Major Continental Rift Research Articles

Evidence of Fluid Induced Earthquake Swarms From High Resolution Earthquake Relocation in the Main Ethiopian Rift

AbstractFluid overpressure and fluid migration are known to be able to trigger or induce fault slip. However, relatively little is known about the role of fluids on generating earthquakes in some of the major continental rifts. To address this, we investigate the interaction between fluids and faults in the Main Ethiopian Rift (MER) using a large seismicity catalog that covers both the rift axis and rift margin. We performed cross‐correlation analysis on four major earthquake clusters (three within the rift and one on the rift margin) in order to significantly improve accuracy of the earthquake relative relocations and to quantify families of earthquakes in which waveforms are similar. We also analyzed variation of seismicity rate and seismic moment release through time for the four clusters. The major results are that for all four clusters the earthquake relocations are 5–15 km deep, aligned to clear N‐NNE striking, steeply (>60°) dipping planes. For the three clusters within the rift, the cross‐correlation analysis identifies earthquake families that occur in short swarms during which seismic rate and moment release increases. Together, this space and time pattern of the seismicity strongly points toward them being fluid induced, with fluid likely sourced from depth such as mantle derived CO2. In contrast, the seismicity on the rift margin lacks earthquake families, with occurrence of earthquakes more continuous in nature, which we interpret as pointing toward tectonic stress‐driven microseismic creep. Overall, our results suggest that deep sourced fluid migration within the rift is an important driver of earthquake activity.

Chapter 1.3 Antarctic volcanism: petrology and tectonomagmatic overview

Abstract Petrological investigations over the past 30 years have significantly advanced our knowledge of the origin and evolution of magmas emplaced within and erupted on top of the Antarctic Plate. Over the last 200 myr Antarctica has experienced: (1) several episodes of rifting, leading to the fragmentation of Gondwana and the formation by c. 83 Ma of the current Antarctica Plate; (2) long-lived subduction that shut down progressively eastwards along the Gondwana margin in the Late Cretaceous and is still active at the northernmost tip of the Antarctic Peninsula; and (3) broad extension across West Antarctica that produced one of the Earth's major continental rift systems. The dynamic tectonic history of Antarctica since the Triassic has led to a diversity of volcano types and igneous rock compositions with correspondingly diverse origins. Many intriguing questions remain about the petrology of mantle sources and the mechanisms for melting during each tectonomagmatic phase. For intraplate magmatism, the upwelling of deep mantle plumes is often evoked. Alternatively, subduction-related metasomatized mantle sources and melting by more passive means (e.g. edge-driven flow, translithospheric faulting, slab windows) are proposed. A brief review of these often competing models is provided in this chapter along with recommendations for ongoing petrological research in Antarctica.

Chapter 1.2 Antarctic volcanism: volcanology and palaeoenvironmental overview

Abstract Since Jurassic time ( c. 200 Ma), Antarctica has had a greater diversity of volcanism than other southern continents. It includes: (1) voluminous mafic and felsic volcanism associated with the break-up of Gondwana; (2) a long-lived continental margin volcanic arc, including back-arc alkaline volcanism linked to slab rollback; (3) small-volume mafic alkaline volcanism associated with slab-window formation; and (4) one of Earth's major continental rift zones, the West Antarctic Rift System (WARS), with its numerous large alkaline central volcanoes. Several of Antarctica's volcanoes are still active. This chapter is a review of the major volcanic episodes and their principal characteristics, in their tectonic, volcanological and palaeoenvironmental contexts. Jurassic Gondwana break-up was associated with large-scale volcanism that caused global environmental changes and associated mass extinctions. The volcanic arc was a major extensional arc characterized by alternating volcanic flare-ups and lulls. The Neogene rift-related alkaline volcanism is dominated by effusive glaciovolcanic eruptions, overwhelmingly as both pāhoehoe- and ‘a‘ā-sourced lava-fed deltas. The rift is conspicuously poor in pyroclastic rocks due to the advection and removal of tephra erupted during glacial intervals. Volcanological investigations of the Neogene volcanism have also significantly increased our knowledge of the critical parameters and development of the Antarctic Ice Sheet.

The role of sub-continental mantle as both “sink” and “source” in deep Earth volatile cycles

The extent to which Earth’s sub-continental lithospheric mantle modulates the flux of volatile elements from our planet’s deep interior to its atmosphere (via volcanism) is poorly constrained. Here, we focus on “off-craton” sub-continental lithospheric mantle because this long-lived reservoir potentially acts as both a volatile “sink” and “source” during major heating and rifting events. The sub-continental lithospheric mantle is primarily formed of peridotites with subordinate amounts of pyroxenites. While both lithologies are dominated by nominally-volatile-free mantle minerals, some of these phases have been shown to contain non-negligible amounts of H2O (e.g. 100’s of ppmw in clinopyroxene). Data for volatile elements other than Li are, however, limited. We present new, high-precision, in-situ Secondary Ion Mass Spectrometry analyses of H, F, Cl, Li and B in olivine and pyroxenes from well-characterised garnet- and spinel-bearing peridotites and pyroxenites (from southern Patagonia and the Antarctic Peninsula). Our study confirms that clinopyroxene is the main host of H2O and F. The maximum F contents we report (up to 154 ppmw) are higher than those in previous studies and occur in Ti-Cr diopsides in highly-metasomatised peridotites and Ti-Al augites from clinopyroxenite veins. Water contents of clinopyroxenes (up to 615 ppmw) are within the range previously published for continental mantle. Lithium concentrations are low (<5 ppmw) in all analysed phases and both Cl and B are below detection levels (14 ppmw and 0.03 ppmw, respectively).Unique to our study is the large variation in major- and trace-element concentrations of the clinopyroxenes, which allows us to place quantitative constraints on how volatiles are stored in the mantle. We demonstrate that: (i) F contents of clinopyroxenes closely correlate with Ti and (ii) DHCpx-Opx and DFCpx-Opx is systematic and inversely correlated with temperature. Despite the redistribution of volatiles during sub-solidus re-equilibration, we show that the first order control on the concentration of volatiles in clinopyroxene is the style of metasomatism, i.e. channellised flow versus reactive percolation. The mean bulk volatile contents of peridotites from Pali Aike and the Antarctic Peninsula (H2O = 89 ± 31 ppmw, F = 16 ± 11.2 ppmw and Li = 2 ± 0.7 ppmw) are within the range previously published for continental “off-craton” mantle. The pyroxenites have significantly higher mean bulk concentrations of H2O (260 ± 59 ppmw), F (86 ± 43 ppmw) and Li (1.0 ± 0.35 ppmw). While the greater capacity of mantle pyroxenites to host H2O relative to the associated peridotites has previously been observed in global “off-craton” mantle xenolith suites (e.g. Oahu, Hawaii; eastern China and the Rio Grande Rift, SW USA), here we show for the first time that pyroxenites are also major hosts of F (but not Cl, Li or B). Because of their relatively low solidus temperatures, pyroxenites in “off-craton” settings will be readily re-mobilised during lithospheric extension (and heating). We suggest these pyroxene-rich mantle lithologies may be responsible for the elevated concentrations of H2O and F observed in basalts and volcanic gasses from major continental rift zones and flood basalt provinces, and hence an important consideration in models of global volatile cycles.

Volcanogenic CO2 Degassing in the Songliao Continental Rift System, NE China

The Wudalianchi monogenetic volcanic field (WMVF) is located in the Songliao basin within a major continental rift system in NE China. Bubbling springs and diffuse degassing from soils are typical features of the WMVF. Chemical compositions and C-He isotope analyses revealed that the cold spring gases might originate from the enriched upper mantle (EM), which resulted from the mixing between slab materials (subducted organic sediments and carbonates) in the mantle transition zone (MTZ) and the ambient depleted mantle. These EM-derived volatiles experienced variable degrees of crustal input, including both continental organic metasediments and crustal carbonates during their ascending path to the surface. The estimated results of the degassing CO2 fluxes, combined with previous geophysical evidence, suggest that the CO2 degassing activities become weaker from early to late in Quaternary.

Crustal structure beneath the Malawi and Luangwa Rift Zones and adjacent areas from ambient noise tomography

Crustal shear wave velocity structure beneath the Malawi and Luangwa Rift Zones (MRZ and LRZ, respectively) and adjacent regions in southern Africa is imaged using fundamental mode Rayleigh waves recorded by 31 SAFARI (Seismic Arrays for African Rift Initiation) stations. Dispersion measurements estimated from empirical Green's functions are used to construct 2-D phase velocity maps for periods between 5 and 28 s. The resulting Rayleigh wave phase velocities demonstrate significant lateral variations and are in general agreement with known geological features and tectonic units within the study area. Subsequently, we invert Rayleigh wave phase velocity dispersion curves to construct a 3-D shear wave velocity model. Beneath the MRZ and LRZ, low velocity anomalies are found in the upper-most crust, probably reflecting the sedimentary cover. The mid-crust of the MRZ is characterized by an ~3.7% low velocity anomaly, which cannot be adequately explained by higher than normal temperatures alone. Instead, other factors such as magmatic intrusion, partial melting, and fluid-filled deep crustal faults might also play a role. Thinning of the crust of a few kilometers beneath the rifts is revealed by the inversion. A compilation of crustal thicknesses and velocities beneath the world's major continental rifts suggests that both the MRZ and LRZ are in the category of rifts beneath which the crust has not been sufficiently thinned to produce widespread syn-rifting volcanisms.

Large igneous provinces linked to supercontinent assembly

Models for the disruption of supercontinents have considered mantle plumes as potential triggers for continental extension and the formation of large igneous provinces (LIPs). An alternative hypothesis of top-down tectonics links large volcanic eruptions to lithospheric delamination. Here we argue that the formation of several LIPs in Tarim, Yangtze, Lhasa and other terranes on the Eurasian continent was coeval with the assembly of the Pangean supercontinent, in the absence of plumes rising up from the mantle transition zone or super-plumes from the core–mantle boundary. The formation of these LIPs was accompanied by subduction and convergence of continents and micro-continents, with no obvious relation to major continental rifting or mantle plume activity. Our model correlates LIPs with lithospheric extension caused by asthenospheric flow triggered by multiple convergent systems associated with supercontinent formation.

Timing of Iapetus Ocean rifting from Ar geochronology of pseudotachylytes in the St. Lawrence rift system of southern Quebec

Laser ablation 40 Ar/ 39 Ar step-heating analyses for encapsulated and unencapsulated pseudotachylytes from a Neoproterozoic normal fault belonging to the St. Lawrence rift system (Canada) preserve the absolute timing of rifting and initial opening of the Iapetus Ocean. The total gas and retention ages for encapsulated pseudotachylytes from the Montmorency fault (Quebec City) are 610.3 ± 4.6 Ma and 619.0 ± 2.5 Ma. Ten unencapsulated analyses from two pseudotachylyte veins with varying matrix/clast ratios yield total gas ages of 634.7 ± 1.6–663.9 ± 1.8 Ma. These ages show an excellent linear relationship with the proportion of clast inclusions, resulting in lower intercept ages (i.e., no host rock) of 613.3 and 614.2 Ma. These statistically indistinguishable ages constrain major seismic faulting along the St. Lawrence rift system and significantly improve prior estimates for late Neoproterozoic rifting of Iapetus. The upper intercepts, reflecting host-rock ages, match cooling ages of Grenville basement in the area. We conclude that the time of major continental rifting along the northern Laurentian margin and initiation of the Iapetus Ocean occurred at 613–614 Ma, coeval with emplacement of the 615 Ma Long Range dikes of Labrador. This study also demonstrates that Ar geochronology of pseudotachylytes using varying clast/matrix ratios is a robust method to date ancient faulting.

Mantle transition zone discontinuities beneath the Baikal rift and adjacent areas

Like most other major continental rifts, the Baikal rift zone (BRZ) in Siberia is presumably underlain by a hot and partially molten mantle, which has a reduced seismic velocity relative to surrounding areas. Recent seismic tomography studies, however, gave conflicting results about the depth extent and even the existence of the low‐velocity anomaly beneath the BRZ, suggesting that additional constraints are needed. Here we present results from stacking of about 1700 radial P‐to‐S receiver functions from a single long‐running seismic station, TLY, located at the SW tip of Lake Baikal. A clear uplift of the 410 km discontinuity (d410) with a magnitude of about 47 km relative to the south margin of the Siberian platform is observed beneath the rift. Currently available seismic results suggest that the uplift is unlikely to be caused by addition of water to mantle transition zone (MTZ) silicates but is the result of a 550°C reduction in temperature in the vicinity of the d410. In addition, the 660 km discontinuity (d660) shows a downward trend toward the rift from the south, suggesting that the entire MTZ might have a low temperature beneath the rift. The thickening of the MTZ suggests a high‐velocity anomaly of about 2% in the MTZ, and rules out the possibility that the rifting is caused by a mantle plume originated in or beneath the mantle transition zone.

The phylogenetic history and biogeography of the frankincense and myrrh family (Burseraceae) based on nuclear and chloroplast sequence data

Generalized hypotheses for the vicariant, Gondwanan origin of pantropically distributed eudicotyledon families must be refined to accommodate recently revised dates that indicate major continental rifting events predate the evolution of many tricolpate angiosperm clades. Here, we use molecular phylogenies of an eudicotyledon family previously hypothesized to have a Gondwanan origin, the Burseraceae, to test this and other alternative biogeographical hypotheses in light of recalibrated geological events. Phylogenies based on nuclear and chloroplast data were reconstructed for 13 of the 18 genera (50 spp. total) of Burseraceae using parsimony, maximum likelihood, and Bayesian methods. Ages of all lineages were estimated using penalized likelihood and semiparametric rate smoothing [Bioinformatics 2003 (19) 301], which allows the user to calibrate phylogenies based on non-clock-like DNA sequence data with fossil information. Biogeographical hypotheses were tested by comparing ages of species and more inclusive lineages with their extant and most parsimonious ancestral distributions. Our data support a North American Paleocene origin for the Burseraceae followed by dispersal of ancestral lineages to eastern Laurasia and Southern Hemisphere continents.

Seismic tomography of continental rifts revisited: from relative to absolute heterogeneities

Tomographic images for four major continental rift zones, namely the southern Rhine Graben (SRG, Germany/France), the Gregory rift (Kenya) which is the central part of the East African rift system, the Rio Grande rift (RGR) in the United States and the Lake Baikal rift zone (LBR) in Russia have been revisited by calculating and comparing absolute velocity models. The four rifts exhibit strong structural differences in the uppermost mantle down to more than 300-km depth, suggesting major differences in their geodynamic evolution albeit their similarity in age and similar surface expression. The comparative analysis suggests that tomographic images of rift zones can be used to characterize continental rifts, once the corrections to obtain absolute velocities have been carried out. Our results suggest that while the Kenya and the Rio Grande rift may be considered active with large upwelling plumes being the main controlling factor in the evolution, the southern Rhine Graben and the Lake Baikal rift are more likely passive rifts, where complex regional stress fields and inherited structures play the governing role in the evolution.

The Rio Grande rift: A geological and geophysical overview

The Rio Grande rift is a major structural element of the Southern Rocky Mountain region. During the last 20 years this feature has become widely recognized as a major Cenozoic continental rift zone. During this time we have learned much about the structure and evolution of the rift. However, many gaps in our knowledge remain that prevent us from fully understanding its evolution and the processes that formed it. The rift should not be studied in isolation, because the Laramide orogeny, uplift, complex magmatism, and extension that have occurred since the Late Cretaceous are all to some extent related.

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.

Tectonic evolution of the Aravalli orogen (NW India): an inverted Proterozoic rift basin?

The Aravalli mountain range (AMR) in the northwestern part of the Indian Peninsula consists of two main Proterozoic metasedimentary and metaigneous sequences, the Aravalli and Delhi Supergroups, respectively, which rest over the Archaean gneissic basement. A synthesis and reinterpretation of the available geological, geochronological and geophysical data, including results of own field work and geophysical interpretations pertaining to the AMR, indicate its origin as an inverted basin: rifting into granitoid basement began ca. 2.5; Ga ago with Aravalli passive rifting (ca. 2.5–2.0 Ga) and Delhi active rifting (ca. 1.9–1.6 Ga). Associated mafic igneous rocks show both continental and oceanic tholeiitic geochemistry and are comparable with Phanerozoic, rift-related magmatic products. Available data showed no conclusive evidence for oceanic lithoshere and island-arc/active margin magmatic activity in the AMR. Subsequent inversion and orogeny (Delhi orogeny, ca. 1.5-1.4 Ga) lead to complex deformation and metamorphism. Only in the western and central zones has the basement been involved in this mid-Proterozoic (Delhi) deformation, whereas it is unaffected in the eastern part, except for local shear zones mainly along the basement/cover interface. The grade of metamorphism increases from the greenschist facies in the east to the amphibolite facies in the west with local HP assemblages. These latter are explained by rapid burial and exhumation of thin and cool continental lithosphere. Subsequently, during a final, mild phase of inversion, the Vindhyan basins consisting mainly of sandstones, limestones and shales, flanking the AMR formed which are comparable to foreland basins. The tectonic evolution of the AMR is therefore interpreted as an example of a major inverted continental rift and of a Proterozoic intra-continental orogen.

The origin of hydrothermal and other gases in the Kenya Rift Valley

The Kenya Rift Valley (KRV) is part of a major continental rift system from which much outgassing is presently occurring. Previous research on gases in the KRV has tended to concentrate on their geothermal implications; the present paper is an attempt to broaden the interpretation by consideration of new data including helium and carbon isotope analyses from a wide cross-section of sites. In order to do this, gases have been divided into categories dependent on origin. N 2 and noble gases are for the most part atmospherically derived, although their relative concentrations may be altered from ASW ratios by various physical processes. Reduced carbon (CH 4 and homologues) appears to be exclusively derived from the shallow crust, with thermogenic δ 13C values averaging −25‰ PDB for CH 4. H 2 is likely also to be crustally formed. CO 2, generally a dominant constituent, has a narrow δ 13C range averaging −3.7‰ PDB, and is likely to be derived with little modification from the upper mantle. Consideration of the ratio C/ 3He supports this view in most cases. Sulphur probably also originates there. Ratios of 3He/ 4He reach a MORB-like maximum of 8.0 R/R A and provide the best indication of an upper mantle source of gases beneath the KRV. A correlation between 3He/ 4He and the hydrocarbon parameter log (C 1/ΣC 2–4) appears to be primarily temperature related. The highest 3He/ 4He ratios in spring waters are associated with basalts, perhaps because of the leaching of basalt glasses. There may be a structural control on 3He/ 4He ratios in the KRV as a whole.

Sedimentology of the Permian Radok Conglomerate in the Beaver Lake area of MacRobertson Land, East Antarctica

AbstractThe mid- to Upper Permian Radok Conglomerate, the lowermost formation of the Permo-Triassic Amery Group, crops out in the Beaver Lake area of the northern Prince Charles Mountains, East Antarctica. Outcrop is confined to a north-south elongate, fault-bounded corridor interpretedas a remnant of a continental extensional basin formed during Late Palaeozoic times. This basinforms a small part of the much larger Lambert Graben, a major continental rift system. The RadokConglomerate consists of interbedded conglomerates, argillaceous sandstones, siltstones, and minor, thin carbonaceous siltstones and coals. Textural, petrographic, palaeocurrent and other data suggestlocal derivation from Precambrian massifs to the immediate west, during a period of fault activity.The unit is a minimum of 400 m thick, the base being unexposed, and grossly fines upward. It isabruptly overlain by quartzo-feldspathic sandstone-dominated rocks of the Upper Permian Bainmedart Coal Measures. Seven recurrent lithofacies have been recognized with the Radok Conglomerate, and are interpreted as the products of poorly-confined stream flow, sheet flow and sediment gravity flow processes, suspension fallout in shallow standing water, and organic sediment accumulation in peat-forming wetlands. The unit as a whole is interpreted as having accumulated as a coarse alluvial apron along the western margin of a ?graben extensional trough. Similar, though poorly exposed, facies are exposed on the eastern margin of the basin and may reflect similar depositional systems. Towards the top of the Radok Conglomerate, typical Radok lithologies are interbedded with quartzo-feldspathic sandstones derived from the south, precursors of the overlying Bainmedart Coal Measures. Interference between transverse (Radok) and axial (Bainmedart) drainage is possibly related to progressive infilling of extensional topography, thereby allowing axially flowing rivers to avulse increasingly into the Beaver Lake region from the main Lambert Graben.

Metallogeny of the midcontinent rift system of North America

Abstract The 1.1 Ga Midcontinent rift system of North America is one of the world's major continental rifts and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long rift are exposed only in the Lake Superior region. In the Lake Superior region, the rift cuts across Precambrian basement terranes ranging in age from ∼ 1850 Ma to more than 3500 Ma. Where exposed, the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles. This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. Rift-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments; (2) sediment-hosted copper sulfide and native copper; (3) copper sulfide veins and lodes hosted by rift-related volcanic and sedimentary rocks; and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks. Magmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the rift and include CuNiPGE and TiFe (V) in the Duluth Complex, U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from country rocks; the interaction between magma and country rocks was important in generation of the magmatic CuNi sulfide deposits. A mantle plume origin has been proposed for the formation of the Midcontinent rift. More than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift, providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines. These brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume. Hydrothermal deposits were emplaced for at least 30–40 m.y. after rift magmatism and extension ceased. This time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift, or may be due to the timing of post-rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift.

Variations across and along a major continental rift: An interdisciplinary study of the Basin and Range Province, western USA

Geological, geochemical, and geophysical data gathered within the central part of the Basin and Range and adjacent areas of the western USA suggests that considerable heterogeneity characterizes Cenozoic extension in this region. Good exposure and an abundance of pre-rifting markers indicate 250 km of extension of the upper crust over the past 16 m.y. Extension of several hundred percent has occurred in two distinct deformational domains, Death Valley and Lake Mead, separated by a relatively unextended block, the Spring Mountains. The limited topographic differences between extended and unextended regions imply that material with a crustal density has been added to the extended regions. Although igneous activity can provide some of this added material, kinematics of extension within the Death Valley region suggest that lateral flow of the middle and lower crust into the extended areas accounts for much of the needed material. Such flow is consistent with geochemical analysis of intermediate to silicic volcanic rocks in the Death Valley area. These volcanic rocks contain isotopic and geochemical trends similar to Mesozoic plutonic rocks from the western part of the Sierra Nevada, about 150–200 km to the west, thus suggesting that the upper crust has moved by that amount relative to deeper crustal levels. Geochemical analyses of basaltic magmas in the region indicate that two mantle reservoirs are present: an OIB-type asthenosphere, and an old, Precambrian continental lithosphere. The ancient lithospheric mantle is preserved beneath the Central Basin and Range, but to the west and north the basaltic rocks have a signature compatible with an asthenospheric origin. These differences indicate that the degree of thinning and removal of the mantle lithosphere varies considerably across the Central Basin and Range. These differences are compatible with the inference from geological and geophysical arguments that thinning of the mantle lithosphere at the latitude of the Central Basin and Range is localized beneath the Sierra Nevada. Geophysical measurements have shown that the thickness of the crust varies little from a mean of about 30 km over the entire Basin and Range; the crust under the high Sierra Nevada to the west might have about the same thickness. Estimates of the buoyancy of the crust and mantle based on P-wave crustal structures suggest that the most buoyant, and thus probably the warmest, mantle lies under the Sierra Nevada and not under areas of strongly thinned upper crust of the Death Valley and Lake Mead regions to the east. Similar analyses indicate that the extended upper crust of the Northern Basin and Range overlies an upper mantle more buoyant than that of the Southern and Central Basin and Range; this is in accord with geochemical and seismological inferences. Thus, the style of lithospheric extension varies considerably both along and across the strike of the Basin and Range.

Heat flow and hydrothermal vents in Lake Baikal, U.S.S.R.

The processes that generate thinning, extension, and finally rifting of continental crust are often assumed to be either “active” or “passive.” There are two major continental rifting provinces active in the world today: the East African Rift and the Baikal Rift in Siberia. Many authors suspect that the Baikal Rift formed in response to the India‐Eurasia collision, which deformed the lithosphere in central Asia creating the Himalayas in the south and an extensional province in Siberia [Molnar and Tapponier, 1975; Tapponier and Molnar, 1979; Zonenshain and Savostin, 1981]. Others believe that the Baikal Rift is primarily an active phenomenon, citing updoming around Lake Baikal and the presence of Cenozoic volcanics as an example of slightly elevated partial melting in the asthenospheric material underneath the lake [Artemyev and Artyushkov, 1971; Zorin, 1971, 1981; Puzyrev et al., 1974; Logatchev et al., 1983].

A geophysical model of the Española Basin, Rio Grande rift, New Mexico

A model of the subsurface structure of the eastern part of the Española Basin in the northern Rio Grande rift of New Mexico was constructed from geophysical data obtained since 1983 by the Summer of Applied Geophysical Experience (SAGE) field course. Approximately 742 new gravity observations, 1276 ground magnetic stations, 30 km of seismic refraction lines, 19 km of seismic reflection lines, 22 magnetotelluric stations, and several Schlumberger and dipole‐dipole resistivity lines were established. Our studies provide new information on one boundary of a major continental rift and on the depositional and structural style of an extensional basin within the rift. Integration of these data sets into a single transect indicates that the Española Basin is asymmetrical with approximately 2 to 3 km of sediments and sedimentary rocks near the center, thinning eastward to the Precambrian outcrop of the flanking Sangre de Cristo uplift. Several minor faults with throws of less than 200 m were found, but no major eastern bounding fault was observed. Thus, the Española Basin could be an asymmetrical, west‐dipping half‐graben. However, major fault offset, down toward the basin axis, may occur within Precambrian rocks of the Sangre de Cristo uplift. In either case, the geometry of the basin does not agree well with current models for the structural evolution of continental rifts, which emphasize low‐angle detachment faults which create asymmetrical, hinged half‐grabens. These models predict that major shoulder uplift should occur adjacent to the side of the graben bounded by a listric master fault rather than adjacent to the hinged side. In contrast, for the Española Basin major uplift occurred adjacent to the eastern side, which could be the hinged side of the basin. A thick wedge of older sedimentary rocks with high P‐wave velocity (4.4 km/s) and low electrical resistivity (5 Ω⋅m) was discovered under the younger Tertiary sediments and sedimentary rocks near the center of the basin. This wedge has maximum thickness of 1.2 km at the western end of the profile and thins eastward. The physical properties suggest this layer could be older Tertiary, or possibly a Mesozoic‐Paleozoic, section of rocks. If the latter, it has potential economic importance because of the possible presence of a Cretaceous section which is known to produce oil and gas in the Albuquerque Basin to the south and the San Luis Basin to the north. However, based on data from the Yates La Mesa no. 2 well, 10 km south of the transect, this wedge is likely middle Tertiary lacustrine deposits (NMOCD, 1986). The great thickness of lake deposits may represent a major lacustrine facies of the Eocene Galisteo and El Rito formations, exposed around the southern, southwestern, and northwestern margins of the basin. Magnetotelluric data suggest the crystalline basement underlying the central Española Basin may be more conductive than near the eastern margin. The entire Española Basin is also underlain by a highly conductive layer of about 1 Ω⋅m at a depth of 15 km. Both the shallow and deep low‐resistivity zones may result from hot, saline fluids. Such fluids deep within the crust may reduce the shear strength of the crust significantly and concentrate crustal extension on the west side of the rift.