Regional metamorphic zones: tectonic controls

Abstract

Factors governing the development of well-characterized metamorphic zones in orogenic belts showing moderate to high-pressure regional metamorphism are considered. Features recorded in the rocks concerning: variations of temperature with pressure (both across metamorphic belts and in single rocks), ages of metamorphism and uplift history, are compared with those predicted by the elegant quantitative thermal models of Richardson, England and co-workers. Across metamorphic zones in the well-known Scottish Dalradian and New England Acadian terranes the rocks show much more complex histories and relationships than those expected from simple extrapolation of one-dimensional thermal models across the terranes, and it is clear that consideration must be given to the influence of tectonic movements and other controls in creating lateral variations in heat flux across sets of metamorphic zones. In the Lepontine region of the Alps, present-day differential uplift must be affecting the metamorphic mineral assemblages presently ‘freezing in’ at depth. Qualitative models of the effect of tectonic displacement show that the ‘ freezing in' of the thermal effects of such displacements must potentially always be occurring at some depth in a terrane undergoing metamorphism. Tectonic movements will pass from postmetamorphic in shallower levels to synmetamorphic in deeper levels, and repeated or continuous tectonic movement may be potentially frozen into a considerable thickness of rock. Contrasts in thermal controls (such as heat generation) of provinces on either side of a tectonic boundary of long standing, may augment the lateral heat flux caused by tectonic dislocation in creating well-developed metamorphic zones that are not purely depth related. Such an origin is suggested for prominent sets of metamorphic zones seen adjacent to major dislocations in both the Scottish Highlands and New England. Within the Scottish Dalradian, tectono-metamorphic domains may be recognized at whose margins there are often changes in the character and pattern of metamorphic zones. It is concluded that prominent metamorphic zones are often the product of tectonic boundary conditions, and that the thermal history of the major part of an extensive orogenic segment is best seen away from such zones and within the interior portions of tectono-metamorphic domains where metamorphic grade is often relatively uniform.