Stratigraphy in the continental crust: lithologic and tectonic records

Guido Gosso(*) & Maria Iole Spalla(*)
(*) Dipartimento di Scienze della Terra «A. Desio», Università degli Studi di Milano and C.N.R.-I.D.P.A. Via Mangiagalli, 34 - 20133 Milano, Italy. guido.gosso@unimi.it, iole.spalla@unimi.it

Preservation of stratigraphic settings in continents is mostly confined to uppermost crustal levels, in which prevailing deformation is translational and internal strain of tectonic units is weak. This does not specially depend on association of metamorphism to tectonic processes, but rather on mechanical properties of deforming multilayers. Scattered findings of sedimentary features, not supported by structural investigation on the actual nature of lithologic layering, may lead to failure in determining size of preserved sedimentary sequences. In the intermediate and lower crust, interaction of metamorphism and deformation down to granular scale facilitates construction of new types of lithologic layers and segregation of thick differentiated mineral layerings that mimic stratigraphic sequences. Critical examples of difficulties encountered in different tectonic contexts when assessing the sedimentary origin and related stratigraphic meaning of variously deformed layered sequences are summarised. In deep subduction-collision zones similar or contrasted lithostratigraphies are of little help in definition of tectonic units; the structural and metamorphic reworking of rocks of contrasted origin constructs tectonic units that repeatedly couple and decouple from similar adjacent sequences and their actual relative mechanical paths may be disclosed by a combined structural and petrologic analytical tool delimiting volumes that experienced equivalent structural histories and metamorphic signatures (contouring of tectono-metamorphic units=TMUs); these units constitute valuable elements of correlation in metamorphic belts and for the investigation of mechanisms of lithosphere dynamics.

Tectonics, metamorphism, stratigraphy, structural correlation.