World Stress Map Project Research Articles

Tectonic stress in the Earth’s crust: advances in the World Stress Map project

Abstract Tectonic stress is one of the fundamental data sets in Earth sciences comparable with topography, gravity, heat flow and others. The importance of stress observations for both academic research (e.g. geodynamics, plate tectonics) and applied sciences (e.g. hydrocarbon production, civil engineering) proves the necessity of a project like the World Stress Map for compiling and making available stress data on a global scale. The World Stress Map project offers not only free access to this global database via the Internet, but also continues in its effort to expand and improve the database, to develop new quality criteria, and to initiate topical research projects. In this paper we present (a) the new release of the World Stress Map, (b) expanded quality ranking schemes for borehole breakouts and geological indicators, (c) new stress indicators (drilling-induced fractures, borehole slotter data) and their quality ranking schemes, and (d) examples for the application of tectonic stress data.

New aspects of tectonic stress inversion with reference to the TENSOR program

Abstract Analysis of tectonic stress from the inversion of fault kinematic and earthquake focal mechanism data is routinely done using a wide variety of direct inversion, iterative and grid search methods. This paper discusses important aspects and new developments of the stress inversion methodology as the critical evaluation and interpretation of the results. The problems of data selection and separation into subsets, choice of optimization function, and the use of non-fault structural elements in stress inversion (tension, shear and compression fractures) are examined. The classical Right Dihedron method is developed in order to estimate the stress ratio R , widen its applicability to compression and tension fractures, and provide a compatibility test for data selection and separation. A new Rotational Optimization procedure for interactive kinematic data separation of fault-slip and focal mechanism data and progressive stress tensor optimization is presented. The quality assessment procedure defined for the World Stress Map project is extended in order to take into account the diversity of orientations of structural data used in the inversion. The range of stress regimes is expressed by a stress regime index R’ , useful for regional comparisons and mapping. All these aspects have been implemented in a computer program TENSOR, which is introduced briefly. The procedures for determination of stress tensor using these new aspects are described using natural sets of fault-slip and focal mechanism data from the Baikal Rift Zone.

Recent tectonic stress and crustal deformation in and around the Pannonian Basin: data and models

Abstract Recent (active) tectonics of the Pannonian Basin and its surroundings has been investigated using data from over 900 earthquake focal mechanism solutions, 200 borehole breakout analyses, some in-situ stress measurements and by applying finite element modelling technique. We have established a database for indicators of recent stress, and analysed the stress state of the region by the methods of the World Stress Map project. The alignments of the largest horizontal stresses have been mapped and the tectonic regimes were also determined. We present a map of seismoactive faults and seismic energy release combining historical and modern seismicity data and results of local seismotectonic studies. The pattern of earthquake slip vectors and the style of faulting are summarised in order to characterise the active deformations. Our results show that the alignment of the largest horizontal stress exhibits a radial pattern around the Adriatic sea. In the Southern Alps and northwestern Dinarides the largest horizontal stress (SH) is aligned N-S and thrust faulting is dominant. Along the southern Dinarides and the Dalmatian coast thrusting with strike-slip component can be observed. Here the trajectories of SH are aligned NE-SW. E-W aligned SH trajectories and normal faulting are characteristic of the Rhodope Massif. Thrust faulting of the Vrancea region seems to be distinct from the compressive regime around the Adriatic sea. In the Pannonian Basin borehole breakout analyses show that the direction of largest horizontal stress is changing from N-S in the western part to NE-SW in the east. Most of focal mechanisms and available hydraulic fracturing measurements indicate strike-slip and thrust faulting inside the basin. The lack of normal faulting mechanisms indicates that the extension of the basin has been terminated and a new compressive stress regime prevails. The crustal deformation of the area is controlled by the counterclockwise rotation of Adria with respect to Europe around a pole at the 45°N latitude and 6–10°E longitudes, which is inferred from satellite geodesy and supported by earthquake slip vectors. This movement can explain the shortening of the Southern Alps, and squeezing eastward the region between the Adriatic sea and the Mur-Murz line. Rotation of Adria generates thrusts along the Dalmatian coast, and this compressive deformation extends into the land far from the coastline, and leads to squeezing of the Pannonian Basin from the southwest. The seismicity pattern in the Pannonian Basin shows that earthquakes are restricted to the crust and the control by pre-existing (mostly Miocene) fault zones is strongly masked by random activity due to general weakness of the lithosphere. Although earthquakes are of small to medium magnitude (M ≤ 6), the cummulative energy release is remarkably higher than in the surrounding Carpathian arc. The Vrancea zone is the only exception, where high energy release in the crust and down to 200 km depth is associated with a relict subducted slab. Finite element stress modelling has been performed in order to simulate the observed stress pattern and, hence, to understand the importance of different possible stress sources in and around the Pannonian Basin. The observed radial stress pattern of the region can be well explained by the counterclockwise rotation of the Adriatic microplate as a first-order stress source. Additional boundary conditions, such as the active deformation at the Vrancea zone and the role of rigid crustal blocks at the Bohemian Massif and the Moesian Platform, can significantly effect the style of deformation and the alignment of the largest horizontal stress. Furthermore, our calculations show that differences in the crustal thickness and the presence of large scale fault zones in the Pannonian region have only local influence on the model results.

Tectonics, sedimentation and volcanism in the East African Rift System: introduction

1997). This issuepresents new results mainly from geologicalinvestigations concentrated in the westernbranch of the East African Rift System andconducted in the framework of differentresearchprojects on the East African Rift. An additionalpaper is related to the seismicity ofthe southernpart of the Kenya Rift.IGCP400 PROJECT GEODYNAMICS OFCONTINENTAL RIFTINGRationaleThe presence of active intracontinental riftsystems with deep water basins, such as thoseof East Africa and Baikal, gives rise to severalproblems regarding the mechanism of basinformation in plate interiors. For example, one ofthe most important results of the ILP WorldStress Map project is that the interior ofcontinental lithospheric plates are undercompression (Zoback, 1992). Central Asia isunder compression resulting from the India­Eurasia convergence and collision; and Africa issurrounded by mid-oceanicspreading ridges anda continental collision to the north. Afundamental question therefore is how do suchdeep extensional rift basins develop in acompressional environment? To address thisproblem, it is necessary to investigate not onlythe rift basins themselves, but also theirlithospheric structure and their general intraplatetectonic setting.The aim of IGCP400 (1996-2000)Geodynamics of Continental Rifting is tounderstand the causes and effects of continentalrifting by making comparisons of the deepstructure and the geophysical, tectonic,kinematic and magmatic processes responsiblefor the initiation and development ofintracontinental rifts, particularly the Afro­Arabian, East African, Baikal, Rio Grande andEuropean Cenozoic Rift Systems. For a betterunderstanding of the evolution of riftingprocesses with time, the project also includesthe North Sea Rift and rifted passive margins.Present-dayknowledge of rifts is uneven andit is necessary to promote further investigationwhere information is lacking, and also to developnew ideas and methodologies to address crucialrift problems. Models of origin resulting fromthe comparative studies can be tested using datafrom geodesy, tectonics and seismology forkinematics; gravimetry, seismictomography, andheat flow for rheology; and gravimetry andfission tracks for the thermomechanicalbehaviour of the lithosphere. The project alsoaddresses related topics including environmentalissues, natural hazards, palaeoclimatic andpalaoenvironmental changes, and lacustrineenvironmental protection. It also promotesmultinational north-south and east-westscientific and technological cooperation.Recent activitiesThe IGCP400 project officially started in July1996. Three business meetings were organisedduring the first six months to disseminateinformation on the new project, to recruitmembers and to assemble a pool of specialists.Business meetings were held during the 21thEGS General Assembly in The Hague, TheNetherlands (6-10 May 1996), during thescientific meeting of the INTAS-93-134projectContinental rift tectonics and sedimentary basinevolution, in Novosibirsk, Russia (22-24May

A theory of intraplate tectonics

The theory of plate tectonics assumes a rigid behavior of tectonic plates and therefore fails to account for observed intraplate deformation. A new theory of intraplate tectonics is developed to calculate the first‐order intraplate deformation induced by horizontal displacement of deformable plate boundaries. It is based on simple assumptions that link the well‐established directions of relative plate motion to the displacement and deformation fields within a plate interior adjacent to three types of deformable plate boundaries: inward‐, outward‐, and tangential‐displaced boundaries. The theory predicts the direction of intraplate displacement, displacement rate, strain, and stress fields in terms of small circles, great circles, and 45° loxodromes around the pole of rotation of two adjacent plates. The 45° loxodromes are two orthogonal sets of directions, clockwise and counterclockwise, that intersect both small and great circles at 45°. The principal axis of the maximum horizontal stress follows small circles for inward‐displaced boundaries, great circles for out ward‐displaced boundaries, and loxodromes for tangential‐displaced boundaries. The theoretical predictions are systematically compared with more than 4000 reliable observed directions of maximum horizontal stress provided by the world stress map project [Zoback, 1992]. The theory indicates that the first‐order intraplate deformation is predominantly induced by horizontal forces acting on plate boundaries and by buoyancy forces that arise from lateral density variations between mid‐ocean ridges and plate interiors (ridge push). The simplicity of the predictions and their good agreement with observations suggests that intraplate deformation should be investigated in the pole of rotation spherical coordinate system.

Origins of the European regional stress field

We present a study of the origins of the European regional stress field. Intraplate stresses due to various plate boundary forces and intraplate stress sources were predicted using an elastic finite element analysis. The finite element mesh consists of about 1629 nodes in a network of 3117 triangular elements which provide a spatial resolution of about 1°. The study builds on the previous modelling studies through consideration of a wide range of plausible forces acting along the collisional eastern and southern plate boundaries and, importantly, through the inclusion of forces due to other lateral density variations within the lithosphere, such as those associated with the continental margins and high topography. The modelling was constrained by more than 1800 stress indicators from the World Stress Map Project which provide information about the orientation of the observed maximum horizontal compressive stress, S H. The torque associated with the ridge push force is well constrained and the principal source of uncertainty relates to the boundary conditions used to represent the collisional resistance acting along the eastern and southern boundaries. A gradient in the collisional resistance increasing in magnitude from west to east is assumed to act along the southern plate boundary, whereas the eastern boundary is modelled as a pinned margin at 46°E. The S H orientation and magnitude of the regional stress field was found to be largely invariant to the boundary conditions used to represent the tectonic forces acting along the eastern and southern margins, and is characterized by a nearly uniform NW-SE compression with a magnitude in the range of 10–20 MPa averaged over a 100 km thick lithosphere. Lateral density variations within the continental areas of Europe were found to reduce the magnitude of the predicted stresses but do not have a significant effect on the orientation of the regional stress field.

Analysis of the South American intraplate stress field

The first‐order South American intraplate stress field was modeled through a finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources. The finite element mesh consisted of 3100 nodes in a network of 5993 equal‐area triangular elements which provided a spatial resolution of about 1° at the equator. An important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the Mid‐Atlantic Ridge (e.g., ridge push), the continental margins along the east coast of Brazil and Argentina, and the elevated continental crust (e.g., the Andean Cordillera). Predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated: pinned collisional boundaries and applied collisional boundary forces. Constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress, SHmax, provided by 217 stress indicators from the World Stress Map Project as well as by SHmax magnitude estimates and torque information from previous investigations. Our modeling results demonstrate that the first‐order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate. The predicted intraplate stress field is characterized by a nearly uniform E‐W SHmax orientation throughout most regions of the plate, with stress magnitudes generally less than 20 MPa averaged over a 100‐km‐thick lithosphere. Significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the Andes. Although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained, we estimate a plausible upper bound on the force per unit length acting along the Peru‐Chile Trench to be about 2.5 × 1012 N m−1. While some of our models are consistent with a driving basal drag to balance the torques acting on the plate, the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force.

Tectonic stresses in the African plate: Constraints on the ambient lithospheric stress state

An elastic finite-element analysis of the African intraplate stress field is used to determine constraints on the stress state resulting from variations in the gravitational potential energy of the lithosphere ( U 1) produced by lateral density variations. The modeling is constrained by 150 stress indicators extracted from the World Stress Map Project data set. Lateral variations in U 1 are calculated by using a simple lithospheric density model that is consistent with observed geoid anomalies across mid-ocean ridges and continental margins. Predicted tectonic stresses in the oceanic regions of the African plate range from tension along the mid-ocean ridges (9 MPa) to compression in the ocean basins (10 MPa). Continental regions near sea level are in a near-neutral state of stress. There are large extensional stresses present in the Ethiopian highlands (15 MPa), the East African rift (9 MPa), and southern Africa (8 MPa). The general agreement between the predicted and the observed stress fields suggests that the principal long-wavelength features of the intraplate stress field, including the observed extension in eastern and southern Africa, can be explained in terms of stresses arising from lithospheric density variations without appeal to poorly determined sublithospheric processes. The state of stress in continental regions with elevations greater than 70 m is predicted to be extensional, providing an alternative source of continental tension that has important implications for the dynamics of continental breakup.

Utilizing in-situ stress data for seismic hazard assessment: the World Stress Map Project's contribution to GSHAP

Utilizing in-situ stress data for seismic hazard assessment: the World Stress Map Project's contribution to GSHAP

The origin of sedimentary basins: State of the art and first results of the task force

The origin of sedimentary basins is a key element of the geological evolution of the continental lithosphere. During the last decade substantial progress has been made in understanding the thermo-mechanical aspects of sedimentary basin formation and the isostatic response of the lithosphere to surface loads such as basins. Most of this progress has been made not so much in terms of the development of new modelling methodologies and insights into the rheological makeup of the lithosphere but rather in the processing of substantial, new, high-quality data sets from previously unexplored areas of the globe. Virtually all modelling carried out so far has been in terms of lithospheric displacements, thus refraining from a full examination of dynamic controls exerted by lithospheric stresses. This is because stresses are very sensitive to adopted lithospheric rheologies and these rheologies have been, by convention, unrealistically simple. This is true of models for both extensional and compressional sedimentary basins. For example, most models for extensional basin formation (Roberts et al., 1991) are keyed to lithospheric strain due to an unknown and unspecified stress field rather than to the strain response of the lithosphere to a known and/or realistic stress state. Moreover, changes in plate tectonic regimes and associated stress fields have been shown to be quite important in controlling the subsidence record and stratigraphic architecture of extensional basins. Similarly, models of basins developed in compressional environments (e.g., McClay, 1991) have been conventionally related to flexure profiles (displacement patterns), again not invoking the dynamic control of the compressional stresses intrinsic to this particular tectonic setting. Another reason that the relationship between lithospheric stresses and displacements in tectonic modelling has not received full attention is because little has been known about the actual stress state in the lithosphere. This situation has recently changed drastically as the result of the successful World Stress Map Project carried out in the framework of the International Lithosphere Program @back, 1992). Further, the application of structural techniques to establish the temporal evolution of paleo-stress fields has begun in a number of sedimentary basins (Letouzey, 1990). Not only has the dynamic element of lithospheric deformation been inadequately addressed but present quantitative models of the origin of basins are incapable of solving problems related to subsequent structural developments that may be intrinsically coupled with the basin formation. For example, late-stage compression during the post-rift evolution of extensional basins can largely explain current discrepancies between estimates of crustal thinning derived from structural analyses and subsidence data of rifted basins (Kooi and Cloetingh, 1989). Further progress in understanding the role of extensional faults in offshore areas like the North Sea requires detailed structural studies and modelling of exposed successions such as those found in intramontane transpressional basins. Post-rift compression of extensional or transtensional sedimentary basins leads to complex near-surface deformation patterns. In such cases of structural inversion, inher-

First- and second-order patterns of stress in the lithosphere: the world stress map project : Zoback, M L J Geophys ResV97, NB8, July 1992, P11703–11728

First- and second-order patterns of stress in the lithosphere: the world stress map project : Zoback, M L J Geophys ResV97, NB8, July 1992, P11703–11728

First‐ and second‐order patterns of stress in the lithosphere: The World Stress Map Project

To date, more than 7300 in situ stress orientations have been compiled as part of the World Stress Map project. Of these, over 4400 are considered reliable tectonic stress indicators, recording horizontal stress orientations to within <±25°. Remarkably good correlation is observed between stress orientations deduced from in situ stress measurements and geologic observations made in the upper 1–2 km, well bore breakouts extending to 4–5 km depth and earthquake focal mechanisms to depths of ∼20 km. Regionally uniform stress orientations and relative magnitudes permit definition of broad‐scale regional stress patterns often extending 20–200 times the approximately 20–25 km thickness of the upper brittle lithosphere. The “first‐order” midplate stress fields are believed to be largely the result of compressional forces applied at plate boundaries, primarily ridge push and continental collision. The orientation of the intraplate stress field is thus largely controlled by the geometry of the plate boundaries. There is no evidence of large lateral stress gradients (as evidenced by lateral variations in stress regime) which would be expected across large plates if simple resistive or driving basal drag tractions (parallel or antiparallel to absolute motion) controlled the intraplate stress field. Intraplate areas of active extension are generally associated with regions of high topography: western U.S. Cordillera, high Andes, Tibetan plateau, western Indian Ocean plateau. Buoyancy stresses related to crustal thickening and/or lithospheric thinning in these regions dominate the intraplate compressional stress field due to plate‐driving forces. These buoyancy forces are just one of several categories of “second‐order” stresses, or local perturbations, that can be identified once the first‐order stress patterns are recognized. These second‐order stress fields can often be associated with specific geologic or tectonic features, for example, lithospheric flexure, lateral strength contrasts, as well as the lateral density contrasts which give rise to buoyancy forces. These second‐order stress patterns typically have wavelengths ranging from 5 to 10+ times the thickness of the brittle upper lithosphere. A two‐dimensional analysis of the amount of rotation of regional horizontal stress orientations due to a superimposed local stress constrains the ratio of the magnitude of the horizontal regional stress differences to the local uniaxial stress. For a detectable rotation of 15°, the local horizontal uniaxial stress must be at least twice the magnitude of the regional horizontal stress differences. Examples of local rotations of SHmax orientations include a 75°–85° rotation on the northeastern Canadian continental shelf possibly related to margin‐normal extension derived from sediment‐loading flexural stresses, a 50°–60° rotation within the East African rift relative to western Africa due to extensional buoyancy forces caused by lithospheric thinning, and an approximately 90° rotation along the northern margin of the Paleozoic Amazonas rift in central Brazil. In this final example, this rotation is hypothesized as being due to deviatoric compression oriented normal to the rift axis resulting from local lithospheric support of a dense mass in the lower crust beneath the rift (“rift pillow”). Estimates of the magnitudes of first‐order (plate boundary force‐derived) regional stress differences computed from modeling the source of observed local stress rotations magnitudes can be compared with regional stress differences based on the frictional strength of the crust (i.e., “Byerlee's law”) assuming hydrostatic pore pressure. The examples given here are too few to provide a definitive evaluation of the direct applicability of Byerlee's law to the upper brittle part of the lithosphere, particularly in view of uncertainties such as pore pressure and relative magnitude of the intermediate principal stresses. Nonetheless, the observed rotations all indicate that the magnitude of the local horizontal uniaxial stresses must be 1–2.5+ times the magnitude of the regional first‐order horizontal stress differences and suggest that careful evaluation of such local rotations may be a powerful technique for constraining the in situ magnitude stress differences in the upper, brittle part of the lithosphere.

The dynamics of motion of the South American Plate

Observations of the intraplate stress field, as compiled and evaluated in the context of the World Stress Map Project, can be compared with stress modelling results to increase our understanding of the dynamics of motion of the lithospheric plates. In this paper we investigate the driving and resistive plate tectonic forces acting on the South American plate in order to establish a detailed basis for future stress modelling. Our basic assumption is that the plate is in mechanical equilibrium; that is, the net torque of the forces acting on the plate vanishes. The complexity of the forces exerted along the margins of the South American plate requires the dynamical analysis to be performed step‐wise. Incorporating the forces which are expected to be of major importance, a first‐order model is defined. The effect of additional plate boundary forces is subsequently examined in terms of superpositions onto the first‐order model. With the first‐order model the relation between three resistive forces and shear stress on the base of the lithosphere is derived. The ridge push force, driving the motion of the South American plate, can be quantified independently and is included in the calculations as a known quantity. Solutions are obtained using four different determinations of the Euler vector of absolute motion of the South American plate, which is taken to define the orientation of the basal shear stress. The results demonstrate that the concept of a basal shear stress actively driving plate motion cannot be discarded on the basis of a dynamical analysis. Further, with the assumption that basal drag is entirely concentrated below the continental pan of the lithosphere, estimates of the maximum amount of continental basal shear resistance are derived. Evaluation of an alternative model for the South American plate, excluding the northernmost pan of the Andes, demonstrates the significance of the resistive force exerted on the South American plate due to convergence with the Caribbean plate. It is to be expected that comparison of theoretical distributions of intraplate stress, computed on the basis of the present dynamical analysis, with observations of the actual state of stress will further constrain the range of feasible force models.

Stress magnitudes in the crust: constraints from stress orientation and relative magnitude data

The World Stress Map Project is a global cooperative effort to compile and interpret data on the orientation and relative magnitudes of the contemporary in situ tectonic stress field in the Earth's lithosphere. Horizontal stress orientations show regionally uniform patterns throughout many continental intraplate regions. These regional intraplate stress fields are consistent over regions 1000-5000 km wide or ca . 100 times the thickness of the upper brittle part of the lithosphere ( ca . 20 km) and about 10-15 times the thickness of typical continental lithosphere ( ca . 150-200 km). Relative stress magnitudes or stress regimes in the lithosphere are inferred from direct in situ stress measurements and from the style of active faulting. The intraplate stress field in both the oceans and continents is largely compressional with one or both of the horizontal stresses greater than the vertical stress. The regionally uniform horizontal intraplate stress orientations are generally consistent with either relative or absolute plate motions indicating that plate-boundary forces dominate the stress distribution within the plates. Since most regions of normal faulting occur in areas of high elevation, the extensional stress régimes in these areas can be attributed to superimposed bouyancy forces related to crustal thickening and/or lithosphere thinning; stresses derived from these bouyancy forces locally exceed mid-plate compressional stresses. Evaluating the effect of viscous drag forces acting on the plates is difficult. Simple driving or resisting drag models (with shear tractions acting parallel or antiparallel to plate motion) are consistent with stress orientation data; however, the large lateral stress gradients across broad plates required to balance these tractions are not observed in the relative stress magnitude data. Current models of stresses due to whole mantle flow inferred from seismic tomography models (and with the inclusion of the effect of high density slabs) predict a general compressional stress state within continents but do not match the broad-scale horizontal stress orientations. The broad regionally uniform intraplate stress orientations are best correlated with compressional plate-boundary forces and the geometry of the plate boundaries.

Stress measurements and tectonic implications for Fennoscandia

The main results from the Fennoscandian Rock Stress Data Base (FRSDB) are presented. Rock stress data from Sweden are now being selected to enter the data base of the World Stress Map Project of the International Lithosphere Programme. It is suggested that ridge push from the Mid-Atlantic Ridge is the major stress-generating mechanism. The scatter in stress magnitude and direction for Fennoscandia is due to the change in the stress field near faults. This is illustrated by means of simple 2-dimensional distinct element analyses of blocky rock masses subjected to a homogeneous far-field stress regime. Earthquakes are assumed to appear at fault intersections where stresses are concentrated.