Lithospheric structure of the Rio Grande rift (original) (raw)
Related papers
Whole-Lithosphere Shear During Oblique Rifting
2021
Processes controlling the formation of continental whole-lithosphere shear zones are debated, but their existence requires that the lithosphere is mechanically coupled from base to top. We document the formation of a dextral, whole-lithosphere shear zone in the Death Valley region (DVR), southwest United States. Dextral deflections of depth gradients in the lithosphere-asthenosphere boundary and Moho are stacked vertically, defining a 20-50-kmwide, lower lithospheric shear zone with ∼60 km of shear. These deflections underlie an upper-crustal fault zone that accrued ∼60 km of dextral slip since ca. 8-7 Ma, when we infer that whole-lithosphere shear began. This dextral offset is less than net dextral offset on the upper-crustal fault zone (∼90 km, ca. 13-0 Ma) and total upper-crustal extension (∼250 km, ca. 16-0 Ma). We show that, before ca. 8-7 Ma, weak middle crust decoupled upper-crustal deformation from deformation in the lower crust and mantle lithosphere. Between 16 and 7 Ma, detachment slip thinned, uplifted, cooled, and thus strengthened the middle crust, which is exposed in metamorphic core complexes collocated with the whole-lithosphere shear zone. Midcrustal strengthening coupled the layered lithosphere vertically and therefore enabled whole-lithosphere dextral shear. Where thick crust exists (as in pre-16 Ma DVR), midcrustal strengthening is probably a necessary condition for whole-lithosphere shear.
Continental rifting as a function of lithosphere mantle strength
Tectonophysics, 2008
a b s t r a c t The role of the uppermost mantle strength in the pattern of lithosphere rifting is investigated using a thermo-mechanical finite-element code. In the lithosphere, the mantle/crust strength ratio (S M /S C ) that decreases with increasing Moho temperature T M allows two strength regimes to be defined: mantle dominated (S M N S C ) and crust dominated (S M b S C ). The transition between the two regimes corresponds to the disappearance of a high strength uppermost mantle for T M N 700°C. 2D numerical simulations for different values of S M /S C show how the uppermost mantle strength controls the style of continental rifting. A high strength mantle leads to strain localisation at lithosphere scale, with two main patterns of narrow rifting: "coupled crust-mantle" at the lowest T M values and "deep crustal décollement" for increasing T M values, typical of some continental rifts and non-volcanic passive margins. The absence of a high strength mantle leads to distributed deformations and wide rifting in the upper crust. These numerical results are compared and discussed in relation with series of classical rift examples.
A mechanism to thin the continental lithosphere at magma-poor margins
Nature, 2006
Where continental plates break apart, slip along multiple normal faults provides the required space for the Earth's crust to thin and subside 1 . After initial rifting, however, the displacement on normal faults observed at the sea floor seems not to match the inferred extension 2 . Here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults. Our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust (<10 km) by exhumation of mid-crustal and mantle material. This transitional crust is capped by sub-horizontal detachment surfaces over distances exceeding 100 km with little visible deformation. Our rift model is based on numerical experiments constrained by geological and geophysical observations from the Alpine Tethys and Iberia/Newfoundland margins 3-9 . Furthermore, we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust. The presence of such lower crustal gabbros is well constrained for the Alpine Tethys system 4,9 . Initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution. This is achieved in our models by the formation of weak ductile shear zones.
Upper mantle deformation and seismic anisotropy in continental rifts
Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy, 2000
We investigate the seismic anisotropy signature of the continental rifting process. Severa! sources of anisotropy are considered: the lithospheric deformation, the asthenospheric flow, and the occurrence of oriented melt pockets in the asthenospheric mande. Our results show that contrasted anisotropy patterns should be associated with the various conceptual models of rifting. Thus seismic anisotropy measurements may allow one to discriminate between these models. Anisotropy measurements in the Rio Grande, East-African and Rhine rifts suggest that these rifts formed by a transtensional deformation of the lithospheric mande rather than by homogeneous extension of the lithosphere. Alignment of melt-lenses in the asthenospheric wedge may also account for a significant part of the seismic anisotropy recorded in the internai domains of these rifts.
Earth and Planetary Science Letters, 2001
Analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process. Rift propagation is not random, but tends to follow the trend of the orogenic fabric of the plates, systematically reactivating ancient lithospheric structures. Continental rifts often display a clear component of strikeŝ lip deformation, in particular in the early rifting stage. Moreover, although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process, there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts. These observations suggest that the deformation of the lithosphere, especially during rifting, is controlled by its preexisting structure. On the other hand, (1) the plasticity anisotropy of olivine single crystal and aggregates, (2) the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs, and (3) seismic anisotropy data, which require a tectonic fabric in the upper mantle coherent over large areas, suggest that preservation within the lithospheric mantle of a lattice preferred orientation (LPO) of olivine crystals may induce a large-scale mechanical anisotropy of the lithospheric mantle. We use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process. We assess the deformation of an anisotropic continental lithosphere in response to an axi-symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head. Model results show that a LPO-induced mechanical anisotropy of the lithospheric mantle may result in directional softening, leading to heterogeneous deformation. During continental rifting, this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique (305 5³) to the preexisting mantle fabric. This directional softening associated with olivine LPO frozen in the lithospheric mantle may also guide the propagation of the initial instability, that will follow the preexisting structural trend. The preexisting mantle fabric also controls the deformation regime, imposing a strong strike^slip shear component. A LPOinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting (structural inheritance), the plume^rift paradox, and the onset of transtension within continental rifts.
Geochemistry Geophysics Geosystems, 2009
1] Oblique rifting is investigated through centrifuge experiments that reproduce extension of a continental lithosphere containing a preexisting weakness zone. During extension, this weakness localizes deformation, and different rift obliquity is obtained by varying its trend with respect to the stretching direction. Model results show that deformation is mostly controlled by the obliquity angle a (defined as the angle between the orthogonal to the rift trend and the extension direction). For low obliquity (a < 45°), rifting is initially characterized by activation of large, en echelon boundary faults bordering a subsiding rift depression, with no deformation affecting the rift floor. Increasing extension results in the abandonment of the boundary faults and the development of new faults within the rift depression. These faults are orthogonal to the direction of extension and arranged in two en echelon segments linked by a complex transfer zones, characterized by strike-slip component of motion. In these models, a strong strain partitioning is observed between the rift margins, where the boundary fault systems have an oblique-slip motion, and the valley floor that away from the transfer zones is affected by a pure extension. Moderate obliquity (a = 45°) still results in a two-phase rift evolution, although boundary fault activity is strongly reduced, and deformation is soon transferred to the rift depression. The fault pattern is similar to that of low-obliquity models, although internal faults become slightly oblique to the orthogonal to the direction of extension. Deformation partitioning between the rift margins and the valley floor is still observed but is less developed than for low-obliquity rifting. For high obliquity (a > 45°), no boundary faults form, and the extensional deformation affects the rift depression since early stages of extension. Dominance of the strike-slip motion over extension leads to the development of oblique-slip and nearly pure strike-slip faults, oblique to both the rift trend and the orthogonal to the extension direction, with no strain partitioning between the margins and the rift floor. These results suggest that oblique reactivation of preexisting weaknesses plays a major role in controlling rift evolution, architecture, and strain partitioning, findings that have a significant relevance for natural oblique rifts.
Tectonophysics, 1990
As part of a major cooperative seismic experiment, a series of seismic refraction profiles have been recorded in south -central New Mexico with the goal of determining the crustal structure in the southern Rio Grande rift. The data gathered greatly expand the seismic data base in the area and consist of three interlocking regional profiles: a reversed E-W line across the rift, an unreversed N-S axial line, and an unreversed SW-SE line. The reversed E-W line shows no significant dip along the Moho ('32 km thick crust) and a 7.7 km/s Pn velocity. Results from the N-S axial line and the NW-SE line indicate an apparent Pn velocity of 7.95 km/s and significant dip along the Moho with crustal thinning toward the south and southeast. When interpreted together, these data indicate a crustal thinning in the southern rift of 4-6 km with respect to the northern rift and the adjacent Basin and Range province and establish the regional Pn velocity to be approximately 7.7 km/s. These results suggest that the Rio Grande rift can be identified as a crustal feature separate and distinct from the Basin and Range province. Gas Company, Dallas, Texas. 3Department of Geosciences, Purdue University, West Lafayette, Indiana. Laramide orogenic activity [Chapin and Seager, 1975]. From late Eocene through most of the Oligocene, voluminous calcalkalic volcanism took place throughout most of the southwestern United States as a result of subduction of the Farallon plate beneath the North American plate [Atwater, 1970; Lipman et al., 1972; Coney and Reynolds, 1977]. In the rift area, this activity is evidenced by the formation of the San Juan, Datil-Mogollon, and Davis Mountains, as well as other smaller volcanic fields that flank the rift [Chapin and Seager, 1975]. By late Oligocene to early Miocene time, magmatism changed from predominantly calc-alkalic compositions to more basic compositions [Seager and Morgan, 1979; Chapin, 1979]. This change in volcanism in the rift area was contemporaneous with the transition from a subduction regime to a regime of regional extension [Lipman and Mehnert, 1975] and the early initiation of rifting. The basaltic-andesitic volcanism lasted to about 20 Ma in the northern rift and to about 26 Ma in the southern rift and was followed by a magmatic lull which lasted until 13 Ma [Chapin, 1970; Seager and Morgan, 1979]. Following this lull, basaltic-rhyolitic volcanism started with basaltic activity peaking around 5 Ma [Seager andMorgan, 1979]. Basaltic fields and individual flows dotted the floor of the Rio Grande rift from southern Colorado to the Mexican border [Seager and Morgan, 1970; Chapin and Seager, 1975]. The major volcanic activity was concentrated in the Jemez Mountains-Taos Plateau area, the Socorro Peak area, and the Magdalena Peak area where major northeast trending lineaments intersect the rift [Chapin, 1971]. About 7-4 Ma, the Southern Rocky Mountains and adjacent areas were strongly uplifted due to mantle upwelling [Eaton, 1979]. In the southern Rio Grande rift, Seager et al. [1984] interpreted the change from basaltic andesite to alkali-olvine basalt to represent two different but transitional extension regimes. In the early regime (starting around 28 Ma), extension developed in a back arc setting and resulted in the eraplacement of basaltic andesite, the formation of broad NW trending basins, and the uplift of some of the region's fault-block mountains. The later 6143 helped with the CARDEX experiment, especially L.W. Braile and Carl
Geochemistry, Geophysics, Geosystems, 2012
1] We study the propagation of a continental rift and its interaction with a continental margin utilizing a 3-D lithospheric model with a seismogenic crust governed by a damage rheology. A long-standing problem in rift-mechanics, known as the tectonic force paradox, is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism. Our modeling results demonstrate that under moderate rift-driving tectonic forces the rift propagation is feasible even in the absence of magmatism. This is due to gradual weakening and "long-term memory" of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks. We show that the style, rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces, but also on the rate of healing. Accounting for the memory effect provides a feasible solution for the tectonic force paradox. Our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin. Thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin. In such a case, a new fault system is created perpendicular to the direction of the rift propagation. These results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern.