Intraplate Stress Research Articles

Intraplate stresses and plate‐driving forces in the Philippine Sea Plate

We use a spherical shell elastic finite element analysis to characterize intraplate stresses and plate‐driving forces in the Philippine Sea Plate. Our finite element mesh is comprised of 489 nodes and 914 elements, and provide a spatial resolution of about 1°. The Philippine Sea Plate is unusual in that it does not have large numbers of seismic or tectonic stress indicators, and there are no midplate borehole stress data which can be used as constraints in this finite element study. However, it does possess a unique boundary configuration which potentially can lend more insight to plate forces and stresses than can similar studies on plates with more seismically active interiors. Because the plate's boundary consists primarily of subduction zones with roughly equal proportions of boundary on the overlying and downgoing sides of trench, the resultant plate stress field is very sensitive to subduction‐related forces. As a result, borehole stress measurements in certain parts of the plate would be useful in further constraining these forces. We find that trench pull forces can explain a large component of the velocity of the Philippine Sea Plate. However, trench pull has somewhat less overall effect on the stresses across the plate than do the lateral density and topographic variations within the plate (i.e., gravitational or potential energy force), and trench suction forces are probably about an order of magnitude smaller than the net trench pull forces. Collisional forces may dominate the stress fields near Taiwan and the Izu Peninsula, but they are of only secondary importance in determining the motion and deformation of the Philippine Sea Plate. The forces that generate stress orientations most closely matching the observed stress indicators produce a lithospheric stress field across the plate that averages about 15 MPa over a 70 km thick lithosphere, scaling inversely with lithosphere thickness. Values up to four times as great are produced locally.

Intraplate stresses and plate-driving forces in the Philippine Sea Plate

Intraplate stresses and plate-driving forces in the Philippine Sea Plate

Mechanical controls on collision-related compressional intraplate deformation

Intraplate compressional features, such as inverted extensional basins, upthrust basement blocks and whole lithospheric folds, play an important role in the structural framework of many cratons. Although compressional intraplate deformation can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the development of the most important compressional intraplate structures. These can occur at distances of up to ±1600 km from a collision front, both in the fore-arc (foreland) and back-arc (hinterland) positions with respect to the subduction system controlling the evolution of the corresponding orogen. Back-arc compression associated with island arcs and Andean-type orogens occurs during periods of increased convergence rates between the subducting and overriding plates. For the build-up of intraplate compressional stresses in fore-arc and foreland domains, four collision-related scenarios are envisaged: (1) during the initiation of a subduction zone along a passive margin or within an oceanic basin; (2) during subduction impediment caused by the arrival of more buoyant crust, such as an oceanic plateau or a microcontinent at a subduction zone; (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism, and convergence rates and directions; (4) during post-collisional over-thickening and uplift of an orogenic wedge. The build-up of collision-related compressional intraplate stresses is indicative for mechanical coupling between an orogenic wedge and its fore- and/or hinterland. Crustal-scale intraplate deformation reflects mechanical coupling at crustal levels whereas lithosphere-scale deformation indicates mechanical coupling at the level of the mantle-lithosphere, probably in response to collisional lithospheric over-thickening of the orogen, slab detachment and the development of a mantle back-stop. The intensity of collisional coupling between an orogen and its fore- and hinterland is temporally and spatially variable. This can be a function of oblique collision. However, the build-up of high pore fluid pressures in subducted sediments may also account for mechanical decoupling of an orogen and its fore- and/or hinterland. Processes governing mechanical coupling/decoupling of orogens and fore- and hinterlands are still poorly understood and require further research. Localization of collision-related compressional intraplate deformations is controlled by spatial and temporal strength variations of the lithosphere in which the thermal regime, the crustal thickness, the pattern of pre-existing crustal and mantle discontinuities, as well as sedimentary loads and their thermal blanketing effect play an important role. The stratigraphic record of collision-related intraplate compressional deformation can contribute to dating of orogenic activity affecting the respective plate margin.

Late Miocene to Recent tectonic evolution of Crete (Greece): geological observations and model analysis

Using a numerical model, we focus on the late Middle Miocene to Recent kinematic evolution of the Cretan segment of the Hellenic Arc. Geological observations of Crete are given a quantitative interpretation in terms of tectonic mechanisms controlling the evolution of the active arc boundary. This was achieved by calculating intra-plate stress fields for various possible distributions of forces and comparing the models with observations. The models address specific questions concerning important changes in the observed horizontal stress patterns. We deal with the question of what caused extension to initiate. Modelling results of the collapse of an earlier formed topography and extensional forces acting on the plate boundary are compared with a newly reconstructed tectonostratigraphy of Crete. Our results suggest that arc-normal pull is the dominant force that generates the Late Miocene extension in the Cretan segment of the overriding plate, although arc-normal pull in combination with intra-plate spreading forces cannot be excluded. The observed transform motions in the Pliny and Strabo trenches led us to incorporate experiments with additional resistance on the eastern (Levantine) segment of the Hellenic Arc. The models performed with a transform resistance along the trenches are in agreement with the Cretan deformation for the Pliocene to Recent period. The stress fields for the short duration compressional periods around the Middle–Late Miocene and the Miocene–Pliocene boundary are modelled by assuming resistive instead of tensional forces acting at the overriding margin.

Post-early Messinian counterclockwise rotations on Crete: implications for Late Miocene to Recent kinematics of the southern Hellenic arc

Most geodynamical models for the kinematics of the central Mediterranean recognise that major tectonic rotations must have played an important role during the Neogene. The Hellenic arc is believed to have been subjected to clockwise rotations in the west and counterclockwise rotations in the east, while the southern part (Crete) shows no rotations ( Kissel and Laj, 1988). Many qualitative and quantitative models are based on the idea that Crete did not rotate. We present new palaeomagnetic data which show that post-early Messinian counterclockwise rotations have occurred on Crete. The amount of counterclockwise rotation generally varies between 10° and 20°, but in central Crete much larger rotations (up to ∼40° counterclockwise) were found. Only a few sections did not show any rotation. The anisotropy of magnetic susceptibility (AMS) shows lineations, which are consistently WNW–ESE throughout Crete, indicating post-rotational WNW–ESE extension, or NNE–SSE compression. The observed counterclockwise rotations are consistent with the results of tectonic modelling by Ten Veen and Meijer (1998). The latter study compares the late-Middle Miocene to Recent kinematics with modelled intra-plate stresses for various possible distributions of plate boundary forces. Observations reveal that motion along left-lateral and right-lateral faults occurred during the Pliocene. The model analysis shows these motions to be consistent with transform resistance along the eastern segment of the overriding margin. The counterclockwise block rotations observed by us are probably a consequence of displacements along the left-lateral and right-lateral faults and could reflect a similar tectonic regime that involved transform resistance.

Recent intraplate stress field in the eastern Duero Basin (N Spain)

Palaeostresses inferred from brittle mesostructures in the eastern Duero Basin show a recent stress field characterized by an extensional regime, with local strike‐slip and compressional stress states. Orientations of the maximum horizontal stress ( SHmax) show a relative scattering with two main modes: NNE to NE–SW and NW–SE. These orientations suggest the existence of two stress sources responsible for the dominant directions of the maximum horizontal stress in northeastern Iberia. Extensional structures within a broad‐scale compressional stress field can be related to both the decrease in relative stress magnitudes from active margins to intraplate regions and rifting proccesses ocurring in eastern Iberia. Stress states with NW–SE‐trending SHmax are compatible with the dominant pattern established for western Europe. NE–SW orientations of SHmax suggest the occurrence of tectonic forces coming from the Pyrenean zone. Geological and geophysical data indicate the existence of both orientations from the upper Miocene to the present‐day in NE Iberia.

Collisional intraplate deformation

Intraplate compressional structures, such as inverted extensional basins, upthrusted basement blocks and whole lithospheric folds, play an important role in the tectonic framework of many cratons. Although compressional intraplate structures can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the most important compressional intraplate deformations. Such structures can occur at distances of up to 1600 km from a collision front, both in back-arc (hinterland) and fore-arc (foreland) positions with respect to the subduction system controlling the respective orogen. The stratigraphic record of collision-related intraplate compressional deformation can contribute to the dating of orogenic activity affecting the respective plate margin. Four collision-related scenarios are envisaged for the build-up of intraplate compressional stresses: (1) during the initiation of subduction zones along passive margins or within oceanic basins, (2) during impedance of subduction processes caused by the arrival of an oceanic plateau, transform ridge or microcontinent at a subduction zone, (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism and convergence rates and direction, (4) during post-collisional over-thickening of an orogenic wedge. Crustal-scale foreland deformation reflects mechanical coupling between the orogenic wedge and its foreland at crustal levels; build-up of high fluid pressures in subducted sediments may account for their decoupling. Lithosphere-scale foreland deformation reflects mechanical coupling between the orogenic wedge and its forelands at the level of the mantle-lithosphere, possibly related to collisional lithospheric over-thickening and the development of mantle back-stops. Processes governing mechanical coupling/decoupling of an orogen and its forelands are still poorly understood.

Focal mechanisms of small earthquakes in the southeastern Brazilian shield: a test of stress models of the South American plate

Focal mechanisms of small earthquakes with magnitudes of about 3 in the SE Brazilian shield are calculated using S/P amplitude ratios. Low attenuation (Qp from 400 to 800) in the shield upper-crustal layers allowed sharp S arrivals to be recorded up to distances of 100km. Besides P-wave polarities, SH-wave first motions were also used to constrain the nodal-plane orientations. Normal and reverse faulting mechanisms with strike-slip components were found. The inversion of four mechanisms to estimate the stress tensor indicated a strike-slip stress regime with roughly E–W-orientated σ1 and N–S σ3. Both the orientations and the shape factor (φ=0.7) of the inverted stress are in excellent agreement with theoretical predictions for that part of Brazil from the driving-force model of Coblentz & Richardson (1996). Good agreement with the nature of the stress, as well as its orientation, was also found for the model of Meijer (1995). Both of these theoretical models include spreading stresses along the continent/ocean lithospheric transition. Because the earthquakes are more than 300km from the continental shelf they should not be affected by the local flexural forces caused by sediment load in the marginal basins. The agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses.

The dynamics of Cenozoic and Mesozoic plate motions

Our understanding of the dynamics of plate motions is based almost entirely upon modeling of present‐day plate motions. A fuller understanding, however, can be derived from consideration of the history of plate motions. Here we investigate the kinematics of the last 120 Myr of plate motions and the dynamics of Cenozoic motions, paying special attention to changes in the character of plate motions and plate‐driving forces. We analyze the partitioning of the observed surface velocity field into toroidal (transform/spin) and poloidal (spreading/subduction) motions. The present‐day field is not equipartitioned in poloidal and toroidal components; toroidal motions account for only one third of the total. The toroidal/poloidal ratio has changed substantially in the last 120 Myr with poloidal motion decreasing significantly after 43 Ma while toroidal motion remains essentially constant; this result is not explained by changes in plate geometry alone. We develop a self‐consistent model of plate motions by (1) constructing a straightforward model of mantle density heterogeneity based largely upon subduction history and then (2) calculating the induced plate motions for each stage of the Cenozoic. The “slab” heterogeneity model compares rather well with seismic heterogeneity models, especially away from the thermochemical boundary layers near the surface and core‐mantle boundary. The slab model predicts the observed geoid extremely well, although comparison between predicted and observed dynamic topography is ambiguous. The midmantle heterogeneities that explain much of the observed seismic heterogeneity and geoid are derived largely from late Mesozoic and early Cenozoic subduction, when subduction rates were much higher than they are at present. The plate motion model itself successfully predicts Cenozoic plate motions (global correlations of 0.7–0.9) for mantle viscosity structures that are consistent with a variety of geophysical studies. We conclude that the main plate‐driving forces come from subducted slabs (>90%), with forces due to lithospheric effects (e.g., oceanic plate thickening) providing a very minor component (<10%). For whole mantle convection, most of the slab buoyancy forces are derived from lower mantle slabs. Unfortunately, we cannot reproduce the toroidal/poloidal partitioning ratios observed for the Cenozoic, nor do our models explain apparently sudden plate motion changes that define stage boundaries. The most conspicuous failure is our inability to reproduce the westward jerk of the Pacific plate at 43 Ma implied by the great bend in the Hawaiian‐Emperor seamount chain. Our model permits an interesting test of the hypothesis that the collision of India with Asia may have caused the Hawaiian‐Emperor bend. However, we find that this collision has no effect on the motion of the Pacific plate, implying that important plate boundary effects are missing in our models. Future progress in understanding global plate motions requires (1) more complete plate reconstruction information, including, especially, uncertainty estimates for past plate boundaries, (2) better treatment of plate boundary fault mechanics in plate motion models, (3) application of numerical convection models, constrained by global plate motion histories, to replace ad hoc mantle heterogeneity models, (4) better calibration of these heterogeneity models with seismic heterogeneity constraints, and (5) more comprehensive comparison of global plate/mantle dynamics models with geologic data, especially indicators of intraplate stress and strain, and constraints on dynamic topography derived from the stratigraphic record of sea level change.

Seismicity and stresses in the Brazilian passive margin

Abstract Seismicity and stresses along the Brazilian continental margin show different patterns between the northeastern and southeastern regions. In the northeastern margin, earthquakes tend to occur onshore under a strike-slip regime with horizontal compression parallel to the northern coast line. In the southeastern margin, higher seismicity is observed offshore, in areas where the continental crust was highly extended during the South Atlantic rifting in the Mesozoic. Three new events in the southeast continental shelf are shown to have reverse-faulting mechanisms, from waveform modeling of short-period teleseismic P waves. Multiple depth phases (reflections from the top and bottom of the water layer and double reflections in the water layer) could be identified and better constrained the hypocenters to middle and upper crustal depths. In South America, models of intraplate stresses caused by plate boundary forces and spreading effects due to the continental/oceanic crustal transition indicate higher compressional stresses in the SE offshore area, as compared with the continental area, in agreement with the observed higher seismicity and reverse-faulting mechanisms. The combination of regional stresses, local flexural effects from thick sedimentary loads, and a presumably weaker crust from Mesozoic thinning explains the main patterns of seismicity in the northeastern and southeastern Brazilian margins.

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.

Analysis of erosion events and palaeogeothermal gradients in the North Alpine Foreland Basin of Switzerland

Abstract For basin modelling purposes, uplift and erosion analysis is as important as subsidence analysis. Within the North Alpine Foreland Basin of Switzerland (NAFB) three major interregional unconformities separate the sedimentary succession: the late Palaeozoic, the base Tertiary and the base Quaternary unconformity. The amount of missing section depends on the palaeogeographic basin position and on different inversion mechanisms (thermal uplift, intraplate stress, tectonic uplift or isostatic rebound movements). Several techniques exist to determine the magnitude and timing of maximum palaeotemperatures and palaeogradients, facilitating direct estimation of missing section. In our study coalification and fission track data are analysed and compared with recently published models concerning erosion in the NAFB. The results indicate that each inversion episode is accompanied by significant erosion (up to several km). The following range of removed section values were reconstructed: (i) late Palaeozoic unconformity: 1000–1200 m; (ii) base Tertiary unconformity: 800–1800 m; (iii) base Quaternary unconformity: 1500–3000 m in the SW, up to 700 m in the NE and 4100–4400 m in parts close to the Alpine front. The reconstruction techniques provide only rough estimates as each approach is based on certain assumptions and calibration uncertainties. Also, the obtained values are higher than those expected from simple evaluation of the preserved stratigraphic record. Calculated palaeogeothermal gradients are very high (80–90 °C km −1 ) during the late Palaeozoic, slightly higher than normal during the late Cretaceous (30–50 °C km −1 ) and lower than normal (15–30 °C km −1 ) during the Cenozoic.

A new class of transform plate boundary

The theory of plate tectonics postulates that the relative motion between two neighboring plates occurs along three types of boundaries: divergent (spreading center, rift), convergent (subduction, collision), and horizontal (transform). Because the theory assumes rigid behavior of plates, transform plate boundaries must lie along small circles around the pole of rotation of relative motion between two neighboring plates. However, global models of current plate motion (e.g., NUVEL-1A) show that several boundaries with significant horizontal motion (i.e., the Dead Sea Fault and the Eastern Andean Frontal Fault Zone) do not lie along small circles but rather intersect the circles at 45°. The orientation of these faults can be explained by a new theory of intraplate tectonics, which predicts the first-order intraplate stress field in terms of small circles, great circles, and spiral lines that intersect both sets of circles at 45°. According to the theory, these transform faults are situated along the 45° spiral lines and follow the direction of maximum horizontal shear stress. The theory also predicts that the direction of interseismic relative plate motion between the two plates should be oriented at 45° to the transform plate boundary; this prediction can be tested within a few years using space geodesy. The alignment of these faults along spiral lines is explained by the theory's predicted stress field and a plasticity (von Mises) yield stress criterion for earthquake rupture. It is suggested that these faults represent a new class of transform plate boundary between large deformable plates and not between rigid sub-plates, as formerly postulated.

Paleostress reconstructions and geodynamics of the Baikal region, Central Asia, Part 2. Cenozoic rifting

Investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses. It appears that changes of stress field with time are an important factor that either controls or results from the rifting process. The object of this paper is to study the changes of fault kinematics and paleostress field with time in the Baikal Rift System during the Cenozoic. Reduced paleostress tensors were determined by inversion from fault-slip data measured in the central part of the rift and its southwestern termination, between 1991 and 1995. Results show that the stress field varies as well in time as in space. Two major paleostress stages are determined, corresponding broadly to the classical stages of rift evolution: Late Oligocene-Early Pliocene and Late Pliocene-Quaternary. The first paleostress stage is related to the rift initiation and the second to the major stage of rift development. Similarities between the recent paleostress field and the present-day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active. Therefore, we propose to use ‘proto rift’ for the Late Oligocene-Early Pliocene stage and ‘active rift’ for the Late Pliocene-Quaternary stage of rift development. During the ‘proto rift’ stage, the stress field was characterized by a compressional to strike-slip regime. A progressive change from transpression to transtension is suspected for the central part of the rift (Baikal and Barguzin basins) during this period. In the western termination of the rift (Sayan Massif, Tunka depression), a strongly compressional stress field with oblique thrusting kinematics is well constrained in the Late Miocene-Early Pliocene interval. The ‘active rift’ stage was initiated by a marked change in fault kinematics and stress regime in the Late Pliocene. In the central part of the rift, the stress regime changed into pure extension, while in the southwestern extremity, it changed into pure strike-slip. Fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far-field compressional stress on a mechanically heterogeneous crust, with the southwards-pointing wedge of the Siberian Craton acting as a passive indentor. The Cenozoic time-space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere, on intraplate stresses generated by the India-Eurasia convergence and the West-Pacific subduction.

Stress orientations in Brazilian sedimentary basins from breakout analysis: implications for force models in the South American plate

Regional patterns of crustal stresses in Brazil were studied with a detailed breakout analysis performed in 541 wells distributed throughout the country: 481 from basins along the continental margin, and 60 from intracratonic basins. A total of about 591 km of cumulative well length, digitally sampled at 0.15 m intervals, was analysed. Most intervals (80 per cent) are confined between −400 and −4000 m. Only wells that deviate less than 10° from the vertical were considered. For a given well (1) the mean orientation and the standard deviation of each interval with breakouts were calculated, (2) all intervals with mean breakout orientations less than 10° from the orientation of the hole deviation were discarded, (3) all intervals with standard deviations less than 12° were selected, (4) the selected breakouts were weighted by their interval length to calculate the mean orientation and standard deviation of the whole well, and (5) the final results were classified according to the World Stress Map criteria. Thus, 16 wells were classified as A (a total of 7.2 km of breakouts), 42 as B (7.1 km), 78 as C (7.4 km), 205 as D (3.3 km), 184 as E (0.7 km) and 16 were discarded. In most basins, the breakout orientations from different wells were usually consistent for qualities A, B and C, allowing a good estimate of the regional maximum horizontal stress (SHmax); wells classified as quality D showed a large scatter. The regional SHmax determined from breakouts is generally in good agreement with the available nearby focal mechanisms. The main trends of SHmax are (1) NW-SE, parallel to the coast, along the equatorial marginal basins, (2) E-W in the Alagoas Basin, (3) NNE-SSW, parallel to the coast, in the Sergipe and Reconcavo basins, and (4) NW-SE in the Middle Amazon Basin. In the equatorial and eastern continental margins, north of 15° S, breakouts and focal mechanism measurements indicate that the SHmax orientation is remarkably parallel to the coastline, following a 90° bend of the coast in northeastern Brazil. Because the theoretically predicted intraplate stresses in northern Brazil trend about WNW to ENE (e.g. Meijer 1995; Coblentz & Richardson 1996), we interpret our observations as indicating that local sources of stress at the continental margins (e.g. flexural stresses and lateral density contrasts) dominate the plate-wide stresses and may have been underestimated in theoretical stress models of the South American plate. The new stress data (136 A-C quality out of a population of 541 wells) cover a region of mid-plate South America where only 23 data points had been compiled previously in the World Stress Map database. The more detailed observed patterns of the intraplate stress field should be helpful in better constraining future models of the forces driving the South American plate.

Intraplate stress distribution induced by topography and crustal density heterogeneities beneath the Killari, India, region

The Deccan Traps covered region of the Indian peninsular shield experienced a devastating earthquake of magnitude 6.3(mb&) on September 29, 1993, with the epicenter (18.07°N, 76.45°E) near Killari(Latur). The event had a focal depth of 7 km, and its teleseismic moment tensor analysis suggests reverse faulting characterized by 120°strike, 46°dip, and 100°rake. The fault plane solution of the rnainshock also correlates the reverse faulting with NW‐SE trending nodal plane. In order to gain more insight into understanding the causative factors, the three‐dimensional intraplate lithospheric stresses associated with the topography and subsurface mass hetrogeneities are estimated up to a depth of 15 km beneath a region surrounding the epicenter and lying between 17°–20°N and 75°–78°E. A two‐layered lithospheric model with irregular interface of small amplitude at the Moho has been used for elastostatic stress calculations. The plate boundary force induced stresses are superimposed on the computed stress regime to assess their contribution vis‐a‐vis local causes. The estimated local stresses alone show a subdued northeasterly extensional regime at hypocentral depth. However, the superimposition of an assumed 30 MPa ridge push trending N10°E leads to an enhanced concentration of total stresses beneath the region near the hypocenter. Its maximum value at Killari is 31 MPa at 2 km depth, and it reduces to 29 MPa at 15 km depth. The total deviatoric stresses are also found maximum near Killari. Analysis of computed stresses indicates that the orientation of total principal stresses favors a strike‐slip movement at the focal depth with a NE trending maximum compression. The shear component of these stresses when resolved on the fault plane of the Killari mainshock [U.S. Geological Survey, 1993], however, suggests a thrust movement at 7 km depth beneath the Killari region. This study thus suggests that concentration of total stresses along some kind of crustal weak zones beneath the epicentral region could be an important factor for the reactivation of faults and therefore the Killari earthquake.

Cutting of the European continental lithosphere: Plasticity theory applied to the present Alpine collision

Presently active European Alpine collision has a history of more than 100 Myr convergence. We investigate the collisional contact problem of an apparently rigid African promontory (currently the Italy/Adria block (IAB)) steadily penetrating at 1 cm/yr, over at least the past 45 Myr, into the deformable European plate. We assume that neither continental subduction nor delamination occurs presently in the 300‐km‐wide contact zone enabling use of the indenter‐indentee approach. A key parameter is the aspect ratio h/a, defined by the width of the deformable foreland over the width of the indenter itself. This parameter controls the activation of two fundamentally different modes of deformation: (1) classical indentation mode for h/a≥4.4, described by previous authors, and having effect only in a portion of the deformable plate in the vicinity of the indenter and (2) cutting mode for h/a<4.4 giving plane strain longitudinal splitting of the entire deformable plate and plastic separation of the two halves. We use a matrix operator method to solve for slip lines in the mixed boundary value problem (obliquity, velocity, friction, stress) of the European Alpine case. Implications are as follows: (1) plane stress, vertical lithospheric thickening is the first indentation mode of continental collision; and (2) for the present ratio of h/a≈3 deformation is in the cutting mode and slow plane strain intraplate extension (rifting) is predicted. This mode explains the intraplate stress field obtained from borehole and earthquake data. Considering that the paleoindenter probably has been larger than the IAB it is inferred that h/a has been below 4.4 since at least the end of the Eocene and that consequently the intraplate graben system (Lower Rhine, Rhine, Bresse, Limagne) is due to the cutting of the European plate by Alpine collision.

Mechanical stability of the Redbank Thrust Zone, Central Australia: Dynamic and rheological implications

Deep seismic reflection profiling and other studies indicate that the Moho has been significantly displaced by intracratonic deformation on the Redbank Thrust Zone in central Australia. A two‐dimensional finite‐element model of the thick‐skinned crustal structure of the Redbank Thrust Zone shows that the gravitational stresses generated by the distribution of crustal masses are in agreement with the tectonic and seismic quiescence of the area. Rock failure is concentrated in the downthrusted, and therefore Theologically weakened, crustal segments. Conversely, the upthrusted slices of lower crust and upper mantle undergo little deformation and no failure and thus constitute a mechanically strong whole‐crustal core. The presence of north‐south oriented intraplate stresses derived from far‐field plate boundary forces does not contribute to the inferred mechanical equilibrium of the crust in central Australia. The results may have implications for the mechanical stability of intracratonic thick‐skinned crustal structures elsewhere, and for the long‐term strength of continental lithosphere in general. The models, however, do predict the possibility of intraplate seismicity in response to small changes in tectonic stress, since parts of the crust are already at or close to the failure limit.

Forces controlling the present-day state of stress in the Andes

The present-day state of stress in the Andes is expected to be controlled primarily by two different types of forces: (1) the resistive force exerted on the western overriding margin of the South American plate, and (2) forces that arise from the thickened crust of the Andes (i.e., the effect of topography and its compensating crustal root). We have studied these forces on the basis of a model for the dynamics of the entire South American plate in which the Andes are embedded. In this model a given set of forces is constrained by the criterion that the net torque on the plate should vanish. A thin elastic shell representation is used to calculate the intra-plate stress field associated with the various force distributions. We define a reference model for the present study that incorporates a uniform magnitude for the resistance associated with convergence along the western plate margin (F pcr, plate contact resistance) and does not include the effects of topography. Subsequently, we investigate the effect of lateral variation in the magnitude of F pcr and add the topography-related forces. The main results are: (1) A uniform magnitude of F pcr leads to a better match with the observations than a magnitude that is a function of the dip of the lithosphere subducting below the western plate margin. (2) The amount of horizontal compression across the Andes, found in the case that ridge push is considered to be the only force driving the South American plate, is small compared to the value required to “sustain” the Andes.

Modelling the contemporary stress field and its implications for hydrocarbon exploration

The forces that act on the Earth’s 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.