TECTONIC EVOLUTION OF THE THETFORD MINES OPHIOLITIC COMPLEX AND THE ST.-DANIEL MÉLANGE, THETFORD MINES, QUÉBEC: NEW RESULTS BASED ON FIELD MAPPING AND GEOPHYSICAL INVERSION OF AEROMAGNETIC DATA

In southern Québec, the Thetford-Mines ophiolitic complex (TMOC) and the St-Daniel Mélange occur in the hanging wall of the St-Joseph fault, a major SE-dipping normal fault that bounds the eastern flank of the Notre-Dame Mountains anticlinorium (NDMA). Structural analysis and Ar isotopic dating in the NDMA indicate polyphased metamorphism and deformation involving: Ordovician crustal thickening and Silurian/Early Devonian backthrusting and extension; superimposed by Middle Devonian folding. Field mapping in the TMOC and the St-Daniel Mélange suggests a broadly similar scenario. (i) NW-SE to N-S trending faults that are associated with "late" ultramafic intrusions crosscut the ophiolitic lower crust, and are inferred to represent pre-obduction structures formed during syn-oceanic extension. This implies that some of the dislocation seen in the TMOC pre-dates obduction. (ii) Ordovician structures, attributed to ophiolite obduction, are recorded by syn-metamorphic fabrics that are dominant towards the base of the TMOC and in the underlying continental margin rocks, but which are absent in the upper part of the TMOC and the overlying St-Daniel Mélange. Detailed mapping suggests that the contact between the TMOC and the St-Daniel Mélange is an erosional unconformity, and that the St-Daniel probably represents a syn-emplacement piggy-back basin. (iii) Silurian/Early Devonian structures consist of SE-verging faults and folds in both units, and produced a stratigraphic inversion in the TMOC. (iv) Middle Devonian folding is associated with Type 1 and 2 interference patterns and high-angle reverse faults. Inversion of aeromagnetic data from the TMOC successfully identifies the basal contact of the ophiolite against Laurentian margin metasediments. Furthermore, the data show that crustal and mantle rocks of the TMOC have a distinct magnetic signature, the mantle rocks showing up as lows in comparison to the cumulates. The aeromagnetic map pattern implies that the crust/mantle contact is oriented approximately N-S, at 90o to what is shown on existing geology maps; and that the ophiolite pseudostratigraphy is inverted, which is consistently with our proposed structural evolution.