Philip England

Some thermal and tectonic models for crustal melting in continental collision zones

Summary Calculated geotherms and the pressure-temperature-time (PTt) paths followed by rocks during continental thickening episodes are interpreted with respect to the volumes of crustal melt that may be formed during orogenesis in the absence of heat transfer by mantle-derived melts. Particular attention is paid to a tectonic history that may characterize wider orogenic belts, such as are represented most obviously at present by Tibet. This comprises a period of crustal thickening, followed by an interval during which the crust is thinned by extensional strain, rather than by erosion. The amount of crustal melt produced depends strongly on the amount of water (free, and in hydrated minerals) contained in the lower crust. However, we may expect several (1–5) km3 of crustal melt per km2 of orogen if a crust of around average continental surface heat flux (60–70 mW m−2) is thickened by a factor of two. For the lower surface heat flux, partial melting of a sedimentary source would produce predominantly S-type granites and, with slightly higher geotherms, doubling of crustal thickness can lead to partial melting of amphibolites to give I-type granitic activity and calc-alkaline volcanism.

Vertical averages of rheology of the continental lithosphere: relation to thin sheet parameters

Abstract We calculate vertically-averaged rheologies for continental lithosphere subject to biaxial states of stress corresponding to extensional, compressional, and strike-slip regimes. We assume that brittle deformation occurs by Byerlees Law and that ductile deformation occurs by steady-state creep mechanisms. Under these conditions, the vertically-averaged rheology may be approximated by a single power law over the range of strain rates from 10−13 to 10−17 s−1. This is true not only when either brittle or ductile behavior dominates the strength of the lithospere, but also when the contributions of brittle and ductile behavior are comparable. The value of the power law exponent,n, and the strength of the lithosphere depend on: the depth and stress difference at the brittle-ductile transition, the temperature at the Moho, and the geothermal gradient. In the range of vertically-integrated stress between 1012 and 1013 N m−1n reaches a minimum of between 5 and 10; under other circumstancesn increases rapidly as brittle behavior dominates over ductile behavior. The strength of the lithosphere depends most strongly on the thermal state of the upper mantle but also is influenced by the stress supported in brittle portions of the lithosphere.

Diffuse continental deformation: length scales, rates and metamorphic evolution

In contrast with the oceanic portions of the plates, continents may deform hundreds of kilometres away from any plate boundary. Calculations that treat the continents as continuous media suggest that the across-strike length scale of deformation associated with a convergent boundary is proportional to the along-strike length of the boundary, the constant of proportionality depending on the rheology of the lithosphere. The dependence on convergent boundary length implies that, for fixed velocity of convergence, strain rates decrease with increasing length of the orogen. The dependence on rheology is less straightforward, but it appears from laboratory determination of flow laws that the scale length of compressional deformation decreases as the fraction of the lithospheric strength supported by friction on faults increases; that fraction depends on the thermal profile of the continental lithosphere and will change during the evolution of an orogenic belt. The thermal development of a diffusely deforming compressional belt is nearly independent of its strain history unless the strain rates are less than about 10-15 s-1. If erosion terminates the heating phase of such an orogenic belt, the dominant control on the metamorphic history is the heat supply to the continental lithosphere. In contrast, if extension of the thickened crust ends metamorphism, peak metamorphic conditions depend only on the rheological properties of the lithosphere. Metamorphism in crust that is subject to major extension following compressional orogenesis should be nearly independent of the initial thermal conditions of the crust, and may be distinguished from metamorphism terminated by erosion by a final stage of isobaric cooling from temperatures close to the maximum experienced.

On the geodynamic setting of kimberlite genesis

The emplacement of kimberlites in the North American and African continents since the early Palaeozoic appears to have occurred during periods of relatively slow motion of these continents. The distribution of kimberlites in time may reflect the global pattern of convection, which forces individual plates to move faster or slower at different times. Two-dimensional numerical experiments on a convecting layer with a moving upper boundary show two different regimes: in the first, when the upper boundary velocity is high, heat is transferred by the large-scale circulation and in the second, when the upper boundary velocity is lower, heat is predominantly transferred by thermal plumes rising from the lower boundary layer. For a reasonable mantle solidus, this second regime can give rise to partial melting beneath the moving plate, far from the plate boundaries. The transition between these modes takes place over a small range of plate velocities; for a Rayleigh number of 106 it occurs around 20 mm yr−1. We suggest that the generation of kimberlite magmas may result from thermal plumes incident on the base of a slowly moving plate.

Metamorphic pressure estimates and sediment volumes for the Alpine orogeny: an independent control on geobarometers?

Theories which regard the deep erosion (20–30 km) of orogenic belts as an important factor in the thermal development of the continents rely on pressure estimates from metamorphic geobarometers to give an estimate of the depth scale for this process. The imprecision of geobarometers, and the apparent lack of sediments that would result from this amount of erosion, are frequently-made arguments for much smaller erosion depths. The Alps are a well-studied young mountain belt and afford the opportunity to compare overburden masses inferred from geobarometry with sediment masses in the surrounding sedimentary basins. This comparison suggests that metamorphic geobarometry has not significantly overestimated burial depths, and that the average erosion to date has been over 15 km, with a maximum of 25 km or more; it is probable that the total erosion in the belt will eventually average 20–25 km, with a maximum of around 40 km. A significant feature of the sediment distribution is that over 50% of the volume is outside the neighbouring along-strike sedimentary basins. This fact, which accounts for previous low estimates of Alpine sediment volumes, follows from the response of the lithosphere to loading by continental thickening and may be a common feature of post-orogenic sedimentation.

Migration of the seismic-aseismic transition during uniform and nonuniform extension of the continental lithosphere

We investigate the thermal and seismic implications of two extreme configurations of lithospheric extension. One extreme treats the extension as being approximately uniform with depth, the upper crust extending by the rotation of fault-bounded blocks above a diffusely deforming lower crust; in the other extreme, crustal extension occurs only above a low-angle fault (dip less than about 10°). Very low-angle shear zones that initially move aseismically within a uniformly extending lithosphere may be raised through the depth range that is normally seismically active without cooling appreciably; the condition for this to occur is that extension should take place faster than about 10−14/s. In contrast, extension accommodated entirely above a very low-angle shear zone rapidly brings that zone to temperatures at which it should be seismically active, even if initial movement on it were aseismic. The different thermal histories implied by the two configurations of extension offer the possibility of distinguishing between modes of extension by observations of temperature-time and pressure-temperature paths on rocks close to low-angle faults.