Detailed structural mapping shows that units both above and below the unconformity are folded around the orocline, indicating that at least part of the oroclinal deformation has occurred after the deposition of the Early Permian rocks. In addition, Early Permian (298–290Ma) granitoids are aligned parallel to the oroclinal structure, further indicating that deformation partly occurred during or after the Early Permian. Pre-oroclinal isoclinal folds (F1) and related axial plane cleavage (S1) are well developed in the Texas beds and are curved around the oroclinal structure. Syn-oroclinal structures are characterised by minor kink folds and disharmonic folding (F2) in the core of the orocline. A later phase of kink folding corresponding to a ~N–S shortening direction does not seem to be related to the orocline, and is interpreted as post-oroclinal deformation. A secondary penetrative fabric parallel to the axial plane of the orocline was not observed, indicating low contractional shortening across the orocline (<30%). We demonstrate that the observed strain is too low to account for oroclinal bending during dextral transpression, as previously proposed. We suggest an alternative model involving an initial curved structure, probably related to subduction rollback or a pre-existing curvature in the palaeomargin of eastern Australia, which was amplified by dextral transpression and subsequent E–W contraction.

Geochemical evolution of Dalradian metavolcanic rocks: implications for the break-up of the Rodinia supercontinent

Abstract: Neoproterozoic basaltic magmatism in the Dalradian Supergroup of Scotland and Ireland was associated with the break-up of the Rodinia supercontinent. Magmas were erupted in rift-related basins along a strike length of at least 700 km and during a time period of c . 80 Ma. New major and trace element analyses of metabasalts from several formations are presented to trace the variations in magma compositions in time and space. The primary magmas resulted from variable degrees of mixing of melts derived from mantle sources similar to those of normal and enriched mid-ocean ridge basalts; some younger lavas also show evidence of contamination with continental crust. In contrast to speculations about magmatism elsewhere in Rodinia, the evidence here suggests that there was no involvement of a mantle plume in basalt generation. For example, the Scottish promontory of Laurentia drifted rapidly southwards through c . 25° over the duration of the magmatism, with no evidence of significant elevation above sea level, as might be expected from involvement of a plume. Generation of the primary magmas might have taken place predominantly through decompression melting in depleted upper mantle containing enriched streaks and blobs. Both the Dalradian lithostratigraphy and the metabasaltic compositions are consistent with extreme lithospheric stretching and possibly rupture during the earliest phase of magmatism, whereas generation of later magmatism appears to have been associated with major fault systems, possibly on a foundering continental margin. Supplementary material: Chemical analyses of Dalradian metavolcanic rocks (major elements recalculated to 100%, anhydrous) are available at www.geolsoc.org.uk/SUP18468 .

Provenance of the Novo Oriente Group, southwestern Ceará Central Domain, Borborema Province (NE-Brazil): A dismembered segment of a magma-poor passive margin or a restricted rift-related basin?

Integrated field, LA-MC-ICP-MS geochronology and geochemical data from the Novo Oriente Group have been investigated in order to determine their provenance and possible tectonic setting. This group in the southwestern portion of the Ceará Central Domain, is a well-preserved metavolcanosedimentary sequence, in part exhalative, composed of two distinct formations. The proximal coastal Bonsucesso Formation comprises mainly quartzite and minor basic metavolcanic rocks and is in gradational contact with the distal Caraúbas Formation which constitutes a metapelitic–volcanic-carbonate sequence, including metabasic rocks with pillow structure and sheared serpentinized ultrabasic rocks. The youngest detrital zircon found in the quartzite of the Bonsucesso Formation yielded an age of ca. 2.1 Ga and well-defined peaks around 2.2, 2.3–2.4 and 2.5 Ga. In addition, zircons from a related metabasalt yielded an upper intercept age of 2083 ± 28 Ma. We interpret the zircons found in this metabasalt as inherited from the host quartzite. This fact is supported by depleted-mantle Nd model ages ( T DM age) from correlated metabasic and metaultrabasic rocks from other places in the Novo Oriente sequence, which have yielded ages between 1.36 and 1.69 Ga, that is younger than the zircon age obtained in the metabasalt extruded over the Bonsucesso quartzite. Geochemical data of the metasedimentary rocks from the Caraúbas Formation indicates a provenance composed of a mixture of felsic and intermediate sources typical of old stable cratons and older continental roots of active tectonic settings. The rocks of the Granja (Médio Coreaú Domain) and Bacajá Complexes (southeastern Amazonian Craton) exposed to the west of the Novo Oriente Group, and largely covered by the Phanerozoic Parnaíba intracratonic basin, are the most likely source rocks. Since the precise age of the Novo Oriente Group is not yet well constrained, two distinct settings may be proposed: (1) it may represent a more evolved basin developed from extensional processes during the Mesoproterozoic (ca. 1.5–1.3 Ga); and (2) it could constitute part of the passive margin systems developed during the break-up of the Rodinia crown copyright supercontinent (ca. 0.95–0.8 Ga), associated with a pre-Brasiliano/Pan-African ocean (e.g. Pharusian Ocean).

Lithostratigraphy, facies, and deposition environment of the lower Jurassic Ammonitico Rosso type sediments (ARTS) in the Gümüşhane area, NE Turkey: Implications for the opening of the northern branch of the Neo-Tethys Ocean

Ammonitico Rosso type sediments (ARTS) form the bottom level of the Şenköy Formation in the Gümüşhane area. The formation accumulated in the rift-related basins triggered by extensional tectonic regimes in the Early Jurassic times. Six different sections were studied in this area in order to interpret the lithostratigraphy, facies, and deposition environments of the ARTS. Two basic lithofacies were distinguished, nodular-marly and nodular-calcareous. These facies provide useful information regarding depositional conditions in the area and environmental energy. These factors were mainly related to the relative sea-level that in turn was controlled by tectonic and eustasy conditions. In addition, seven different types of microfacies were recognized and described as a result of the detailed analysis of the sections. The results of the analysis show that these sediments in the Gümüşhane area were developed in transgressive phases/events. These sediments share characteristics with other peri-Mediterranean regions and sea-level curves, and show a similarity to the major transgressive pulse during the Pliensbachian stage. Deposition of ARTS was controlled by syn-depositional extensional movements, leading to their accumulation on top of tilted blocks, slopes and horsts in an open marine environment. The ARTS were developed during the rifting of the continental margins pointing up the phases of the Tethys opening. Neptunian dykes verifying this opening developed at different levels of the ARTS and were filled with overlying sediments. These results support the hypothesis that the ARTS were formed in tectonically active environments where extensional tectonic movements were continuously improved during the deposition of the ARTS.

Enhanced depositional and AVO models for lithologically complex sandstones in the Santos Basin, offshore Brazil

While exploring for hydrocarbons in rift-related basins, volcanic and volcaniclastic rocks and their erosional products are commonly encountered lithologies. At the reservoir level, the presence of rock types derived from volcanism or affected by post-volcanic re-deposition leads to highly varied complex lithologies with wide ranges of inherent rock properties. In the Santos Basin, a number of exploration wells have encountered basalts emplaced along the basin hinge line that have controlled the sedimentation and basin geometry. The main reservoir units of the basin consist of complex Upper Cretaceous sandstones deposited during periods of active volcanism. For one of the main exploration targets, the Senonian Ilhabela Formation, integrated geological and geophysical interpretation of recently acquired seismic and well data has led to a refined sedimentological model, an enhanced AVO model as well as a deeper understanding of the petroleum system. The encountered sandstones are of shallow-marine and deltaic origin, with local occurrence of mafic volcanic and subvolcanic rocks. Dependent on the presence of volcanic rocks in the vicinity of the particular reservoir, two different types of sandstones with different AVO characteristics have been identified. The presence of volcanic rocks may be interpreted from seismic data by their distinct geometry, usually high amplitudes and unique AVO behaviour, expressed as a strengthening of positive amplitudes with increasing offset.

Structure and evolution of the continental crust in the Baikal Fold Region

A summary of original Nd isotopic data on granitoids, silicic volcanics, and metasediments of the Baikal Fold Region is presented. The available Nd isotopic data, in combination with new geological and geochronological evidence, allowed recognition of the Early Baikalian (1000 ± 100 to 720 ± 20 Ma) and Late Baikalian (700 ± 10 to 590 ± 5 Ma) tectonic cycles in the geological evolution. The tectonic stacking, deformation, metamorphism, and granite formation are related to orogenic events that occurred 0.80–0.78 Ga and 0.61–0.59 Ga ago. The crust-forming events dated at 1.0–0.8 Ga and 0.70–0.62 Ga pertain to each cycle. The Early Baikalian crust formation developed largely in the relatively narrow and spatially separated Kichera and Param-Shamansky zones of troughs in the Baikal-Muya Belt. The formation and reworking of the Late Baikalian continental crust played the leading role in the Karalon-Mamakan, Yana, and Kater-Uakit zones and in the Svetlinsky Subzone of the Anamakit-Muya Zone in the Baikal-Muya Belt. In general, three large historical periods are recognized in the evolution of the Baikal Fold Region. The Early Baikalian period was characterized by prevalence of reworking of the older continental crust. The Late Baikalian-Early Caledonian period is distinguished by more extensive formation and transformation of the juvenile crust. The third, Late Paleozoic period was marked by reworking of the continental crust with juxtaposition of all older crustal protoliths. Two models of paleogeodynamic evolution of the Baikalian fold complexes are considered: (1) the autochthonous model that corresponds to the formation of suboceanic crust in rift-related basins of the Red Sea type and its subsequent reworking in the course of collision-related squeezing of paleorifts and intertrough basins and (2) the allochthonous model that implies the formation of fragments of the Baikal-Muya Belt at the shelf of the Rodinia supercontinent, their subsequent participation in the evolution of the Paleoasian ocean, and their eventual juxtaposition during Late Baikalian and Early Caledonian events in the structure of the Caledonian Siberian Superterrane of the Central Asian Foldbelt.

Role of rifting in evaporite deposition in the Great Kavir Basin, central Iran

Abstract The thick middle Miocene evaporites of the Great Kavir Basin, Central Iran, are classified as MgSO 4 -poor potash-bearing deposits. In the northern domain of the Great Kavir Basin these sediments occur as 50 very large salt diapirs, as well as outcrops in several areas, including the Melheh salt pit to the south of Semnan, north Central Iran. Based on the excellent preservation of layering in these sequences, along with well-preserved bottom nucleated primary textures within the halite and sylvite, they are interpreted as largely primary deposits. They also contain some secondary minerals such as langbeinite, d'ansite, polyhalite and anhydrite. The Br content in the halite and sylvinite layers falls within the range of marine evaporites. These MgSO 4 -poor potash-bearing evaporites deposits, along with other marine and continental sediments, appear to have been precipitated in the marginal marine basins around continental rifts. Seemingly, these evaporites formed in marginal marine rift-related basins which also received input from deep, circulating hydrothermal CaCl 2 -rich brines. These hydrothermal brines were also rich in Fe and Zn and apparently were driven upward along faults and fracture zones by the thermal gradient along the axis of the extensional trough.

Mesoproterozoic rifting and Pan-African continental collision in SE India: evidence from the Khariar alkaline complex

The suture zone between the Bhandara craton and the granulite-facies rocks of the Eastern Ghats Province in SE India contains a number of deformed alkaline and tholeiitic intrusives. The Khariar alkaline complex is one of the several occurrences which intruded in the Mesoproterozoic (1,480±17 Ma, 2σ) and was deformed during the Pan-African tectonothermal event. The geochemical signatures indicate a rift-related setting for the magmatic activity. The nepheline syenite parent magma may have been produced by in-mantle fractionation of clinopyroxene and Ti-rich amphibole from a basanitic primary magma derived from an enriched spinel lherzolite mantle source in the sub-continental lithosphere. Geochemical variations in the Khariar alkaline suite can be modeled by the fractionation of clinopyroxene, amphibole, titanite, zircon, apatite and allanite. The Mesoproterozoic alkaline magmatism at Khariar marks the initiation of a NE-SW rift which formed several craton margin basins and opened an ocean towards the south. The sediments of the cratogenic basins and the Eastern Ghats Province were deposited in these rift-related basins. A K-Ar age of 1,330±53 Ma from glauconites in sandstone suggests that the NW-SE trending Godavari–Pranhita graben formed at approximately the same time as the rift at the craton margin. If the two are related, the Godavari–Pranhita graben may represent the failed arm of a rift system in which the NE-SW arm was the active segment. The granulite-facies deformation and metamorphism of the Eastern Ghats Province sediments may be related to an episode of Grenvillian basin inversion. The Mesoproterozoic rifting and Grenvillian basin closure may thus represent two well-defined parts of a Wilson cycle i.e. the opening and closure of an ocean. The Khariar and other alkaline bodies were, however, deformed during a Pan-African collisional event associated with the westward thrusting of the Eastern Ghats Province granulites over the cratonic foreland.

On the Stratigraphy of the Palaeoproterozoic Deweras and Lomagundi Groups (Magondi Supergroup) in Zimbabwe

The Magondi Belt at the northwest rim of the Archaean Zimbabwe Craton extends over a length of 250 km from the Munyati River area in the south to the Shamrocke Mine area in the north. It is composed of the rocks of the Palaeoproterozoic Magondi Supergroup, which is subdivided into the Deweras, Lomagundi and Piriwiri Groups. The predominantly continental sediments and basalts of the Deweras Group were deposited in a rift-related basin margin environment marked by crustal instability. The depositional environment of the overlying Mcheka and Nyagari Formations of the Lomagundi Group is more stable and reflects shelf margin conditions. The Deweras sediments and basic volcanic rocks occurring in the southern part of the belt show a good stratigraphic correlation with those of the Silverside Mine area located between the middle and the northern part of the belt. A correlation of the Silverside Deweras rocks with the successions of the northern part of the belt (Mhangura and Doma West areas) becomes more difficult due to facies change, the presence of two impersistent amphibolite sills in different stratigraphic positions of the Deweras Group, and the distinct transgressive character of the marine Lomagundi rocks. Based on field relations, deformation and similar chemical composition it is suggested that the amphibolite sills of the Doma West area are equivalents of the Deweras metabasalts occurring in the Silverside area and in the southern part of the belt. The amphibolite sills were folded together with the enclosing Deweras sediments during the Magondi Orogeny that occurred between 2000 and 1800 Ma. Refolding of the Deweras rocks took place around 530 Ma during the Pan-African Orogeny. In the northernmost part of the belt (Shamrocke and Makuti-East areas), deformed amphibolites are also found in the Lomagundi Group. Using the immobile trace elements Zr/TiO 2 versus SiO 2 and Nb/Y versus SiO 2 of whole rock geochemical data the Deweras metabasalts of the Silverside area were classified as subalkaline basalts. The plot coincides with that of Deweras basic lavas of the Munyati, Mupfure and Alaska areas (southern part of the belt) and amphibolite sills occurring in the Deweras Group of the Doma West area (northern part of the belt). The metabasalts of the Silverside, Munyati, Mupfure and Alaska areas are characterised by a similar trend of their primitive-mantle normalised element patterns.

Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin (offshore Portugal)

The Lusitanian Basin is one of a series of rift-related basins developed during the rifting that led to the break-up of Iberia and Canada. The strong influence of halokinesis on the depositional evolution of its northern sector provides an analogue for the development of other evaporite-prone rift basins on Atlantic-type passive margins. In this paper, the pre-Valanginian evolution of the Northern Lusitanian Basin is documented based on regional seismic stratigraphy, isochron and time-structure maps tied with sonic and stratigraphic data from 10 exploration wells. The pre-salt structure of the study area is marked by two sets of faults: (i) a N–S to NE–SW striking set active throughout the Jurassic period and (ii) a NW–SE striking set active during the Late Jurassic. The latter defines five Late Jurassic sub-basins northeast of a basin-margin structure, the Marinha-Grande Fault. The development of half-graben basins in the Northern Lusitanian Basin during the Jurassic was overprinted by halokinesis over the Marinha-Grande Fault and other major marginal basement faults. Halokinesis climaxed during the Oxfordian–Kimmeridgian rifting with the formation of a 10 km long salt ridge over the Marinha-Grande Fault. The ridge limited the southward progradation of fluvial/deltaic units derived from hanging-wall drainage systems and restricted the syn-rift sediments to a 15 km long×5 km wide bowl-shaped depocentre located 10 km west of the present coastline. In order to better characterise the Jurassic tectono-sedimentary evolution of the Northern Lusitanian Basin, the interpreted seismic data is compared with physical models for extensional forced folds above active normal faults and with stratigraphic information from well and outcrop.

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

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

Apports de l'etude isotopique Sr et Nd des sediments cenozoiques de Limagne a la comprehension du fonctionnement du rift du Massif central francais

Abstract Radiogenic isotope geochemistry of shales, siliciclastics and carbonates potentially provides important information on the evolution of reservoirs such as continental crust or seawater. The aim of this study is to show that Sr and Nd isotope compositions of Cenozoic sediments and fossil remains in rift-related basins of the Massif Central can be used to contribute to a better understanding of rifting processes. Two questions are of particular relevance: firstly, did the volcanic activity which started during Eocene in the Massif Central continue through Oligocene, if so it would only be visible in sediments? Secondly, did the sedimentation occur in basins directly connected with seawater or in isolated lakes?

Detrital heavy-mineral characteristics of rift-related sediments from the Dragon-1 Well in the Cheju Basin (NE East China Sea), Korea

The rift-related depositional system developed in the divergent plate-margin basin where sedimentary heavy-minerals preserve a record of the tectonic elements. This paper presents how heavy-mineral assemblages and concentrations serve as rifting criteria in rift-related basin. From the Dragon-1 well samples, distinctive change of heavy-mineral assemblages could be used to distinguish different rifting stages and to subdivide a changing tectonic setting. Heavy-mineral concentrations are mainly controlled by the amount of heavy-mineral supply from source area and depositional slope angle in a half-graben.

Sedimentation rates, basin analysis and regional correlations of three Neoarchaean and Palaeoproterozoic sub-basins of the Kaapvaal craton as inferred from precise U–Pb zircon ages from volcaniclastic sediments

Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoic siliciclastic and chemical sediments covering the Kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents. Zircons from tuff beds in carbonate facies of the Campbellrand Subgroup in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensitive High Resolution Ion Microprobe II (SHRIMP II). Dates of 2588±6 Ma and 2549±7 Ma for the middle and the upper part of the Nauga Formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below-wave-base Stratifera and clastic carbonate facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m/Ma, when not corrected for times of erosion and non-deposition. Dates of 2516±4 Ma for the upper Gamohaan Formation and 2555±19 for the upper Monteville Formation, indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 Ma to 62 Ma on the Ghaap Plateau. For these predominantly peritidal stromatolitic carbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf facies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B. For the Kuruman Banded Iron Formation a decompacted sedimentation rate of up to 60 B can be calculated. Thus, for the entire examined deep shelf to tidal facies range, Archaean and Phanerozoic chemical and clastic sedimentation rates are comparable. Four major transgressive phases over the Kaapvaal craton, followed by shallowing-upward sedimentation, can be recognized in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, and partly also in the Transvaal basin, and are attributed to second-order cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be ascribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Proterozoic basins allows discussion of general Precambrian basin development. Siliciclastic and carbonate sedimentation rates of Archaean and Palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical, chemical and biological processes were active in the Precambrian as found for the Phanerozoic. Particularly for stromatolitic carbonates, matching modern and Neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the Malmani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are significantly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap Plateau.

The sediment-hosted Ngwako Pan copper deposit, Botswana

The strat-bound Ngwako Pan copper deposit is located in the northeastern part of the 250-km-long Neoproterozoic Ghanzi Ridge in northwestern Botswana. The sulfide mineralization occurs in the lowermost 25 m of the Middle Ghanzi Formation, which is composed of reduced facies (gray and greenish-gray) siliciclastic and subordinate carbonate metasediments. The 1- to 3.5-km-thick red-bed metasediments of the Lower Ghanzi Formation underlie the mineralized horizon. The distribution of copper ore in the Middle Ghanzi Formation shows a close spatial correlation with the occurrence of basaltic rocks in the Kgwebe Formation underlying the Lower Ghanzi Formation. The mineralized horizon is hosted by lacustrine or lagoonal sediments that were deposited on a paleohigh in a rift-related basin. The sediments were metamorphosed at lower greenschist facies conditions and tightly folded. The homogenization temperatures of fluid inclusions in secondary quartz are in the range of 57 degrees to 254 degrees C, the median being 179 degrees C. The major sulfide minerals, in order of decreasing abundance, are chalcocite- djurleite, bornite, chalcopyrite, pyrite, sphalerite, and galena. They occur as disseminations related to the intergranular porosity and permeability of the sediment and those related to fracture or shear porosity and permeability. Various load features indicate that at least part of the disseminated mineralization or its precursor sulfide minerals was deposited before major compaction was completed. Sulfurization of the sediment and possibly introduction of copper occurred early in the diagenetic history of the sediment. The mineralizing fluids were probably derived from oxidized ground water passing through red-bed footwall sediments. These fluids were enriched in copper where the aquifer was copper rich, i.e., where high Cu basaltic rocks constitute part of the source region of the siliciclastic sediments.