The Earth as a Dynamic Open System
Tectonic phenomena that affect the lithosphere can occur at a wide variety of rates (Price 1975) from 100 to 10-18 s-1 for the processes described in previous chapters. The Soft Plate approach was proposed mainly for processes on the slow side of the spec
- PDF / 2,008,044 Bytes
- 25 Pages / 439.32 x 666.12 pts Page_size
- 59 Downloads / 248 Views
5.1 Introduction: Impact Tectonics
Tectonic phenomena that affect the lithosphere can occur at a wide variety of rates (Price 1975) from 100 to 10- 18 S-1 for the processes described in previous chapters. The Soft Plate approach was proposed mainly for processes on the slow side of the spectrum of strain rates but also has implications on the fast side; in fact, viscoelastic rebound has deep implications in earthquake faulting. Can we extend the range of behaviour of the lithosphere by studying processes that operate at an even faster rate than earthquake faulting, if such processes exist? Only by exploring the whole range of possible behaviours can we propose a coherent theory for the central problem in geodynamics, the rheology of the lithosphere. In fact, there is a geological process that operates at a faster strain rate, -10 6-10 9 S-I, than earthquake faulting. It is impact cratering; and it can be used to probe planetary lithospheres, besides the case of the earth (Thomas et al. 1997). The physical conditions for impact cratering are known from shock metamorphism (see Grieve 1990; Grieve and Pilkington 1996 for a synthesis). Pressures rise from 10 to 100 Gpa and temperatures from 1000 to 10,000 DC; under these conditions crystals successively develop planar features, diapletic glasses by disorganisation of the lattice, melts and vaporisation. This field is well beyond the fast geological processes such as earthquake faulting and volcanism, with temperatures below 1000 DC but with pressures between 1 and 0.1 Gpa. Theoretical energy considerations confirm the observations: a stony chondritic body at 25 km S-1 gives a peak pressure of -900 Gpa at the impact point in granite. The direct tectonic effects of impact are known from observation of known impact craters and from analogies with nuclear explosions (Melosh 1989; Price and Cosgrove 1990). Different stages in the cratering mechanism such as contact, compression, excavation, modification, in this order, produce: the crater itself, tectonic inversion attaining recumbent folding below the crater rim, radial fractures and a circumferential graben outside the crater rim. Therefore, compressive stresses produce the recumbent folding and tensional stresses, during the modification stage, they produce the tensional structures by collapse. All these effects are scaled to the energy of the impactor, depending on its mass and velocity. In the case of Chicxulub the impactor has a 12 km diameter, the multi-ring crater has a 195 km diameter and the Moho at 35 km is displaced 3 km by normal faults (Morgan et al. 1997). A. Ribeiro, Soft Plate and Impact Tectonics © Springer-Verlag Berlin Heidelberg 2002
246
CHAPTER
5
The Earth as a Dynamic Open System
Other direct effects of impact on the lithosphere are well known and documented. They include, besides morphology and tectonics, a petrological signature in the form of shock metamorphism, already referred to, geophysical signature (Grieve and Pilkington 1996) and geochemical signature. The importance of impact crateri
Data Loading...
