Continental Rift Systems Research Articles

Human evolution and possible forcing in continental rift system

Human evolution and possible forcing in continental rift system

Rapid rotation of normal faults due to flexural stresses: An explanation for the global distribution of normal fault dips

AbstractWe present a mechanical model to explain why most seismically active normal faults have dips much lower (30–60°) than expected from Anderson‐Byerlee theory (60–65°). Our model builds on classic finite extension theory but incorporates rotation of the active fault plane as a response to the buildup of bending stresses with increasing extension. We postulate that fault plane rotation acts to minimize the amount of extensional work required to sustain slip on the fault. In an elastic layer, this assumption results in rapid rotation of the active fault plane from ~60° down to 30–45° before fault heave has reached ~50% of the faulted layer thickness. Commensurate but overall slower rotation occurs in faulted layers of finite strength. Fault rotation rates scale as the inverse of the faulted layer thickness, which is in quantitative agreement with 2‐D geodynamic simulations that include an elastoplastic description of the lithosphere. We show that fault rotation promotes longer‐lived fault extension compared to continued slip on a high‐angle normal fault and discuss the implications of such a mechanism for fault evolution in continental rift systems and oceanic spreading centers.

Roadmap to continental rupture: Is obliquity the route to success?

Continental rift systems progress to plate rupture and seafloor spreading, or fail. The separation of Africa and South America is no exception: ruptured Mesozoic rift systems define their irregular coastlines, with basins overlying thinned crust of failed rift sectors. Heine and Brune (2014, p. 211

Geochemical evolution of southern Red Sea and Yemen flood volcanism: evidence for mantle heterogeneity

The Red Sea is part of the Afro-Arabian rift system, the world’s largest active continental rift system. The early opening phases of the Red Sea Rift were accompanied by continental flood magmatism. Large volumes of flood basalts emplaced in the Oligocene through to the present time at discrete eruptive centres along the western margin of the Arabian plate. Some of these rocks, in Southern Yemen, were investigated by geochemistry and K/Ar whole rock (WR) geochronology. In addition, the Jabal At-Tair (JAT) volcano, in the Red Sea trough, was investigated by geochemistry, with particular concern to the lavas of the last eruption of September 2007. The magmatism of Yemen is divided in: Oligocene–Early Miocene trap series (YOM), Tertiary intrusive rocks, and Late Miocene–recent volcanic series (YMR). YOM and Tertiary intrusions yielded K/Ar WR ages mostly in the range 31.6–16.6 Ma. Three older ages of 34.6, 35.4 and 49.0 Ma, if confirmed by further investigation, could suggest an Eocenic pre-trap phase of magmatic activity. YMR samples yielded K/Ar WR ages between 2.52 and 8.14 Ma. Both YOM and YMR basalts are alkaline, but YMR tend to be richer in alkalis than YOM. JAT basalts have subalkaline tholeiitic character, are geochemically homogeneous, and in the hygromagmaphile element spidergrams display increasing normalised concentrations from Cs to Ta, then decreasing up to Lu, with negative spikes of Nb, K and Pb. YOM have patterns almost identical to those of JAT, whereas YMR have higher normalized concentrations of all trace elements, but REE. The geochemical characteristics of JAT, YOM and YMR, framed in the broader context of the Red Sea Rift, are mostly consistent with a model of continental uplift and magmatism occurring across a linear, north–south axis of mantle upwelling, which intersects the Red Sea axis at the initiation site of axial seafloor spreading. The symmetrical propagation of the rift system to opposite sides of the N–S lineament, along the Red Sea axis, resulted in the observed symmetrical distribution of geochemical signatures of the Red Sea basalts and Yemen continental magmas.

Rift‐related origin of the Paleoproterozoic Kuncha Formation, and cooling history of the Kuncha nappe and Taplejung granites, eastern Nepal Lesser Himalaya: a multichronological approach

AbstractIn order to decipher the origin and tectonothermal history of the Kuncha nappe, we undertook a geological investigation in the Taplejung window in eastern Nepal, and carried out multichronological analyses of zircon, apatite, and mica of the Kuncha Formation and Taplejung granites. Three granite bodies that intrude into the Kuncha Formation show fission‐track (FT) ages of 6.2 to 4.8 Ma for zircon and 2.9 to 2.1 Ma for apatite, although the granites yielded zircon U–Pb ages of 1852 ± 24 Ma and 1877 ± 21 Ma, and muscovite 40Ar–39Ar ages of ca 1650 Ma. The U–Pb intercept age of detrital zircons from the Kuncha schist is 1888 ± 16 Ma, and this age represents a probable depositional age of the lower part of the Kuncha Formation. The FT ages of zircon and apatite from the same schist are found to be 5.4 and 2.5 Ma, respectively. No zircon younger than 1.6 Ga was identified from any of these samples. It means that the Kuncha nappe has never undergone thermal events after 1.6 Ga until the Himalayan orogeny in Miocene. The Kuncha Formation and overlying Kali Gandaki Supergroup as well as the Taplejung granites can be correlated with the Coronation Supergroup and the Hepburn intrusives in the Wopmay orogen, northwest Canada. They are interpreted to be deposits in the basins of continental rift system and subsequent passive‐margin settings. All the FT age data of zircon and apatite provide evidence that both the Kuncha nappe and overlying Crystalline nappe seem to have cooled laterally from the nappe front to the north. The FT ages from the front to central part of the nappe in Taplejung suggest that isotherm of 240°C, closure temperature of zircon FT, retreated toward north at about 10 mm/y during the middle to late Miocene and Pliocene.

A Review of Carbonatite Occurrences in Italy and Evaluation of Origins

This review and evaluation seeks to clarify the controversial origins of the Umbria-Latium Ultra-alkaline District (ULUD) and the Vulture carbonatitic occurrence (Intramontane Ultra-alkaline Province, IUP) and their relation to the Roman Comagmatic Province (RCP). Generally, the geochemical and isotopic features of the IUP can be linked to those of the RCP. Hence, the rocks of the ULUD district, together with part of the Tuscan and Roman Province generated in the last 2 Ma can be ascribed to a complex interplay of two subduction events related to magmatism associated with the European and Adria slabs associated with the effect of a slab window below the Italian Peninsular. Carbonate sediments together with pelagic-terrigenous sediment played a major role in the metasomatism of the mantle wedge beneath the IUP, and perhaps all along the transect from southern Italy (Eolian Islands magmatism) to north-central Italy (Tuscan and Umbria magmatism). A diffuse CO2 + H2O metasomatic front produced the condition necessary for the formation of carbonatitic magmatism. However, even where carbonatites are related to continental rift system (i.e., OIB), an origin for MORB-sediment convective recycling melting in the mantle has been hypothesized [i.e., 1].

Geochronology and geochemistry of Cenozoic basalts from eastern Guangdong, SE China: constraints on the lithosphere evolution beneath the northern margin of the South China Sea

The 40Ar–39Ar dating reveals three episodes of basaltic volcanism in eastern Guangdong of SE China since the late Eocene (i.e., 35.5, ~20 and 6.6 Ma). The Miocene alkali olivine basalts (~20 and 6.6 Ma) have OIB-like trace element characteristics, which is coupled with low (87Sr/86Sr)i, high eNd(t), and high eHf(t). In contrast, the late Eocene basalts (35.5 Ma) have overall characteristics of “Island Arc” basalts with strong negative Ta–Nb–Ti anomalies in the primitive mantle-normalized multi-element diagram with high (87Sr/86Sr)i, negative eNd(t), and relatively low eHf(t). All basalts have unexpectedly high 207Pb/204Pb and 208Pb/204Pb, delineating a DUPAL signature in the sources. The late Eocene Arc-like basalts may reflect contributions of relict ancient metasomatized mantle lithosphere that melted as the result of extension-induced asthenospheric upwelling and heating, whereas the Miocene OIB-like basalts may represent partial melting of the asthenospheric mantle beneath the thickened lithosphere. We propose that the Cenozoic basaltic volcanism in eastern Guangdong records an overall lithospheric thickening process beneath SE China, that is, a continental rift system from its maximum extension in the late Eocene to its waning in the Miocene. This interpretation is consistent with the evolution of the South China Sea, whose origin is most consistent with the development of a passive continental margin. The seafloor spreading of the South China Sea during ~ 32–16 Ma may not result from the effect of the “Hainan” mantle plume, but rather played a positive role in allowing the mantle plume to express on the surface.

Scientific Drilling

The East African Rift lakes offer unparalleled opportunities to investigate fundamental climate, environmental, biological, and geological processes through deep coring into the lake bed. Their sediments hold signals of the evolution of tropical rainfall, temperatures, and winds across 20 of latitude on both sides of the equator from the Miocene to the present. Fossil material in these basins chronicles the development of the East African landscapes in which our own species evolved, and records the explosive evolutionary radiation of literally thousands of species of fish, snails, and other aquatic organisms endemic to East Africa’s lakes. Drilling these sediments will also provide insights into the tectonic processes that shape the largest active continental rift system on Earth today.

Spatial variation of crustal strain in the Kachchh region, India: Implication on the Bhuj earthquake of 2001

The Kachchh province of Western India is a major seismic domain in an intraplate set-up. This seismic zone is located in a rift basin, which was developed during the early Jurassic break-up of the Gondwanaland. The crustal strain determined from the GPS velocity data of post-seismic time period following the 2001 Bhuj earthquake indicates a maximum strain rate of ∼266×10−9 per year along N013°. Focal mechanism solutions of the main event of 26 January 2001 and the aftershocks show that the maximum principal stress axis is close to this high strain direction. Maximum shear strain rate determined from the GPS data of the area has similar orientation. The unusually high strain rate is comparable in magnitude to the continental rift systems. The partitioning of the regional NE–SW horizontal stress (SHmax) by the pre-existing EW-striking boundary fault developed the strike–slip components parallel to the regional faults, the normal components perpendicular to the faults, NE-striking conjugate Riedel shear fractures and tension fractures. The partitioned normal component of the stress is considered to be the major cause for compression across the regional EW faults and development of the second-order conjugate shear fractures striking NE–SW and NW–SE. The NE-striking transverse faults parallel to the anti-Riedel shear planes have become critical under these conditions. These anti-Riedel planes are interpreted to be critical for the seismicity of the Kachchh region. The high strain rate in this area of low to moderate surface heat flow is responsible for deeper position of the brittle–ductile transition and development of deep seated seismic events in this intraplate region.

Crustal architecture of the Almada Basin, NE Brazil: an example of a non-volcanic rift segment of the South Atlantic passive margin

Abstract The Almada Basin, located in the southern Bahia State segment of NE Brazil, shares a similar sedimentation history and stress regime with the other eastern Brazilian basins. But when considering the composition of the transitional crust, a remarkably different behaviour is observed between the Almada Basin and the other eastern Brazilian basins. The architectural elements of the basin such as the reflection Moho discontinuity, the oceanic and continental basements, and the sedimentary section, as well as the tectonic style, are discussed in this study using gravity, seismic, regional geology and well-drilling results. The Almada Basin is part of a continental rift system that developed during the Early Cretaceous, heralding the Gondwana break-up and subsequently evolving into a passive divergent margin. Deep seismic profiles show the progressive thinning of the continental crust but there is no clear evidence of mantle exhumation. The Almada Basin is the conjugate margin of the South Gabon Basin in West Africa. Although both basins display similar stratigraphic record and geological evolution, the rifting mechanism resulted in a considerable asymmetric break-up. The Almada Basin is floored by the Itabunas–Salvador–Curaçá Belt of the São Francisco Craton and is characterized by Palaeoproterozoic granulites in a high-angle east-dipping thrust fault regime. However, the South Gabon Basin lies on the Neoproterozoic West Congolian Belt that is made up of low- to medium-grade metamorphic rocks in a low-angle thrust tectonic regime. The Precambrian basement fabric of both the Almada and South Gabon basins is marked by lithospheric-scale discontinuities that controlled the implantation of the rift zone and the extension style during the Mesozoic. The strong basement structural inheritance can be recognized in both the geological and geophysical records. The key architectural elements of a volcanic margin, such as large igneous provinces, seaward-dipping reflectors and the basinal syn-rift magmatism, are not recognized in the Almada Basin. Although the South Atlantic margin is mostly volcanic in the southernmost segment, the presence of these two non-volcanic segments – the southern Bahia State and the southern Gabon – are important as a record of the differences in the geotectonic process that governed the formation of the South Atlantic divergent margins.

Sensitivity analysis of rift induced delamination with application to Rwenzori Mountains

SUMMARY Rift induced delamination (RID) was favoured as a geodynamic process explaining the special situation of an extremely elevated old crustal block located in a continental rift system, the Rwenzori Mountains. The concept of RID is an upwelling asthenosphere encircling stiff lithosphere. If temperature- and stress-dependent viscosity of the undermost lower crust is reduced and strength fails, the detachment of cold and dense mantle-lithosphere root is trigged. Surface expressions are propagating rift tips surrounding an uplifting less dense crustal block. This study was performed to test the scope of application of RID hypothesis. Characteristic features of the numerical models are kept constant, whereas parameters controlling the initial temperature anomaly, rheological properties and the background geotherm are varied systematically. The system answer is a bifurcation, either the anomaly cools down or the central mantle lithosphere detaches and sinks down, or, rather an unsuccessful and successful RID. The excess temperature of the prescribed initial anomaly is most sensitive and serves at the transition from cooling to delamination as a critical temperature. Delamination works only in certain ranges. The anomaly full width ranges between 20 and 50km, the offset of the symmetric anomalies lies between 64 and 180km and the top anomaly has to reach into the lower crust. The corresponding geometric characteristics of the Rwenzori region always lies inside the found parameter ranges for the delamination regime, even near the optimum. Quantitative parameters concerning rheology are uncertain but, qualitatively, conform to the available findings. Thus, we establish RID as a possible mechanism coinciding roughly with the local observations, such as, an old crustal horst inside a young rift system, a missing mountain root, the high seismicity, low seismic velocities in the lower crust, minor volcanism and extreme topography. If RID is valid for the very special situation of the Rwenzoris and by now the sole example, one ought to generalize it and the parameter regimes explored can be used as a pattern for searching similar geological situations.

Monthly monitoring of gas and isotope compositions in the free gas phase at degassing locations close to the Nový Kostel focal zone in the western Eger Rift¸ Czech Republic

We report new data on gas and isotope compositions of mantle-derived exhalations from five locations in the Vogtland (Germany)/NW Bohemia (Czech Republic) area, close to the Nový Kostel focal zone, recorded by monthly sampling of gases over a three year period. This region is the locus typicus for the term “earthquake swarm”; since 1985/86 the largest numbers of earthquake swarms in the Vogtland/NW Bohemia have occurred in the Nový Kostel focal zone. Mantle-derived degassing has been studied at four locations within the Cheb Basin degassing centre, and at the Wettinquelle spring to the north on the edge of the Cheb Basin. The Bublák and U Mostku locations are on the Počatky Plesná Fault Zone (PPZ) while the Kopanina and Dolni Častkov locations are on the Mariánské Láznĕ Fault (MLF). The mantle-derived helium content at locations along the PPZ covers the range of the sub-continental lithospheric mantle (SCLM). Along the MLF the 3He/4He ratios are with up to 5.4Ra a little bit lower as at the PPZ locations. The CO2/3He ratios point to a predominantly magmatic source. Depending on the magnitude of gas flux the monthly sampling results indicated various major influences on the fluid signatures at the monitored locations due to seasonal cycles based on the different strong influence of CO2/water interaction. Micro-seismicity occurred repeatedly during the monitoring period and seismically triggered geochemical anomalies were repeatedly observed at the locations close to the focal zone. The 3He/4He ratios drop from 5.9Ra (mean) up to 5.6Ra at Bublák and from 5.6Ra up to 5.3Ra at U Mostku. A decrease in 3He/4He ratios was observed prior to seismic events due to stress accumulation before rupturing, and also after the events due to the release of crustal-derived components within the area of the focal zone. Due to the higher ‘helium baseline content’ of the Kopanina gas the 3He/4He ratios decreased there only from 4.6 to 4.5Ra. At the Wettinquelle, the most distant from the focal zone, the 3He/4He ratios ranged between 2.3 and 2.5Ra altogether. Superimposed to the seismically induced anomalies a clear three month-lasting increase of mantle-derived helium was observed at the locations on the PPZ. The 3He/4He ratios increased to 6.3Ra at Bublák and to 6.0Ra at U Mostku. An increase in the 3He/4He ratios was also noted at Kopanina (to 4.7Ra) and Dolni Častkov (to 5.4Ra), interpreted as indicating an ascent of less degassed magma (dyke intrusions) from the deeper lithospheric mantle reservoir. Superimposed geochemical effects initiated by the temporal and spatial evolution of a hidden magmatic-driven geodynamic process have therefore been observed in a continental rift system for the first time.

The structures, stratigraphy and evolution of the Gulf of Corinth rift, Greece

SUMMARY A multichannel seismic and bathymetry survey of the central and eastern Gulf of Corinth (GoC), Greece, reveals the offshore fault geometry, seismic stratigraphy and basin evolution of one of Earths most active continental rift systems. Active, right-stepping, en-echelon, north-dipping border faults trend ESE along the southern Gulf margin, significantly overlapping along strike. The basement offsets of three (Akrata-Derveni, Sithas and Xylocastro) are linked. The faults are biplanar to listric: typically intermediate angle (∼35° in the centre and 45–48° in the east) near the surface but decreasing in dip and/or intersecting a low- or shallow-angle (15–20° in the centre and 19–30° in the east) curvi-planar reflector in the basement. Major S-dipping border faults were active along the northern margin of the central Gulf early in the rift history, and remain active in the western Gulf and in the subsidiary Gulf of Lechaio, but unlike the southern border faults, are without major footwall uplift. Much of the eastern rift has a classic half-graben architecture whereas the central rift has a more symmetric w- or u-shape. The narrower and shallower western Gulf that transects the >40-km-thick crust of the Hellenides is associated with a wider distribution of overlapping high-angle normal faults that were formerly active on the Peloponnesus Peninsula. The easternmost sector includes the subsidiary Gulfs of Lechaio and Alkyonides, with major faults and basement structures trending NE, E–W and NW. The basement faults that control the rift architecture formed early in the rift history, with little evidence (other than the Vrachonisida fault along the northern margin) in the marine data for plan view evolution by subsequent fault linkage. Several have maximum offsets near one end. Crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the Derveni Fault. The dominant strikes of the Corinth rift faults gradually rotate from 090–120° in the basement and early rift to 090–100° in the latest rift, reflecting a ∼10° rotation of the opening direction to the 005° presently measured by GPS. The sediments include a (locally >1.5-km-) thick, early-rift section, and a late-rift section (also locally >1.5-km-thick) that we subdivide into three sequences and correlate with seven 100-ka glacio-eustatic cycles. The Gulf depocentre has deepened through time (currently >700 mbsl) as subsidence has outpaced sedimentation. We measure the minimum total horizontal extension across the central and eastern Gulf as varying along strike between 4 and 10 km, and estimate full values of 6–11 km. The rift evolution is strongly influenced by the inherited basement fabric. The regional NNW structural fabric of the Hellenic nappes changes orientation to ESE in the Parnassos terrane, facilitating the focused north-south extension observed offshore there. The basement-penetrating faults lose seismic reflectivity above the 4–14-km-deep seismogenic zone. Multiple generations and dips of normal faults, some cross-cutting, accommodate extension beneath the GoC, including low-angle (15–20°) interfaces in the basement nappes. The thermally cool forearc setting and cross-orogen structures unaccompanied by magmatism make this rift a poor analogue and unlikely precursor for metamorphic core complex formation.

Crustal Strain Patterns in the Satpura Mountain Belt, Central India: Implications for Tectonics and Seismicity in Stable Continental Regions

The Satpura Mountains of central India represents an ancient orogenic belt of the Mesoproterozoic time. It has a distinct sygmoidal (S-shaped) geometry with long EW and short NE–SW alignments. The mountain belt has been affected by tectonic activities throughout the geological past. The association of high seismicity, high heat flow and high Bouguer gravity anomaly with high topography of the region is a very distinct feature of the mountain range. Present analysis demonstrates that the average velocity for central India has a value of ~54 mm/year towards N050°. The velocity field vector can be partitioned into an eastward component parallel to the Satpura Mountain Belt and a northward component across the belt. The partitioned components provide evidence for sinistral strike–slip deformation of anomalously high shear strain rate of ~3 × 10−9/year in the region. Similar high shear strain rates are also found from the strain determined by GPS data. An extremely high rate of extensional strain (~600 × 10−9/year), which is comparable to that of the continental rift systems, is recorded from geodetic data of the Satpura Mountain Belt and the adjacent regions. Regional sinistral shape of the Satpura Mountains involved in a sinistral-slip transtension regime is interpreted to be the cause of high extensional and shear strain regime of the area. The occurrence of normal faulting detected in several deep seismic sounding profiles, the Moho upwarp, crustal thinning, high heat flow and high seismicity of the Satpura region are explained by this tectonic model.

New insights on the 1.7–1.0 Ga crustal evolution of the central Grenville Province from the Manicouagan – Baie Comeau transect

Integrated field relationships and U–Pb geochronology of rocks exposed along the southern part of the Manicouagan reservoir (central Grenville Province, Quebec) provide insights into protolith formation and thermal events that range in age from ca. 1.7 to 0.9 Ga and share common features with the rest of the Grenville Province and beyond. From presently lower to higher structural levels, main findings include the recognition of: (a) ca. 1.70–1.75 Ga granitic magmatism and metamorphism in the southern part of the parautochthonous Gagnon terrane, correlative in age with late stages of magmatism in the Makkovik Orogen to the northeast; (b) a ca. 1.69 gabbro-anorthosite suite (Island domain) which is inferred to represent the southwesternmost exposure of Labradorian crust in the central Grenville Province; (c) a 1.4 Ga volcaniclastic sequence and associated plutonic rocks (Canyon domain), which based on lithological association and age, is inferred to represent the NNE extension of the Montauban island arc; and (d) a ca. 1.2 Ga bimodal felsic-mafic volcanic to intermediate volcaniclastic sequence (Banded complex), inferred to have originated in an intra-continental rift system, contemporaneously with the Composite Arc Belt (southwestern Grenville Province) and with widespread extensional magmatism elsewhere in the Canadian Shield. In addition, 1.3 Ga granitoid plutons are documented to intrude Labradorian units at the NW boundary of Canyon domain. The significance of these plutons is ambiguous, as they may represent either stitching plutons, marking the accretion of the 1.4 Ga Canyon domain to the Laurentian margin, or the first stage in the development of the ca. 1.2 Ga continental rift system. The hinterland units (Island domain, Canyon domain and Banded complex) were metamorphosed under medium-pressure granulite facies conditions during the culmination of the Grenvillian orogeny between 1.08 and 1.04 Ga. This metamorphism was accompanied by ca. 1.07 Ga A-type granitic magmatism in Canyon domain. In addition, locally abundant ca. 0.98 Ga late-tectonic felsic pegmatite and ultrapotassic dykes occur in Canyon domain. This event is coeval with titanite ages documented in a wide range of units. The data on the Grenvillian evolution suggest mantle-derived magmatism and associated metamorphism in the hinterland during both the culmination and the waning stages of the Grenvillian orogeny.

Deep slab subduction and dehydration and their geodynamic consequences: Evidence from seismology and mineral physics

We present new pieces of evidence from seismology and mineral physics for the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the Pacific and Philippine Sea slabs under western Pacific and East Asia. The Pacific slab is subducting beneath the Japan Islands and Japan Sea with intermediate-depth and deep earthquakes down to 600 km depth under the East Asia margin, and the slab becomes stagnant in the mantle transition zone under East China. The western edge of the stagnant Pacific slab is roughly coincident with the NE–SW Daxing'Anling-Taihangshan gravity lineament located west of Beijing, approximately 2000 km away from the Japan Trench. The upper mantle above the stagnant slab under East Asia forms a big mantle wedge (BMW). Corner flow in the BMW and deep slab dehydration may have caused asthenospheric upwelling, lithospheric thinning, continental rift systems, and intraplate volcanism in Northeast Asia. The Philippine Sea slab has subducted down to the mantle transition zone depth under Western Japan and Ryukyu back-arc, though the seismicity within the slab occurs only down to 200–300 km depths. Combining with the corner flow in the mantle wedge, deep dehydration of the subducting Pacific slab has affected the morphology of the subducting Philippine Sea slab and its seismicity under Southwest Japan. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent small mantle plumes, or hot upwelling associated with the deep slab subduction. Slab dehydration may also take place after a continental plate subducts into the mantle.

Stratigraphy and palaeoenvironments of the Tanque Basin, southeastern Brazil

Stratigraphic and paleopalynological studies in Cenozoic deposits of the eastern region of the state of Sao Paulo led to the identification of two sedimentary episodes during the Paleogene and Neogene times. The analyses also permitted the reconstruction of the predominant climatic and paleoenvironmental conditions during the time of deposition. The first sedimentation phase developed during the Late Eocene, under humid and subtropical climatic conditions, with cold and dry winters. The second episode occurred during the Late Miocene – Pliocene probably under a subtropical to temperate climate, with well-defined seasons. The findings of this study permit us to suggest that these sediments as well as their ages are strongly correlated to the adjacent deposits of the Sao Paulo and Taubate sedimentary basins belonging to the continental rift system of southeastern Brazil.

40Ar– 39Ar ages of intrusions in East Greenland: Rift-to-drift transition over the Iceland hotspot

Sixteen 40Ar– 39Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56–54 Ma, 50–47 Ma and 37–35 Ma. In the Kangerlussuaq Fjord, which coincides with a major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and within the continental flood basalt succession, alkaline intrusions of the north–south trending Wiedemann Fjord–Kronborg Gletscher lineament range from 52 to 36 Ma. We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord–Kronborg Gletscher lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland–Iceland Rise and are explained by mantle melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50–47 Ma) and later by uplift associated with regional plate-tectonic reorganization (37–35 Ma). The Iceland mantle plume was instrumental in causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and asymmetry in the magmatic history of the conjugate margins of the central Northeast Atlantic.

Regional Metallogenic Controls of Small-intrusion-hosted Ni-Cu (PGE) Ore Deposits in China

Results of research indicate that Ni-Cu (PGE) ore deposits hosted in small intrusions in China mainly occur in three geologic settings: (1) within paleo-continents, (2) in orogenic belt, and (3) continental rift system such as the Panxi Rift system where a large number of flood basalts and associated mafic-ultramafic intrusive are present. Typical examples of these ore deposits are Jinchuan, Kalatongke, Baimazhai, respectively. Based on three controlling factors: (1) sulfide saturation in magma, (2) enough mass ratio between sulfide and magma, (3) sulfide concentration in limited areas, and case studies, we develop a genetic model that emphasizes geological process indicating sulfide segregation at depth before final emplacement. We discuss controls at different scales. These are (1) three geologic settings, (2) close to deep seated fault, (3) distribution of ore bodies in three different types of intrusions in different tectonic settings, (4) multiple intrusions or injections, (5) distal areas of final chamber, and (6) presence of flow differentiation and gravitational concentration.

Composition and pre-metamorphic geodynamic setting of the ultrahigh-pressure metabasic rocks from Dabie Shan, E-China

As part of the Yangtze plate, segments of the Dabie Shan terrane of Central China underwent ultra-high pressure metamorphism during Triassic subduction. We studied the geochemistry of the abundant eclogites to evaluate the nature of the protoliths and their geodynamic setting. Although some previous geochemical work exists, the analyses and interpretation herein are based on a new subdivision of the ultra-high pressure sequence into basement and cover units (Changpu and Ganghe Unit), revealing new and important results. In addition, eclogites of the so-called HP Unit south of the UHP units were studied. Whereas the large ion lithophile elements indicate postmagmatic, metasomatic changes of some samples, the high-field strength elements and the rare earth elements display original magmatic trends. The geochemical characteristics of the eclogites of the ultra-high pressure areas display a strong dependence on their “structural” and geographic position. The eclogites of the basement and the Changpu Unit indicate melt intrusion and extrusion in a continental rift system, i.e. during extensional tectonics. In contrast, the Ganghe Unit is characterized by a pronounced chemical homogeneity. The composition of the eclogites indicates generation from a mantle source highly influenced by slab-derived fluids. Those of the HP Unit show similar characteristics. Magmatism of the Ganghe and HP Unit probably occurred in a continental arc setting. A similar age for both units, geographically and/or tectonically separated, is possible. The geodynamic interpretation based on the geochemistry of the four units points to a Neoproterozoic scenario in which the protoliths of the HP and the cover units could have been of similar age and deposited in one evolving geological system. A rift-related larger-scale basin might have formed, e.g. a continental back-arc basin behind a magmatic arc after or simultaneous to sedimentation and magmatism in the magmatic arc. Alternatively, magmatism occurred in independent geodynamic settings, distinct in time and space. The units were juxtaposed during exhumation, after subduction to varying depths.