David Coblentz

ORIGINS OF THE EUROPEAN REGIONAL STRESS FIELD

Abstract 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, SH. 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 SH 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.

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.

Small-scale convection at the edge of the Colorado Plateau: Implications for topography, magmatism, and evolution of Proterozoic lithosphere

The Colorado Plateau of the southwestern United States is characterized by a bowl-shaped high elevation, late Neogene–Quaternary magmatism at its edge, large gradients in seismic wave velocity across its margins, and relatively low lithospheric seismic wave velocities. We explain these observations by edge-driven convection following rehydration of Colorado Plateau lithosphere. A rapidly emplaced Cenozoic step in lithosphere thickness between the Colorado Plateau and adjacent extended Rio Grande rift and Basin and Range province causes small-scale convection in the asthenosphere. A lithospheric drip below the plateau is removing lithosphere material from the edge that is heated and metasomatized, resulting in magmatism. Edgedriven convection also drives margin uplift, giving the plateau its characteristic bowl shape. The edge-driven convection model shows good consistency with features resolved by seismic tomography.

Model for tectonically driven incision of the younger than 6 Ma Grand Canyon

Accurate models for the incision of the Grand Canyon must include characterization of tectonic influences on incision dynamics such as active faulting and mantle to surface fluid interconnections. These young tectonic features support other geologic data that indicate that the Grand Canyon has been carved in the past 6 Ma. New U-Pb dates on speleothems are reinterpreted here in terms of improved geologic constraints and understanding of the modern aquifer. The combined data suggest that Grand Canyon incision rates have been relatively steady since 3–4 Ma. Differences in rates in the eastern (175–250 m/Ma) and western (50–80 m/Ma) Grand Canyon are explained by Neogene fault block uplift across the Toroweap-Hurricane system. Mantle tomography shows an abrupt step in mantle velocities near the Colorado Plateau edge, and geodynamic modeling suggests that upwelling asthenosphere is driving uplift of the Colorado Plateau margin relative to the Basin and Range. Our model for dynamic surface uplift in the past 6 Ma contrasts with the notion of passive incision of the Grand Canyon due solely to river integration and geomorphic response to base-level fall.

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.

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.

Mantle-driven dynamic uplift of the Rocky Mountains and Colorado Plateau and its surface response: Toward a unified hypothesis

The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that significant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6–10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500–1000 m may account for ∼25%–50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

On the gravitational potential of the Earth's lithosphere

The mean potential energy of the lithosphere is useful for defining the tectonic reference state (TRS) of the Earth and can be used to constrain the ambient state of stress in the plates. In the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the TRS would remain undeformed. Thus the difference between the potential energy of a lithospheric column and the TRS determines whether the column is in an extensional, joeutral, or compressional state of stress. We evaluate and intraplate variations about this mean, using a simple, first-order lithospheric density model. This model assumed that the continental geotherm is linear, and density variations below a depth of 125 km have negligible influence on , and is consistent with observed geoid anomalies across continental margins. is estimated to be 2.379 × 1014 N m−1, which is equivalent to the potential energy of both near sea level continental lithosphere (−160 to +220 m for an assumed crustal density, ρc, in the range 2800–2700 kg m−3) and cooling oceanic lithosphere at a depth of 4.3 km. With the exception of Eurasia, which has anomalously high mean potential energy ( = 2.383 × 1014 N m−1), the mean potential energies of the continental plates are nearly identical to the global mean . The mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere. Both the global and plate mean potential energies are relatively insensitive to a wide range in ρc. The potential of the mid-ocean ridges ( ), 2.391 × 1014 N m−1, is greater than the global mean, which is consistent with the divergent nature of the ridges. Elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to , suggesting that in the absence of external forces, continental regions will be in a slightly extensional state of stress. The importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the South American, Nazca, Indo-Australian, and Pacific plates.

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.

Statistical trends in the intraplate stress field

The World Stress Map (WSM) database contains thousands of intraplate stress indicators, with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion. Previous studies, which relied almost exclusively on visual inspection of the data, established the existence of broad regions of uniform maximum horizontal compressive stress orientation (SHmax) and stress regimes on a regional scale. In the present study, we present a statistical analysis of the WSM stress indicators with the aim of quantifying trends in both the SHmax orientations and stress regimes. The analysis was carried out within 5° × 5° bins which provide a resolution of several hundred kilometers. Only the 4537 high-quality WSM indicators with rating of A to C were used in the analysis. We present results for two types of analysis on the information contained within the bins. First, we evaluate the spatial distribution of the average stress regime (normal, strike-slip, or thrust). Second, we apply the Rayleigh test, a standard statistical method in the analysis of directional data, to the distribution of SHmax orientations to test the null hypothesis that the orientations are random. An important aspect of our study is the quantification of the conclusions drawn from visual inspection of the World Stress Map. Our results indicate that broad regions of uniform SHmax orientations exist in most continental regions at high confidence levels (90% and 95%) and are less robust in the slowest moving continental plates. We also quantify the predominance of strike-slip and compressional stress regimes in continental regions. Importantly, our analysis provides information about trends in the SHmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data, for example, in western North America and continental Australia. Furthermore, we find a strong correlation between average SHmax orientations and both the ridge push torque and the absolute plate velocity azimuths. Our observation that a greater number of SHmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force.