Geological Evidences For Late Paleozoic Intra-pangea Dextral Shear

The Wegenerian configuration of Pangea at Jurassic times, also known as Pangea A, is not questione damong Earth scientists. Debate exists on its pre-Jurassic configu- ration since Ted Irving (1977 Nature) introduced twenty-five years ago Pangea B by placing Gondwana to the East by 2500km with respect to Laurasia on the basis of paleomagnetic data. Pangea B is mainly necessitated by paleolatitude discrepancy be- tween Africa (Gondwana) and North America (Laurasia), which would overlap by about 1700km if reconstructed in a Pangea A configuration at Early Perian times. West Gondwana/Adria poles support Pangea B in the Early Permian, but allow transi- tion to Pangea A by the end of the Permian in the late. stages of the Variscan/Hercynian orogeny (Muttoni et al., 1996 EPSL). Importantly, some of the Early Permian pale- opoles are in igneous rocks and/or from low paleolatitude sites (e.g., Adria), hence making inclination error less likely as an explanation for Pangea B as envisaged by Rochette and Vandamme (2001 Annali di Geofisica). A few Earth scientists tried to seek geological evidence for the pre-Jurassic dextral mega-shear between Gondwana and Laurasia necessary to go from Pangea B or similar configuration to the Pangea A configuration at the time of Atlantic opening in the Jurassic (e.g., Ricou, The Ocean Basins and Margins, Vol. 8, Plenum Press, 1996). Nevertheless, it is fair to say that Pangea B and its tectonic implications have not been broadly accepted but this leaves the problem of the supportive paleomagnetic data unresolved. The ultimate option is to abandon the geocentral axial dipole (GAD) hypothesis by introducing a non-dipole component in the Late Paleozoic paleomagnetic field to explain the paleolatitude dis- crepancy between Africa and North America (Van der Voo and Torsvik, 2001 EPSL). We are not so distressed by the geological consequences of Pangea B to abandon the GAD hypothesis which has been the basis for virtually all tectonic interpretations us- ing paleomagnetic data. We present new paleomagnetic data from Adria (i.e., West Gondwana) to verify whether Pangea B is paleomagnetically defensible, and review recent geological data from the literature in support of any pre-Jurassic intra-Pangea dextral mega-shear which could be reconciled with the necessary Pangea B to A tran- sition. Relevant geological lines of evidence for Gondwana vs. Laurasia Pangean mo- tions which build on the interpretations of late Hercynian faults sensu Arthaud and Matte (1977 GSA Bulletin) are as follows: (i) Late Paleozoic mantle-scale dextral 1 shear with simultaneous emplacement of orogen-scale granitoids characterized late Hercynian Pyrenees D2 deformation (e.g., Leblanc et al., Tectonophysics, 261, 1996; Gleizes et al., J. Struct. Geol., 20, 1998, Druguet and Hutton, J. Struct. Geol., 20, 1998). (ii) The Hercynian orogen of Gondwanan France and Iberia has been recently interpreted as the result of the juxtaposition of far-travelled terranes bounded by Late Paleozoic mainly dextral transpressive faults with up to 2000km of dextral displace- ment (Shelley and Bossiere, J. Struct. Geol., 22, 2000). (iii) the Alps preserve evidence for an orogen-scale Permian (trans)tensional event characterized by the emplacement of mafic and ultramafic rocks at the crust-mantle boundary (the Ivrea-Verbano Zone: e.g., Handy and Zingg, Geol. Soc. Am. Bull., 103, 1987; Handy and Streit, Earth Pl. Sc. Lett., 165, 1999; the Malenco Unit: e.g., Trommsdorff et al., Ofioliti, 24, 1999). A similar scenario characterized also the northern Apennines (Marroni et al., Tectono- physics, 292, 1998). Permian magmatic underplating is accompanied by coeval em- placement of granitic and rhyolitic magmatism in shallower levels of the (southern) Alpine crust (e.g., Bozen porphyrites), and volcanosedimentary sequences deposited in dextral pull-apart basins (i.e., Collio Fm.). (iv) central Europe is occupied by ca. 48000km3 of Permian volcanics erupted in a general trans(tensional) regime proba- bly along mantle-scale faults (Beneck et al., Tectonophysics, 266, 1996). These and other geological evidences suggest that docking of Gondwana to Laurasia occurred in a general dextral tectonic regime involving lithosphere-scale processes such as ana- tectic melting and/or magma channeling along shear zones, terranes tectonics, as- thenosphere unroofing and magma underplating which we reconcile with Pangea B to Pangea A transformation in the Late Paleozoic (and therefore not in the Triassic as proposed by Torcq et al. (1997 EPSL). Finally, we would point out that where de- formation is known to be dominated by continental large-scale transcurrent motions (e.g., San Andreas fault system, N. America Cordilleran terranes, New Zealand Alpine fault), it is difficult to find field evidence for more than half of the total displacement of juxtaposed plates/terranes (Umhoefer 2000 Tectonophysics).