The Moho and the Lower Crust


In the early 1960s there was considerable discussion between petrologists and geophysicists as to the nature of the MOHO. The P-wave velocity of most regions of the uppermost mantle beneath both continents and oceans lies in the range 8.2 0.2 km/sec. This in itself would restrict the composition of the mantle below the Moho to some combination of the following minerals (which have the appropriate properties):

Olivine Pyroxenes Garnet (minor spinel, hornblende, phlogopite)

The two principal rock types carrying these minerals are: PERIDOTITE (olivine + pyroxenes) and ECLOGITE (pyroxene + garnet), which are of ultramafic and mafic composition respectively.

The nature of the lower crust is less certain. Exhumed Precambrian high-grade granulite-facies rocks (as in Figs. 2 & 3), which have equilibrated at depths of 25-30 km, have an intermediate (dioritic) bulk composition. But deep crustal xenoliths brought up in volcanic breccia pipes tend to be more mafic (gabbroic) in composition. So has the continental crust in part been underplated subsequently by mafic magma? Also, recent deep seismic investigations have revealed strong horizontal reflections in the deep (mainly post-Archaean) crust - what is the cause of these reflections? Do they represent mafic intercalations, differences in fluid content, crustal viscosity, or the bottoming out of shear zones (cf. Kusznir & Matthews, 1988; Meissner & Kusznir, 1987; Warner, 1990; Reston, 1990a). Comparison of crustal reflection profiles across the various tectonic zones of Europe (Wever et al., 1987; Sadowiak et al., 1991) has identified several different types of deep crustal structure: (a) abundant lamellae above the Moho as in BIRPS SWAT 4, (b) bands of reflectors as in BIRPS WINCH 3, (c) hyperbola-like diffractions as in BIRPS SWAT 6-9, (d) "crocodile" diverging reflectors observed in old collision zones, but not so far in the UK, (e) "ramp and flat" stuctures as in BIRPS SWAT 4 2/3, and (f) "fishbone" pattern observed across the Brabant massif. These features (cf. Meissner, 1989) are thought to represent different types of crustal structure. However it is felt that lower crustal viscosity (Meissner & Kusznir, 1987) is a more important factor controlling development of reflectors than is composition, and the current view is that good lower crust reflectors might characterise mature crust, but that this pattern could be destroyed by either compression/collision ("crocodiles", etc.), igneous intrusions or significant extension.

Nonetheless it is commonly assumed that the lower crust is gabbroic in composition, either igneous (gabbro), or metamorphic amphibolite (wet) or granulite (dry).