Richard W. Allmendinger

Andean tectonics related to geometry of subducted Nazca plate

Seismological and geological data show that tectonic segmentation of the Andes coincides with segmentation of the subducted Nazca plate, which has nearly horizontal segments and 30° east-dipping segments. Andean tectonics above a flat-subducting segment between 28°S to 33°S are characterized by (from west to east): (1) a steady topographic rise from the coast to the crest of the Andes; (2) no significant Quaternary, and possibly Neogene, magmatism; (3) a narrow belt of eastward-migrating, apparently thin-skinned, Neogene to Quaternary shortening of the Andes; and (4) Plio-Pleistocene uplift of the crystalline basement on reverse faults in the Pampeanas Ranges. From about 15° to 24°S, over a 30°-dipping subducted plate, a west to east Andes cross section includes: (1) a longitudinal valley east of coastal mountains; (2) an active Neogene and Holocene andesitic volcanic axis; (3) the Altiplano-Puna high plateau; (4) a high Neogene but inactive thrust belt (Eastern Cordillera); and (5) an active eastward-migrating Subandean thin-skinned thrust belt. Tectonics above a steeply subducting segment south of 33°S are similar west of the volcanic axis, but quite different to the east. Early Cenozoic tectonics of western North America were quite similar to the Neogene Andes. However, duration of segmentation was longer and the width of deformation was greater in the western United States. Patterns of crustal seismicity are systematically related to Plio-Quaternary structural provinces, implying that current deformational processes have persisted since at least the Pliocene. Horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench. Above flat-subducting segments, crustal seismicity occurs over a broad region, whereas over steep segments, it is confined to the narrow thrust belt. Strain patterns in the forearc region are complex and perhaps extensional, and a broad region of the Altiplano-Puna and Eastern Cordillera appears to be aseismic.

Kinematic analysis of fault-slip data

Abstract An array of graphical and numerical techniques facilitate qualitative and quantitative kinematic analysis of fault-slip data. Graphical contouring and Bingham statistics of the shortening and extension axes for kinematically scale-invariant faults characterize the distributions and orientations of the principal axes of average incremental strain. Numerical analysis by means of moment tensor summation yields the orientations and magnitudes of the principal strain axes as well as rotational information. Field data can be weighted for moment tensor summation using measurements of fault gouge thickness and/or fault plane width, from which average displacement and fault area can be estimated. The greatest uncertainties of kinematic analysis derive from assumptions about the weighting of the data, the effects of post-faulting rotation on the data, the degree to which sampling is representative of the entire fault population, and the spatial homogeneity of strain. These assumptions can be evaluated for a specific data set. Geometric criteria can distinguish the kinematic heterogeneities produced by triaxial deformation, anisotropy reactivation, strain compatibility constraints and/or multiple deformations. Strain compatibility, material anisotropy and heterogeneity may be characterized by integrating the results of kinematic and dynamic fault-slip analyses.

Inverse and forward numerical modeling of trishear fault‐propagation folds

Fault-propagation folds commonly display footwall synclines as well as changes in stratigraphic thickness and dip on their forelimbs, features that cannot easily be explained by simple parallel kink fold kinematics. An alternative kinematic model, trishear, can explain these observations, as well as a variety of other features which have long intrigued structural geologists. Trishear has received little attention until recently, in part because it must be applied numerically rather than graphically. A new computer program has been developed to analyze trishear and hybrid trishear-fault-bend fold deformation. Trishear fold shape can vary considerably by changing the apical angle of the trishear zone and/or the propagation to slip ratio (P/S) during the evolution of the structure. Breakouts, anticlinal and synclinal ramps, and inversion structures can also be modeled, tracking the kinematics with growth strata. Strain within trishear zones can be used to predict fracture orientations throughout the structures as demonstrated by comparison with analog clay models. Also presented is a method for inverting data on real structures for a best fit trishear model by performing a grid search over a six-parameter space (ramp angle, trishear apical angle, displacement, P/S, and X and Y positions of the fault tip line). The inversion is performed by restoring a key bed to a planar orientation by least squares regression. Because trishear provides a bulk kinematic description of a deforming zone, it is complementary to, rather than competing with, other kinematic models.

Estimates of strain due to brittle faulting: sampling of fault populations

Abstract The geometry of sampling domains is a first-order consideration in the characterization of brittle fault populations. In most cases, descriptions of fault size distributions based on map, cross-section, traverse or borehole data systematically underestimate the number of small faults present in a volume. The geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length. Estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process.

Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah, from COCORP seismic-reflection data

COCORP seismic-reflection data collected from the eastern Basin and Range in west-central Utah provide information on Cenozoic extensional tectonics, Mesozoic thrusting, and their interrelationships. Those data show a series of remarkably continuous, low-angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15–20 km. Over that distance, none of these events are significantly cut by any high-angle normal faults. A major detachment beneath the Sevier Desert can be traced from a surface zone of normal faulting to a depth of 12–15 km, with a regional apparent westward dip of 12°. Tentative correlation of upper- and lower-plate events suggests 30–60 km of extensional displacement on this detachment. Whether this structure is a reactivated Mesozoic thrust is uncertain. West-steepening splays off the end of the detachment reach depths of 20 km and may represent a major Mesozoic ramp or zones of distributed ductile shearing during extension. Some events are interpreted to be Mesozoic thrusts, of which at least one (beneath the House Range) has been reactivated during the Cenozoic. The Snake Range decollement dips gently east and has a sense of Cenozoic displacement opposite to that of other Cenozoic detachments farther east. Deep events are most numerous beneath the east side of the Sevier Desert where they occur to depths of 30 km, at the top of or perhaps partly within the anomalously low velocity upper mantle.

Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, northwestern Argentine Andes

Abstract Kinematic analysis of ∼ 1500 fault-slip measurements from the Puna plateau and adjacent foreland of northwestern Argentina suggests that two regional kinematic regimes characterize late Cenozoic deformation: a thrust phase with ∼WNW-ESE shortening and subvertical extension followed by a strike-slip phase with ∼ENE-WSW shortening and ∼NNW-SSE extension. Radiometric dating combined with field relationships demonstrate that thrust faulting started by 13 Ma and lasted, at least locally, until 1 Ma, and that strike-slip faulting started by 2 Ma and is still active. The shortening direction of the thrust phase, which accounts for most of the Andean shortening, differs from the coeval plate tectonic convergence direction and probably cannot be explained by later oroclinal bending. Paleostructural control of deformation kinematics and/or strike-slip faulting along the thermally weakened volcanic arc might explain the Mio-Pliocene shortening direction. The subhorizontal extension direction of the strike-slip phase is evident at all elevations studied, suggesting that local body forces do not drive it. A decrease in South America-Nazca plate convergence rate and/or complex three-dimensional effects, possibly including kinematic variation with depth in the crust, might provide a satisfactory explanation.

Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt

The COCORP 40°N Transect of the Cordillera of the western United States crosses tectonic features ranging in age from Proterozoic to Recent and provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion. The key features of the transect, centered on the Basin and Range Province, include (1) asymmetric seismic fabrics in the Basin and Range, including west-dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west; (2) a pronounced reflection Moho at 30 ± 2 km and locally as deep as 34 km in the Basin and Range with no clear sub-Mono reflections; and (3) complex-dipping reflections and diffractions locally as deep as 48 km in the Colorado Plateau and Sierra Nevada. The eastern part of the transect, shot above known and inferred Precambrian crystalline basement, probably records features related to the entire history of the orogen, locally perhaps as old as 1800 Ma. In this region, major paleotectonic features probably controlled subsequent structural development. In title western half of the transect, however, most reflectors are probably no older than Mesozoic. Within the Basin and Range Province, there appears to be a strong Cenozoic overprint that is characterized by asymmetric half-grabens, low-angle normal faults, and a pervasive subhorizontal system of reflections in the lower crust; no one model of intracontinental extension is universally applicable. Processes that produce or are accompanied by thermal anomalies (magmatism, enhanced ductility, and extension) appear to be essential in developing a highly layered lower crust.

Inversion tectonics and the evolution of the High Atlas Mountains, Morocco, based on a geological-geophysical transect

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 1998, AGU. See also: http://www.agu.org/pubs/crossref/1999/1998TC900015.shtml; http://atlas.geo.cornell.edu/morocco/publications/beauchamp1999.htm

Andean reactivation of the Cretaceous Salta rift, northwestern Argentina

Abstract Throughout the Andes, foreland geometries are correlated with the orientation of the subducting Nazca Plate: fold-and-thrust belts with steep subduction and basement uplifts with flat. The geometries observed in the southern Cordillera Oriental and northern Sierras Pampeanas do not fit this pattern. Instead, inversion of the Cretaceous Salta Rift Basin and mechanical differences between rift and non-rift domains are proposed as the primary controls on both the timing of late Tertiary uplift and deformation, and foreland geometries. The influence of the rift basin is documented through field observations of structures and lithologies, kinematic analysis of minor fault data, and published data on local stratigraphy. The southern Cordillera Oriental developed within the southwestern subbasin of the Salta rift and is a basement-involved fold-and-thrust belt. The Sierras Pampeanas developed to the south of the rift and are basement uplifts. Dominant structures in both regions are N/S-trending reverse or thrust faults. They are cut by oblique strike-slip faults. Older deformation is Mio-Pliocene in age and is characterized by thrust kinematics with E-W to NW-SE shortening. Younger deformation is Plio-Quaternary in age and is characterized by strike-slip kinematics with NE-SW shortening, except along the boundary between the Cordillera Oriental and the Sierras Pampeanas where thrust kinematics with N-S shortening prevail. The similar kinematics but different geometries in the two provinces during Mio-Pliocene deformation and the anomalous thrust kinematics observed during Plio-Quaternary deformation suggest that the Salt rift is the main control on structural geometries. A rift inversion model is developed and applied to the southern Cordillera Oriental.

Pure and simple shear plateau uplift, Altiplano-Puna, Argentina and Bolivia

Abstract The concepts of pure and simple shear strain provide a meaningful way of discussing the alternative modes of lithospheric shortening which produced uplift of the central Andean plateau. Since 10 Ma, discrete segments of the plateau exemplify pure or simple shear modes. Characteristics of the simple shear mode are a flanking thin-skinned thrust belt, and subdued relief and lack of significant Late Miocene to Recent deformation within the plateau. The pure shear mode is characterized by more rugged internal relief and active neotectonic deformation within the plateau and thick-skinned foreland deformation involving seismic deformation of the crust to 30 km or more. These modes of crustal thickening correlate with changes in lithosphere thickness, modes of isostatic compensation, broad wavelength topography and back-arc magmatism. The correlation of the boundary between pure and simple shear segments with major changes in pre-existing anisotropies in the foreland of the orogen suggests that events which long pre-date the Andean orogeny indirectly controlled the along-strike variation in the magnitude and style of shortening in the central Andes and may have influenced the development of flat subduction.