The tectonic and rheological evolution of an attenuated cross section of the continental crust: Ivrea crustal section, southern Alps, northwestern Italy and southern Switzerland M. R. HANDY Geologisches Institut, Universität Bern, Baltzerstrasse 1, 3012 Bern, Switzerland A. ZINGG Geologisches Institut, Universität Basel Bernoullistrasse 32, 4056 Basel Switzerland ABSTRACT The tectonic and rheological evolution of the southern Alpine The Ivrea crustal cross section in northwestern Italy and southern continental crust is reconstructed from structural, petrological, and Switzerland (Fig. 1) is an excellent area to test geophysical models of the radiometric studies in the Ivrea and Strona-Ceneri basement units. continental crust. The section actually consists of two basement units, the The deep crust of the southern Alps acquired its present compositional Ivrea zone and the Strona-Ceneri zone, which represent thinned lower to and metamorphic zonation during Paleozoic magmatism and amphib- intermediate continental crust of the southern Alps (reviews in Zingg, olite-to granulite-facies regional metamorphism. Inferred strength con- 1983; Boriani and Origoni Giobbi, 1984; Zingg and others, 1990). The trasts between lower crustal and upper mantle rocks in the Ivrea zone Permian-Mesozoic sedimentary cover of the Strona-Ceneri zone crops out are low at the high temperatures of regional metamorphism. Late to the south of the basement section (Fig. 1). The history of the Ivrea and Paleozoic transtension and basic to intermediate magmatism in all Strona-Ceneri zones is loosely subdivided into three tectonometamorphic crustal levels preceded extensional faulting associated with the forma- episodes (Fig. 2). Large-scale magmatism and regional metamorphism tion of a passive continental margin during early Mesozoic time. Ex- during the Paleozoic era (episode 1) are responsible for the general compo- tensional uplift and cooling of the basement section is coupled with sitional and metamorphic zonation presently observed in the crustal sec- crustal-scale trends of increasing rheological stratification, grain size tion. Crustal attenuation arid rifting during late Paleozoic and early reduction, and strain localization. Mylonitic shear zones in the lower Mesozoic time (episode 2) thinned the crust to a thickness of 10 to 20 km crust and a broad zone of noncoaxial shear at the base of the interme- and brought the basement section to shallower crustal levels. The final diate crust in the Ivrea zone are inferred to have controlled the high- emplacement and steepening of the Ivrea crustal section in the upper crust strain rheology of the attenuating southern Alpine crust. The deep (episode 3) resulted from polyphase Alpine transpressional tectonics crust in the thinned Ivrea crustal cross section transferred noncoaxial (Laubscher, 1971; Schmid and others, 1987). strain from the upper mantle to the intermediate and upper crust. Final We use a combination of structural, petrologic, and radiometric in- uplift and exposure of the deep southern Alpine crust occurred during formation to reconstruct the tectonic and rheological evolution of the Ivrea Late Cretaceous to Tertiary Alpine thrusting. crustal cross section. The main purpose is to show how the kinematic and thermal history of the continental crust is iinked to its high-strain rheology. INTRODUCTION The deep crustal pre-Alpine structures and high-grade metamorphism are remarkably well preserved in the Ivrea and Strona-Ceneri zones because Much of what is currently known about lithospheric rheology stems the obliterating effects of Alpine metamorphism and penetrative deforma- from a combination of geophysical surveys (reviews in Barazangi and tion are restricted to the Insubric line and to the Penninic and Austro- Brown, 1986; Mooney and Brocher, 1987) and laboratory experiments on Alpine basement nappes to the north and west (Fig. 1). Alpine folds and rocks and minerals designed to simulate physical conditions in the litho- faults transect the crustal section and modify the original crustal configura- sphere (Carter, 1976; Tullis, 1979; Kirby, 1983; Carter and Tsenn, 1987). tion (Fig. 1), but these structures do not disrupt the general continuity The correlation of experimentally predicted strength-depth profiles and among the different crustal levels. deformational modes with crustal seismicity is generally consistent with the concept of a compositionally and Theologically stratified continental lithosphere (Brace and Kohlstedt, 1980; Kirby, 1980; Meissner and Streh- lau, 1982; Sibson, 1982, 1983; Smith and Brun, 1984; Ranalli and Murphy, 1987). Existing models adequately describe low-strain intraplate rheology (for example, Kuznir and Park, 1986), but lithospheric rheology Figure 1. Tectonic map of the Ivrea crustal cross section arid at active plate margins is much more complex due to changing thermo- neighboring units in the western part of the southern Alps. Movement barometric conditions during deformation as well as to the marked sense of faults compiled from Bertotti (1990), Handy (1987), Schmid lithological and structural heterogeneity in the crust prior to deformation. and others (1987), and our own observations. Geological Society of America Bulletin, v. 103, p. 236-253, 11 figs., February 1991. 236 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/2/236/3381146/i0016-7606-103-2-236.pdf by guest on 26 September 2021 Schematic Restored TECTONIC MAP OF THE IVREA Crustal Section CRUSTAL CROSS-SECTION Upper wyq Sediment Crust & adjacent unitsN Middle Crust Ivrea Zone ultrabasite metabasite paragneiss & schist In Insubric line Strona-Ceneri Zone P Pogallo line & shear zone gneiss unit LM Lago Maggiore line amphibolite LG Lugano-Grona line schist unit Cr Cremosina line orthogneiss VCu Val Cuvia line Symbols VCo Val Colla line Basement east of Episode 3 thrust CMB Cossato-Mergozzo- the Strona-Ceneri Zone (Al ne) -Brissago line |l1 i VJ gneiss & schist sediments ^ P' ^O^fZf Episode 2 shear zones Post-Variscan rocks (Late Paleozoic and Early Mesozoic) +. +i Permian intrusive Episode 3 r relative movement Permian volcanic rock 10 km 0 —^ Episode 2 Mesozoic sediment Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/2/236/3381146/i0016-7606-103-2-236.pdf by guest on 26 September 2021 EPISODE AGE EVENTS FABRICS EVENTS FABRICS high-pressure./low-temperature event: d age and extent unknown an s Ordovician granitoids s : PALEOZOIC polyphase deformation, 1 genesi e l amphibolite to granulite facies annealed polyphase folding and shearing, annealed regional metamorphism, medium to coarse grained amphibolite facies regional fine to coarse grained anatexis metamorphism, anatexis (annealed mylonitic fabrics) Episod orogenesi Crusta Late Paleozoic magmatic activity in all crustal levels l PERMIAN - high-temperature shearing mylonitic SW-rim of the Strona-Ceneri zone: crusta erosion and transtensional faulting cataclastic d ? TRIASSIC ? locally annealed an g NW-rim of the Strona-Ceneri zone: n localized retrograde lower mylonitic, : 2 coolin amphibolite to greenschist facies locally cataclastic e l TRIASSIC - retrograde amphibolite and mylonitic, shearing LIASSIC greenschist facies shearing locally cataclastic (extensional faulting in shallower cataclastic (Pogallo shear zone) Episod Overal attenuatio crustal levels, e.g. sediments) formation of the geophysical l Ivrea body steepening of the Ivrea crustal cataclastic localized thrusting, folding and cataclastic CRETACEOUS - section, strike-slip faulting under anchizone TERTIARY : localized folding and thrusting conditions or lower 3 s e transpressiona e NW-rim of the Ivrea zone: mylonitic greenschist facies shearing tectonic Alpin Episod along the Insubric line IVREA ZONE STRONA-CENERI ZONE Figure 2. Tectonometamorphic history of the Ivrea and Strona-Ceneri basement units. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/103/2/236/3381146/i0016-7606-103-2-236.pdf by guest on 26 September 2021 TECTONIC AND RHEOLOGICAL EVOLUTION OF IVREA CRUSTAL SECTION, SOUTHERN ALPS 239 STRUCTURAL STYLE AND INFERRED RHEOLOGY 3d-3f) to kilometer scale (Schmid, 1967; Steck and Tieche, 1976). Epi- sode 1 structures in the Ivrea and Strona-Ceneri zones indicate the follow- A Field and Microscopic Approach to Rheology ing sequence of increasing flow strength: schist < paragneiss < metabasite s peridotite < pyroxenite. The predominance of boudinage structures with The magnitude, distribution, and style of strain are used to infer the moderate to high aspect ratios (Figs. 3g-3i) suggests that high-grade re- competence contrast among different rocks and minerals. We note at the gional metamorphism in the intermediate to lower continental crust is outset that most of our structural observations come from the central to associated with low strength contrasts among all rocks. In addition, the northeastern part of the Ivrea and Strona-Ceneri zones (between the Val observations indicate the importance of grain size as a determinant of d'Ossola and the northeastern end of the Lago Maggiore, Fig. 1). This relative flow strength at high strains (Fig. 3h and discussion in next informational bias also reflects the general distribution of published struc- section). tural work in the area. Episode 1 microstructures are strongly annealed under the conditions Broad constraints on the relative viscosity of different rock layers are of regional metamorphism. This annealing obscures most traces of the obtained