Room: 407 Tel: Patrice Rey

Lecture 6: Structural geology of divergent plate boundaries Aims: To characterize deformation at divergent plate boundaries

The Wilson cycle: Rifting and the formation of continental margins As the concept of sea floor spreading gained acceptance in the late 60's, the consequences for geology gradually began to dawn. One of the first to recognise how plate tectonics could be applied to the geological record was J. Tuzo Wilson. If continents rift apart to form ocean basins, other oceans must close. This may be repeated throughout Earth history. This cycle is known as the Wilson Cycle: (1) Rifting of continents by mantle diapirism (2) Continental drift, seafloor spreading & formation of ocean basins (3) Progressive closure of ocean basins by subduction of ocean lithosphere (4) Continental collision and final closure of ocean basin The two diagrams below illustrate some simple concepts of continental rifting (e.g. the Gondwana continent) at the start of the Wilson Cycle. Uprising plume causes doming of crust with magma chamber developing underneath. As extension continues, an ocean basin forms, and thick sedimentary sequences develop at continental margins as rivers dump sediments in deep water. However in reality may be a bit more complex... From Tarney, Leicester Uni.

Driving force for extension Passive rifting: Plate boundary forces drive the stretching and thinning of the continental lithosphere. Active rifting: Body forces related to the thermal thinning of the lithospheric mantle drive thinning.

Development of Continental Rifts... Early ideas on the development of rifts are conceptualised in the diagram below. This is based on the African rift system, where there is significant rift magmatism. There is notable extension, shown by the widening of the diagram block by at least 50 km. At the same time there is uplift or ascent of the more ductile mantle, especially the asthenosphere. The crust, and particularly the upper crust, is assumed to act in a brittle fashion. Progressive formation of a rift valley through extension of the lithosphere and continental crust (by about 50 km). Note that uprise and decompression of the underlying asthenosphere results in magma formation. The crust responds by brittle fracture. Early rift sediments are faulted and move down into the developing rift (graben). Erosion takes place on the sides of the rift valley. The first stage assumes that symmetric normal faults begin to form in the brittle crust. The second stage shows simultaneous necking of the lithosphere with uprise of an asthenosphere diapir. The decompression associated with the latter causes melting of the mantle to give basaltic magmas. Pre-existing sediments are downfaulted into the graben. The third stage is accompanied by significant extension and by more uprise of the asthenosphere. The latter causes doming of the crust. New sediments are deposited within the graben as a result of erosion of the uplifting sides of the graben. So there are both pre-rift and syn-rift sediments within the developing rift valley, but sediments on the flanks are progressively eroded away.

Development of Continental Margins The fourth stage shows the actually rifting-apart of the continent, so the asthenosphere rises towards the surface, causing decompression and extensive melting. New basaltic oceanic crust is formed. Finally, sea-floor spreading takes over as the ocean basin widens. The rift sedimentary sequence is buried beneath younger marine sediments.

Evolution of a passive continental margin: An example Rifting: Middle Triassic (210Ma) to Early Jurassic (~190Ma) Drifting: Beginning Early (~190Ma) continuing today. Rift basins bounded by moderately dipping normal and oblique-slip faults Inactive rift basins Seafloor-spreading centers Producing oceanic crust

Continental Shelf Sediments The figure below is representative of a number of crustal cross-sections across the continental shelf of the eastern Atlantic seaboard of North America. The critical point is the huge thicknesses of Mesozoic and Tertiary sediments, here shown as almost 15 km, but in other cross-sections this can be even thicker. Note that at the bottom of this pile are volcanics and volcanogenic sediments, and evaporites, which most likely are shallow water. Also, massive carbonate reef structures, which must also be shallow water, but also must indicate progressive subsidence slow enough that shallow water sedimentation can keep pace with it.

Rift basins are simply normal-fault bounded sedimentary basins. Rift basins are a fundamental manifestation of continental extension, they are prime repositories for sediment accumulation, and account for significant accumulations of hydrocarbons. Asymmetric basin subsidence Antithetic fault Subsidence accelerates Structural Geology of Rift Basins

A/ Domino fault block model in which both the faults and the intervening fault blocks rotate during extension. B1/ Planar kink fault geometry and B2/ Listric fault-subhorizontal décollement. In both B1 and B2 horizontal displacement (h) on the décollement fault creates a potential void bewteen the hanging wall and footwall which is erased by the collapse of the hanging wall along antithetic faults dipping at 45º in B1, and vertical faults in B2. A B1 B2 Contrasted models of extensional basin.

The formation of a roll-over anticline in a sand box experiment... D. Jarmusewski. 1998, Wuersburg

Normal faults in sand-box experiments McClay et al. More at: Physical models (=analogue experiments) allow to investigate simplified geological systems. Using colored layers of sand, as an analogue for the upper crust, and applying various boundaries conditions, McClay’s team (Univ. London) can successfully create, in their laboratory, structures observed in nature. Pictures on the right represent various geometry of sedimentary basins, from symmetrical extension (top), to highly asymmetrical extension (bottom).

Rift Basin Inversion It has long been recognized that the many rift basins in the world contain a wide variety of post-rift compressional structures. Many of these features involve reutilization of normal fault that were active during extension, and hence they are referred to as inversion structures. Inversed basin are they are prime target for oil and gas exploration. Roll over fold, onlap, and pinchout The development of extensional basin above a décollement is associated with the formation of a roll over anticline. The progressive rotation of the surface of the basin is responsible for the “onlap” deposition of layer. In contrast drop of sea level creates “pinchout” Onlap Pinchout Some structural and sedimentary features of extensional basin.

Extension of stable continental lithosphere: Numerical experiments Generalized model of continental lithosphere. Mechanical and thermal boudary conditions on the left reflect symmetry. A uniform force is applied to the right boundary (dynamic condition), or alternatively a constant strain rate (kinematic condition), the top of the lithosphere is at 0ºC, and a constant basal heat flow q b is applied to the lower boundary. Isostatic rebound forces act on density interfaces. Horizontal components of gravitational forces associated with the vertical displacement of density interfaces are neglected.

Govers and Wortel, cm/y; Extension: 470km; 5Ma. Extension rate fast relative to thermal reequilibration 6cm/y; Extension: 340km; 8Ma. Extension rate slow relative to thermal reequilibration Extension of stable continental lithosphere: Numerical experiments

Wernicke simple shear model Many steeply dipping normal faults are actually listric faults. As the lithosphere is stretched during continental extension, the deeper crust thins by plastic deformation, while the upper crust is broken up and pulled apart by listric faults which 'bottom out' in the ductile layer. This is the essence of McKenzie-type models of basin formation. As the lithospheric mantle is thinned by stretching it is of course partly replaced by hotter asthenosphere. This will gradually cool on a time scale of the order of m.y., and as it cools it becomes denser and the shallow basin above gradually subsides and is progressively filled with shallow-water sediment. The amount of subsidence will depend on the initial amount of stretching. Note that subsidence occurs in two stages: (1) as a result of tectonic stretching – on a short time scale, (ca. 10 my), and (2) as a result of thermal subsidence – long time scale, (ca my). Further development have been proposed in particular by Wernicke. The important difference is in that stretching is accommodated by a single low-angle detachments system which produces a strong asymmetry. Basins associated with the thermal subsidence phase may be offset from the thin-skinned basins associated with the initial rifting. Magmatic effects (melting resulting from the uprising asthenosphere) may be offset from the main sedimentary basins. Because of the asymmetry, the continental margins on the two sides of an opening ocean may have very different profiles. Extension of stable continental lithosphere: Conceptual models

More at: L. Moresi CSIRO Particle in cell finite element code is a new generation of computer code for modelling fluid of history dependent properties. Applications of the code include smaller-scale geological studies of crustal deformation, geomechanical problems such as slope stability, engineering-scale models such as the flow of granular materia, and fluid-dynamics problems where free-surfaces, interfaces and suspended particles are included. One of the most attractive aspect of this code is its ability to account for both brittle and plastic deformations. This type of code could possible simulate both McKenzie and Wernicke type of lithospheric extension. Some promising developments in numerical modelling techniques...

Structural characteristics of divergent margins Rifting, sedimentary and oceanic basins, and the formation of continental margins result from processes that tend to thin and extend the continental crust. Normal faults in the upper brittle crust and flat foliations in the ductile lower crust result from of a stress regime in tension. The combined action of normal faulting and horizontal flattenning is responsible for thinning and extension of the continental crust. As seafloor spreading occur, stretching and thinning of the continental margin stops. Thermal subsidence follows so that the sedimentary pile deposited on the margins consist of pre-, syn- and post- rift packages.