Dennis S. Hodge

Mechanism of subsidence of ancient cratonic rift basins

Cratonic basins commonly occur over ancient rift zones. These inactive rift basins are recognizable by a positive linear Bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys. Many of these basins undergo discrete periods of increased subsidence rates, or reactivations, long after the mass excess responsible for the linear gravity high was emplaced. Furthermore, the reactivation of many cratonic basins occurs simultaneously with large-scale compressional tectonics. It is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess. The subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear Maxwell viscoelastic rheology. Solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time. The subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved. Since ancient rift mass excesses may be uncompensated over long geologic time intervals, the early thermal and structural evolution of rifts may not significantly influence later basin subsidence. The models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material, allowing the uncompensated basin to settle toward an isostatic-compensation depth faster than normal. Since viscosity is a strong function of temperature, reactivation by a world-wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent. An increase of 15% in the geothermal gradient, for example (from 16.5°–18.9°K/km), will cause about a 5% increase in subsidence. This increase in heat flow, however, seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100% increase in subsidence might occur. Since the rheology of the lithosphere is nonlinear, effective viscosity is also a strong nonlinear function of stress. The presence of a regional compressive stress during periods of tectonism of 1.1 · 108 Pa (about 2.8% of the buckling strength of the lithosphere) produces a short period of reactivated subsidence (≅ 105 yr). During the reactivated subsidence, the newly-imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation. This suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere. This may be accomplished by an intermittent application of the regional stress over time.

Thermal control of low-pressure fractionation processes

Abstract Thermal models detailing the solidification paths for shallow basaltic magma chambers (both open and closed systems) were calculated using finite-difference techniques. The total solidification time for closed chambers are comparable to previously published calculations; however, the temperature-time paths are not. These paths are dependent on the phase relations and the crystallinity of the system, because both affect the manner in which the latent heat of crystallization is distributed. In open systems, where a chamber would be periodically replenished with additional parental liquid, calculations indicate that the possibility is strong that a steady-state temperature interval is achieved near a major phase boundary. In these cases it is straightforward to analyze fractionation models of the basaltic liquid evolution and their corresponding cumulate sequences. This steady thermal fractionating state can be invoked to explain large amounts of erupted basalts of similar composition over long time periods from the same volcanic center and some rhythmically layered basic cumulate sequences.

Melt segregation in anatectic granites: a thermo-mechanical model

Abstract The generation and initial migration of magma produced by crustal anatexis involves partial melting and at least partial separation of melt from residual solid. The generation of anatectic granitic magma is thus governed by the flow of heat, melt and residual solid. A model which couples two-phase flow in porous media and heat flow was developed to simulate the evolution of anatectic melt around a mantle-derived mafic heat source. Results of our modelling suggest that emplacement of a mafic intrusion in the lower crust may result in partial melting of the crustal rocks and, if sufficient porosity is produced by melting, in migration and segregation of the anatectic melt. Segregation of the melt and its residuum was significant in models in which at least 25% melting of the host rock occurred. As an example, our calculations indicated that a 5-km-thick mafic intrusion emplaced in country rocks with an initial temperature of 800°C will create a zone of partial melting 5 km thick above and below the intrusion. In this case migration and segregation of the melt will produce a layer of magma approximately 1 km thick above the intrusion, and an even larger volume of melt below the intrusion. The anatectic magma moves upward by disaggregating the partially melted country rocks which form the roof of the evolving magma chamber. Upward migration by this process is limited to the portion of the crust which is partially melted by heat from the intrusion. The process of melt migration will modify the melt composition by both zone-refining and fractional crystallization in the initial stages of melt accumulation.

Mechanics of emplacement of basic intrusions

Abstract The sunken nature of many basic intrusions can be explained by simple models for flexure of the lithosphere subjected to loading by a magma. The lithosphere is divided into the strata above the magma which deforms as a stack of elastic plates, and the substratum below the magma which is modelled as an elastic plate overlying a weak fluid asthenosphere. Plane-strain plate theory is used to calculate shape and size of plutons. Significant mechanical parameters controlling the shape are: (1) depth of emplacement; (2) width of intrusion; (3) total lithospheric thickness; (4) magma density; (5) effective thickness of the overburden; and (6) magmatic pressure. In areas with lithospheric thickness of 50–100 km, basic intrusions emplaced in the upper crust (

Geochemistry and petrogenesis of the Laramie anorthosite complex, Wyoming

Abstract A geochemical investigation of the Laramie anorthosite complex determined that monsonite associated with the complex are characterized by positive Eu anomalies and display a regular variation in composition with distance from the monzonite/county rock contact. Anorthositic rocks have major and trace element abundance typical of similar complexes. The internal variations in the monzonite were produced by in situ fractionation and contamination. The data indicate that anorthosite and monzonite cannot be comagmatic. It is proposed that the anorthosite and monzonite of the complex evolved from two distinct magmas, and that two stages of anatectic melting contributed to the evolution of the monzonite. An initial stage of partial melting was induced by intrusion of a gabbroic anorthosite magma into the lower crust; a second partial melting event occurred after emplacement where heat from the intrusions melted country rocks resulting in extensive contamination ofthe monzonite.

Far-field flexural response of Lake Bonneville from paleopluvial lake elevations

May, G.M., Bills, B.G. and Hodge, D.S., 1991. Far-field flexural response of Lake Bonneville from paleopluvial lake elevations. Phys. Earth Planet. Inter., 68: 274-284. There are 18 paleopluviallakes in east-central Nevada that were contemporaneous with ancient Lake Bonneville. Shoreline elevations from three of these lakes (lakes Waring, Clover and Franklin) have been measured at 201 locations. The measurements reveal that these shorelines record the far-field flexural response of an isostatically rebounding Lake Bonneville. Linear regression analysis on each shoreline data set yields west-dipping slope values of 0.23 m km- 1, 0.19 m km-1 and 0.11 m km -1 for lakes Waring, Clover and Franklin, respectively. These values decrease in magnitude with increasing distance from the point of maximum uplift and are consistent with flexure theory. An east-west profile of the ancient shoreline elevations provides an extended lithospheric deflection datum for comparison of various lithospheric models. The new data support the results of Bills and May and suggest that the lithosphere underlying the Bonneville basin is 21-25 km thick and overlies mantle with an effective viscosity of (1.0-1.4) X 10 20 Pa s.

Finite-element method in modeling geologic transport processes

Deterministic mathematical modeling of complex geologic transport processes may require the use of odd boundary shapes, time dependency, and two or three dimensions. Under these circumstances the governing transport equations must be solved by numerical methods. For a number of transport phenomena a general form of the convective-dispersion equation can be employed. The solution of this equation for complicated problems can be solved readily by the finite-element method. Using quadrilateral isoparametric elements or triangular elements and a computational algorithm based on Galerkins procedure, solutions to unsteady heat flux from a dike and seawater intrusion in an aquifer have been obtained. These examples illustrate that the finite-element numerical procedure is well suited for solving boundary-value problems resulting from modeling of complex physical phenomena.

Creep strain-heating due to folding

Abstract Intense deformation of the rocks in the lower crust is a common feature of regional metamorphic terrain. The conversion of some mechanical energy to thermal energy by strain-heating may make a significant contribution to the thermal regime. This strain-heating contribution is studied by the development of a single-layer viscous fold model that couples the effects of strain-heating with folding. The results indicate that the temperature rise from strain-heating shows a linear increase for the initial period of folding, but at later times the bending effect of folding begins to dominate the strain-heating and an exponential temperature rise with time is experienced. In addition, strain-heating effects are increased if the viscosity contrast of the layers is increased. The results suggest that in a continental region thermal temperature increases between 25 to 100°C might be achieved assuming reasonable conditions during deformation of a regional metamorphic terrain.

Investigations of low-temperature geothermal potential in New York State

Temperature gradient map and published heat flow data indicate a possible potential for a geothermal resource in western and central New York State. A new analysis of bottom-hole temperature data for New York State confirms the existence of three positive gradient anomalies: the East Aurora, Cayuga, and Elmira anomalies, with gradients as high as 32/sup 0/C/km, 36/sup 0/C/km, and 36/sup 0/C/km, respectively. Ground waters from two of these anomalies are enriched in silica relative to surrounding areas. Heat flows based on silica geothermometry are 50 to 70 mWm/sup -2/ for the anomalies and 41.4 mWm/sup -2/ for bordering regional flux. A correlation between Bouguer gravity anomalies and the temperature gradient map suggests that the geothermal anomalies may occur above radioactive granites in the basement.

Gravity Study of a Hypersthene Syenite in the Laramie Anorthosite Complex, Wyoming

On the northwest border of the Laramie anorthosite complex, 236 gravity stations were located approximately 1 per square km over a hypersthene syenite body and country rock. The hypersthene syenite has a maximum width of about 6 km, extends about 18 km, and is bordered principally by anorthosite, hornblende syenite, and granite gneiss. Three 15 to 18-mgal anomalies with maximum gradients of about 4 mgal/km are centered over the hypersthene syenite. Bouguer anomaly values range from a mean of about —150 mgal in the anorthosite to a high of —128 mgal over the hypersthene syenite to Bouguer mean values of —150 mgal over granite gneisses to the west. Strikingly, isoanomaly lines locally crosscut geologic contacts. Computed minimum density contrasts of about δσ = .23 indicate that a subsurface mafic mass, possibly norite, is the principal disturbing mass and not the hypersthene syenite. Models indicate that this mafic mass is within .5 km of the surface in places and extends to a depth of about 5 km. The geometry of the hypersthene syenite is difficult to ascertain.