John R. Booker

Partially molten middle crust beneath southern Tibet : Synthesis of project INDEPTH results

INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.

Aftershocks Caused by Pore Fluid Flow

Large shallow earthquakes can induce changes in the fluid pore pressure that are comparable to stress drops on faults. The subsequent redistribution of pore pressure as a result of fluid flow slowly decreases the strength of rock and may result in delayed fracture. The agreement between computed rates of decay and observed rates of aftershock activity suggests that this is an attractive mechanism for aftershockss.

Rapid inversion of two‐ and three‐dimensional magnetotelluric data

We have developed an efficient iterative inversion method applicable to both two-dimensional (2D) and three-dimensional magnetotelluric data. The method approximates horizontal derivative terms with their values calculated from the fields of the previous iteration. The equations at each horizontal coordinate then become uncoupled. At each iteration this allows separate inversions for the improved conductivity profile beneath each measurement site. Resultant profiles are interpolated to form a new multidimensional model for which the fields for the next iteration are calculated. The method is extremely fast, and tests with 2D data show very promising results.

Onset of convection in a variable-viscosity fluid

The Rayleigh number R, in a horizontal layer with temperature-dependent viscosity can be based on the viscosity at T 0 , the mean of the boundary temperatures. The critical Rayleigh number Roc for fluids with exponential and super-exponential viscosity variation is nearly constant at low values of the ratio of the viscosities at the top and bottom boundaries; increases at moderate values of the viscosity ratio, reaching a maximum at a ratio of about 3000, and then decreases. This behaviour is explained by a simple physical argument based on the idea that convection begins first in the sublayer with maximum Rayleigh number. The prediction of Palm (1960) that certain types of temperature-dependent viscosity always decrease Roc is confirmed by numerical results but is not relevant to the viscosity variations typical of real liquids. The infinitesimal-amplitude state assumed by linear theory in calculating Roc does not exist because the convection jumps immediately to a finite amplitude at R 0c . We observe a heat-flux jump at R 0c exceeding 10% when the viscosity ratio exceeds 150. However, experimental measurements of R 0c for glycerol up to a viscosity ratio of 3400 are in good agreement with the numerical predictions when the effects of a temperature-dependent expansion coefficient and thermal diffusivity are included.

Electrically Conductive Crust in Southern Tibet from INDEPTH Magnetotelluric Surveying

The crust north of the Himalaya is generally electrically conductive below depths of 10 to 20 km. This conductive zone approaches the surface beneath the Kangmar dome (dipping north) and extends beneath the Zangbo suture. A profile crossing the northern Yadong-Gulu rift shows that the high conductivity region extends outside the rift, and its top within the rift coincides with a bright spot horizon imaged on the INDEPTH CMP (common midpoint) profiles. The high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region.

Internal structure of the San Andreas fault at Parkfield, California

Magnetotelluric and seismic reflection surveys at Parkfield, California, show that the San Andreas fault zone is characterized by a vertical zone of low electrical resistivity. This zone is 500 m wide and extends to a depth of 4000 m. The low electrical resistivity is attributed to high porosity of saline fluids present in the highly fractured fault zone. The occurrence of microearthquakes and creep in the low resistivity zone is consistent with suggestions that seismicity at Parkfield is fluid driven.

Thermal convection with strongly temperature-dependent viscosity

This paper experimentally investigates the heat transport and structure of convection in a high Prandtl number fluid layer whose viscosity varies by up to a factor of 300 between the boundary temperatures. An appropriate definition of the Rayleigh number R uses the viscosity at the average of the top and bottom boundary temperatures. With rigid boundaries and heating from below, the Nusselt number N normalized with the Nusselt number N 0 of a constant-viscosity fluid decreases slightly as the viscosity ratio increases. The drop is 12% at a variation of 300. A slight dependence of N/N 0 on R is consistent with a decrease in the exponent in the relation N ∝ R β from its constant-viscosity value of 0·281 to 0·25 for R [lsim ] 5 × 10 4 . This may be correlated with a transition from three- to two-dimensional flow. At R ∼ 10 5 and viscosity variation of 150, the cell structure is still dominated by the horizontal wavelength of the marginally stable state. This is true with both free and rigid upper boundaries. The flow is strongly three-dimensional with a free upper boundary, while it is nearly two-dimensional with a rigid upper boundary.

Low electrical resistivity associated with plunging of the Nazca flat slab beneath Argentina

Beneath much of the Andes, oceanic lithosphere descends eastward into the mantle at an angle of about 30° (ref. 1). A partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material. This wedge is the ultimate source for magma erupted at the active volcanoes that characterize the Andean margin. But between 28° and 33° S the subducted Nazca plate appears to be anomalously buoyant, as it levels out at about 100 km depth and extends nearly horizontally under the continent. Above this ‘flat slab’, volcanic activity in the main Andean Cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge. But it is unknown where slab volatiles go once this happens, and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward. Here we present results from a magnetotelluric profile in central Argentina, from which we infer enhanced electrical conductivity along the eastern side of the plunging slab, indicative of the presence of partial melt. This conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth, or that melt is supplied from the 410 km discontinuity, consistent with the transition-zone ‘water-filter’ model of Bercovici and Karato.

Magnetotelluric inversion for minimum structure

Structure can be measured in terms of a norm of the derivative of a model with respect to a function of depth f(z), where the model m(z) is either the conductivity σ or log σ. An iterative linearized algorithm can find models that minimize norms of this form for chosen levels of chi‐squared misfit. The models found may very well be global minima of these norms, since they are not observed to depend on the starting model. Overfitting data causes extraneous structure. Some choices of the depth function result in systematic overfitting of high frequencies, a “blue” fit, and extraneous shallow structure. Others result in systematic overfitting of low frequencies, a “red” fit, and extraneous deep structure. A robust statistic is used to test for whiteness; the fit can be made acceptably white by varying the depth function f(z) which defines the norm. An optimum norm produces an inversion which does not introduce false structure and which approaches the true structure in a reasonable way as data errors decrease...

Optimal one-dimensional inversion and bounding of magnetotelluric apparent resistivity and phase measurements

Abstract The properties of the log of the admittance in the complex frequency plane lead to an integral representation for one-dimensional magnetotelluric (MT) apparent resistivity and impedance phase similar to that found previously for complex admittance. The inverse problem of finding a one-dimensional model for MT data can then be solved using the same techniques as for complex admittance, with similar results. For instance, the one-dimensional conductivity model that minimizes the χ2 misfit statistic for noisy apparent resistivity and phase is a series of delta functions. One of the most important applications of the delta function solution to the inverse problem for complex admittance has been answering the question of whether or not a given set of measurements is consistent with the modeling assumption of one-dimensionality. The new solution allows this test to be performed directly on standard MT data. Recently, it has been shown that induction data must pass the same one-dimensional consistency test if they correspond to the polarization in which the electric field is perpendicular to the strike of two-dimensional structure. This greatly magnifies the utility of the consistency test. The new solution also allows one to compute the upper and lower bounds permitted on phase or apparent resistivity at any frequency given a collection of MT data. Applications include testing the mutual consistency of apparent resistivity and phase data and placing bounds on missing phase or resistivity data. Examples presented demonstrate detection and correction of equipment and processing problems and verification of compatibility with two-dimensional B-polarization for MT data after impedance tensor decomposition and for continuous electromagnetic profiling data.