Guido Schreurs

The numerical sandbox: comparison of model results for a shortening and an extension experiment

Abstract We report results of a study comparing numerical models of sandbox-type experiments. Two experimental designs were examined: (1) A brittle shortening experiment in which a thrust wedge is built in material of alternating frictional strength; and (2) an extension experiment in which a weak, basal viscous layer affects normal fault localization and propagation in overlying brittle materials. Eight different numerical codes, both commercial and academic, were tested against each other. Our results show that: (1) The overall evolution of all numerical codes is broadly similar. (2) Shortening is accommodated by in-sequence forward propagation of thrusts. The surface slope of the thrust wedge is within the stable field predicted by critical taper theory. (3) Details of thrust spacing, dip angle and number of thrusts vary between different codes for the shortening experiment. (4) Shear zones initiate at the velocity discontinuity in the extension experiment. The asymmetric evolution of the models is similar for all numerical codes. (5) Resolution affects strain localization and the number of shear zones that develop in strain-softening brittle material. (6) The variability between numerical codes is greater for the shortening than the extension experiment. Comparison to equivalent analogue experiments shows that the overall dynamic evolution of the numerical and analogue models is similar, in spite of the difficulty of achieving an exact representation of the analogue conditions with a numerical model. We find that the degree of variability between individual numerical results is about the same as between individual analogue models. Differences among and between numerical and analogue results are found in predictions of location, spacing and dip angle of shear zones. Our results show that numerical models using different solution techniques can to first order successfully reproduce structures observed in analogue sandbox experiments. The comparisons serve to highlight robust features in tectonic modelling of thrust wedges and brittle-viscous extension.

Analogue benchmarks of shortening and extension experiments

Abstract We report a direct comparison of scaled analogue experiments to test the reproducibility of model results among ten different experimental modelling laboratories. We present results for two experiments: a brittle thrust wedge experiment and a brittleviscous extension experiment. The experimental set-up, the model construction technique, the viscous material and the base and wall properties were prescribed. However, each laboratory used its own frictional analogue material and experimental apparatus. Comparison of results for the shortening experiment highlights large differences in model evolution that may have resulted from (1) differences in boundary conditions (indenter or basal-pull models), (2) differences in model widths, (3) location of observation (for example, sidewall versus centre of model), (4) material properties, (5) base and sidewall frictional properties, and (6) differences in set-up technique of individual experimenters. Six laboratories carried out the shortening experiment with a mobile wall. The overall evolution of their models is broadly similar, with the development of a thrust wedge characterized by forward thrust propagation and by back thrusting. However, significant variations are observed in spacing between thrusts, their dip angles, number of forward thrusts and back thrusts, and surface slopes. The structural evolution of the brittle-viscious extension experiments is similar to a high degree. Faulting initiates in the brittle layers above the viscous layer in close vicinity to the basal velocity discontinuity. Measurements of fault dip angles and fault spacing vary among laboratories. Comparison of experimental results indicates an encouraging overall agreement in model evolution, but also highlights important variations in the geometry and evolution of the resulting structures that may be induced by differences in modelling materials, model dimensions, experimental set-ups and observation location.

Experiments on strike-slip faulting and block rotation

Analogue model experiments investigating strike-slip faulting in zones of distributed shear deformation show that fault orientations and fault evolution indicate modifications of the stress field with increasing bulk shear strain. Dextral bulk shear deformation at low strain is accommodated dominantly by synthetic strike-slip faults (Riedel shears). At higher strain, secondary antithetic and synthetic faults develop mostly between earlier formed major Riedel shears. Closely spaced parallel antithetic faults (cross faults) delimit domains that rotate about vertical axes with continuing shear deformation. Rotation of fault-bounded domains results in sigmoidal antithetic faults that have a dip-slip component and a dip direction that changes along strike. There is good agreement between models and natural examples of distributed shear zones where block rotations about vertical axes have been documented.

Analogue modelling of faulting in zones of continental transpression and transtension

Abstract Experiments were performed to simulate deformation in zones of continental transpression and transtension. Stratified models consisted of brittle analogue materials overlying a thin layer of viscous material. Oblique deformation was obtained by combining a basal, distributed strike-slip shear component with either transverse shortening (transpression) or transverse extension (transtension). In transpression experiments the imposed ratio of shear strain rate and shortening strain rate exerts an important control on initial fault evolution in the brittle layers of the model. In those experiments with a relatively high strain rate ratio (≥3.6), subvertical, en echelon strike-slip faults develop first, striking at angles of 25–37° to the shear direction. With increasing strain several convergent strike-slip fault zones form displaying positive flower structures. In low strain rate ratio experiments (≤2.7), gently dipping (30–45°), downward converging thrust faults accommodate initial failure. They bound pop-up structures that strike parallel to the shear direction. Increasing strain results in a fault pattern dominated by oblique-slip reverse faults. Partial partitioning of fault motion occurs at late stages of strain when strike-slip faults form within popup structures. The strike-slip faults merge at depth with confining oblique-slip reverse faults, have a curved shape in plan view and a dip direction which changes along strike. Fault patterns can be used as kinematic indicators. En echelon strike-slip faults initially accommodate deformation in a transtension experiment and strike at low angles (6–10°) to the shear direction. With increasing strain, normal faults form parallel to older strike-slip faults. They develop as a result of partitioning of fault motion and gravity failure. There is good agreement between experimental results and natural examples of continental transpressional and transtensional tectonics.

Age constraints on the tectonic evolution of the Itremo region in Central Madagascar

The Itremo region in Central Madagascar comprises a deformed metasedimentary sequence (Itremo Group) that has undergone greenschist to lower amphibolite facies metamorphism. During a first phase of deformation (D1) Itremo Group sediments were deformed into a fold-and-thrust belt and transported toward the E to NE on top of migmatitic gneisses rocks of Anatananarivo block. A second phase of deformation (D2) affected both the fold-and-thrust belt and structurally underlying units, and formed large-scale N-S trending folds with steeply dipping axial planes. A Late Neoproterozoic Th–U–Pb XRF monazite age (565±17 Ma) dates the emplacement of a granite that truncates first-phase structures in the Itremo Group, and indicates that the fold-and-thrust belt formed prior to ≈565 Ma. Th–U–Pb electron microprobe dating was applied to elongated monazites that lie within the first-phase foliation of Itremo Group metapelites. The detrital cores of zoned monazites reveal two distinct age populations at ∼2000 and 1700 Ma, the latter age giving a maximum depositional age for the Itremo Group. Statistical analysis of ages determined from the rims of zoned monazites and from unzoned monazites indicates three Late Proterozoic–Early Paleozoic monazite growth events at about 565–540, 500 and 430 Ma. The oldest age population is contemporaneous within error, with the intrusion of the dated granite. The two younger age populations are found both in the Th–U–Pb and Ar–Ar data; together with the perturbation of the Rb–Sr system we interpret both ages as due to alteration related to fluid circulation events, possibly connected to the emplacement of pegmatite fields in Central Madagascar. Syn-D1 tectonic growth of contact metamorphism minerals such as andalusite has been observed locally in metapelites along the margin of Middle Neoproterozoic (≈800 Ma) granites, suggesting that D1 in the Itremo Group is contemporaneous with the intrusion of granites at ≈800 Ma. The N-S trending D2 folds are associated with ≈E-W shortening during the final assembly of Gondwana in Late Neoproterozoic–Early Cambrian times.

Late Palaeozoic to Neogene geodynamic evolution of the northeastern Oman margin

When the highlands of Arabia were still covered with an ice shield in the latest Carboniferous/Early Permian period, separation of Gondwana started. This led to the creation of the Batain basin (part of the early Indian Ocean), off the northeastern margin of Oman. The rifting reactivated an Infra-Cambrian rift shoulder along the northeastern Oman margin and detritus from this high was shed into the interior Oman basin. Whereas carbonate platform deposits became widespread along the margin of the Neo-Tethys (northern rim of Oman), drifting and oceanization of the Batain basin started only in Late Jurassic/Early Cretaceous time. Extensional tectonics was followed in the Late Cretaceous by contraction caused by the northward drift of Greater India and Afro-Arabia. This resulted in the collision of Afro-Arabia with an intra-oceanic trench and obduction of the Semail ophiolite and the Hawasina nappes south to southwestward onto the northern Oman margin ~ 80 m.y. ago. During the middle Cretaceous, the oceanic lithosphere (including the future eastern ophiolites of Oman) drifted northwards as part of the Indian plate. At the Cretaceous–Palaeogene transition (~ 65 Ma), oblique convergence between Greater India and Afro-Arabia caused fragments of the early Indian Ocean to be thrust onto the Batain basin. Subsequently, the Lower Permian to uppermost Maastrichtian sediments and volcanic rocks of the Batain basin, along with fragments of Indian Ocean floor (eastern ophiolites), were obducted northwestward onto the northeastern margin of Oman. Palaeogene neo-autochtonous sedimentary rocks subsequently covered the nappe pile. Tertiary extensional tectonics related to Red Sea rifting in the Late Eocene was followed by Miocene shortening, associated with the collision of Arabia and Eurasia and the formation of the Oman Mountains.

West-northwest directed obduction of the Batain Group on the eastern Oman continental margin at the Cretaceous-Tertiary boundary

The Batain coast area in eastern Oman is dominated by allochthonous Permian to Late Maastrichtian sedimentary and volcanic rocks (Batain Group), unconformably overlain by neoautochthonous Tertiary sediments. The allochthonous rocks of the Batain coast were previously attributed to the Hawasina complex, the Permian to Coniacian/Santonian sedimentary infill of the neo-Tethyan Hawasina basin off northern Oman. Previous structural interpretations suggested that the Batain Group, along with the Hawasina complex and the Semail ophiolite, was obducted in the Coniacian to Campanian from NE to SW onto the northern Oman continental margin. Results of our work in the Batain area differ from previous interpretations, with most significant differences concerning timing and direction of obduction. Our results show that WNW directed tectonic movements formed a fold-and-thrust belt and led to the obduction of allochthonous rocks onto the east Oman continental margin during latest Maastrichtian/earliest Paleocene times. This is coeval with emplacement of ophiolitic fragments along the eastern coast of Oman (eastern ophiolite belt) but is about 15–20 Myr later than emplacement of Hawasina complex and Semail ophiolite in northern Oman. Postemplacement structural evolution during the Tertiary involved intraplate extension, possibly reflecting the Red Sea/Gulf of Aden opening, and late Tertiary shortening related to convergence between Arabia and Eurasia. Late Tertiary contractional deformation resulted in refolding of the Batain nappes and in folding of the overlying Tertiary sediments. A palinspastic reconstruction of the Batain area indicates that the Permian to Upper Cretaceous sediments were formerly deposited in the Batain basin, a part of the proto-Indian Ocean, along the present-day eastern Oman margin. This leads us to propose that Permian breakup of Gondwanaland created both continental margins of Oman and led to the opening of two major basins: the neo-Tethyan Hawasina basin in the north and the proto-Indian Ocean Batain basin in the east, the latter separating Arabia from greater India.

Discovery of thrust klippen, northwest of Mary Kathleen, Mt Isa Inlier, Australia

In the northeastern part of the Mary Kathleen 1:100 000 sheet, a thick pile of Early to Middle Proterozoic metasediments, metavolcanics and intrusive rocks were deformed by an early thrusting event and later by east‐west shortening. D1 produced a thrust complex, containing imbricate stacks of thrust sheets and a sequence of foliations, changing in orientation and style: S1A is a phyllonitic fabric on the major thrusts; Sib is the axial‐plane foliation to west‐verging folds in the immediate vicinity of the major thrusts; and S1C is the axial‐plane foliation of northeast‐southwest upright folds, locally developed within the thrust sheets, caused by shortening parallel to the transport direction. Two thrust systems have been studied in detail. In the first, in the Deighton area, the geometry of the imbricates, the extension lineations on S1A and the orientation of D1C folds all point towards a westward or northwestward movement direction. In contrast, the West Leichhardt imbricates may have moved southward. ...

Fault development and interaction in distributed strike-slip shear zones: an experimental approach

Abstract Analogue model experiments using both brittle and viscous materials were performed to investigate the development and interaction of strike-slip faults in zones of distributed shear deformation. At low strain, bulk dextral shear deformation of an initial rectangular model is dominantly accommodated by left-stepping, en echelon strike-slip faults (Riedel shears, R) that form in response to the regional (bulk) stress field. Push-up zones form in the area of interaction between adjacent left-stepping Riedel shears. In cross sections, faults bounding push-up zones have an arcuate shape or merge at depth. Adjacent left-stepping R shears merge by sideways propagation or link by short synthetic shears that strike subparallel to the bulk shear direction. Coalescence of en echelon R shears results in major, through-going faults zones (master faults). Several parallel master faults develop due to the distributed nature of deformation. Spacing between master faults is related to the thickness of the brittle layers overlying the basal viscous layer. Master faults control to a large extent the subsequent fault pattern. With increasing strain, relatively short antithetic and synthetic faults develop mostly between old, but still active master faults. The orientation and evolution of the new faults indicate local modifications of the stress field. In experiments lacking lateral borders, closely spaced parallel antithetic faults (cross faults) define blocks that undergo clockwise rotation about a vertical axis with continuing deformation. Fault development and fault interaction at different stages of shear strain in our models show similarities with natural examples that have undergone distributed shear.

Analysis of analogue models by helical X-ray computed tomography

Abstract The aim of analogue model experiments in geology is to simulate structures in nature under specific imposed boundary conditions using materials whose rheological properties are similar to those of rocks in nature. In the late 1980s, X-ray computed tomography (CT) was first applied to the analysis of such models. In early studies only a limited number of cross-sectional slices could be recorded because of the time involved in CT data acquisition, the long cooling periods for the X-ray source and computational capacity. Technological improvements presently allow an almost unlimited number of closely spaced serial cross-sections to be acquired and calculated. Computer visualization software allows a full 3D analysis of every recorded stage. Such analyses are especially valuable when trying to understand complex geological structures, commonly with lateral changes in 3D geometry. Periodic acquisition of volumetric data sets in the course of the experiment makes it possible to carry out a 4D analysis of the model, i.e. 3D analysis through time. Examples are shown of 4D analysis of analogue models that tested the influence of lateral rheological changes on the structures obtained in contractional and extensional settings. Supplementary material: The nineteen movies referred to in the article are available at https://doi.org/10.6084/m9.figshare.c.4788519