Ancient Rift Zones Research Articles

Emplacement of the Argyle diamond deposit into an ancient rift zone triggered by supercontinent breakup

Argyle is the world’s largest source of natural diamonds, yet one of only a few economic deposits hosted in a Paleoproterozoic orogen. The geodynamic triggers responsible for its alkaline ultramafic volcanic host are unknown. Here we show, using U-Pb and (U-Th)/He geochronology of detrital apatite and detrital zircon, and U-Pb dating of hydrothermal titanite, that emplacement of the Argyle lamproite is bracketed between 1311 ± 9 Ma and 1257 ± 15 Ma (2σ), older than previously known. To form the Argyle lamproite diatreme complex, emplacement was likely driven by lithospheric extension related to the breakup of the supercontinent Nuna. Extension facilitated production of low-degree partial melts and their migration through transcrustal corridors in the Paleoproterozoic Halls Creek Orogen, a rheologically-weak rift zone adjacent to the Kimberley Craton. Diamondiferous diatreme emplacement during (super)continental breakup may be prevalent but hitherto under-recognized in rift zones at the edges of ancient continental blocks.

In situ evidence of earthquakes near the crust mantle boundary initiated by mantle CO2 fluxing and reaction-driven strain softening

This study aims to understand the process behind the worldwide connection between deep crustal/upper mantle earthquakes and CO2 emissions along faults in rift zones. We do this by studying CO2-induced mineral reactions that facilitate strain localization in peridotites from an ancient rift zone in the Seiland Igneous Province (SIP), North Norway.Strain localization in association with hydration processes is well documented in all types of tectonic settings and has major implications for rheological behavior in active plate margin processes. The implications of CO2-bearing fluids are less studied, though experiments have shown how CO2 can influence the flow laws of olivine by imposing a brittle and more localized type of deformation.This study documents narrow shear zones observed within ultramafic rocks from the Seiland Igneous Province (SIP) comprising large volumes (>20,000 km3) of mafic, ultramafic, silicic and alkaline melts that were emplaced into the lower continental crust (25–30 km) between 570 and 560 Ma under an extensional regime. The extensional shear zones are mm cm-scale and contain extremely fine-grained material with a distinct shape preferred orientation (SPO), but weak to absent crystallographic preferred orientation. The shear zones offset dykes across numerous micro-faults that are documented in areas close to a major fault zone cutting through the area. Within the shear zones, olivine and clinopyroxene react to form orthopyroxene and dolomite at approximately 11 kb and 850°C according to the reaction:2 Olivine + Clinopyroxene + 2 CO2 = Dolomite + 2 OrthopyroxeneThis reaction formed coronas of orthopyroxene and dolomite between olivine and clinopyroxene in the shear zones. In addition, large olivine grains proximal to the shear zones show a microfabric with subgrain walls decorated by rounded grains of dolomite and more irregular and elongated grains of orthopyroxene. Clinopyroxene grains are separated from the enstatite and dolomite by at least hundreds of microns, suggesting material transport within the shear zone. The shear zones thus provide a unique insight into the interplay between CO2-metasomatism and reaction accommodated strain softening. Carbonation-driven cracking and mineral reaction also serves to reduce grain size, making grain boundary sliding an efficient process, further enhancing the rheological contrast between the shear zone and the host rock. The sudden decrease in rock strength could lead to rapid deformation and triggered pseudotachylite formation during earthquake events in the near proximity of the micro-shear zones. Our observations match the relations between CO2 emissions and earthquakes observed in present rift environments such as the East African rift and in New Zealand, and underline the importance of active shear zones as fluid conduits in the lower crust and upper mantle.

SYSTEM ORGANIZATION OF DISJUNCTIVE TECTONICS IN CONSOLIDATED BASEMENT OF DNIPRO-DONETS PALEORIFT

Formulation of the problem. The article is the second in trilogy, which exposes geological structure and geodynamic evolution of Proterozoic crystalline basement of Dnieper-Donets paleorift. Knowledge of strike-slip tectonics of intraplate geostructures with sedimentary basins belonging to them, such as Dnieper-Donets paleorift with cognominal hydrocarbon bearing basin within it, has important applied significance, owing to structure-froming and strcuture-controlling role of strike-slip faults, that cause spatial distribution of various tectono-magmatic centers, zones of fold uplifts series, structural-facial zones, salt-folds subbasins, etc., within which oil & gas fields are localised. The subject of the research is structural appearance of strike-slip tectonics in precambrian crystalline basement in the form of secondary deformed structures of rift evolution stage. The subject of studies has been structural-paleotectonic and peleogeodynamic conditions of Dnieper-Donets rift formation.The purpose of the article. Purpose of the research has been study of structure-forming processes at the rifting stage, for which the following tasks have been resolved:− study of territory expansion in the ancient rift zones of horizontal strike-slip dislocations;− reconstruction of tectonical position of paleogeodynamic field’s main parameters in paleotensions of rift evolution stage;− study of typical structural-kinematic paragenesis in identified zones of strike-slip dislocations;− modelling of Dnieper-Donets rift formation kinematics.Methods. Methodical approach in researches of rift stage tectonic evolution has been indentified by complexing of structural-kinematic and structural-paragenenetic methods of tectonophysical analysis for study and diagnostic of morphogenetic type of Precambrian basement dislocaion structures. Actual curvlinearity of basement faults has been converted to discrete assemblage of lineaments with overall number of 900 items, which filled output database for tectono-physics analysis of disjunctive systems.Results. Spatial dislocation, paleotectonic position and paleogeodynamic conditions of ancient riftogenic fault systems formation has been studied. Modelling of rift formation kinematics has been carried-out based on reconstruction of main parameters of earth crust tension-deformation state at the stage of rifting. It has been shown that rifting was occurring in overall plate collision geodynamic environment under interference of local horizontal strike-slip field of paleotensions from tangential componental stretching. Under these geodynamic conditions in zones of dynamic influence of almost all regional systems of disjunctive faults, linear zones of concentration of horizontal strike-slip dislocations, creating within them typical ensembles of secondary deformed strike-slip structural-kinematic paragenesis, were forming. Elastic deformation with divergence of "cold" continental earth crust was grounded by the rifting mechanism. The main role in rifting mechanism was played by transforming faults formed following structural directions of transverse transregional fault systems. They served as unique tectonic “rails” by which opposite margins of megasegments – pull-apart basins were spreading, which consequently joined and tuned into one rift system.Scientific novelty. The main factor and cause of continental rifting should be considered stretching deformations of earth crust in strike-slip field of geodynamic tesnsions, owing to that "embryo" subbasins, following mechanism of pull-apart basin was formed. They created "windows" for injection of mantle substance by newly built weaken zones in earth crust, and as a result they formed a steady-state "mantle window" in lithosphere, that served as distributary channel in the rift system of mantle fluids.

Supracrustal suite of the Precambrian crystalline crust in the Ghor Province of Central Afghanistan

The Proterozoic pre-Ediacaran metamorphic basement of the southern Tajik (North Afghanistan) continental block and the adjacent Band-e-Bayan zone is exposed in the Ghor Province of Central Afghanistan. It is predominantly composed of the EW-striking supracrustal succession consisting of interbedded felsic schists and gneisses (metapsammites), amphibolites (metabasalts), calcite and dolomite marbles. The metamorphic facies changes from greenschist in the Band-e-Bayan zone to amphibolite facies in the Tajik block. The supracrustal rocks of the Band-e-Bayan zone and Tajik block possess common features suggesting that the former represents a tectonized part of the latter. The geochemical characteristics of metapsammites indicate derivation of the clastic material from a continental arc and, partly from a passive continental margin, whereas the composition of metabasalts suggests their possible formation in a continental rift basin. The tectonic setting of supracrustal unit could be interpreted as a back-arc type basin. We presume that the Tajik microcontinent split off the Gondwana supercontinent along an ancient rift zone during the late Paleozoic.

Crustal velocity structure associated with the eastern Tennessee seismic zone: Vp and Vs images based upon local earthquake tomography

AbstractWe present three‐dimensional P and S wave velocity models for the active eastern Tennessee seismic zone (ETSZ) using arrival time data from more than 1000 local earthquakes. A nonlinear tomography method is used that involves sequential inversion for model and hypocenter parameters. We image several velocity anomalies that persist through most of the inversion volume. Some anomalies support the presence of known features such as an ancient rift zone in northern Tennessee. Other anomalies reveal the presence of basement features that can be correlated with regional gravity and magnetic anomalies. We image a narrow, NE‐SW trending, steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the prominent New York‐Alabama magnetic lineament. The low‐velocity zone may have an apparent dip to the SE at depths exceeding 15 km. The low‐velocity zone is interpreted as a major basement fault juxtaposing Granite‐Rhyolite basement to the NW from Grenville southern Appalachian basement to the SE. Relocated hypocenters align in near‐vertical segments suggesting reactivation of a distributed zone of deformation associated with a major strike‐slip fault. We suggest that the ETSZ represents reactivation of an ancient shear zone established during formation of the super continent Rodinia.

Seismic structure of the Central US crust and shallow upper mantle: Uniqueness of the Reelfoot Rift

Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben—Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200km depth. This LVZ is of variable width, being relatively narrow (∼50km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

Rift zone abandonment and reconfiguration in Hawaii: Mauna Loa's Ninole rift zone

A new onshore-offshore three-dimensional seismic velocity model for the Island of Hawaii reveals a massive buried rift zone within Mauna Loa9s southeast flank, introduced here as the Ninole rift zone. This feature extends more than 60 km south of Mauna Loa9s summit, spans a depth range of ∼2–14 km below sea level, and is the probable source of the 100–200 ka Ninole volcanics in several prominent erosional hills. The ancient rift zone may stabilize Mauna Loa9s southeast flank, focusing recent volcanic activity and deformation onto the unbuttressed west flank. The upper portion of the Ninole rift zone appears to have migrated westward over time, possibly triggered by landsliding, causing its eventual abandonment in preference to Mauna Loa9s present-day southwest rift zone. Subsequently, the lower southwest rift zone broke away, tracking rift intrusions along the trace of the Kahuku detachment fault. Similar rift zone migration is thought to be under way at Kilauea volcano, and may one day lead to the abandonment of the east rift zone. Such rift zone reconfiguration is a reflection of changing stress conditions within growing volcanoes; it is probably much more common than previously assumed, and may enable the growth of large volcanic edifices such as Mauna Loa.

Hydrothermal polycyclic aromatic hydrocarbons in marine and lagoon sediments at the intersection between Tjörnes Fracture Zone and recent rift zone (Skjálfandi and Öxarfjörður bays), Iceland

Polycyclic aromatic hydrocarbons (PAHs) in marine and lagoon sediments, and thermal waters were studied in the northern part of the Icelandic recent rift zone and Tjörnes Fracture Zone. All samples analyzed represent slightly altered clastic deposits and hot water discharging from springs and wells. Data obtained show that hydrocarbons are associated mainly with hydrothermal minerals and deep hot thermal ground waters, suggesting that PAH distribution is closely related to the dynamics of hydrothermal environment. Deep emanation of PAHs in present day gas emissions was studied with diatomite absorbent traps. Traps were left exposed at Skógar and Skógalón localities (recent lagoon sediments, Öxarfjörður geothermal field) for 15 and 30 days. Content and distribution of PAHs associations in surface sediments can be used as indicators of hydrocarbon migration and possible zones of hydrocarbon accumulation in land and marine sedimentary basins in active and ancient rift zones of Iceland.

Polycyclic aromatic hydrocarbons as evidence of hydrocarbon migration in marine and lagoon sediments of a recent rift zone (Skjálfandi and Öxarfjörður), Iceland

Polycyclic aromatic hydrocarbons (PAHs) content and distribution was studied in the northern part of modern rift zone. All samples analysed represent slightly altered clastic deposits and hot water discharging in springs and from wells. To study PAH in present-day gas emission traps with diatomite absorbent were installed in the Skógalón geothermal field. All samples were analysed by the method of Shpol'sky spectroscopy. Hydrothermally altered deposits show the highest amount of PAH. Data obtained allow to believe that the distribution of PAH is closely associated with the dynamics of the hydrothermal environment. The fissure formation and temperature fluctuation in hydrothermal systems can govern both the processes of PAH synthesis and sorption. The evidences of PAH migration up through the Skjálfandi and Öxarfjörður sedimentary basins show that hydrocarbons associate mainly with hydrothermal minerals and deep ground waters. Studying PAH associations can be used as an indicator of hydrocarbons distribution, ways of migration and possible zones of accumulation in the land and marine sedimentary basins in active and ancient rift zones of Iceland.

Key Late Cenozoic fluvial archives of eastern Europe: the Dniester, Dnieper, Don and Volga

This paper reviews the occurrence, structure, stratigraphy and development of the principal deposits of the Rivers Dniester, Dnieper, Don and Volga. Such archives occur as alluvial accumulations (infills, ‘sheets’ and fans/deltas), sometimes filling great buried incisions and giving rise, in the modern relief, to plains, valleys and terraces. These accumulations consist of alluvial suites, each with unique lithofacies characteristics. Correlation is based on marker beds from marine transgressions, waste mantles and glaciations, together with mammalian and molluscan biostratigraphy from the fluvial deposits themselves. Overprinted onto a background of predominant incision (typically 160–200 m), within the middle reaches of Dnieper, Don and Volga there are two complete major erosion/aggradation cycles between the Miocene and the late Middle Pleistocene, followed by a younger erosion phase that continues to the present. In contrast, within the Middle Dniester, one such cycle is recorded in the Late Miocene-Early Pliocene, followed by 250–270 m of incision. Most valleys served as arterial channels for intracontinental sediment transport but, where they coincide with ancient rift zones, they represent basins of alluvial accumulation. Neotectonic movements, the dynamics of which have differed between the East European Platform and the transitional zone between this platform and the Carpathian Orogenic Belt, probably controlled the general incision and erosion/aggradation cycles. Climatic oscillations, however, have influenced the structure and composition of the alluvial suites.

A historical approach to continental flood basalt volcanism: insights into pre-volcanic rifting, sedimentation, and early alkaline magmatism

Continental flood basalts are widely thought to be produced from mantle plume heads. However, plume theories do not observe any role for lithospheric rifting before flood basalt events, and consider irrelevant the fact that most, if not all, continental flood basalts have erupted through deep rifts containing thick sedimentary sequences. At best, plume theories invoke selective capture of such deep rifts or lithospheric thinspots by rising mantle plume heads. However, the fact that CFBs of the world erupted through deep, ancient rift zones, and alternative dynamical considerations of flood basalt genesis, directly lead towards a new, historical approach to flood basalt emplacement. This approach takes cognizance of the basic unity of geological history and processes, satisfactorily explains pre-volcanic rifting, sedimentation, mantle metasomatism, and early, pre-tholeiite, enriched alkaline magmatism for tens of millions of years. Both incubating and impacting plume heads ought to lead to pre-volcanic lithospheric doming which is usually not observed in flood basalts. Continental or oceanic flood basalt events instead seem to be derived by convective partial melting during sudden lithospheric pull-apart (splitting) along pre-existing lithospheric discontinuities such as deep rifts or fracture zones.

GEOPHYSICAL CRITERIA FOR“RESUSCITATING”OF THE PANXI ANCIENT RIFT TECTONIC ZONE

Deep structure and properties and features of the geophysical fields of the Panxi region, in the middle of the Kang-Dian (Kangding-Yunnan) tectonic belt, are studied as a whole. The study is based on the detection of magnetic field (including paleomagnetic dating, aeromagnetics), geothermal field, natural earthquake, plate motion, explosion seismology and magneto-electrotelluric sounding.The results show that the Panxi tectonic zone is neither a completely withered ancient rift nor a modern rift system in the heyday having come into being since the Mesozoic or the Cenozoic era, but it is an active tectonic zone (i. e. a "resuscitating" ancient rift zone) formed and reformed by magma activity and tectonic movements in adjacent regions.

Influence of Precambrian Rift Trends on Pre-Devonian Deposition in Southern Northwest Territories, Canada: ABSTRACT

A study incorporating all available outcrop, well, and geophysical data shows that pre-Devonian deposition in the southern part of the Northwest Territories of Canada was strongly influenced by a weakened basement framework of Precambrian horsts and grabens which developed in response to continental breakup and sea-floor spreading in the proto-Pacific. Major horsts continuously rose to form arches that shed clastics into the downdropping basins. As the highs were worn down, the deposits changed from dominantly clastic to largely carbonate and the highs were ultimately transgressed. Within the area the pre-Hadrynian basement consists of granite on the east, metamorphics and minor sediments throughout the central part, and unaltered sediments on the west. The three trends are believed to define a westerly transition from the old craton through an ancient rift zone to the proto-Pacific. To define the history, the pre-Devonian section was broken into major time-rock sequences which represent separate transgressive-regressive cycles of deposition terminated either by local or regional uplift and erosion or by renewed transgression. Within each sequence most lateral changes from thinner intertidal deposits to thicker deeper water ones were seen to occur in narrow transition zones, indicating that the cycles were dominated by differential vertical subsidence alongmore » major hinge lines. This Precambrian framework runs the length of western North America and controlled much of the early Paleozoic sedimentation along it. It also greatly influenced later Laramide thin-skin thrusting wherein the horst-cored arches acted as buttresses to the eastward-moving blocks.« less

Mechanism of subsidence of ancient cratonic rift basins

Cratonic basins commonly occur over ancient rift zones. These inactive rift basins are recognizable by a positive linear Bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys. Many of these basins undergo discrete periods of increased subsidence rates, or reactivations, long after the mass excess responsible for the linear gravity high was emplaced. Furthermore, the reactivation of many cratonic basins occurs simultaneously with large-scale compressional tectonics. It is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess. The subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear Maxwell viscoelastic rheology. Solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time. The subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved. Since ancient rift mass excesses may be uncompensated over long geologic time intervals, the early thermal and structural evolution of rifts may not significantly influence later basin subsidence. The models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material, allowing the uncompensated basin to settle toward an isostatic-compensation depth faster than normal. Since viscosity is a strong function of temperature, reactivation by a world-wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent. An increase of 15% in the geothermal gradient, for example (from 16.5°–18.9°K/km), will cause about a 5% increase in subsidence. This increase in heat flow, however, seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100% increase in subsidence might occur. Since the rheology of the lithosphere is nonlinear, effective viscosity is also a strong nonlinear function of stress. The presence of a regional compressive stress during periods of tectonism of 1.1 · 10 8 Pa (about 2.8% of the buckling strength of the lithosphere) produces a short period of reactivated subsidence (≅ 10 5 yr). During the reactivated subsidence, the newly-imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation. This suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere. This may be accomplished by an intermittent application of the regional stress over time.

The role of rifting in the tectonic development of the midcontinent, U.S.A.

Recent studies have proposed the existence of several major ancient rift zones in the midcontinent region of North America. Although the dating of some of these rifts (and even the rift interpretations) are subject to question, an analysis of these “paleo-rifts” reveals three major episodes of rifting: Keweenawan (~ 1.1 b.y. B.P.), Eocambrian (~ 600 m.y. B.P.), and early Mesozoic (~ 200 m.y. B.P.). The extent of these events documents that rifting has played a major role in the tectonic development of the midcontinent region. This role goes well beyond the initial rifting event because these features display a strong correlation with Paleozoic basins and a strong propensity for reactivation. For example, the Eocambrian Reelfoot rift was reactivated in the Mesozoic to form the Mississippi embayment and is the site of modern seismicity which suggests reactivation in a contemporary stress field of ENE compression. Even though the importance of rifting can be established, recognition of rifts and delineation of their complexities remain a major problem which requires more study.

Geophysical Studies of Basement Geology of Southern Peninsula of Michigan

Only fragmentary direct information is available on the basement complex of the Southern Peninsula of Michigan because of limited and poorly distributed basement drill holes. This has encouraged the use of geophysical methods, primarily gravity and magnetic, to study the Precambrian formations. A basement configuration map prepared from magnetic depth estimates and basement drill tests confirms that the basement surface under the Southern Peninsula has the form of an oval depression reaching a maximum depth of more than 15,000 ft (4.5 km) below sea level on the western shore of Saginaw Bay. A basement topographic high is associated with the Howell anticline and a roughly north-south-striking basement trough plunges into the basin from the common boundary point of Indiana, Ohio, and Michigan. Aeromagnetic and Bouguer gravity anomaly maps, together with isotopic dates of samples obtained from basement drill holes and extrapolation of known Precambrian geologic trends, indicate that four basement provinces underlie the Southern Peninsula. The Penokean province can be traced geophysically from Lake Michigan into the Southern Peninsula, where it is characterized by east-southeast-striking geophysical anomalies. Central and southwestern Michigan is underlain primarily by felsic rocks correlating with the Central province. Basement rocks in southeastern Michigan strike north-northeast and are interpreted to be metamorphosed intrusive and extrusive rocks and mafic and felsic gneisses of the Grenville province. The Grenville front strikes south-southwest from Sagi aw Bay to west of the Howell anticline and from there due south to the Michigan Ohio boundary. A Keweenawan rift zone characterized by mafic intrusive and extrusive rocks and by uplifted gneisses transects all of the provinces and extends from Grand Traverse Bay to southeastern Michigan. Another subparallel ancient rift zone may be present in southwestern Michigan. These zones were formed during an episode of crustal extension in Keweenawan time. Subsequent deformation of the sedimentary rocks within the basin generally has been associated with movement along lines of basement weakness apparently related to the rift zone and Penokean structural trends.