Hidden Terranes in the Northern Apennines, Italy: A Record of Late Cretaceous-Oligocene Transpressional Tectonics 1 Michele Marroni and Benedetta Treves 2 Dipartimento di Scienze della Terra, Universita’ di Pisa, 56100, Italy ABSTRACT We propose that a very oblique or transpressional tectonic regime was dominant during the early (pre-collisional) orogenic evolution of the Northern Apennines (Late Cretaceous to Early Oligocene). This hypothesis resolves many inconsistencies in the previous reconstructions of this orogenic belt, which were based on a classic model of orthogonal convergence between the European and Adriatic plates. The crucial lines of evidence that point to a major role of oblique tectonics in the structuring of the Northern Apennines are: (1) the plate tectonics framework, that indicates left-lateral oblique convergence along the Europe/Adria plate margin; (2) the lack of a magmatic arc during the entire pre-collisional convergent history of the chain (a time span 45 m.y., from Late Cretaceous to Early Oligocene); (3) the long (20 m.y.) residence time of turbidite sequences in the trench (the ‘‘dormant’’ trench); (4) the multiple source areas of turbidites from both sides of the basin, and the associated coarse gravity deposits; (5) the opposite vergence of deformations in some oceanic units; (6) the unmatching stratigraphic features, distinct deformation and metamorphic histories between adjacent overthrust oceanic units (Ligurids), here interpreted as tectonostratigraphic terranes. Specific aspects of Apennine stratigraphy and tectonics and the geometry and structure of the contacts be- tween the Ligurid Units suggest the existence of a number of terranes juxtaposed by transpression during the early (Late Cretaceous to Early Oligocene) orogenic evolution of the chain. Introduction and Geological Outline The Northern Apennines fold and thrust belt (fig- orthogonal convergence between the Iberian-Euro- pean and Adriatic plate margins (Elter and Pertusati ure 1) evolved through the suturing of the Jurassic Ligurian ocean by convergence of the Adria sub- 1973; Boccaletti et al. 1980; Principi and Treves 1984; van Wamel 1987). In the paleogeographic plate with the Iberian-European plate. The present structural setting includes several thrust units de- reconstructions so far proposed, the various overthrust units are retrodeformed across the chain rived from two paleogeographic domains: the Ligu- rian oceanic basin and the Adriatic continental and are located as adjacent basins originally in se- quence along the shortening axis. These models, margin. The oceanic (Ligurid) units form the upper- most structural nappes of the chain, which were de- however, fail to explain several major large-scale features of the chain, including the lack of a mag- formed during the Late Cretaceous-Eocene oceanic closure and successively thrust eastward onto the matic arc and the relationships of the plate mo- tions; moreover, many problems arise in inter- continental margin units (Sub-Ligurian and Tuscan Units) during collision in the late Tertiary (Bocca- preting, in a pure convergence context, the stratigraphic and structural relationships among letti et al. 1971; Elter 1975; Abbate and Sagri 1982; Abbate et al. 1980, 1986, 1994; Boccaletti et al. the Ligurid thrust units. These inconsistencies can be addressed by postulating a major role of oblique 1980, 1984; Treves 1984). The Apennines orogenic evolution has been pre- convergence (transpression). In this paper, we discuss several large-scale indi- viously analyzed in a simple cylindrical frame of cations of transpression, and we describe in detail some aspects of the Ligurid Units that suggest the 1 Manuscript received April 8, 1997; accepted October 7, existence of a number of terranes juxtaposed during 1997. the early (Late Cretaceous to Early Oligocene) evo- 2 Centro di Studio per la Geologia dell’ Appennino, Uni- versita’ di Firenze, 50121, Italy. lution of the Northern Apennines chain. [The Journal of Geology, 1998, volume 106, p. 149–162]  1998 by The University of Chicago. All rights reserved. 0022-1376/98/10602-0005$01.00 149