M. H. P. Bott

The mechanics of oblique slip faulting

Oblique slip faulting could develop under the influence of an oblique stress system or as a result of inhomogeneity of strength or elastic properties. The dynamics of fracture within a preferred plane is studied, and the direction of movement immediately after fracture has been found to coincide with the direction of maximum shearing stress within the fracture plane. The direction of this stress within a given plane subjected to a general stress system has been derived. For a stress system of given orientation it is found that the initial slip may occur in any possible direction within the plane, the direction depending on therelative values of the three principal pressures. Theoretically, if a preexisting fault is subjected to a reoriented stress system, oblique movement will occur after fracture.

Evolution of young continental margins and formation of shelf basins

Abstract It is suggested that tectonic development of young continental margins, after their initiation by continental splitting, is caused by progressive loss of gravitational energy associated with the juxtaposition of continental and oceanic crust. Subsidence of the shelf may occur as isostatic response to crustal thinning caused by hot creep of lower continental crustal material towards the suboceanic mantle. Sedimentary basins on the shelf, in particular, may result from the interaction of normal faulting in the upper crust with oceanward creep in the lower crust.

Stress concentration in the upper lithosphere caused by underlying visco-elastic creep

Abstract The implications of subdividing the lithosphere into upper elastic and lower viscoelastic layers are investigated by finite-element analysis. Application of uniform horizontal boundary stresses at the ends of a lithospheric plate leads to amplification of the stress in the elastic layer by a factor about equal to the ratio of lithospheric to elastic layer thicknesses, and the visco-elastic layer becomes nearly stress-free except near its ends. The time constant for approach to equilibrium is proportional to viscosity, being for our model 0.21 My for 10 23 N s m −2 , and there is some accompanying flexure of the lithosphere. Local variation in the thickness of the elastic layer causes inverse variation in the stress, in part explaining the stability of shield regions and the tectonic activity of hot plateau uplift regions. It is shown that stress amplification also occurs where the stresses arise from body forces such as differential loading and isostatic compensation across continental margins.

Crustal doming and the mechanism of continental rifting

Abstract Bott, M.H.P., 1980. Crustal doming and the mechanism of continental rifting. In: J.H. Illies (Editor), Mechanism of Graben Formation. Tectonophysics, 73: 1–8. A mechanical explanation of the relationship between crustal uparching and graben formation is suggested, based on the geologically supported assumption that doming precedes rifting. Rifting and graben formation can occur under conditions of crustal tension by the Vening Meinesz wedge subsidence mechanism modified to apply to the uppermost 20 km or thereabouts of the continental crust. If sediment-filled troughs of around 5 km depth are to be formed by this mechanism, then a persistent tension of about 200 MPa (2 kbar) is required. Such a stress system may result from the combined effect of the topographic load of the uplifted region and the upthrust caused by the underlying low-density upper mantle region below. If the upper 10 to 20 km of the crust is elastic but the underlying region can deform slowly by visco-elastic creep, then stress differences of the order of 200 MPa can occur in the upper elastic crust by this process. It is suggested that such a stress system, rather than one arising from bending of the uparched crust or from plate boundary stresses, may be the primary cause of the rifting.

The origin of tectonic stress in the lithosphere

Abstract The sources of lithospheric stress and their distinctive features are briefly reviewed. It is suggested that there are two main categories of lithospheric stress: renewable stress which persists despite continuing stress relaxation and non-renewable stress which can be dissipated by relief of the initial strain. The two most important types of renewable stress arise from plate boundary forces and from isostatically compensated loads. Non-renewable stress systems include bending stresses, membrane stresses and thermal stresses. An important phenomenon generating large stresses at shallow depth is stress amplification caused by lower lithospheric creep. This applies to renewable stresses but not to the non-renewable type. It is suggested that only renewable stresses contribute significantly to tectonic activity. However, bending and thermal stresses are locally important in subducting lithosphere.

Stresses and plate boundary forces associated with subduction plate margins

Primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the Earth and associated topographical loading. When such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere, plate boundary forces develop and modify the plate interior stresses. In this paper, unite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries, with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked. In such regions, several types of horizontal deviatoric stress may occur, including (1) local compression in the trench-arc region caused by the dense sinking slab and the associated surface downflexure; (2) plate interior tension which occurs when this compression is intersected by a weak subduction fault; (3) local tension associated with thickening of the crust at the arc and elsewhere; (4) local tension in the back arc region produced by the underlying low density upper mantle; and (5) downbending stresses in the subducting slab, thermal stresses, and transmitted ridge push, which are not included in the modelling here. A gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked. It is, however, the intersection of the local compression (1) by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces. The state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs. Plate boundary forces have been evaluated for each of the models. It is shown that slab pull and trench suction may be of comparable magnitude.

Modelling the plate-driving mechanism

This paper demonstrates how sub-lithospheric loading gives rise to plate interior stresses and plate boundary forces. The stressing of the lithosphere resulting from plume heads in the upper mantle (hot spots), from low density asthenospheric upwelling beneath ocean ridges and from dense subducting slabs has been modelled by finite element analysis. It is first demonstrated how a sub-lithospheric load, exemplified by a sub-continental plume head, produces local deviatoric stress in the strong upper lithosphere within a plate interior region. It is shown that a 500 km wide region of anomalous density averaging −8 kg m −3 between 100 km and 400 km depths gives rise to a deviatoric tension of 75 MPa in a 20 km thick strong layer forming the upper lithosphere above the deep buoyant load. The large stresses in the strong layer are caused by the shear drag and excess pressure exerted by the buoyant load. When such a stress system is cut across by a zone or plane of weakness, then plate boundary forces and distant plate interior stresses are produced. A weak zone beneath the crest is incorporated in a model of a normal ocean ridge, and this yields a ridge push force of 2.5 × 1012 Nm−1 referenced to old ocean floor and deviatoric compression of about 40 MPa in old ocean floor. A model of a ridge underlain by an anomalously low density upper mantle (plume head) yields a much larger ridge push force, and large deviatoric compressions of nearly 100 MPa extend into the bordering continents. Subduction pull (slab pull and trench suction) of about −4.5 × 1012Nm−1 is modelled for a 300 km deep slab separated from the overriding plate by a weak fault, and collision pull is shown to result from downbulging lower lithosphere beneath collision mountain ranges.

Plate boundary forces at subduction zones and trench-arc compression

Abstract Viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone. The subduction fault is included using the dual node technique. It is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench-arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates. This may be masked at the trench by bending stress. It is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces. Slab pull and trench suction were found to be of comparable magnitude within the range 1.0 to 4.0 × 10 12 N/m in models studied with vertical subduction, and there are indications that this may also apply when the slab dips at 45° as a result of viscous flow induced by rollback. When the slab dips beneath the arc-backarc region, it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction. This suggests that backarc tension associated with Marianas type trench-arch systems may be related to the nearly vertical slab whereas backarc compression in the Chilean type may result from the low dip and small downpull of the slab. It is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors.

Ridge push and associated plate interior stress in normal and hot spot regions

Abstract The stress distribution and plate boundary force produced by ridge push are modelled by elastic/viscoelastic finite element analysis in oceanic lithosphere and in an adjacent continent separated by a passive margin. Models are presented for both a normal slow-spreading ridge and for one underlain by an anomalously low density upper mantle associated with a hot spot which gives rise to 1 km extra elevation of the sea floor. The weak zone at the ridge crest is simulated by the viscoelastic elements extending up to the surface beneath the immediate crest. Using a standard thermal and rheological model of the oceanic lithosphere, a normal ridge gives rise to horizontal deviatoric compression of about 40 MPa in the 25–30 km elastic layer beneath older ocean floor. A much smaller modelled compression of 9 MPa occurs in the 20 km thick elastic layer of the adjacent upper continental crust, as a result of the superimposed tension produced by the thick low-density continental crust (in reality this compression may be up to 30 MPa higher if the sub-continental mantle is denser than that beneath old ocean floor, as the geoid anomalies possibly suggest). A fourfold increase in spreading rate has an insignificant effect on the stresses. Much larger compressions develop adjacent to the modelled hot spot ridge in normal oceanic and especially continental lithosphere, these reaching 100 MPa beneath the old ocean floor and 90 MPa in the adjacent continent. The stresses determined by the modelling are comparable in magnitude but generally slightly lower than values estimated using the density moment function. The ridge push force referenced to 90 Ma old oceanic lithosphere is estimated to be 2.5 × 1012 N/m for the modelled normal ridge and 6.2 × 1012 for the modelled hot spot ridge. The modelling shows that the ridge push is produced by the high pressure and shear drag exerted by the asthenosphere on the lithosphere. The high pressure effect predominates at normal ridges where the plates are forced apart by gravitational wedging. The shear drag effect contributes substantially at hot spot ridges. The modelling helps to explain the prominent compressional stress in the continents bordering the North Atlantic, in terms of the hot spot activity beneath the mid-Atlantic ridge.

The Cenozoic uplift and earthquake belt of mainland Britain as a response to an underlying hot, low-density upper mantle.

A belt of hot, low-density uppermost mantle underlying mainland Britain down to at least 200 km depth, revealed by seismic tomography, may be the prime cause of the Cenozoic uplift and exhumation. We use finite-element modelling to demonstrate how isostatic uplift can occur in response to such a low-density hotspot beneath continental crust. To explain the narrow width of the uplift of northern Britain, the lower crust must be ductile (power-law rheology assumed) and the asymmetrical uplift may be bounded at least on the west side by a pre-existing fault or faults of appropriate polarity. Faulting has probably been reverse under NW–SE regional compression since the onset of the Cenozoic. With the assistance of continuing denudation, inferred gross Cenozoic exhumation of up to 3000 m can be explained. British earthquakes concentrate along a similar north–south belt, with the strongest events in the west. We suggest that the earthquakes result from the continuing tectonics associated with the hot upper mantle, the uplift it causes, and the weakened crust. The underlying low-density region gives rise to tensional loading stress in all directions and bending stresses are associated with the upper-crustal flexuring accompanying uplift. These large stresses supplement NW–SE regional compression. Available earthquake mechanisms are approximately consistent with this stress environment.