Publisher Summary This chapter deals with the Basin and Range province defies generalities. Extension has widened the zone of high elevation, causing minor subsidence in the northern Basin and Range, and major subsidence including the opening of a new ocean basin in the southern range. Basin and Range extension is largely confined to the orogenic crust of the western Cordillera. A complicated series of events led to the extensional stresses, beginning with back-arc extension that occurred as isolated core complexes in the Pacific Northwest region during Eocene and early Oligocene time. Over-thickened lithosphere may have been weakened by the orogenic process itself, a flare-up in arc-related magmatism associated with slowing subduction, or heat transferred from the younger, and hotter oceanic lithosphere that was subducted beneath it during the latest Eocene. Geologic and geophysical probing of the Basin and Range lithosphere indicate a province that is highly variable at the surface, while being far more uniform at depth.
This chapter focuses on the Oslo rift formed in response to a combination of regional stretching caused by dextral strike-slip movements along the Sorgenfrei-Tornquist Zone and a positive temperature anomaly in the asthenosphere. The earliest manifestation of the Oslo rifting event was the formation of a shallow depression in late Carboniferous time. Subsequent erosion removed the upper 1-3 km of rift related and prerift rocks in the northern, exposed part of the rift, whereas in the southern, submerged part of the rift, rift-related deposits are buried under younger sediments. Seismic, gravity and geochemical data imply that the rifting event caused significant modifications of the lithosphere. Petrological and geochemical data suggest that massive layer represents dense cumulates and gabbroic rocks formed by fractional crystallization of mantle-derived magmas in deep crustal. The storage of hot, mafic magmas in the deep crust also caused anatexis of Permian gabbros and, to a lesser extent, Precambrian country-rocks, and transport of light, syenitic and granitic components to the upper crust, whereas dense residues are left in the deep crust together with the dense cumulates. The mafic magmatism associated with the rift appears to have originated in different parts of a heterogeneous upper mantle, which before the rifting event, belonged to the subcontinental lithosphere. Important unsolved problems in the Oslo rift are the relative timing of tectonic activity and magmatism between the exposed Oslo Graben, the submerged Skagerrak Graben, and the intensity of magmatism in the Skagerrak Graben as compared to the Oslo Graben.
This chapter discusses the theoretical studies relevant to the geodynamics of continental rifting, omitting the petrological intricacies of magma genesis and ascent. Hypotheses for the initiation of rift structures depends on the primary source of extensional stress which initiates the faulting. The passive hypothesis attributes the primary extension to plate interior deviatoric tension of distant origin, such as produced by the trench suction plate boundary force, with volcanism and vertical movement being a secondary response to lithospheric extension. The active hypothesis attributes volcanism, doming and faulting to the influence of an underlying hot upper mantle which may be generated by a plume. Finite element modeling is used to show that quite a small upper mantle hot spot gives rise to substantial deviatoric tension in the strong regions of the lithosphere. The present stress regime is applicable during the Tertiary, indicates low-lying continental regions are subject to compression rather than tension. As a consequence, the active hypothesis may be relevant to the Tertiary rift systems, whereas the passive hypothesis may have been dominant during the more tensional stress regime of the Mesozoic. Lithospheric boundinage may occur where the deformation is periodic. Thinning of the lithosphere may also take place without stretching as a result of conversion to asthenosphere; a number of mechanisms are proposed based on thermal, magmatic, and diapiric processes.
This chapter describes the formation of extensional stresses in the Earth's lithosphere that is a global tectonic phenomenon, occurring in all plate settings. When initiated in continental lithosphere, extension give rise to a range of tectonic features called “continental rifts.” Continental rifts preserve the most complete, and possibly the only, record of critical structures and of transient processes associated with the incipient stages of continental breakup. Continental rifts have existed at least since the Proterozoic Eon, and their effects on continental lithosphere are both short- and long term. Modification of the crust occurs through deformational processes associated directly with rifting or with post-rifting stress regimes. The timing of uplift, faulting, and magmatism, are of key importance in understanding processes of lithospheric extension.
This chapter deals with rifled passive margins. Passive continental margins mark the juxtaposition of continental and oceanic lithosphere within plate interiors. Passive margins originate by the continental break-up process, and are divisible into rifted and sheared (offset) types. Rifted margins form where the initial plate separation is approximately perpendicular to the rupture. They show a gradational transition, and their morphology can be subdivided into continental shelf, continental slope and continental rise. Sheared margins form when the initial split is along a transform fault; they generally show a much sharper transition and a much smaller thickness of sediments. There are several recent compilations dealing with passive margins.
This chapter discusses the principal petrologic techniques that have been applied to igneous rocks in rifts. The objective is to provide the necessary background for the comparative analysis of rifts. Petrological and geochemical studies of rift-associated igneous rocks can provide information on pressure, temperature, compositions of crust and mantle sources, and processes involved in magma ascent and evolution. Production of large amounts of magma is mechanistically unlikely because it requires extensive heat transfer by thermal conduction. It is appropriate to reconcile small volumes of melt and melts generated by small degrees of partial melting may not be a major process of magma genesis because the fraction of melt that is generated is proportional to the amount of source rock compositional modification that occurs. Mechanism cannot account for large volumes of melt because it requires that either temperatures in the source region be very close to solidus temperatures or that pressure decreases be unrealistically large. It is the most effective way to obtain large volumes of melt and melts requiring large degrees of partial melting. Geothermometers are used for Iherzolitic mineral assemblages including olivineorthopyroxene-c linopyroxene +/- spinel+/-garnet. The most commonly used geothermometers are based on either chemical exchange reactions or on solvus equilibria.
Publisher Summary This chapter discusses the Rio Grande rift, which is a part of a broad region of the western United States, including the Basin and Range province, that has undergone lithospheric thinning and crustal extension during the middle to late Cenozoic. The present extensional setting of the rift is generally related to plate boundary forces acting along the southwestern edge of the North American plate, a transform boundary along which right-lateral slip occurs. The rift follows a zone of crustal deformation formed during the Laramide and Ancestral Rocky Mountains orogenic events. Tertiary sedimentation in the rift area is characterized generally by continental basinal deposits, including interbedded epi- and pyroclastic rocks.
This chapter discusses the principal technique used to derive the deep subsurface distribution of electrical resistivity—namely, the magnetotelluric (MT) method. MT exploration of the Earth is evolved to the status of a mature science during the past two decades with the advent of digital instrumentation, real-time processing, and sophisticated, robust analysis techniques showing increasing promise for interpreting complex, three-dimensional (3-D) electrical structures in the Earth. Magnetellurics is the recording and study of naturally occurring electric and magnetic fields at the Earth's surface. There are several electromagnetic geophysical techniques that use transmitted energy from man-made sources. The chapter deals with the physics of these consequences and how they are used in practice to apply the MT method. Basic MT principles are applied both to synthetic and to actual continental rift data to illustrate the value and limitations of the method.
This chapter gives an overview of continental rifts. A rifts is a major elongate tectonic depressions bounded by normal faults, with no implication for the mode of development or for the mechanism of formation of these depressions. The common association of rifts with volcanism, high heat flow, anomalous crust and upper mantle structure, and seismicity provides compelling evidence that rifts are not confined to upper crustal levels, but that they are linked to dynamic processes in the lithosphere and asthenosphere. Highly extended terranes are characterized by upper crustal extension of the order of 100%, perhaps an order of magnitude greater than is typical in rifts. Rifts are associated with an extensional stress field. The chapter focuses on an improved understanding of the fundamental lithospheric processes of rifting, deep structures and processes associated with rifting, and this have not attempted to elaborate upon near surface geological details and upper crustal extensional styles particularly for petroleum exploration applications.
This chapter discusses the applicability and limitations of potential field methods. Gravity is used to delineate hidden rifts, to determine the shape of rift troughs, including the nature of faulting, to investigate crustal and upper mantle structure in association with explosion and earthquake seismology, and to ascertain the isostatic state of rifts, including flexural studies. The lateral variations of the Earth's gravitational field are determined either by measuring the vertical component of gravitational attraction using a gravity meter or by using satellites to determine the variation in geoid height, which is proportional to the anomalous gravitational potential. There are three main approaches to the interpretation of a residual anomaly: (1) the indirect method, (2) the parametric method, and (3) the direct method.
This chapter provides an overview of the features of the West and Central African rift system(WCARS). The WCARS is a very large scale feature which is distinctive in the sense that it traverses the entire continent. Its complex history involved extension, shearing, and compression over a period extending from the early Cretaceous into the early Tertiary. Portions of its history correlate with changes in plate movements, and the basins which contained major petroleum resources. It is characterized by an unusual amount of subsidence over most of its extent which obscured many rift structures. The region is an ideal place to investigate the interaction of extension and shear during major rifting events.
Publisher Summary This chapter discusses heat flow data, which are a determination of near-surface conductive heat flow. Temperature data are usually collected in near-vertical bore holes, or in near vertical probe penetrations of lake sediments, and hence essentially only the vertical component of heat flow is determined. The most important limitation of heat flow data for analysis of rift processes is that the data represent only the near-surface conductive heat flow. The surface heat flow pattern in rift zones represents a number of components: (1) heat generated by radioactive decay of unstable isotopes in the crust, (2) heat conducted into the crust from the underlying mantle, (3) heat refracted in the crust by thermal conductivity structure, (4) heat advected into the crust by magmatism, (5) heat advected within the crust by tectonic deformation, and (6) heat redistributed in the uppermost crust by ground water flow.
This chapter discusses that East African rift system (EARS), which displays dramatic subfacial, crustal, and upper mantle contrasts, some of which have been delineated in detail by geological and geophysical studies. Crustal models derived from recent seismic studies in the Kenya rift show no evidence for major intrusions and densification of the crust beneath the rift as it inferred primarily on the basis of gravity models. The Kenya rift also displays striking east west symmetry. The presence of zones of very low seismic velocity in the upper mantle directly beneath rifts and associated crustal thinning suggest that active mantle processes are the primary cause of rifting in Kenya and Ethiopia. The EARS provides classic examples of continental rifts in Ethiopia, Kenya, and the Western rift. There are strong similarities between these rifts and other continental rifts such as the Rio Grande rift and the Baikal rift. Rifts, which have relatively narrow geologic features in their surface expression, may be underlain by a relatively narrow, hot, and upper mantle upwarp.
This chapter deals with the capabilities and limitations of seismic methods and seismic data in studies of continental rifts. Substantial structural and velocity variations are expected, and are commonly observed, in continental rifts because of the effects of extension, magmatism, and high heat flow associated with the rifting process. The chapter describes the seismic methods that have been used in rift studies, followed by a review of seismic properties of rocks emphasizing effects of temperature. The extensive crustal velocity data available for North America, which identifies the characteristic crustal properties associated with continental rifts, are also utilized. Each method has its own capabilities and limitations in terms of the depth range of investigation, degree of resolution attainable, and the physical properties determined. At present, shear-wave velocity, anisotropy, and Poisson's ratio data from continental rift areas are relatively sparse. Similarities between the crustal velocity structures observed in North America and east Africa suggest the common characteristics of the stable continental crust, and the processes and effects of continental rifting.
This chapter discusses the Midcontinent Rift system (MCR). The majority of the rift's igneous rocks are derived from a melting mantle plume and the overlying lithosphere over a short span of less than 20 m.y. The MCR is a profound disruption of the crust, as indicated by its anomalous geophysical signatures. The geophysical data, together with limited outcrops of the rift rocks and a few deep drill holes, suggest a general consistency in structural style, tectonic evolution, and age along the length of the rift system. The pre-rift crust is thinned, in places, to less than one-third of its original thickness. The crust beneath the rift is anomalously thick due to the addition of up to 20 km of mantle-derived volcanic rocks and as much as 10 km of post-volcanic elastic sedimentary strata. Additional isotopic dating and paleomagnetic measurements should result in a better understanding of the relative ages of the rift units along the entire length of the MCR. Efforts are directed toward mapping the finer structures within the rift and expanding and refining the gross structural interpretations. Integrated models should consider the relative ages of the individual rift segments and their structural relations to one another which will lead to a better understanding of the spatial and temporal evolution of the MCR.
This chapter focuses on the European Cenozoic Rift system (ECRS) extending from the Mediterranean to the North Sea over a distance of some 1100 km. It is a continuous system of rift structures from the Rhine depression in southeastern France to the Leine graben in northern Germany and through the Lower Rhine Embayment in western Germany into the central graben in the North Sea. Taphrogenesis is additionally influenced by erosional processes along the elevated graben shoulders and the consequent transport of debris into the graben depressions. Several thousand meters of sedimentary infill deposited in the fault troughs correspond to proportional denudations of the elevated flanks.
This chapter deals with the Southern Oklahoma aulacogen, which is a major structural element of interior North America. As a rift, this feature involved extensive volcanism and modification of the crust. When reactivated by later events at the continental margin, vertical displacements alone are approximately 15 km. This aulacogen provides the structural framework for a major portion of the central U.S.; some related faults have been experienced by recent movements, which can pose an earthquake hazard. There is a considerable amount of geophysical data available in the southern Oklahoma aulacogen area. The gravity data provides significant control on deep structure and show that the aulacogen is associated with a huge positive gravity anomaly, which requires the presence of a major crustal anomaly in the vicinity of the Wichita uplift.
This chapter describes the Baikal rift system, which is 1800 km to 2400 km long and is situated at the boundary between the Siberian Platform to the northwest and the Caledonian Sayan-Baikal fold belt to the southeast. This rift system has been the object of intensive study by Soviet scientists for many years. The Baikal rift system is composed of fifteen individual topographic depressions which are associated with an approximately 1500-km long domal uplift. The central portion of Baikal Rift system is almost entirely located on the relatively weak and anisotropic basement of the Sayan-Baikal fold belt. The sub-vertical crustal boundary between the Siberian Platform and the fold belt forms an abrupt western boundary for the central portion of the rift system and its domal uplift, and in particular runs along the west side of the Lake Baikal depressions.
Peridotite xenoliths from the Bereya alkali picrite tuff in the Vitim volcanic province of Transbaikalia consist of garnet lherzolite, garnet–spinel lherzolite and spinel lherzolite varieties. The volcanism is related to the Cenozoic Baikal Rift. All peridotites come from pressures of 20–23 kbar close to the garnet to spinel peridotite transition depth, and the presence of garnet can be attributed to cooling of spinel peridotites, probably during formation of the lithosphere. The peridotites show petrographic and mineral chemical evidence for infiltration by an alkaline silicate melt shortly before their transport to the Earth's surface. The melt infiltration event is indicated petrographically by clinopyroxenes which mimic melt morphologies, and post-dates outer kelyphitic rims on garnets which are attributed to an isochemical heating event within the mantle before transport to the Earth's surface. Single-mineral thermometry gives reasonable temperature estimates of 1050±50°C, whereas two-mineral methods involving clinopyroxene are falsified by secondary components in clinopyroxene introduced during the melt infiltration event. Excimer Laser–ICP-MS analysis has been performed for an extensive palette of both incompatible and compatible trace elements, and manifests the most thorough dataset available for this rock type. Orthopyroxene and garnet show only partial equilibration of trace elements with the infiltrating melt, whereas clinopyroxene and amphibole are close to equilibration with the melt and with each other. The incompatible element composition of the infiltrating melt calculated from the clinopyroxene and amphibole analyses via experimental mineral/melt partition coefficients is similar to the host alkali picrite, and probably represents a low melt fraction from a similar source during rift propagation. The chemistry and chronology of the events recorded in the xenoliths delineates the series of events expected during the influence of an expanding rift region in the upper mantle, namely the progressive erosion of the lithosphere and the episodic upward and outward propagation of melts, resulting in the evolution of the Vitim volcanic field.
The Balmuccia alpine lherzolite massif is a fragment of subcontinental lithospheric mantle emplaced into the lower crust 251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists largely of lherzolite, with subordinate harzburgite and dunite, and an array of dike rocks formed in the mantle before crustal emplacement. Dike rocks include websterite and bronzitite of the Cr-diopside suite, spinel clinopyroxenite and spinel-poor websterite of the Al-augite suite, gabbro and gabbronorite of the late gabbro suite, and hornblendite of the hydrous vein suite. The dike rocks display consistent intrusive relationships with one another, such that Cr-diopside suite dikes are always older than dikes and veins of the Al-augite suite, followed by dikes of the late gabbro suite and veins of the hydrous vein suite. Phlogopite (phl) veinlets that formed during interaction with the adjacent crust are the youngest event. There are at least three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a polybaric fractionation series from spinel clinopyroxenite to websterite and feldspathic websterite, which crystallized from aluminous alkaline magmas at relatively high pressures. The late gabbro suite of dikes intruded at lower pressures, where plagioclase saturation occurred before significant mafic phase fractionation. Hornblendite veins have distinct compositional and isotopic characteristics, which show that they are not related to either the Al-augite suite or to the late gabbro dike suite. Cr-diopside suite dikes have Nd and Sr isotopic compositions similar to those of the host lherzolite and within the range of compositions defined by ocean–island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic compositions similar to those of Cr-diopside suite lherzolite and websterite. The late gabbro dikes have Nd and Sr isotopic compositions similar to mid-ocean ridge basalt (MORB) asthenosphere. Lead isotopic compositions for all of the samples fall in the present-day MORB field on the 208Pb/204Pb vs. 206Pb/204Pb diagram but are displaced above this field on the 207Pb/204Pb vs. 206Pb/204Pb diagram. There is overlap in the data between the Cr-diopside suite and the Al-augite and hydrous vein suites, with the exception that the Cr-diopside websterite dikes have more radiogenic Pb than any of the other samples. In Pb–Pb space as well, the late gabbro suite has the least radiogenic isotopic compositions, reflecting a change in magma source region during uplift. These data show that tectonic thinning of subcontinental lithospheric mantle during extension caused a change in the source regions of mantle-derived magmas from an ocean island basalt (OIB)-like lithosphere to the underlying MORB asthenosphere. They also demonstrate that the upper mantle acquires its heterogeneous isotopic character through several different processes, including in situ radiogenic growth, addition of asthenospheric melts, dike-wall rock ionic exchange, redistribution of the lithospheric dike and vein materials by melting, and in the late stages of emplacement, assimilation of crustal materials.
