Shaohua Zhou

Topography, boundary forces, and the Indo-Australian intraplate stress field

The relative contribution of topographic (e.g., ridge push, continental margins, and elevated continental crust) and plate boundary (e.g., subduction and collisional) forces to the intraplate stress field in the Indo-Australian plate (IAP) is evaluated through a finite element analysis. Two important aspects of the IAP intraplate stress field are highlighted in the present study: (1) if substantial focusing of the ridge push torque occurs along the collisional boundaries (i.e., Himalaya, New Guinea, and New Zealand), many of the first-order features of the observed stress field can be explained without appealing to either subduction or basal drag forces; and (2) it is possible to fit the observed SHmax, (maximum horizontal stress orientation) and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes (e.g., tens of megapascals, averaged over the thickness of the lithosphere) throughout the plate. This study therefore presents a plausible alternative to previous studies of the IAP intraplate stress field, which predicted very large tectonic stress magnitudes (hundreds of megapascals) in some parts of the plate. In addition, topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental India and Australia, and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions. A central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin. We note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field, and this nonuniqueness continues to make modeling the IAP stress field problematic. While our study is an important step forward in understanding the sources of the IAP intraplate stress field, a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin.

The origins of the intraplate stress field in continental Australia

The ridge push force acting on the Indo-Australian plate exerts a significant torque (8.5 × 1025N m) about a pole at 30.3°N, 34.5°E. The angular difference between this torque pole and the observed pole of rotation for the plate (19.2°N, 35.6°E) is less than 12° and suggests that the ridge push force plays an important role in the dynamics of the Indo-Australian plate. We have used an elastic finite-element analysis to study the predicted intraplate stress field in continental Australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate. The modeling indicates that a number of important features of the observed stress field within the Australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the Himalaya, Papua New Guinea, and New Zealand collisional boundaries segments. These features include NS-to NE-SW-oriented compression in the northern Australia and E-W-oriented compression in southern Australia. Our analysis also shows that subduction processes along the northern and eastern boundaries provide only second-order controls on the intraplate stress field in continental Australia.

Mechanical consequences of granite emplacement during high-T, low-P metamorphism and the origin of “anticlockwise”PT paths

High-temperature (T), low-pressure (P) metamorphic belts preserve evidence of metamorphism under extremely perturbed, and hence transient, thermal regimes.PT paths are commonly “anticlockwise”; that is, the maximumT was attained prior to or at the same time as the maximumP with cooling at near constant or increasingP, and, in many terrains, peak metamorphic temperatures prevailed during convergent deformation. Using a simple coupled thermal-mechanical model that assumes a strongly temperature-dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high-T, low-P terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent. The model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation,Tm˚crit, the depth of emplacement of the magmas and, most importantly, the granite forming mechanism. During the prograde cycle material points attain maximum temperatures during a transient ( < 3 Ma) high strain rate pulse withe up to ∼ 10−14 s−1. At levels near the site of granite emplacement, the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression.

On the stability of isostatically compensated mountain belts

The formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence. The evolution of potential energy stored in an orogen is dependent on (1) the density structure, (2) the thermal evolution, and (3) the way convergent deformation is partitioned between crust and mantle lithosphere. It is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension, or collapse, may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened (e.g., England, 1987). In this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the “Brace-Goetze” model for the rheology of the lithosphere. The reference lithosphere, defined to be in potential energy and isostatic balance with the mid-ocean ridges, changes with different thermal parameters of the lithosphere. The instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts (measured relative to the reference state) with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range. It is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates (greater than a few times 10−16 s−1) and leading to significant finite extension providing the Moho temperatures exceed about 650–700°C, a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust. Consequently, processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse. The calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening (i.e., fl ≤ fc/2, where fl and fc are the lithospheric and crustal thickening factors).

On the timing relationship between fluid production and metamorphism in metamorphic piles: Some implications for the origin of post-metamorphic gold mineralisation

Abstract The relative timing of metamorphism at different crustal levels is quantified for a range of metamorphic environments associated with convergent deformation. We characterise different metamorphic environments by making simple assumptions about the nature of heat input and the nature of heat withdrawal from the terrane. If internal heat production is the major heating agent during crustal thickening, then deep crustal levels reach their metamorphic temperature peak later than shallow levels. In terranes heated from below, such relationships apply only if erosion commenced early in the thermal history or if erosion rates are fast. Quantitative knowledge of these relations can be used together with simple assumptions about prograde fluid release to predict the likely timing of fluid passage through rocks higher in the column. In terranes with deep-later characteristics the evidence for fluid penetration may be preserved in the form of mineralised quartz veins or late stage alteration. Within our simple parameterisation of crustal heat sources we can use the grade of a metamorphic rock together with field evidence on the timing of metamorphism, igneous intrusion and deformation in order to constrain the time scale of the underlying thermal perturbation and erosion process for a given terrane. We demonstrate that time intervals up to several tens of million years may separate metamorphism and fluid emplacement in model evolutions that are scaled to describe greenschist facies metamorphic terranes that formed at low pressure. Such timing relationships are in fact recorded in many greenschist facies metamorphic terranes that host late stage mesothermal gold deposits. We suggest therefore that there is no conflict between the observation that the emplacement of many gold deposits occurred demonstrably after peak metamorphism of their host rocks and the interpretation that metamorphic fluids are responsible for the mineralisation.

A study of the design of inclined wellbores with regard to both mechanical stability and fracture intersection, and its application to the Australian North West Shelf

Abstract Knowledge of the in-situ stress field can be applied both in planning most stable drilling trajectories and also in maximizing intersection of teh wellbore with open, natural, and hydraulically-induced fractures in the reservoir. Inclined wells drilled with the optimum drilling direction (azimuth) and deviation (from the vertical), at which the shear stress anisotropy around the wellbore wall is minimized, may be more mechanically stable than vertical wells in various stress regimes. Such mechanically stable trajectories may also maximize open fracture intersection. Combining the objectives of maximum mechanical stability and maximum open fracture intersection • • wells in an extensional stress regime should be highly deviated from vertical (at about 55–70°) and drilled along the azimuth of the least horizontal princial stress; • • wells in a strike-slip stress regime should be horizontal and drilled at 55–70° with respect to the major horizontal principal stress; • • wells in a compressional stress regime should be slightly deviated from vertical (at about 20–35°) and drilled along the azimuth of the major horizontal principal stress. Application of this study to the Wanaea/Cossack field of the Australian North West Shelf, where the in-situ stress field has been constrained, suggests that the most stable drilling direction and that which maximizes potential intersection with any open fractures in the reservoir is horizontal, in the azimuth of the least horizontal principal stress, 005–010°N.

An analytical method for determining horizontal stress bounds from wellbore data

Abstract Several issues including control of stress-induced wellbore instability and design of deviated and horizontal wellbores require knowledge of the magnitude and orientation of the in-situ stress field. An analytical method has been developed to determine bounds to the in-situ horizontal stresses from wellbore data for cases where the explicit measurement of stresses is not available. The method has been used to determine the anisotropic stress state at depths between 1000 m and 4000 m in the immediate region of the Cossack and Wanaea fields in the North West Shelf of Australia. The validity of the method is demonstrated by the satisfactory comparison of the determined stress bounds with the minor and major horizontal stresses estimated directly from modified leak-off tests.

Modelling the Contemporary Stress Field and its Implications for Hydrocarbon Exploration

The forces that act on the Earths lithospheric plates are responsible for the stress regime within the plates at a regional scale, and thus influence issues pertinent to hydrocarbon exploration ? such as the nature of fault reactivation, hydraulic seal integrity, natural and induced fracture orientation and wellbore stability. Four models of the intraplate stress field of the Australian continent have been produced by elastic finite element modelling of the forces acting on the Indo-Australian plate. All four models incorporate the push of mid-ocean ridges and of continental margins. In the four models the magnitude of the poorly constrained convergent boundary and basal drag forces are varied within reasonable limits. Despite being based on significantly different force magnitudes, regional stress orientations predicted by the three models that recognise the heterogeneity of forces acting along the convergent northeastern boundary of the Indo-Australian plate are considered reliable because they are consistent over much of the Australian continent and show broad agreement with the available in situ stress measurements. In the absence of in situ stress measurements, for example from borehole breakouts, modelled stress orientations should be incorporated into the assessment of issues pertinent to hydrocarbon exploration that are influenced by the contemporary stress field. In the context of fault reactivation, pre-existing vertical faults striking at 30° to 45° to the maximum horizontal stress direction are the most prone to at least a component of strike-slip motion. Planes dipping in the minimum horizontal stress direction are the most suitably oriented to be reactivated in extension, and planes dipping in the maximum horizontal stress direction are the most suitably oriented to be reactivated in compression. Modelled stress orientations can also be used to predict open natural fracture orientation, with the preferred orientation being normal to the minimum horizontal stress, and to help assess the hydraulic integrity of reservoir seals, with faults and fractures normal to the minimum horizontal stress least likely to be sealing.

Modeling of dynamic uplift, denudation rates, and thermomechanical consequences of erosion in isostatically compensated mountain belts

In this paper we present a simple one-dimensional model to examine the interplay between dynamic uplift and denudation in response to simultaneous surface erosion and lithospheric deformation under the assumption that the surface elevation of a mountain belt is isostatically compensated and the lithosphere undergoes vertically homogeneous deformation. We also discuss the dynamics of isostatically compensated mountain belts in relation to surface erosion, based on a ‘Brace-Goetze’ lithospheric model consisting of a quartz-dominated crust and an olivine-dominated upper mantle. The mechanical strength of such a lithosphere is governed by a combination of brittle failure and power law or Dorn law creep. Assuming a simple linear dependence of erosion rate on elevation, we find that contrary to previous suggestions, the effect of high denudation rates on the mechanical strength of the lithosphere does not involve significant weakening of the lithosphere.

A supplement to ‘A study of the design of inclined wellbores with regard to both mechanical stability and fracture intersection’

In this supplement, it is shown that, by recognizing the need to avoid hydraulically induced horizontal fractures, the predicted upper bound mud weights for mechanical stability of inclined wellbores are lower than our previous predictions (Zhou et al., 1994), and more realistic. The key conclusion remains that the mechanical stability of inclined wellbores can be improved if they are drilled with an optimum drilling direction and deviation angle.