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.

/pdf/first-and-second-order-patterns-of-stress-in-the-lithosphere-595hs86ljh.pdf

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

Global crustal stress pattern based on the World Stress Map database release 2008

Dynamics of intra-plate compressional deformation: the Alpine foreland and other examples

The Atlantic margin of the Norwegian, Faeroese, British and Irish sectors encompasses numerous basins which vary in character, but are related in terms of their evolution as part of a single passive margin. Lineament analysis of the margin shows a predominance of NE–SW, N–S and NW–SE trends, mainly reflecting Mesozoic–Cenozoic extensional faulting. Some major Precambrian and Caledonian structures, principally steeply-dipping shears, were opportunistically reactivated according to the prevalent stress pattern. The extensional history of the margin spanned a c. 350 Ma interval between the close of the Caledonian orogeny and early Eocene break-up. Episodes of Permo-Triassic, (mainly late) Jurassic, Early Cretaceous, ‘middle’ Cretaceous and latest Cretaceous–Early Eocene age can be distinguished from one another in space and time. The anomalous length of the total period of extension prior to continental separation is partly explained by step-wise lateral offsets of the crustal thinning axes towards the line of eventual break-up. The picture is, however, complicated by some changes in extensional style and direction. These include mosaic-like fragmentation of Pangea in the Permo-Triassic, the imposition of more systematic E–W extension by Jurassic times, and the change to NW–SE extension focused on the present margin in the Early Cretaceous (probably Hauterivian). The resulting structural configuration reflects the overprinting of a complex network of Jurassic and older basins by a continuous NE–SW chain of deep Cretaceous-Cenozoic basins. An extensional pulse of latest Cretaceous to earliest Eocene age (best observed in the Norwegian Sea) with extensive basaltic volcanism led to continental break-up at approximately 53 Ma. The margin was structurally modified by some important events postdating the Early Eocene. On breakup, the background stress field changed from extension to mild SE-directed compression, and widespread inversion structures formed in the thick Cretaceous-Cenozoic depocentres. The inversions can best be explained by ridge-push from the adjacent spreading centres, but could also be linked to Tethyan closure events and changes in the North Atlantic spreading vector. Post-break-up extension of the North Atlantic passive margins has been reported in the western Barents Sea, Jan Mayen and East Greenland and (for the first time here) in the northern Voring Basin. We propose that these areas were linked by a single extensional pulse induced by the change to a more ESE-directed relative plate motion in the Oligocene-Miocene. Major uplift and exhumation of peripheral landmasses and inboard basins took place at intervals throughout the Cenozoic. Initial uplift can be attributed to pre-break-up rifting and post-break-up compression, but the most significant event took place in the Plio-Pleistocene and was intimately associated with glacial erosion and isostatic adjustment through repeated glaciations and interglacials. The regional scale of this event and its significance for exploration is widely under-estimated.

Principal tectonic events in the evolution of the northwest European Atlantic margin

A fluid injection-induced seismicity experiment was conducted in the German Continental Deep Drilling Program (KTB) main borehole at 91 km depth (in situ temperature of 260°C) to extend knowledge about stress magnitudes and brittle faulting to depths and temperatures approaching the brittle-ductile transition Almost 400 microearthquakes were induced at an average depth of 88 km by injection of KBr/KCl brine into a ∼70 m open hole section near the bottom of the borehole Although most focal plane mechanisms were poorly constrained due to the very small size of the induced earthquakes, several different clusters of microearthquakes with distinct mechanisms were defined Most of the microearthquakes for which focal plane mechanisms were determined were strike-slip events with a NNW trending P axis, essentially parallel to the direction of maximum horizontal compression observed in the borehole The largest induced earthquake, M 12, occurred 18 hours after injection was started This event was a strike-slip/reverse faulting event which also had a NNW trending P axis Utilization of a precise relative location technique indicates that many of microearthquakes occurred relatively far (50–100 m) from the well bore Modeling of the pore pressure disturbance caused by injection suggests that many of the earthquakes were induced by extremely small pore pressure perturbations (<1 MPa) less than 1% greater than the ambient, approximately hydrostatic pore pressure at depth Thus it is apparent that there are critically stressed, permeable fault zones in the crust, even at great depth and temperature A frictional analysis of the focal plane mechanisms of the induced microearthquakes indicates that fault slip is consistent with the stress magnitudes and orientations determined in situ at depths to 77 km in the borehole and relatively high coefficients of friction (∼06–07) reported by Brudy et al [this issue] This and the observation that very small pore pressure perturbations were able to trigger seismicity appear to confirm the hypothesis that “Byerlee's law” (ie, that differential stresses in situ are limited by the frictional strength of well-oriented, preexisting faults) is valid to great crustal depth and that the crust is in brittle failure equilibrium at depths and temperatures approaching the brittle-ductile transition, even in this relatively stable intraplate area

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JB02814

Injection-induced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site, Germany

Nearly 1500 stress orientation determinations are now available for Europe. The data come from earthquake focal mechanisms, overcoring measurements, well bore breakouts, hydraulic fracturing measurements, and young fault slip studies and sample the stress field from the surface to seismogenic depths. Three distinct regional patterns of maximum compressive horizontal stress (SHmax) orientation can be defined from these data: a consistent NW to NNW SHmax stress orientation in western Europe; a WNW-ESE SHmax orientation in Scandinavia, similar to western Europe but with a larger variability of SHmax orientations; and a consistent E-W SHmax orientation and N-S extension in the Aegean Sea and western Anatolia. The different stress fields can be attributed to plate-driving forces acting on the boundaries of the Eurasian plate, locally modified by lithospheric properties in different regions. On average, the orientation of maximum stress in western Europe is subparallel to the direction of relative plate motion between Africa and Europe and is rotated 17° clockwise from the direction of absolute plate motion. The uniformly oriented stress field in western Europe coincides with thin to medium lithospheric thickness (approximately 50–90 km) and high heat flow values (>80 m W/m2). In western Europe a predominance of strike-slip focal mechanisms implies that the intermediate principal stress is vertical. The more irregular horizontal stress orientations in Scandinavia coincide with thick continental lithosphere (110–170 km) and low heat flow (<50 m W/m2). The cold thick lithosphere in this region may result in lower mean stresses associated with far-field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography. The stress field of the Aegean Sea and western Anatolia is consistent with N-S extension in a back arc setting behind the Hellenic trench subduction zone. The stress field is influenced in places by regional geologic structures, e.g., in the Western Alps, where SHmax directions show a slight tendency toward a radial stress pattern. Not all major geologic structures, however, appear to affect the SHmax orientation, e.g., in the vicinity of the Rhine rift system horizontal stress orientations are continuous.

Regional patterns of tectonic stress in Europe

Stress measurements and tectonic implications for Fennoscandia

New developments in hydrofracturing stress measurements at Luleå university of technology

Estimation of virgin rock stresses from horizontal hydrofractures

Sleeve fracturing : a borehole technique for in-situ determination of rock demformability and rock stresses

Christer Ljunggren

Papers

Regional patterns of tectonic stress in Europe

Stress measurements and tectonic implications for Fennoscandia

New developments in hydrofracturing stress measurements at Luleå university of technology

Estimation of virgin rock stresses from horizontal hydrofractures

Sleeve Fracturing - A Borehole Technique For In-situ Determination of Rock Deformability And Rock Stresses