Thermal consequences of lithospheric extension: Pure and simple (original) (raw)
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Journal of Geodynamics, 1985
. Thermal model for lithospheric thinning and associated uplift in the neotectonic phase of intraplate orogenic activity and continental rifts. Journal of Geodynamics, 3: 137-153. Many regions of recent tectonic activity are characterized by the existence of mantle inhomogeneties which are revealed by various geophysical methods. It is found that the lithosphere beneath the continental rifts and many recent uplifts (high plateaus) is abnormally thin. This thinning of the lithosphere is connected with the process of intensive convective heating. A study of the thermal evolution of the lithosphere, when the convective heat flow is supplied to its base, is performed in the framework of a Stefan's type problem. Solutions are derived giving the relationship between the evolution of the thickness of the lithosphere and the value of the heat flow at the base of the lithosphere at the neotectonic reactivation stage. The results of computations show that the thickness of the lithosphere can be reduced to one half of its initial value in a time interval of an order of several million years when the value of convective heat flow from the anomalous mantle is of an order of 40-100 mW/m 2. Important data on the evolution of the lithosphere can be obtained from the comparative analysis of the composition of volcanic rocks at different stages of continental rifting. It has been found that the source of magmas has a general tendency to migrate to a shallower depth with time. This process suggests successive thinning of the lithosphere. Based on this deduction, we can use these data to evaluate the time dependence of heat flow from the anomalous mantle. The study of this inverse problem leads to the conclusion that the beginning of rifting should be accompanied by a sudden (like a step-function) increase in heat flow supplied by a mantle plume to the base of the lithosphere. The required values of additional heat flow are in the range 55-102 mW/m 2. Vertical crustal movements are caused by a number of physical processes (thermal expansion of the lithosphere, isostatic forces associated with a density inversion at the base of the lithosphere, tension produced by the flow of heated material and finally the regional tectonic stress field). The role of these factors and their interactions in tectonic processes are discussed.
Lithospheric thinning, uplift, and heat flow preceding rifting
Tectonophysics, 1991
Mareschal, J.-C. and Gliko, A., 1991. Lithospheric thinning, uplift, and heat flow preceding rifting. In: A.F. Gangi (Editor), World Rift Systems. Tectonophysics, Conversion of lithosphere into asthenosphere provides an active mechanism of lithospheric thinning and formation of intracontinental rifts. The movement of the lithosphere-asthenosphere boundary (LAB), considered as a phase change, is determined by the solution of a Stefan-like problem. The ascent of the LAB and the surface heat flow perturbation are calculated for different boundary conditions at the base of the lithosphere. The calculations show that the rate of lithospheric thinning is independent of heat conduction across the lithosphere. When the temperature increases at the LAB, the amplitude of thinning is also independent of the thickness of the lithosphere. Lithospheric thinning to the base of the crust could thus take place rapidly if a temperature perturbation on the order of 250 K is maintained at the LAB. The characteristic time depends strongly on the latent heat and on the difference between the geothermal and phase transition temperature gradients; it is estimated to range between 1 and 30 M.y. This mechanism of lithospheric thinning appears to be compatible with petrological and geochronometric data on temporal changes in the depth of origin of magmas from the East African Rift.
Thermal consequences of lithosphere extension over continental margins: the initial stretching phase
Geophysical Journal International, 1984
We compute the thermal evolution of a lithosphere submitted to stretching during a finite duration of time in order to discuss the initial stretching phase of future continental margins. The numerical method developed can handle 2-D laterally variable stretching as well as sedimentation. It is shown that lateral conduction is more important than vertical conduction over most continental margins during their formation by stretching. A simple way to evaluate the relative importance of lateral and vertical conduction effects at the axis of the zone of rifting just prior to oceanization is proposed. A simple way to evaluate the amplitude of the thermal uplift on the edges of the zone of rifting at the end of the stretching phase is also presented. For small width zones of rifting (< 70–100 km) lateral cooling becomes so large as to prevent large-scale melting and, presumably, prevent the transition to oceanization. The effect of highsedimentation rates (100–500 m Myr-1) is to increase the surface temperature of the lithosphere and consequently significantly decrease the surface heat flow.
A convective heat transfer model for lithospheric thinning and crustal uplift
Journal of Geophysical Research, 1979
Lithospheric thickness and crustal uplift dimensions are calculated numerically as functions of time after the onset of a convective , transfer of heat into the lithosphere by magma intrusion. The numerical models show that the rate of lithospheric thinning depends on the rate or magma entry into the lithosphere and the initial temperature regime of the lithosphere. The width and height of uplift for continental regions depends on (1) the apparent flexural rigidity of the lithosphere, (2) the diameter of the hot spot beneath the lithosphere, (3) the density contrast between the new asthenosphere and the original lithosphere, (4) erosion rates, and (5) the rate of magma entry into the lithosphere. For oceanic lithosphere the amount of uplift depends on points 1, 2, and 3 above, on the eruption depth, and on the size of the volcanic load (if any). With reasonable values for these parameters the model produces crustal uplift rates and lithospheric deflections similar to those observed for continental rifts and Hawaii. The model suggests that a fractured lithosphere beneath Hawaii is not necessary. Volcanic loading combined with the uplift produced by lithospheric thinning can produce an arch with an amplitude comparable to that of the propose similar effects. These authors contend that midplate mantle convection produces continental rift systems such as the East African rifts, the Rhine graben, or the Lake Baikal rift, which are characterized by pronounced crustal uplift and abnormally thin lithosphere. Midplate mantle convection can promote crustal uplift and lithospheric thinning by elevating temperatures within the upper mantle. Thermal expansion and phase changes (including partial melting) within the lithosphere produce crustal uplift, while partial melting changes the lithosphere into asthenosphere. Both a heat supply and a heat transfer mechanism are needed to elevate temperatures within the lithosphere. Midplate mantle convection can supply the heat to the base of the lithosphere in the form of hot, partially molten mantle. Heat transfer can occur by either conduction or convection. Crough and Thompson [1976] and Gasset al. [1977] present models where heat is transferred into the lithosphere by conduction. Their models show that with conduction alone, lithospheric thinning is a slow process. Gass et al. conclude that penetrative magmatism may be an important heat transfer mechanism. Penetrative magmatism transfers heat into the lithosphere by convection. Hawaiian arch. The model applied to moving plates Unlike the free mantle convection which carries suggests that the lithosphere/asthenosphere the magma to the base of the lithosphere, magma boundary is asymmetrically distorted in the direc-intrusion is a forced convection caused by the tion of plate movement. density and/or viscosity contrast between the Oklahoma 74150. magma and the surrounding lithosphere. This paper examines the role of forced convective heat transfer in thinning the lithosphere and determines the time-dependent crustal uplift associated with it for both continental and oceanic crust. Lithospheric Thinning The following assumptions are made in the convective heat transfer model: 1. Midplate mantle convection transports hot, partially molten material to the base of the lithosphere. 2. Overburden pressure and/or buoyancy cause this low-density, low-viscosity magma to intrude the overlying lithosphere. Magma entry is random, the rate of magma entry is constant, and the size of each rising accumulation of magma is small in comparison to hot spot dimensions. 3. After entering the lithosphere, the magma cools and solidifies, releasing heat (specific and latent) into the adjacent lithosphere. Heat transfer occurs near the base of the lithosphere. This means that the magma cools rapidly in relation to the lithospheric thinning event (for small accumulations of magma this is true) and that the ascent velocity of the magma is not very large in comparison to the upward velocity of the lithosphere/asthenosphere (L/A) boundary. 4. The heat released by the cooling magma Paper number 8B1354.
The geodynamics of lithospheric extension
Tectonophysics, 2008
Lithospheric extension is a fundamental plate tectonic process controlling the collapse of mountain belts, the break-up of continents and the formation of new oceanic basins. This review summarises major advances on understanding: a) the nature of the ocean-continent transition, b) the origin and evolution of detachment faults, c) the dynamic strength of continental lithosphere during extension, and d) the role of magmatism during extension. Major steps have been made in mapping out the complexities of the ocean-continent transition zone, particularly in magma-poor rifted margins, where extensional structures are not masked by voluminous magmatism. The role of detachment faulting during rifting seems to be crucial and explains observations on depth-dependent stretching of the lithosphere. Despite new insights into the dynamic behaviour of the lithosphere, its rheological response remains a major uncertainty in modelling the evolution of extensional systems. Likewise, the multiple roles that basaltic magmatism may have in modifying extension remain a key question. Future insights into the global behaviour of extensional systems will be gained by constraining lithospheric rheology through integrating structural evolution and magmatism in the field, geodetic measurements of active extension, thermochronology, seismic surveys and drilling programs.
Journal of Geophysical Research: Solid Earth, 2015
Numerical experiments have been used to relate the range in the distribution and the style of deformation observed in rifted margins to localizing/delocalizing thermomechanical processes. The experiments give rise to four end-members of margins for varying initial lithospheric strength and extension rates. The first two end-members are narrow and asymmetric and narrow and near-symmetric, conjugate margins. The third end-member is asymmetric conjugate margins, wherein one side is <100 km wide and the other is >100-300 km wide. Lastly, we explore wide rift systems that may form very asymmetric conjugate margins with one narrow margin and a very wide conjugate, 200 km to > 350 km across. With initial and boundary conditions close to that inferred from the North and South Atlantic margins, we find that not all margins experience a polyphase rifting history of stretching-thinning-exhumation. Instead, the stretching mode can be very short or protracted, and the thinning or the exhumation modes can be incomplete or absent. The deformation localization of the thinning mode is in places associated with the formation of a keystone block or "block H." A new mechanism for the formation of the unstable crustal root under block H is described, wherein the bounding border faults lead to differential thinning of the crust and mantle lithosphere. Nonuniform extension also occurs in both types of wide rift systems and is related to the sequential deformation migration outward of an initial graben, associated with effective lithospheric strengthening that occurs during crustal thinning and bending.
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.
EGU General Assembly Conference Abstracts, 2009
Many passive rifted margins are characterized by depth dependent thinning whereby the mantle lithosphere seems to have been thinned more than the crust. Examples include the Exmouth Plateau and North Atlantic margins. Here we propose an explanation for this observation that is supported by numerical models. Instabilities of the lithosphere develop during the late syn-rift stage below the margins of the rift zone. The instabilities develop preferably around heterogeneities in the lithosphere such as rift margins, where lateral thermal variations promote the development of small-scale convection cells. Our models show that the drips (the instabilities) that are formed consist of lower lithosphere material. When the drips detach and sink into the upper mantle, they actually remove base lithosphere material from the lithosphere. The lithosphere thus experiences additional thinning. Because the drips do not develop until the rift zone is well developed (late syn-rift to early post breakup), continental rifts such as the North Sea Basin did not experience depth dependent thinning. Another consequence of this process is that the detachment of the drips provides a way to recycle lithosphere material into the upper mantle below passive rifted margins. This could help explain the chemistry of some melts that have a lithospheric component.
Flexural Uplift of Rift Flanks Due to Mechanical Unloading of the Lithosphere During Extension
Journal of Geophysical Research, 1989
We suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound. This mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes, and magmatic thickening of the crust. Our hypothesis is based on two critical concepts. First, the lithosphere retains finite mechanical strength or flexural rigidity during extension. Second, isostatic rebound (uplift) of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation. We develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks. We first investigate the isostatic consequences of finite simple slip on an initially planar, dipping normal fault cutting the entire lithosphere. When the lithosphere retains flexural rigidity during extension, the topography resulting from this model resembles a half graben, and the footwall rift flank is flexurally uplifted. This simple normal faulting model explains free-air gravity anomalies and topography observed at rift flanks in oceanic lithosphere (such as Broken Ridge in the eastern Indian Ocean, the Caroline ridges-Sorol Trough in the western equatorial Pacific, and the Coriolis Trough behind the New Hebrides island arc). We then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates. When the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical, the surface of slip can be regarded as a detachment. By simplification, our general model accounts for pure shear extension of the lithosphere that is uniform with depth. In this case, detachments have no meaning in the geologic sense. However, the kinematics of depth-independent pure shear may nevertheless be described in terms of a surface, which we term a kinematic reference surface, at some depth in the lithosphere. We speculate that the depth of this surface may be rheologically controlled. The magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface. In contrast, if local isostasy is assumed when the lithosphere undergoes a given amount of depth-independent pure shear, the resulting topography will be the same regardless of how the kinematics of that extension are formulated. The basin and rift flank topography and free-air gravity anomaly over young continental rifts, such as the Rhine graben, can be satisfied using our general extensional model with a small amount (<5 km) of extension along a listric-shaped detachment soling into the crust-mantle boundary. Because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains, we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension. et al., 1980; McKenzie, 1984; White et al., 1987; Mutter et al., 1988], and (5) dynamic support of rift flank topography during extension [Zuber and Parmentier, 1986; Parmentier, 1987].