Phanerozoic Rift Research Articles

Syn‐Rift Magmatism and Sequential Melting of Fertile Lithologies in the Lithosphere and Asthenosphere

AbstractThe timing and rate of decompression melting of a compositionally heterogeneous mantle during continental rifting are assessed with a new one‐dimensional geodynamic code, MELT1D. MELT1D computes pressure and temperature in the extending lithosphere and rising asthenosphere and calculates the resulting melt fraction and eruption rates for different lithologies. A series of models simulate syn‐rift melt production from (a) dry and wet depleted lherzolite similar to the mantle that underlies most mid‐ocean ridges, (b) dry and wet relatively fertile ultramafic compositions representing plume or primitive mantle material, (c) pyroxenite representing recycled ultramafic oceanic crust or magmatic metasomes, and (d) basalt representing recycled mafic crust or metasomes. The models predict sequential melting of the different compositions that is broadly consistent with basalt eruption histories in many Phanerozoic rifts. Results show a progressive transition in magma sources as the lithosphere thins, beginning with melting of wet mantle and compositionally fertile mafic components near the lithosphere‐asthenosphere boundary during the earliest stages of extension. This transitions to magmatism dominated by melting of relatively fertile ultramafic components (pyrolitic and pyroxenitic compositions) as extension progresses, and finally to melting of ambient lherzolite asthenosphere as lithosphere thinning approaches breakup. Mantle composition, pre‐rift lithosphere thickness, and mantle temperature exert the greatest controls on the timing and volumes of magmas produced from each lithology. In general, a cool or thick lithosphere has a greater capacity to sequester fertile lithologies than thin or warm lithosphere, and thus has a greater capacity to produce early syn‐rift magmas without requiring a hot mantle plume.

Rhabdophane Th-Pb ages indicate reactivation of Mesoarchean structures in west Pilbara Craton during breakup of Greater India and Australia-Antarctica

Abstract Uranium-Th-Pb dating of phosphate minerals in very low-grade metasedimentary rocks from the Archean Pilbara Craton, Western Australia, has revealed a long history of deformation and fluid flow during the Paleoproterozoic. However, this technique has not detected evidence for fluid flow along craton margins during Phanerozoic rifting and breakup. We report the use of in situ Th-Pb geochronology of rhabdophane, a hydrous light rare earth element phosphate, to date fluid flow in shale from the 2.76 Ga Mount Roe Basalt from drill hole number 6 of the Archean Biosphere Drilling Program (ABDP6), northwestern Pilbara Craton. Thorium-Pb dating of rhabdophane in carbonaceous shale yields three main populations with weighted mean 208Pb/232Th ages of 152 ± 6 Ma, 132 ± 4 Ma, and 119 ± 4 Ma, which indicates phosphate growth up to 2.64 b.y. after deposition. The rhabdophane ages are coeval with three major breakup events in eastern Gondwana: separation of Southwest Borneo and Argoland from Australia (ca. 156–152 Ma), breakup of Greater India from Australia (ca. 140–135 Ma), and separation of Greater India/India from Antarctica (ca. 123 Ma). The proximity of drill hole ABDP6 to major Mesoarchean faults and shear zones on the craton margin, which are parallel to rift propagation and basin development, points to episodic reactivation of ancient crustal structures >2.8 b.y. after their formation. Our results also highlight the potential of rhabdophane as a U-Th-Pb geochronometer for dating low-temperature (<200 °C) fluid flow and hydrous alteration. The migration of Mesozoic fluids through Archean shales adds weight to questions about the origin of geochemical signals in ancient altered rocks and how to extract information about the early environment and biosphere.

Variations in crustal and lithospheric structure across the Eastern Indian Shield from passive seismic source imaging: Implications to changes in the tectonic regimes and crustal accretion through the Precambrian

Changes in the dominant tectonic regimes and the modes of formation and maturation of the continental crust during the Precambrian are a subject of active debate. Here, we discuss these issues in terms of new information on the crustal and lithospheric architecture of the different terranes in the Eastern part of the Indian Shield (EIS). We present well-constrained estimates of the crustal and lithospheric thickness characterizing different terranes of the EIS through the joint inversion of P-receiver functions (PRF) and fundamental mode group velocity dispersion data of Rayleigh waves (10–100 s). The lithospheric thicknesses are also computed independently using the S-receiver function (SRF) modeling. The study involved 2167 PRFs and 100 SRFs from 15 broadband stations. Our modeling detects a marked crustal (~4–8 km) and lithospheric (~15–20 km) thinning beneath the Singhbhum-Odisha-Craton (SOC), spatially correlating with the central part of the craton comprising the gneissic terrain contrasted by a crustal thickening (~2–6 km) beneath the horseshoe-shaped Iron Ore Group. We propose that such a crustal configuration beneath the Paleoarchean craton may suggest the dominance of vertical tectonic processes such as thickening of an oceanic mafic plateau and recurrent melting producing the TTGs and the Singbhum Granite suite. Infra-crustal reworking involving Rayleigh-Taylor instabilities and gravitational processes could explain the crustal architecture of the SOC much similar to a few other Paleoarchean cratons such as Pilbara (Australia), Barberton and Kappvaal (South Africa). Our modeling has also revealed a relatively smaller degree of crustal (2–4 km) and lithospheric thinning (4–10 km) beneath the Eastern Ghat Mobile Belt, south of the SOC. We observe a relatively thicker crust (~42 km) characterized by a flat Moho over a large area (~40,000 sq. km) encompassing the Chota Nagpur Granite Gneiss Terrane (CGGT). Such thickening is consistent with the convergence and collision tectonics involving the SOC and CGGT. However, we note a 15–20 km lithospheric thinning associated with the CGGT. Such thinning may mainly relate to Phanerozoic rifting along the Damodar graben and Rajmahal magmatism. Broadly, our model of the crustal and lithospheric architecture of the EIS reflects the imprints of northward subduction of SOC beneath the CGGT.

Appreciation of A.M. Celâl Şengör

Celâl Şengör is one of the leading and most influential geologists of our time. He has made many significant contributions to the understanding of the evolution of our planet in many areas, including continental growth, Phanerozoic orogenic belts, continental rifts, transform faults, and mantle plumes.

Athapuscow aulacogen revisited: Geochronology and geochemistry of the 2046 Ma Union Island Group mafic magmatism, East Arm of Great Slave Lake, Northwest Territories, Canada

The Slave craton underwent widespread extension during the Paleoproterozoic. The East Arm basin of Great Slave Lake formed during this period and preserves a protracted record of sedimentation and volcanism in the southeastern Slave craton. Within the East Arm basin package, the Union Island Group represents voluminous mafic volcanism with subordinate sedimentary strata. We report the first high precision U–Pb zircon and baddeleyite dates for the Union Island Group; 2045.8 ± 1.0 Ma for a volcaniclastic unit from the lower basalt formation and 2042.7 ± 3.0 Ma for a diabase intrusion within volcaniclastic horizons in the lower basalt formation. We fundamentally revise the lower basin stratigraphy and demonstrate that the Union Island Group is the oldest identified supracrustal package in the East Arm basin, ∼120 Myr older than previously thought. This revised stratigraphy is supported by detrital zircon provenance age distribution in two Union Island Group sedimentary samples, which is dominated by 2.76–2.56 Ga ages, reflecting prominent input from the local Archean Slave basement.The Union Island Group contains two geochemically distinct mafic volcanic packages. The lower basalt package is predominantly alkaline, characterized by an OIB-like chemical signature with high and variable incompatible element contents (108–438 ppm Zr, 13–62 ppm Nb). In contrast, the upper basalt package is tholeiitic, contains much lower and uniform incompatible element contents (83–101 ppm Zr, 2–4 ppm Nb), and has an E-MORB-like chemical signature. Both packages display overlapping, depleted time-integrated εNd(i) values (+1.1 to +3.2). Petrological modeling suggests the lower package originated as OIB-like melts from the asthenosphere that interacted with a depleted reservoir; the upper package was produced by larger degree of partial melting of a similar mantle source, in addition to possible lithospheric input. The petrogenesis established in this study is consistent with a passive continental rift origin for the Union Island Group magmatism and is further supported by geochemical similarities with Proterozoic and Phanerozoic rift successions. We therefore propose that the 2046 Ma Union Island Group represents an incipient rift basin sequence formed during Paleoproterozoic extension that marked the initial breakup of the southern margin of the Slave craton from a pre-Laurentia supercontinent.

Re-imagining and re-imaging the development of the East African Rift

The East African Rift (EAR) has fascinated and challenged the geological imagination since its discovery nearly a century ago. A new series of images showing the sequential development of faulting and volcanism along the Rift from 45 Ma to present offers a regional overview of that development. The EAR is the latest phase of the extensive Phanerozoic rifting of the East African continental plate, interwoven with the lithospheric fabrics knitted together during its complex Proterozoic past. South of 5° S, the EAR variously follows or cuts across the Karoo rift trends; north of 5° S, it is almost totally within new or reworked Neoproterozoic terranes, while the Karoo rifts are almost totally outside them. The compilations raise several aspects of rift development seemingly in need of re-imagining, including tight-fit reconstructions of the Gulf of Aden, and the projection of Mesozoic rifts from Yemen to Somalia. Overall, the rifting process does not accord well with a mechanistic paradigm and is better imagined within the Prigoginian paradigm, which accepts instability and disorder within natural processes such as mantle plumes. The structural complexity of Afar and its non-alignment with magnetic anomalies suggests that the seafloor spreading process is, in its beginnings at least, more chaos than order.

A lithosphere-scale structural model of the Barents Sea and Kara Sea region

Abstract. We introduce a regional 3-D structural model of the Barents Sea and Kara Sea region which is the first to combine information on the sediments and the crystalline crust as well as the configuration of the lithospheric mantle. Therefore, we have integrated all available geological and geophysical data, including interpreted seismic refraction and reflection data, seismological data, geological maps and previously published 3-D models into one consistent model. This model resolves four major megasequence boundaries (earliest Eocene, mid-Cretaceous, mid-Jurassic and mid-Permian) the top crystalline crust, the Moho and a newly calculated lithosphere–asthenosphere boundary (LAB). The thickness distributions of the corresponding main megasequences delineate five major subdomains (the northern Kara Sea, the southern Kara Sea, the eastern Barents Sea, the western Barents Sea and the oceanic domain comprising the Norwegian–Greenland Sea and the Eurasia Basin). Relating the subsidence histories of these subdomains to the structure of the deeper crust and lithosphere sheds new light on possible causative basin forming mechanisms that we discuss. The depth configuration of the newly calculated LAB and the seismic velocity configuration of the upper mantle correlate with the younger history of this region. The western Barents Sea is underlain by a thinned lithosphere (80 km) resulting from multiple Phanerozoic rifting phases and/or the opening of the NE Atlantic from Paleocene/Eocene times on. Notably, the northwestern Barents Sea and Svalbard are underlain by thinnest continental lithosphere (60 km) and a low-velocity/hot upper mantle that correlates spatially with a region where late Cenozoic uplift was strongest. As opposed to this, the eastern Barents Sea is underlain by a thicker lithosphere (~ 110–150 km) and a high-velocity/density anomaly in the lithospheric mantle. This anomaly, in turn, correlates with an area where only little late Cenozoic uplift/erosion was observed.

Regional geology and tectonics: v.1A: Principles of geologic analysis; v.1B: Phanerozoic rift systems and sedimentary basins; v.1C: Phanerozoic passive margins, cratonic basins and global tectonic maps

Regional geology and tectonics: v.1A: Principles of geologic analysis; v.1B: Phanerozoic rift systems and sedimentary basins; v.1C: Phanerozoic passive margins, cratonic basins and global tectonic maps

Proterozoic ferroan feldspathic magmatism

The Proterozoic eon is characterized by an unusual abundance of ferroan feldspathic rocks that range in composition from granite to quartz syenites to feldspathoid-bearing syenites. Three associations of ferroan rocks may be recognized on the basis of major element compositions: (1) metaluminous alkalic to alkali-calcic syenites and granites associated with anorthosites and iron-rich basaltic rocks; (2) alkaline intrusions in continental rifts; and (3) calc-alkalic metaluminous to peraluminous granitoid plutons that are less strongly ferroan than the other two groups. The first two groups form by differentiation of tholeiitic to alkali basaltic parental magmas; the third by partial melting of quartzo-feldspathic crust. Although all three associations may form in intraplate settings, only the alkaline intrusions, which typically contain high incompatible trace element abundances, plot entirely within the “within plate granite” fields on trace element discrimination diagrams. Processes of crustal melting and magma mixing have had a greater effect on the trace element compositions of ferroan granitoids from the other two associations, causing some rocks to plot outside this field.Of these three associations, the granitoids associated with anorthosites are the most voluminous and are essentially restricted to the Proterozoic. Special conditions, perhaps related to the presence of a long-lived supercontinent under which widespread mantle upwelling took place, appear required to explain the abundance of these rocks. Ferroan alkaline rocks, although rarely preserved in Archean crust, are present in Proterozoic and Phanerozoic rifts. The earliest alkaline intrusions thus may mark the establishment of rigid continents capable of rifting. The formation of ferroan calc-alkalic granites does not appear to be time-dependent: these granites have been generated throughout Earth history in response to crustal melting.

Magmatism-dominated intracontinental rifting in the Mesoproterozoic: The Ngaanyatjarra Rift, central Australia

The Late Mesoproterozoic (1085–1040Ma) Ngaanyatjarra Rift, previously referred to as the Giles Event, is the dominant component of the Warakurna Large Igneous Province (LIP) that affected much of central and western Australia. This rift is well preserved and provides excellent examples of rift structure at a variety of crustal levels and times in the rift's evolution. Geological knowledge is integrated with geophysical interpretations and models to understand the crustal structure and evolution of this rift. Two phases are identified: an early rift stage (1085–1074Ma) that is characterised by voluminous magmatism within the upper crust and relatively little tectonic deformation; and a late rift stage that is characterised by tectonic deformation, synchronous with the deposition of a thick pile of volcanic and sedimentary rocks (1074–1040Ma). Compared to modern rift examples, this rift is unusual in that the crust was thickened by ~15km and overall extension was very limited. However, its structure and evolution are very similar to the near-contemporaneous Midcontinent Rift, which shows the addition of a similar quantity of magmatic material as well as crustal thickening and limited extension. For these Mesoproterozoic rifts, we suggest that magmatism was the dominant process, and that the extension observed was a response to magmatism-induced crustal thickening and the gravitational collapse of the crustal column. Other Proterozoic rifts show similar characteristics (e.g. Transvaal Rift), whereas most Phanerozoic rifts are dissimilar, showing instead a dominance of extension, with magmatism largely a result of this extension. This change in the style of rifting from the Precambrian to the Phanerozoic may relate to the influence of a typically cooler and stronger lithosphere, which has caused stronger strain localisation and a greater role for extension as the controlling factor in rift evolution.

Deep Norden: Highlights of the lithospheric structure of Northern Europe, Iceland, and Greenland

We present a review of geophysical models of the continental lithosphere of Norden, which includes the Nordic countries (Denmark, Iceland, Finland, Norway, Sweden), Greenland, and the adjacent regions of the neighbouring countries. The structure of the crust and the lithospheric mantle reflects the geologic evolution of Norden from Precambrian terrane accretion and subduction within the Baltic Shield and Greenland to Phanerozoic rifting, volcanism, magmatic crust formation, subduction and continent-continent collision at the edges of the cratons and at the plate boundaries. The proposed existence of a mantle plume below Iceland has not been uniquely demonstrated by the available seismic evidence. Its connection to the break-up of the North Atlantic Ocean c. 65 My ago is uncertain, but the >30 km thick crust in the strait between Iceland and Greenland may indicate the track of the plume. Using the results from seismic (reflection and refraction profiles, P- and S-wave, body-wave and surface-wave tomography), thermal, gravity, and petrologic studies, we review the structure of the crust and the lithospheric mantle of Norden and propose an integrated model of physical properties of the lithosphere of the region, including maps of lateral variation in crustal and lithospheric thicknesses and compositional variation in the lithospheric mantle.

Deep Europe today: geophysical synthesis of the upper mantle structure and lithospheric processes over 3.5 Ga

abstract We present a summary of geophysical models of the subcrustal lithosphere of Europe. This includes the results from seismic (reflection and refraction profiles, P- and S-wave tomography, mantle anisotropy), gravity, thermal, electromagnetic, elastic and petro-logical studies of the lithospheric mantle. We discuss major tectonic processes as reflected in the lithospheric structure of Europe, from Precambrian terrane accretion and subduction to Phanerozoic rifting, volcanism, subduction and continent-continent collision. The differences in the lithospheric structure of Precambrian and Phanerozoic Europe, as illustrated by a comparative analysis of different geophysical data, are shown to have both a compositional and a thermal origin. We propose an integrated model of physical properties of the European subcrustal lithosphere, with emphasis on the depth intervals around 150 and 250 km. At these depths, seismic velocity models, constrained by body- and surface-wave continent-scale tomography, are compared with mantle temperatures and mantle gravity anomalies. This comparison provides a framework for discussion of the physical or chemical origin of the major lithospheric anomalies and their relation to large-scale tectonic processes, which have formed the present lithosphere of Europe.

Persistent fault controlled basin formation since the Proterozoic along the Western Branch of the East African Rift

The Western Branch of the East African Rift System is outlined by elongate sedimentary basins, frequently occupied by Cenozoic rift lakes. The role of the inheritance of the leading rift faults from pre-existing basement structures has often been invoked. Recent studies in western Tanzania confirm the extent of the northwest orientated Palaeoproterozoic Ubende Belt contribution to the Phanerozoic Rift. Attention is drawn here on the occurrence of different Meso- and Neoproterozoic sedimentary basins that developed along the ductile shear belt as a result of repeated sinistral wrench fault reactivation. These basins partly overlap each other and typically bear shallow and weakly evolved sediments. North of the Ubende Belt, the Mesoproterozoic Kibara Belt is inferred to have originated as a basin controlled by the complex termination of the Ubende wrench fault. Phanerozoic rift basins also develop along the northwest orientated Ubende Belt structure. They display the same elongate shape as the Proterozoic basins. In Late Palaeozoic-Early Mesozoic the Karoo rift basins formed from a dextral lateral shear reactivation of the inherited Proterozoic shear faults. During the first phase of development the Lake Tanganyika Basin is belived to bear the same characteristics as all previous basins along the Ubende Shear Belt, mainly controlled by strike-slip movements along pre-existing shear faults. The present Lake Tanganyika Basin is subdivided in two sub-basins, separated by the transverse Mahali Shoal, which is an active structure located on the Ubende Shear. The deep lake basin mainly developed outside the Ubende Belt. The northern sub-basin appears to be structurally controlled by the reactivation of the Mesoproterozoic sinistral wrench fault termination of the Ubende shear faults. Structural control of the Palaeoproterozoic basement is however unclear for the southern sub-basin of Lake Tanganyika: this part of the rift segment is flanked by Palaeoproterozoic basement which has not been affected by the Ubende Shear.

Mechanisms of inheritance of rift faulting in the western branch of the East African Rift, Tanzania

The western branch of the East African Rift system is commonly cited as a result of Phanerozoic reactivation of the Paleoproterozoic Ubendian belt in western Tanzania. Geological evidence is provided to show that prominent mechanical anisotropies successively appeared during Proterozoic evolution of the Precambrian basement and that their different reactivation behavior contributed to the Phanerozoic rift pattern. The Ubende belt (1950–1850 Ma) is a NW oriented, amphibolite facies ductile lateral shear belt in which older (2100–2025 Ma) and complex granulite facies terranes are included along trend. Retrograde multiphase sinistral strike‐slip mylonites developed along the NW oriented ductile shear belt. They reflect persistent Proterozoic wrench fault reactivation of the latter. Shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the Proterozoic wrench fault reactivation. Neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone, which appears as an elongate and NW trending positive flower structure, locally enhanced by late Proterozoic contraction. Phanerozoic rifting is demonstrated by others to occur in three distinct episodes, during which the complex rift segment formed upon the multiphase Proterozoic wrench fault zone. The evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered. The Proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern, which became selectively reactivated by sinistral transpressional ductile‐brittle mylonites. Proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults. According to the position of these mylonites in the center or in the external parts of their NW oriented Neoproterozoic transpression, they reactivate as complex and multiphase rift faults or as normal and recent faults, respectively. The Paleoproterozoic NW oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy. Its reactivation in Phanerozoic stress fields is likely dextral oblique transtension, considered as a leading mechanism of the pluriphase and NW oriented deep rift basins.

Structural controls on sedimentation in continental rifts

Abstract Structure and climate have each been proposed as the primary control on sedimentation in continental rifts. Numerous Phanerozoic rifts spanning a wide range of inferred palaeoclimates have consistent relationships between structure and sedimentation, and between structural evolution and stratigraphic sequences. Six principal structural features strongly influence sedimentation patterns in rifts. (1) Topographically high accommodation zones segment rifts into 50–150 km long structural half-graben, each one a discrete depocentre. (2) Reactivation of pre-existing structural elements commonly controls the location and orientation of accommodation zones. Drainage systems outside the rift then may be captured and diverted towards the rift axis, establishing some accommodation zones as input points for sediment. (3) Footwall uplift at main border faults directs most drainage away from young rifts. Along the rift flanks, uplift is minimized at accommodation zones, enhancing their role as sediment input points. (4) Gentle roll-over of topography on flexural margins produces drainage systems that are generally small, and that do not introduce large amounts of sediment into the rift. (5) Breakup of the basin floor into elongate, rift-parallel fault blocks greatly enhances along-axis sediment transport. (6) Development of fault relay zones and transfer faults localizes down-dip sediment movement. These structural controls occur in the present-day low latitude rifts of East Africa, where climate ranges from arid to humid. Similar features have been identified in ancient rifts that are interpreted to span a comparable range in climates. Clearly, climate is important in determining the sedimentary history of continental rifts. The present study indicates that climatic effects, however, are superimposed on the fundamental structural geometries that evolve during rifting.

Phanerozoic exhumation history of northern Prince Charles Mountains (East Antarctica)

Apatite fission-track data from samples of Precambrian basement, Late Permian Triassic sedimentary rocks and inferred Cretaceous intrusive bodies are used to constrain the low-temperature (i.e. sub ~110°C) thermal history of the northern Prince Charles Mountains, East Antarctica. Two discrete phases of cooling have been identified, both of which are attributed to regional exhumation associated with rifting episodes. A phase of late Palaeozoic cooling, that began during the Carboniferous, is inferred to have been associated with the initial formation of the Lambert Graben. A more recent phase of cooling was initiated during the Early Cretaceous and is estimated to have locally involved the removal of at least 2 km of material using an assumed palaeotemperature gradient of ~25°C km−1 at the time of cooling. This latter phase of exhumation was closely accompanied by the emplacement of a variety of mafic alkaline rocks at ambient palaeotemperatures less than ~60°C and was probably related to renewed extension of the Lambert Graben during the break-up of eastern Gondwana. The results of this study suggest that final exhumation of high-grade Precambrian basement of the northern Price Charles Mountains was largely controlled by Phanerozoic rifting events.

Rift basin evolution in Africa: the influence of reactivated steep basement shear zones

Summary Phanerozoic rift basin evolution in Africa has been strongly influenced by the pre-existing basement structure inherited from Precambrian tectonics. Two modes of basement reactivation may be identified; the largely dip-slip reactivation of gently dipping shear zone and thrust structures and the largely strike-slip reactivation of steep shear zones and faults. In particular, crustal scale steep basement shear zones have exercised a profound control on rift basin formation on a regional scale. Such shear zone structures are seen to govern the location, trend and structural style of rifts and appear to constrain the orientation of displacements generating the rifts. The shear zones are relatively narrow, high strain zones formed during ductile deformation. They occur as steep, crustal scale ramps that define a pronounced mechanical anisotropy within the African continental crust. These structures are difficult to reactivate by dip-slip movement and thus tend to act as major strike-slip transfer zones during later extensional events. This paper discusses examples of the steep shear zone-rift relationship from three of the rift systems of Africa: the Upper Palaeozoic (Karoo) rifts of Central Africa, the Mesozoic rifts of the Sudan and the Cenozoic rifts of East Africa. These discussions indicate that steep shear zones are sites of rift nucleation and play a fundamental role in the mechanisms of rift propagation and subsequent continental breakup.

Crustal structure of the Midcontinent rift system: Results from GLIMPCE deep seismic reflection profiles

Interpretation of Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) seismic reflection profiles indicates that the Midcontinent (Keweenawan,1100 Ma) rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km (about 10.5-s reflection time) along profiles crossing western, central, and eastern Lake Superior and the northern end of Lake Michigan. This area may overlie the greatest thickness of intracratonic rift deposits on Earth. Times to Moho reflections vary along strike from 11.5 to 14 s (about 37-46 km depth) in the west, to 17 s (about 55 km) in the center, and 13 to 15 s (about 42-49 km) in the eastern end of Lake Superior. The prerift crust, however, was thinned 25-30 km beneath the central rift (compared with its flanks), providing evidence for crustal extension by factors of about 3-4. The Midcontinent rift system differs from Phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding (presumably unextended) regions.

Geologic and geochronologic constraints on the evolution of the Red Sea-Gulf of Aden and Afar depression

New K-Ar age determinations show that the development of Southern Afar since 14 Ma ago involved five stages of tectonism and volcanism at 14-11, 11-10, 9-7, 5-4 and post-1.6 Ma ago; while the major phases of rifting for the Red Sea-Gulf of Aden have been shown by geological and geophysical data to be 25, 14, 10 and 4 Ma ago. It is suggested that since 14 Ma ago, identical periods of volcanism preceded by rifting have affected both areas.Continental breakup in the Red Sea region was initiated along large transcurrent faults followed by extension manifested by normal faulting, block tilting in the brittle crustal region and repeated dyke injection. Two large scale transcurrent faults were identified, the Marda (NW-SE) and the Ambo (ENE-WSW) which, it is suggested, controlled the trends of rifting in the Red Sea and Gulf of Aden, respectively. Structural and geological evidence indicate that these faults have had a relatively long history, since they were active in pre-Jurassic times. Because at least the Ambo fault cuts across the N-S trends of the late Proterozoic basement, its orientation must be primarily related to the Phanerozoic rift tectonics.This model avoids overlap of the Arabian plate with the Danakil and Aisha blocks because it involves only limited true oceanic crust. The new age data indicate that the initiation of the Aden and Red Sea rifts was not accompanied by active volcanism on land until the Oligocene-Miocene (30-5 Ma ago). Following that time, volcano-tectonic rifting continued without substantial hiati.

On the nature of greenstone belts in the precambrian

At present there is considerable speculation concerning the origin of greenstone belts. They are compared with Phanerozoic geosynclines, fold belts, modern island arcs or with the rift structures formed above subduction zones in marginal seas. Four stages in greenstone-belt development can be determined (3.8-3.0, 3.0-2.6, 2.6-1.9 and 1.9 b.y. and younger). Sections of early greenstone belts contain all the essential features of the modern oceanic sequences. Their peculiarities are: great thickness, presence of jaspilites and scarce occurrence of very coarse clastics. On the contrary, the latter type of sediment is widespread in younger greenstone belts and in those of the last stage which are completely formed in continental environment. Whole rock chemistry of Precambrian volcanics (more than 1800 analyses all over the world) confirms that alkali-olivine basalts are absent till 1.9 b.y. and andesites are generally rare. Widespread type of Early Precambrian basalts are Ol-Hy and Q normative ones, showing close similarity to typical abyssal tholeiites. The presence of basaltic komatiites is not a unique petrochemical feature of volcanism in the Early Precambrian as one thinks. Now similar rocks have been found in Phanerozoic rift zones. There is no evidence suggesting significant shortening of initial crustal complexes and greenstone belts. These data along with slight metamorphism and above features can only be explained by the formation of greenstone belts in rifting environment. During the early stages (3.8-1.9 b.y.) the thickness of the protocrust was not more than 10–20 km. It allows us to understand the rare occurrence of intermediate volcanics at early stages of the Earth's evolution. In summary it is argued that in the Early Precambrian the spread of primitive oceanic lithosphere was not compensated by the consumption in subduction zones. Consequently, plate tectonics cannot explain the formation of the crust in the early history of the Earth.