Ancient Rift Systems Research Articles

Geochemistry of Paleozoic Dolerite Dikes in the Northeastern Kola Peninsula and Their Relations to Flood-Basalt and Alkaline Magmatism

The paper presents data on dolerite dikes in the Murmansk block in the northeastern Kola Peninsula. The dikes belong to a Middle Paleozoic flood-basalt association, which is a principal element of the large-scale plume–lithospheric process in the area of the Barents Sea. According to their geological, petrological, and geochemical features, the dolerite dikes are classified into two groups: older and younger, which pertain to the initial and final evolution of the large igneous province produced under the effect of the Barents Sea plume at 450–340 Ma. One of the results of the plume–lithosphere interaction was the origin of the Eastern Barents Basin, which is filled with large volumes of basalts, and this predetermined the further subsidence of the area in Meso-Cenozoic time and the accumulation of thick sedimentary sequences with significant hydrocarbon resources. The peripheries of the area with products of basaltic magmatism are marked by alkaline rock complexes. The largest of the latter are the intrusions of the Kola Province and kimberlites in Archangelsk oblast. The dolerites of the dikes abound in xenogenic grains of zircon, which were dated at 2.7, 1.8, 1.3, 0.7 Ga. These age values suggest that the parental melts may have been derived in the lower crust and that the magma-generating processes may have evolved in the root zone of an ancient rift system.

Stress development in heterogenetic lithosphere: Insights into earthquake processes in the New Madrid Seismic Zone

The New Madrid Seismic Zone (NMSZ) in the Midwestern United States was the site of several major M 6.8–8 earthquakes in 1811–1812, and remains seismically active. Although this region has been investigated extensively, the ultimate controls on earthquake initiation and the duration of the seismicity remain unclear. In this study, we develop a finite element model for the Central United States to conduct a series of numerical experiments with the goal of determining the impact of heterogeneity in the upper crust, the lower crust, and the mantle on earthquake nucleation and rupture processes. Regional seismic tomography data (CITE) are utilized to infer the viscosity structure of the lithosphere which provide an important input to the numerical models. Results indicate that when differential stresses build in the Central United States, the stresses accumulating beneath the Reelfoot Rift in the NMSZ are highly concentrated, whereas the stresses below the geologically similar Midcontinent Rift System are comparatively low. The numerical observations coincide with the observed distribution of seismicity throughout the region. By comparing the numerical results with three reference models, we argue that an extensive mantle low velocity zone beneath the NMSZ produces differential stress localization in the layers above. Furthermore, the relatively strong crust in this region, exhibited by high seismic velocities, enables the elevated stress to extend to the base of the ancient rift system, reactivating fossil rifting faults and therefore triggering earthquakes. These results show that, if boundary displacements are significant, the NMSZ is able to localize tectonic stresses, which may be released when faults close to failure are triggered by external processes such as melting of the Laurentide ice sheet or rapid river incision.

Climatic catastrophes in Earth history: two great Proterozoic glacial episodes

Near the beginning and end of the Proterozoic Eon (2.5 Ga–542 Ma) the Earth went through dramatic climatic perturbations. The Palaeoproterozoic (Huronian) glaciations are best known from the Canadian Shield where there is evidence of at least three such episodes. Glacial deposits of comparable age are also known from Fennoscandia, South Africa and Western Australia. In the type area, the Huronian glacial deposits are preserved in an ancient rift system that preceded break‐up of the supercraton, Kenorland, whereas those in the southern hemisphere may have been deposited in a foreland basin setting. Detailed correlations between the two hemispheres must await more geochronological data. Following a long period (~1.5 Ga) with little evidence of glaciation, the climatic upheavals of the Neoproterozoic Era began. The two most widespread glacial events are known as the Sturtian and Marinoan. The Neoproterozoic glaciations also took place on a supercontinent (Rodinia). Some were accompanied by unexpected rock types such as dolomitic cap carbonates and iron formations, both of which show evidence of hydrothermal influence. Major influences on surface temperatures on Earth include solar luminosity (increasing throughout geological history) and the concentration of atmospheric greenhouse gases such as CO2 (generally diminishing with time). It is suggested that the two great Proterozoic climatic oscillation periods resulted from perturbations of the balance between these two variables, triggered by drawdown of atmospheric CO2 during intensive weathering of supercontinents. A weathering‐related negative feedback loop resulted in multiple glaciations with intervening warm periods. Climatic stability only returned after the supercontinent broke apart and reduced continental freeboard moderated continental weathering. Copyright © 2012 John Wiley & Sons, Ltd.

Reactivations of boundary faults within a buried ancient rift system by ductile creeping of weak shear zones in the overpressured lower crust: The 2004 mid-Niigata Prefecture Earthquake

We elucidated fine-scale heterogeneities of the seismogenic structure associated with the 2004 mid-Niigata Prefecture Earthquake (thrust fault), Japan, by deploying a dense portable seismic array in the southwestern edge of the source region to observe the aftershocks. A velocity model inverted from double-difference tomographic analysis with first arrival times shows that most aftershocks were aligned between sedimentary strata in the hanging wall and the basement in the footwall. The basement is characterized by clear step-like and tilted block structures that gradually deepen toward the west. The domino-tilted block structures of the basement reveal evidence of a Miocene rift system buried beneath the thick sedimentary sequence. The aftershocks appear to be aligned roughly along pre-existing boundaries of the step-like array of tilted blocks. In addition, we used the Natural Earthquake Reflection Profile (NERP) method to image reflective zones in the lower crust, defining a set of relatively strong reflectors. Low seismic velocity anomalies combined with high electrical conductivity in the lower crust suggest that a plausible explanation for the reflective zones is the presence of reservoirs of overpressured crustal fluids. Given these considerations, we propose that pre-existing boundary faults within the buried ancient rift system were reactivated through ductile creeping of weak shear zones in the overpressured lower crust.

Reactivation of ancient rift systems triggers devastating intraplate earthquakes

Large historical and recent intraplate earthquakes have been concentrated along a contractional zone at the eastern margin of the Japan Sea back‐arc basin. Here we present high‐resolution three‐dimensional tomographic imaging of seismic velocities in this zone using data from a dense seismograph deployment. We discover that stepwise and tilted block structures of the basement, which are geophysical evidence of a Miocene rift system, are widely distributed beneath the thick sedimentary basin. Most aftershocks following recent intraplate earthquakes align roughly along the tilted block boundaries and are controlled by weaknesses associated with buried rift systems. Furthermore, slow anomalies are localized beneath the seismogenic zones, suggesting that fluids may have locally weakened the strength of the lower crust. We propose that stress loading through ductile creeping of the weak lower crust reactivates pre‐existing weak faults within ancient rift systems, leading to devastating intraplate earthquakes.

The 26 January 2001 "Republic Day" Earthquake, India

The Mw 7.6 Republic Day earthquake that shook the Indian province of Gujarat on the morning of 26 January 2001 was the most deadly in India's recorded history (Figure 1). One month after the earthquake official Government of India figures placed the death toll at 19,727 and the number of injured at 166,000. Preliminary indications are that 600,000 people were left homeless, with 348,000 houses destroyed and an additional 844,000 damaged. The Indian State Department estimates that the earthquake affected, directly or indirectly, 15.9 million people, nearly 50% of the population of Gujarat. More than 20,000 cattle were reported killed. Government estimates place direct economic losses at $1.3 billion, although more recent estimates indicate losses may exceed $5 billion. The Republic Day earthquake occurred on the Kachchh Peninsula (Figure 2), which has a long history of strong earthquakes (Bapat et al. , 1983). The region is bordered to the north and the south by ancient rift systems (Biswas, 1971, 1987, 1989; Srivastava, 1971). Structures within these rift systems (Rajendran and Rajendran, 1998, 2001) and on the Kachchh mainland (Malik et al , 2000) are now subjected to compressional stress and reverse faulting resulting from India's collision with Asia (Gowd, 1996; Chandra, 1977). Compresssional features appear to be of at least two different ages, with folding events occurring prior to 120 Ka and a distinct set of new features activated more recently (Rockwell, personal communication, 2001). The first historical Kachchh earthquake to attract international attention was the 1819 Allah Bund (Dam-of-God) earthquake, which created a 6-m-high, 6-kin-wide natural dam across the Puran or Nara River (which enters the Rann of Kachchh from the north). A lake 30 km in diameter, Lake Sindri, was formed south of the Allah Bund, and although this is now partly filled …

Thermal and Maturation Modeling of the Urengoy Field, West Siberian Basin: Some Special Considerations in Basin Modeling1

Present-day temperature and vitrinite reflectance (Ro) profiles are considered key factors for calibration in numerical basin modeling; therefore, it is important to understand how such profiles are generated. Basin modeling typically considers factors such as rock consolidation, depth-variation of petrophysical characteristics, changes in heat flow, and paleoclimate; however, our basin evaluations have shown that other processes that influence temperature and Ro (vitrinite reflectance) profiles commonly are inadequately studied. Modeling within the Urengoy field of the West Siberian basin has allowed numerical estimates of some of these nonstandard processes. First, a substantial increase in temperature (up to 5-10iC) is expected from changes in heat conductivity due to the formation of significant free gas, gas-condensate, and condensate. Second, climate variations creating numerous periods of formation and degradation of permafrost zones for the last 3.4 m.y. could decrease the modern rock temperatures by 10-17iC in the upper 1.5 km of the sedimentary section and up to 10iC at its base. Due to the short duration of these two processes, their effect on organic matter maturation is minimal; nevertheless, their roles in the formation of present-day temperature profiles necessitate their inclusion in basin modeling. Third, modeling results demonstrated that change in heat conductivity of rocks with dispersed organic matter can increase temperature of sedimentary rocks by 3-5iC and rock maturity by not more than 0.02% (Ro). Finally, the sharp increase in vitrinite reflectance (% Ro) with depth, typical for the deep sedimentary units of the Urengoy field and many other continental rift basins, is possible to explain from hydrothermal activity. The influence of these processes on subsurface temperatures and maturation of organic matter can vary significantly depending on lithology, organic matter content, and the location of the modeled section relative to the ancient rift system. The results of this study can help one estimate the relative influence of these parameters.

Sedimentation and subsidence patterns in the central and north basins of Lake Baikal from seismic stratigraphy

Comparison of sedimentation patterns, basement subsidence, and faulting histories in the north and central basins of Lake Baikal aids in developing an interbasinal seismic stratigraphy that reveals the early synrift evolution of the central portion of the Baikal rift, a major continental rift system. Although there is evidence that the central and northern rift basins evolved at approximately the same time, their sedimentation histories are markedly different. Primary sediment sources for the initial rift phase were from the east flank of the rift; two major deltas developed adjacent to the central basin: the Selenga delta at the south end and the Barguzin delta at the north end. The Barguzin River system, located at the accommodation zone between the central and north basins, also fed into the southern part of the north basin and facilitated the stratigraphic linkage of the two basins. A shift in the regional tectonic environment in the mid Pliocene(?) created a second rift phase distinguished by more rapid subsidence and sediment accumulation in the north basin and by increased subsidence and extensive faulting in the central basin. The Barguzin delta ceased formation and parts of the old delta system were isolated within the north basin and on Academic Ridge. These isolated deltaic deposits provide a model for the development of hydrocarbon plays within ancient rift systems. In this second tectonic phase, the dominant sediment fill in the deeper and more rapidly subsiding north basin shifted from the flexural (eastern) margin to axial transport from the Upper Angara River at the north end of the basin.

Introduction and review

As we are reminded in the opening paper (Cordey) of the Jurassic session, over 50% of the proven hydrocarbon reserves of the UKCS Northern and Central North Sea occur within the Jurassic. More importantly with a view to future exploration, the Jurassic probably contains the majority of the undiscovered reserves in the North Sea basin in subtle structural and stratigraphic traps, many of which may only be denned using novel interpretation techniques, such as sequence stratigraphy and detailed 3D seismic. The Jurassic also provides us with an excellent opportunity to study the evolution and fill of an ancient rift system from both sub-surface and outcrop data. The Jurassic session was therefore specifically targeted at developing new insights into Jurassic stratigraphy at both basin and field scales by utilizing seismic stratigraphy (Vail and Mitchum 1977; Galloway 1989), tectono-stratigraphy (Hubbard et al. 1985), sequence stratigraphy (Van Wagoner et al. 1988, 1990) and some novel techniques such as the application of ichnofabrics as an environmental indicator (Taylor and Gawthorpe). The development of sequence stratigraphy over the past 15 years has contributed greatly to our understanding of basin fill processes but little has been published on its application to the North Sea Jurassic. Sequence stratigraphic models have mainly evolved from the analysis of sedimentary basins with relatively simple subsidence histories, such as post-rift successions of passive continental margins and intra-cratonic basins (Vail and Mitchum 1977; Galloway 1989). These models have proven applicable to the prediction of reservoir, source and seal facies on a field and basin. . .

Modern Continental Rifts: Characteristics and Applications to Recognition of Ancient Rifts: ABSTRACT

Characteristics of modern continental rifts can provide a checklist of expected features in ancient rift systems: (1) distinctive morphology and dimensions as long, linear, sinuous structures up to thousands of kilometers in length and tens of kilometers in width; (2) high heat flow, resulting in extrusive and intrusive igneous bodies and hot-spring deposits; (3) small volcanoes within and large volcanoes outside only certain parts of rift systems; (4) complex stress fields that combine elements of extension, compression, shear, and torsion; (5) complex internal faulting; (6) gravity highs over intrusive mafic rocks and gravity lows over sedimentary rocks; (7) high seismicity, which leaves a record of earthquake-induced sedimentary structures; (8) progressive basin abando ment because of rift-jumping; (9) reactivation through time; (10) individual basins within rift systems formed by complexes of tilted-fault blocks, grabens, and horsts; (11) interfingering sedimentary deposits from slope, riverine, valley-floor, and lacustrine environments; (12) large and small rift-valley lakes, only some of which exhibit high biological productivity and anoxic bottom waters depositing organic-rich muds; and (13) deposits of the precursors of coal, petroleum, gas, and minerals that bear such elements as copper, lead, zinc, phosphorus, barium, or uranium. The combination of these critical features helps to identify ancient rift systems. However, crust weakened by rift-forming processes may be overprinted by later tectonic events that tend to obscure important details in ancient rifts such as the Newark rift system of eastern North America. End_of_Article - Last_Page 1446------------

Recognition of Ancient Rift Systems: Examples From the Proterozoic of South Australia

Rifting is a dynamic process which can lead to crustal separation. An essential feature in the development of rifts is the progressive intrusion of dykes of basic and ultrabasic magma along the rift axis. Different stages in the development of these dykes are reflected by gravity and magnetic anomalies and an understanding of these can facilitate the recognition of ancient rift systems and continental margins. An examination of gravity and magnetic data covering the Proterozoic Adelaide Geosyncline of South Australia has allowed the identification of an Atlantic type passive margin. Sinuous, extensive coincident gravity and magnetic anomalies from the west of South Australia appear to represent an ancient Proterozoic rift system which did not develop to crustal separation.

Accumulation of fossil fuels and metallic minerals in active and ancient rift lakes

A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H 2S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes. Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys.

A gravity survey across the Gardar Igneous Province, SW Greenland

The Gardar Igneous Province is believed to involve the remains of Proterozoic rifting which evolved through repeated phases of activity around 1300 Ma, 1250 Ma and 1170 Ma ago. A gravity survey across part of the province has established the presence of a gravity 'high' of some 300 g u amplitude which is 30 km wide and elongated ENE–WSW, centred on Tunugdliarfik and axially along one of the main zones of Gardar intrusives. This gravity anomaly is similar to that of other ancient rift systems and appears to be a residual of the axial anomaly to be found in modern active rifts. It is interpreted in terms of a basic mass underlying a late Gardar rift, some 50 km long and 25 km wide, intruding the crust. Evidence suggests the Gardar rifting cannot be explained in terms of a major crustal separation as an extension of the mid-continental rift and Keweenawan systems of North America. It is more consistent with crustal stretching of no more than 10 km and possibly diapiric mechanisms. The coastal gravity gradient appears to be due to a change in crustal thickness coincident with the coastline, some 25 km beyond the present ice cap limit. Although Greenland may be in overall isostatic balance, it seems that the coastal zone remains overcompensated.

Tectonics and the gravity field in the continental interior

Gravity fields in the continental interior reveal little relation to late Paleozoic structure and often show an inverse relation to early Paleozoic structure. Because of the inversion, displacements have been interpreted by several investigators as being caused by an isostatic mechanism; however, structural development recorded by geologic evidence indicates a more complex history than simple isostasy would allow. Detailed gravity surveys covering large areas in Illinois are used to determine mechanisms of crustal movements more definitively. From these detailed studies interpretations are extended to other areas having similar geologic-gravity relationships within the midcontinent region. In the interior, a mosaic pattern of gravity highs and lows outlines mass concentrations that formed during the evolution of the craton. The midcontinent gravity high, the central anomaly lying within a linear, symmetrical belt of gravity anomalies, is superimposed over the mosaic pattern and trends through the Lake Superior-Keweenawan rift system. Similar anomalies, although of smaller intensity, trend through centers of intracratonic basins. From the relation between linear, symmetrical anomalies and structure displayed by the Lake Superior rift system, intracratonic basins are interpreted as representing the terminal stage of more subdued rifting. Initial depression of basins is believed to be caused by the collapse of incipient rift systems. The presence of numerous ancient rift systems in the craton suggests that mantle convection played an important part in the deformation of the youthful continental crust. Erosion from uplifts and sedimentation in depressions resulted in further deflections, but of an isostatic nature, with subsequent faulting and plutonism. Subcrustal convection continued throughout Paleozoic time, but its effects on crustal bending became less pronounced. Examples of these structures and related gravity fields are presented.