Behavior of an Archean Granitic Batholith: Discussion

Abstract

In recent years, the term train' has become increasingly important in descriptive structural geology. Expressions like train facies' or 'strain fabric' (formerly Veformation fabric') have been introduced (Halasen 197 1, Clifford 1972), which are exclusively related to tectonic flow, and these are used with increasing frequency, while use of the term strain is comparatively uncommon in papers on non-diastrophic deformation or flow of lava and magma. If this trend' continues 'geologic strain' will become synonymous with 'tectonic flow'. It need hardly be pointed out that strain is not a structural property but rather a general concept of theoretical mechanics, and that it is not the only method of specifying a coptinuous deforn~ation (minus translation) It has become clear, however, that the strain concept, rather than alternative methods, will be an important tool for the modcrn structural geologist (cf. Ramsay 1969). Clifford's ( 1972) study is essentially an application of the strain theory to the wall rock deformation of a granitic batholith. The paper employs such engineering terms as 'pure shear' and 'simple shear', as well as tectonic expressions related to strain. These technical terms have been unequivocally defined in standard books of theoretical mechanics, and/or in recent textbooks of structural geology (Table 1 ). Clifford ( 1972) does not use 'pure shear' and other expressions in their standard sense, neither does he re-define his technical vocabulary. The reader is forced to deduce from the general context which meaning is to be attached to the various terms (Table 1 ). Clifford ( 1972) prescnts convincing evidence that the triaxial strain of the wall rocks is markedly heterogeneous, i.e. that the magnitude of horizontal compressive strain increases toward the contact of the batholith. Me then claims that this inhomogcneous deformation corresponds closely to irrotational strain (labelled 'pure shear in three dimensions', see Table I ) , and attributes it largely to axially symmetric inflakion of thc uiaconsolidatcd batholith. Of gcomctric necessity, most heterogeneous strains involve large components of rigid rotation, i.e. the local strain is generally rotational (Ramsay 1969, p. 52) . Figure 1 illustrates schematically what heterogencous strains might be expected around uniformly expanding circular diapirs. The cross-sectional strain (normal to the axis of diapirs) must be irrotational (Fig. la ) , but large components of rigid rotation occur in other sections (Fig. l b ) . These components will tend to be greatest where geometry of the granite contact is approximately cylindrical. Inflation of noncircular diapirs will generally lead to rotational crosssectional strain in the sul-rounding rocks. Clifford ( B 972) considers two general models of radial horizontal compression of the supracrustal wall rocks, ( 1 ) by emplacement of a conical granitic mass, and (2) by late-stage horizontal inflation' of a batholith. He asserts that the former model is characterized by biaxial strains (labelled 'constrictive', Table 1 ), and that the latter model involves an early phase of subvertical simple shear parallel to the contact. Figure 1 b illustrates that subverti-