Rift Basins Research Papers - Academia.edu (original) (raw)

- by
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- Alkaline silicate rocks, Rift Basins

The estimations of the geothermal histories are essential in terms of gas and oil exploration in the frontier Mannar Basin. Sterane and hopane isomer compositions were examined by gas chromatography and mass spectrometry on forty-two sediments from the Barracuda and Dorado North exploration wells in the offshore Mannar Basin. The C 29 sterane 20S/(20S + 20R) ratios increase with depth in the Barracuda well, and drastically increase in the lowermost Cretaceous sediments of both wells. The biomarker proxies indicate that oil and gas generation threshold has been achieved in the Late Cretaceous sediments of the Barracuda well (3,850-4,741 m). Also, vitrinite reflectance (Ro) values for the Late Cretaceous samples in the Barracuda well were measured from 0.75-0.91%. Geological background of the rifted Mannar Basin can probably indicate higher geothermal anomalies than the present conditions. The standard rifting heat flows can be extrapolated with observed thermal maturity and gas deposit in the Mannar Basin. The kinetic model of the representative Late Cretaceous sediments (4,260-4,470 m) in the Barracuda well may indicate that in-situ gas/ oil generation started since the Early Eocene. The in-situ gas generation was gradually increased and reached peak conditions during the Early Miocene (ca. 20 Ma). However, the Late Cretaceous sediments of the Dorado North well and the Tertiary sediments of the both wells indicate poor cumulative hydrocarbon generation.

- by Amila Sandaruwan Ratnayake
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- Kinetic Modeling, Maturity Model, Hydrocarbon Systems, Rift Basins

Fault-bound half-graben-type Raniganj Basin in eastern peninsular India hosts thick sedimentary succession of the coal-bearing Barakar Formation (Early Permian), which constitutes a part of the Lower Gondwana Supergroup. Sedimentological attributes of the upper Barakar succession are portrayed in two predominant facies associations, viz., (i) the lower part characterized by meandering fluvial facies association with distributary channel systems and (ii) the upper part represented by a transgressive, tide-wave-influenced, fluvio-estuarine facies association. Distinct beds with various soft-sediment deformation structures (SSDSs) manifesting paleoseismic events are reported as seismites from the upper Barakar succession. The SSDSs include convolute laminae, complex deformations with folded strata, syn-sedimentary growth faults, pseudonodules, loop bedding, and various water-escape structures including sand–silt flows/dikes. The seismites are restricted at two distinct stratigraphic horizons (S1andS2) within the upper Barakar succession,separated by undeformed beds in between. The seismites record episodes of paleoearthquake events triggered by syn-sedimentary reactivation of basin marginal faults in the half-graben-type Raniganj Gondwana Basin.These demarcate phases of significant shift in the depositional conditions from fluvial dominated to a transgressive tide-wave-influenced estuarine
system,caused by post-glacial basinal adjustments in the peninsular Indian Gondwanaland.

- by Biplab Bhattacharya
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- Sedimentology, Gondwana, Permian, Basin evolution

The sedimentary successions exposed in northeast Calabria document the Jurassic–Early Cretaceous tectonic– sedimentary evolution of a former segment of the European–Iberian continental margin. They are juxtaposed today to units representing the deformation of the African and Adriatic plates margins as a product of Apenninic crustal shortening. A complex pattern of unconformities reveals a multi-stage tectonic evolution during the Early Jurassic, which affected the facies and geometries of siliciclastic and carbonate successions deposited in syn-and post-rift environments ranging from fluvial to deep marine. Late Sinemurian/Early Pliensbachian normal faulting resulted in exposure of the Hercynian basement at the sea-floor, which was onlapped by marine basin-fill units. Shallow-water carbonate aprons and reefs developed in response to the production of new accommodation space, fringing the newborn islands which represent structural highs made of Paleozoic crystalline and metamor-phic rock. Their drowning and fragmentation in the Toarcian led to the development of thin caps of Rosso Ammonitico facies. Coeval to these deposits, a thick (N 1 km) hemipelagic/siliciclastic succession was sedimented in neighboring hanging wall basins, which would ultimately merge with the structural high successions. Footwall blocks of the Early Jurassic rift, made of Paleozoic basement and basin-margin border faults with their onlapping basin-fill formations, are found today at the hanging wall of Miocene thrusts, overlying younger (Middle/Late Jurassic to Late Paleogene) folded basinal sediments. This paper makes use of selected case examples to describe the richly diverse set of features, ranging from paleontology to sedimentology, to structural geology, which are associated with the field identification of basin-margin unconformities. Our data provide key constraints for restoring the pre-orogenic architecture of a continental margin facing a branch of the Liguria–Piedmont ocean in the Western Tethys, and for estimating displacements and slip rates along synsedimentary faults.

- by Simone Fabbi
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- Sedimentology, Stratigraphy, Tectonics, Jurassic
- by Manuel Pubellier
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- Basin analysis, Rift Basins

High-resolution aeromagnetic surveys in the Nordkapp Basin, western Barents Sea, demonstrate
the capability of modern, high-resolution aeromagnetic surveys to provide an effi cient
and promising tool for mapping features related to salt diapirism. Salt diapirs are clearly
visible by a small, low-amplitude negative round to ellipsoidal magnetic pattern. This pattern
coincides with shallow sedimentary layers deformed by the rising salt during active and passive
diapirism. The dimensions of these features coincide in shape and size with those interpreted
on gravity and seismic surveys.

- by Laurent Gernigon and +3
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- Salt Tectonics, Aeromagnetics, Rift Basins
- by isabelle moretti
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- Geology, Geochemistry, Geophysics, Faulting (Structural Geology)
- by Mazlan Madon
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- Petroleum geology, Petroleum Production and Exploration, Rift Basins, Malay Basin

A new generation of aeromagnetic data documents the post-Caledonide rift evolution of the
southwestern Barents Sea (SWBS) from the Norwegian mainland up to the continent-ocean transition. We
propose a geological and tectonic scenario of the SWBS in which the Caledonian nappes and thrust sheets,
well-constrained onshore, swing from a NE-SW trend onshore Norway to NW-SE/NNW-SSE across the SWBS
platform area. On the Finnmark and Bjarmeland platforms, the dominant inherited magnetic basement
pattern may also reflect the regional and post-Caledonian development of the late Paleozoic basins. Farther
west, the pre-breakup rift system is characterized by the Loppa and Stappen Highs, which are interpreted as a
series of rigid continental blocks (ribbons) poorly thinned as compared to the adjacent grabens and sag
basins. As part of the complex western rift system, the Bjørnøya Basin is interpreted as a propagating system
of highly thinned crust, which aborted in late Mesozoic time. This thick Cretaceous sag basin is underlain by a
deep-seated high-density body, interpreted as exhumed high-grade metamorphic lower crust. The abortion
of this propagating basin coincides with a migration and complete reorganization of the crustal extension
toward a second necking zone defined at the level of the western volcanic sheared margin and proto-breakup
axis. The abortion of the Bjørnøya Basin may be partly explained by its trend oblique to the regional, inherited,
structural grain, revealed by the new aeromagnetic compilation, and by the onset of further weakening later
sustained by the onset of magmatism to the west.

- by Laurent Gernigon and +1
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- Inheritance, Tectonic Evolution of Rifted Continental Margins, Barents Sea, Caledonian tectonics

The Malay Basin has a very high present-day surface heat flow, with an estimated heat flow anomaly of about 33-42 m W m-2 • The heat flow anomaly is interpreted as the result of thinning of the lithosphere during basin formation. The basin is relatively young age (about 35 Ma), which implies that the thermal anomaly due to lithospheric thinning has not dissipated completely; the Basin is still undergoing thermal subsidence. Data from over 60 wells were used in the analysis of subsidence and thermal histories to gain a better understanding of its tectonic evolution. A model of lithospheric stretching was used, whereby rifting occurred over a 10 Ma year period starting 35 Ma ago. The subsidence histories from well data gave stretching factor (13) estimates ranging from about 1.2 on the basin flanks to about 4 in the centre. The basin flanks were uplifted during the initial rifting, causing subsidence to be delayed for about 10 Ma. Flank uplift was probably the result of non-uniform stretching of the lithosphere and horizontal heat loss through the sides of the basin as the lithosphere was being stretched. Heat flows calculated using the 13 estimates agree with those derived from well test data and, therefore, supports the stretching model. These results are comparable with those derived from maturity mode~g using available vitrinite maturity data.

- by Mazlan Madon
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- Tectonics & Sedimentation, Subsidence, Basin analysis, Basin modelling

Forced folds typically develop above the tips of propagating normal faults in rifts that contain thick, prerift salt or mudstone sequences. This structural style is associated with the deposition of wedge-shaped synrift deposits that thin and onlap towards monoclinal growth folds overlying the vertically restricted fault tips. Subtle stratigraphic traps may develop on the flanks of these folds although, due to limited seismic resolution and sparse well data, the architecture, thickness and distribution of early synrift reservoirs are difficult to predict. To improve our understanding of early synrift reservoir development on the flanks of forced folds, we focus on seismic-scale outcrop analogs along the Hadahid Fault System, Suez rift, Egypt. Our data indicate that forced folding dominated during early rifting and that the onset of folding was diachronous along-strike. Fluvial systems incised the rotating monocline limbs, leading to the formation of valley-like erosional relief along the base synrift unconformity. Reservoir-prone fluvial facies are only locally developed along the forced fold flank, with their distribution related to the degree of sediment bypass downdip into the adjacent basin. Early synrift relief not filled by fluvial strata was backfilled by transgressive, tidally influenced, reservoir-prone facies, with carbonates being locally developed in areas of low clastic sediment supply. Further extension and fault-tip propagation led to amplification of the forced folds, and deposition of shallow marine-to-shelf parasequences that became thinner towards the growing folds. Although displaying greater strike continuity than the underlying fluvial or tidal reservoirs, shoreface sandstone reservoirs amalgamate onto the flanks of the forced folds and may be absent towards the fold crest. This outcrop analog helps us better understand the sub-seismic stratigraphic architecture and facies distributions of early synrift reservoirs on the flanks of extensional forced folds.

- by Christopher Jackson and +1
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- Structural Geology, Faulting (Structural Geology), Folding (Structural Geology), Reservoirs

The Malay Basin is a Tertiary transtensional rift basin located in offshore Peninsular Malaysia. A study of the subsurface pressure data has revealed at least two major overpressure compartments that are sealed by regional shale units.... more

- by Mazlan Madon
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- Overpressure, Rift Basins, Malay Basin
- by Emiliano Campos-madrigal
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- Earth Sciences, Tectonics & Sedimentation, Cretaceous, Turbidites

A three-dimensional study of the structure of the Suez rift has been carried out using field and subsurface data, in
an attempt to determine the role of transverse faults and the longitudinal evolution of the rift.
As in most intracontinental rifts, the structure of the Gulf of Suez area is governed by normal faults and tilted
blocks whose crests constitute the main target of exploratory wells. The fault pattern consists of two major sets of
trends: (1) longitudinal faults parallel to the lift axis and created in an extensional regime where o3 was trending
ENE-WSW; and (2) transverse faults with a N-S to NNE-SSW dominant trend. Ihe tiansverse faults are inherited
passive discontinuities, while most of the longitudinal faults were created during Neogene times in a purely extensional
regime Both sets were simultaneously active, producing a zigzag pattern and rhombic-shaped blocks. The transverse
faults can show horizontal strike-slip components and act as relays between major normal faults
Although the Suez rift appears as a simple narrow elongated trough dominated by two almost symmetrical
shoulders, its internal structure is asymmetrical Cross-profiles show that all the major blocks are tilted in the same
direction However, the tilt direction changes twice along the rift To the north and to the south of the rift, the blocks
are tilted eastward, while in the central paît they are tilled westward. To the north the change of dip is accompanied by
a graben-type "twist zone" without transverse faulting, at least in the Neogene series. To the south the change of dip is
accompanied by a more complex structure involving both a major transverse fault and a horst-type "twist zone" In
this latter case the transverse fault does not cut through the entire rift
Balanced cross-sections established from subsurface data show that the tilt angle and the amount of extension
increase from north to south, while the width of the blocks decreases, indicating a pole of opening close to the northern
end of the Gulf Minimum values for the amount of opening range from 5 km in the north to about 20 km in the south..

Gulf of Suez is a prolific petroleum province and probably has the most unique geological setting to study sedimentary responses to tectonics. The short source-to-sink distance (less than 30km), active rifting provides ample opportunity to study the rapid sedimentary response to the interaction between rifting, accommodation space and rate of sediment supply. The earliest evidence of rifting in the Gulf of Suez is documented to be the Oligocene-Miocene, which is responsible for the present-day shape of the Gulf. Much of the rifting was influenced by the structural grain established during the Late Proterozoic to Cambrian. Late Cretaceous Limestone (Brown Limestone) is considered to be the dominant source of hydrocarbons. Factors such as rapid burial of reservoir quality sands from arid provenance, formation of several horst-graben structures and sediment drapes due to differential subsidence and reactivation of deeper rifts, effective trapping by shallower evaporate sequence, all contributed to several classical fields, with millions of barrels of recovered oil, from fields such as Gemsa, Ramadan, and few other well-known fields. These fields produced mostly from the pre-rift clastics and carbonate sequences, in well-defined faulted-block structures. However, much of the future exploration in the Gulf of Suez will depend on understanding the sedimentation trends in syn-rift and post-rift sag periods (Oligocene-Miocene), in highly complex fault systems in the regionally extensive Nukhul, Rudeis, and Kareem formations. These reservoirs are poorly imaged due to the thick overburden of evaporites and sparse well coverage, which contribute to the subsurface uncertainty. Recent work on the Yusr Field, Eastern Desert for an enhanced oil recovery project provides new exploration ideas. This study suggests that locating field-scale transfer or accommodation zones by applying the knowledge gained in East African Rift System and other similar intra-cratonic rifts, and through experimental deformation on scaled, physical models could supplement and possibly compensate for the poorly imaged 3D seismic and help delineate exploration targets. This study focuses on the Rudeis fm. and its hydrocarbon potential. Carefully extracted seismic attributes (Ant Tracking* and Coherence attributes) illuminates fault patterns that are previously not recognized. Preliminary observations indicate that depocenters are created between the transfer zones, with a high chance of up-dip stratigraphic terminations and lateral seals. While better imaged 3D seismic is imperative, intuitive application of knowledge analogues is essential in these challenging areas.

- by Sharma Dronamraju
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- Sedimentology, Stratigraphy, Sequence Stratigraphy, Extensional Tectonics

The relationship between mantle perturbation, penetrative magmatism, crustal uplift and the processes of rifting and rift propagation was studied. Quasi-scale model experiments and theoretical analyses by Boussinesq's three function method were used. Three dimensional stress analyses around a rising mantle intrusion and semiquantitative model experiments on mantle upwelling and magma penetration into brittle and brittle-ductile lithospheric materials indicate the origin of ridges, rifts, and rift-basin structures. These structures are commonly initiated by broad domal or plateau uplift. The magnitude and nature of the uplift, fissure or rift development, branching and pattern(s) are controlled primarily by flexural rigidity, magnitude of stress development due to mantle intrusion and magma penetration, and the degree of preferred orientation of weakness (anisotropy) in the lithosphere. Continued magma penetration into the crust produces en echelon fissures, vertical and lateral dike propagation, graben and graben and horst structures with step or block faulting. This is followed by axial subsidence, fault opening and periodic volcanism with continued crustal extension, stretching and slow spreading. Mantle and magma penetration into the crust are shown to be important in developing in situ lateral compressive stress adjacent to and around the active rift zones. Transverse fissures, dike swarms, volcanoes and anticlines between two rifts (as in Iceland) result from such lateral compressive stress. This stress is also important in the development of marginal fissures and faults (as in fast-spreading centers). Time progressive model experiments on magma formation and magma penetration from the asthenosphere-lithosphere boundary show the importance of magma chambers. These chambers cause thinning and stretching of the composite lithosphere, vertical and lateral propagation of dikes and dike swarms, and initiate rifts with or without significant uplift. This study also suggests that multiple dike wedging with rapid extension fracturing can generate excess magma pressure and push force, and may contribute seismicity to spreading centers.

Combining geophysical, petrological and structural data on oceanic mantle lithosphere, underlying astheno-sphere and oceanic basalts, an alternative oceanic plate spreading model is proposed in the framework of the westward migration of oceanic spreading ridges relative to the underlying asthenosphere. This model suggests that evolution of both the composition and internal structure of oceanic plates and underlying upper mantle strongly depends at all scales on plate kinematics. We show that the asymmetric features of lithospheric plates and underlying upper asthenosphere on both sides of oceanic spreading ridges, as shown by geophysical data (seismic velocities, density, thickness, and plate geometry), reflect somewhat different mantle compositions , themselves related to various mantle differentiation processes (incipient to high partial melting degree, percolation/reaction and refertilization) at different depths (down to 300 km) below and laterally to the ridge axis. The fundamental difference between western and eastern plates is linked to the westward ridge migration inducing continuing mantle refertilization of the western plate by percolation-reaction with ascending melts, whereas the eastern plate preserves a barely refertilized harzburgitic residue. Plate thickness on both sides of the ridge is controlled both by cooling of the asthenospheric residue and by the instability of pargasitic amphibole producing a sharp depression in the mantle solidus as it changes from vapour-undersaturated to vapour-saturated conditions, its intersection with the geotherm at ~90 km, and incipient melt production right underneath the lithosphere-asthenosphere boundary (LAB). Thus the intersection of the geotherm with the vapour-saturated lherzolite solidus explains the existence of a low-velocity zone (LVZ). As oceanic lithosphere is moving westward relative to asthenospheric mantle, this partially molten upper asthenosphere facilitates the decoupling between lower asthenosphere and lithosphere. Thereby the westward drift of the lithosphere is necessarily slowed down, top to down, inducing a progressive decoupling within the mantle lithosphere itself. This intra-mantle decoupling could be at the origin of asymmetric detachment faults allowing mantle exhumation along slow-spreading ridges. Taking into account the asymmetric features of the LVZ, migration of incipient melt fractions and upwelling paths from the lower asthenosphere through the upper asthenosphere are oblique, upward and eastward. MORB are sourced from an eastward and oblique, near-adiabatic mantle upwelling from the lower as-thenosphere. This unidirectional mantle transfer is induced by isostatic suction of the migrating spreading ridge.

- by Mazlan Madon
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- Structural Geology, Malaysia, Petroleum geology, Sedimentary Basins

Lake Navaisha and aquifer surrounding the lake are important water resources in the area and are used extensively for irrigation and domestic water supplies. Continued or increased withdrawals from these sources have the potential to affect water levels in these aquifers. This thesis presents the design of a 3-D conceptual model of ground-water flow, the development and calibration of a numerical model for steady state groundwater simulation.Part of this study also includes updating the three-dimensional hydro-geologic framework model to serve as the foundation for the development of a steady-state regional ground-water flow model on the basis of integration of geology, hydro-geochemistry, geophysics, isotopic analysis and selection of mathematical boundary conditions. Groundwater flow in the Navaisha basin was modelled numerically with the ground water modelling system(GMS 5.0)and is used to simulate ground-water flow in the aquifers and lake–aquifer interaction. A four layer system was designed from which, the upper two layers represent the sediment aquifer, and the lower layers represent the volcanic aquifer. The regional model area was divided into grid blocks 300 meters areal space while the local and site model have 150m and 80m areal grid spacing respectively. The Navaisha lake is considered as an integral part of the ground water flow system since heads and flow patterns in surficial aquifers can be strongly influenced by the surface Navaisha lake that are are direct contact ,vertically and laterally with the aquifer. The lake was simulated by specifying a high hydraulic conductivity for lake-volume grid cells, the “high K” technique. The model was calibrated to static water level measurements in wells. Pilot points were used as a device for characterisation of parameter spatial variation in conjunction with the regularization in the ground water model calibration. Overall, the finite difference groundwater model result was comparable with measured well data .The simulated head and flow distributions mimic the important aspects of the flow system, such as magnitude and direction of the head contours. Also, simulated lake level varies in a manner determined by the water budget computed for the lake in the model grid. This process is crucial in making the model serve as simulator of the response of lake stage to hydraulic stresses applied to the aquifer and variation in climatic condition, a capability desired by resources manager. The sensitivity of lake level computed using high–K method was tested to the choice of K2/K1, where K1 is the hydraulic conductivity of the lake nodes and K1 is the hydraulic conductivity of the aquifer. The results indicate that values of K2/K1 less than 1000 produced a significant head differential across the lake (computing four wells at the lake surface), which could result in erroneous calculations of seepage to and from the lake. A value of K2/K1 greater than 1000 but less than 1,000,000 gave acceptable solution, produced no gradient across the lake. https://www.itc.nl/library/papers_2005/msc/wrem/yohannes.pdf

- by Dr. Yohannes Yihdego Woldeyohannes
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- Geology, Hydrogeology, Structural Geology, Water

Dolomites occur extensively in the lower Cretaceous along syn-sedimentary
fault zones of the Baiyinchagan Sag, westernmost Erlian Basin, within a predominantly
fluvial–lacustrine sedimentary sequence. Four types of dolomite
are identified, associated with hydrothermal minerals such as natrolite, analcime
and Fe-bearing magnesite. The finely-crystalline dolomites consist of
anhedral to subhedral crystals (2 to 10 lm), evenly commixed with terrigenous
sediments that occur either as matrix-supporting grains (Fd1) or as
massive argillaceous dolostone (Fd2). Medium-crystalline (Md) dolomites
are composed of subhedral to euhedral crystals aggregates (50 to 250 lm)
and occur in syn-sedimentary deformation laminae/bands. Coarse-crystalline
(Cd) dolomites consist of non-planar crystals (mean size >1 mm), and occur
as fracture infills cross-cutting the other dolomite types. The Fd1, Md and
Cd dolomites have similar values of d18O (205 to 110& Vienna PeeDee
Belemnite) and d13C (+14 to +45& Vienna PeeDee Belemnite), but Fd2 dolomites
are isotopically distinct (d18O 85 to 23& Vienna PeeDee Belemnite;
d13C +14 to +86& Vienna PeeDee Belemnite). Samples define three
groups which differ in light rare-earth elements versus high rare-earth elements
enrichment/depletion and significance of Tb, Yb and Dy anomalies.
Medium-crystalline dolomites have signatures that indicate formation from
brines at very high temperature, with salinities of 118 to 23 2 eq. wt. %
NaCl and Th values of 167 to 283°C. The calculated temperatures of Fd1 and
Cd dolomites extend to slightly lower values (141 to 282°C), while Fd2 dolomites
are distinctly cooler (81 to 124°C). These results suggest that the dolomites
formed from hydrothermal fluid during and/or penecontemporaneous
with sediment deposition. Faults and fractures bounding the basin were
important conduits through which high-temperature Mg-rich fluids discharged,
driven by an abnormally high heat flux associated with local volcanism.
It is thought that differing amounts of cooling and degassing of
these hydrothermal fluids, and of mixing with lake waters, facilitated the precipitation of dolomite and associated minerals, and resulted in the petrographic
and geochemical differences between the dolomites.

- by Fiona Whitaker
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- Geology, Sedimentology, Cretaceous, Fluid Mixing
- by Elena Centeno-Garcia
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- Earth Sciences, Geology, Tectonics & Sedimentation, Cretaceous

Dolomites occur extensively in the lower Cretaceous along syn-sedimentary fault zones of the Baiyinchagan Sag, westernmost Erlian Basin, within a predominantly fluvial–lacustrine sedimentary sequence. Four types of dolomite are identified, associated with hydrothermal minerals such as natrolite, analcime and Fe-bearing magnesite. The finely-crystalline dolomites consist of anhedral to subhedral crystals (2 to 10 lm), evenly commixed with terrigenous sediments that occur either as matrix-supporting grains (Fd1) or as massive argillaceous dolostone (Fd2). Medium-crystalline (Md) dolomites are composed of subhedral to euhedral crystals aggregates (50 to 250 lm) and occur in syn-sedimentary deformation laminae/bands. Coarse-crystalline (Cd) dolomites consist of non-planar crystals (mean size &gt;1 mm), and occur as fracture infills cross-cutting the other dolomite types. The Fd1, Md and Cd dolomites have similar values of d18O (205 to 110&amp; Vienna PeeDee Belemnite) and d13C (+14 to +45&amp; Vienna PeeDee Belemnite), but Fd2 dolomites are isotopically distinct (d18O 85 to 23&amp; Vienna PeeDee Belemnite; d13C +14 to +86&amp; Vienna PeeDee Belemnite). Samples define three groups which differ in light rare-earth elements versus high rare-earth elements enrichment/depletion and significance of Tb, Yb and Dy anomalies. Medium-crystalline dolomites have signatures that indicate formation from brines at very high temperature, with salinities of 118 to 23 2 eq. wt. % NaCl and Th values of 167 to 283°C. The calculated temperatures of Fd1 and Cd dolomites extend to slightly lower values (141 to 282°C), while Fd2 dolomites are distinctly cooler (81 to 124°C). These results suggest that the dolomites formed from hydrothermal fluid during and/or penecontemporaneous with sediment deposition. Faults and fractures bounding the basin were important conduits through which high-temperature Mg-rich fluids discharged, driven by an abnormally high heat flux associated with local volcanism. It is thought that differing amounts of cooling and degassing of these hydrothermal fluids, and of mixing with lake waters, facilitated the precipitation of dolomite and associated minerals, and resulted in the petrographic and geochemical differences between the dolomites.

- by Fiona Whitaker
- •
- Geology, Sedimentology, Cretaceous, Fluid Mixing
- by Isabelle Moretti (PhD) and +1
- •
- Rift Basins, Tilted Block

While the superficial expression of oceanic ridges is generally symmetric, their deeper roots may be asymmetric. Based on a surface wave tomographic three-dimensional model of the Earth’s upper 300 km, we construct a global cross section parallel to the equator of the net rotation of the lithosphere, the so-called tectonic equator. Shear wave velocities indicate a difference between the western and eastern fl anks of the three major oceanic rift basins (Pacifi c, Atlantic, and Indian ridges). In general, the western limbs have a faster velocity and thicker lithosphere relative to the eastern or northeastern one, whereas the upper asthenosphere is faster in the eastern limb than in the western limb. We interpret the difference between the two fl anks as the combination of mantle depletion along the oceanic rifts and of the westward migration of the ridges and the lithosphere relative to the mantle. The low-velocity layer in the upper asthenosphere at the depth of 120–200 km is assumed to represent the decoupling between the lithosphere and the underlying mantle. It is also well defi ned by the distribution of radial anisotropy that reaches minimum values close to the rifts, but with an eastward offset. These results could be explained in the frame of the westward drift of the lithosphere relative to the underlying mantle.

- by Carlo Doglioni and +1
- •
- Plate Tectonics, Tectonic Evolution of Rifted Continental Margins, Rift Basins, Tethys Oceanic Crust

A new generation of aeromagnetic data documents the post-Caledonian tectonic evolution of the southwestern Barents Sea (SBS) up to the continent-ocean transition. Clear evidence of reactivation of Caledonian structures controlling both, Late Palaeozoic and Mesozoic basins can be observed at the edge of the Hammerfest and Nordkapp basins where reactivated low-angle detachments are observed on seismics. Our new aeromagnetic surveys confirm most of the previous structural elements, but new features appear and illustrate the complexity of the pre-Permian tectonic and the basement architecture in the SBS. We propose an updated tectonic scenario of the SBS where the Caledonian nappes and thrust sheets, well constrained onshore, swing anticlockwise from a NE-SW trend close to the Varanger Peninsula to NW-SE across the Nordkapp Basin and the Bjarmeland Platform. On the Bjarmeland Platform, the dominant magnetic grain is clearly NNW-SSE. We show that this pattern reflects a regional pre-Perm...

- by Aziz Nasuti
- •
- Geology, Geophysics, Tectonics, Gravity

The evolution of the Malay Basin, a Tertiary extensional basin offshore Peninsular Malaysia, is explained in terms of a simple kinematic model that accounts for the following key observations: (1) Major basement faults along the basin axis are E-trending rather than NW-trending, (2) Through-going strike-slip faults are absent from the basin axis and margins, and (3) En echelon fold pattern in postrift strata seems to have been influenced by the geometry of underlying extensional half-grabens. Basin development during late Eocene-early Oligocene began with sinistral transtensional shear of a broad NW-trending shear zone (axial shear zone) which contains pre-existing E-trending basement faults. The shearing caused crustal blocks that are bounded by the faults to rotate anticlockwise and form E-trending half-grabens between them. Reversal of shear during the early to middle Miocene, from sinistral to dextral, caused transpressive deformation and inversion of the half-grabens. The intensity of deformation increases southeastwards towards the West Natuna Basin as a result of the buttressing effect of the Natuna basement ridge which resisted the dextral motion along the axial shear zone.

- by Mazlan Madon
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- Malaysia, Extensional Tectonics, Basin inversion, Rift Basins

1987 in Beaumont and tankard ed, Canadian society of petroleum geologist, memoir 12

thie paper focus on the early structures on young rifts with a special emphasis of the Suez and Corinth graben

The Chivillas Formation is the easternmost record of Mesozoic marine volcanism in Mexico. It consists of thick intervals of pillow lavas interbedded with siliciclastic turbidites, and debrites, containing clasts derived from metamorphic, sedimentary and volcanic sources. Clast composition and detrital zircon geochronology indicate a continental provenance, with sources located south of the studied rocks. Detrital zircon ages range from 1573 ± 60 to 125 ± 1.6 Ma. The probability curves have peaks at 124 to 130 Ma (mean 126 Ma); 188 Ma and 921–1236 Ma (the latter with peaks at ∼ 1022 and ∼ 1157 Ma). Subordinate peaks at 277, 419 and 535 Ma are also present. We interpret the youngest zircon population ∼ 126 Ma (Barremian), as the maximum depositional age. Other peaks suggest Grenvillian-type basement and Permo-Triassic arc sources. Late Jurassic detrital zircons were probably derived from the Sierra de Juárez Mylonitic belt. Pillow lavas are mostly alkaline basalts, with SiO2 46%–53%, and alkali oxide (K2O + Na2O) 5–8 wt.%; all samples have low-TiO2 (< 1.6 wt.%) and low V (180–242 ppm), with Ti/V between 30 and 50. 206Pb/204Pb isotopic ratios are 18.6–20.5, and 208Pb/204Pb are 38.4–40.3, within OIB and MORB ranges. Initial εNd(126) are 0.3 to 4.1, and TDM are 632–1520 Ma. Lava compositions are similar to alkaline basalts along the margins of the Atlantic Ocean, particularly to basalts from the Central Atlantic Magmatic Province (CAMP), and to the Cretaceous Peri-Atlantic Alkaline Pulse (PAAP). Volcano-sedimentary rocks from Chivillas Formation probably formed in a subsiding marine rift basin, at a ridge–transform intersection. This basin received sediments transported throughout longitudinal canyons along the major Jurassic–Cretaceous strike–slip fault. In this model, extension of the rift basin was controlled by right-lateral displacement of the strike–slip fault, which acted as a transform fault at this time. This rift is interpreted as a segment of the rift system of the Gulf of Mexico.

- by Elena Centeno
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- Earth Sciences, Tectonics & Sedimentation, Cretaceous, Turbidites
- by Zeca Oliveira
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- Extensional Tectonics, Rift Basins, Rifts

] Since 60 Myr, Peloponnesus and continental Greece have been affected by the Hellenidean compressional and the Aegean extensional phases. This complex evolution resulted in development of a strongly inhomogeneous crust in the Gulf of Corinth region. To study this area, we use a large strain thermomechanical numerical code PARAVOZ previously used for a number of similar problems such as rifi evolution. Yet, instead of varying boundary and initial conditions applied to a plane-layered model, we use available geophysical constraints on the actual deep structure of the lithosphere to test its different possible initial structures. By varying the position of the initial crustal heterogeneity versus the position of the lithospheric slab, we are able to explain the origin of the internal structures and the kinematics of the Gulf of Corinth. The results suggest that the development of shear zones in the lower crust is favored by the gravitational collapse of the thicker part of the crust, whereas the geometry and the kinematics of these shear zones are controlled by the position of the edge of the slab. Asymmetry is seen in cases when a horizontal shift exists between the edge of the slab and the thicker part of the crust. Our model explains the differences between the northern shore and the southern shore as well as the east west variations observed in the Gulf of Corinth. INDEX TERMS: 8109 Tectonophysics: Continental tectonics-extensional (0905); 8020 Structural Geology: Mechanics; 321 O Mathematical Geophysics: Modeling; KEYWORDS: Gulf of Corinth, rifting, strain localization, back arc extension, Greece. Citation: Le Pourhiet, L., E. Burov, and I. Moretti, Initial crustal thickness geometry controls on the extension in a back arc domain: Case of the Gulf of Corinth

- by Isabelle Moretti (PhD)
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- Rift Basins