SYNCHRONOUS EXTENSION AND CONTRACTION IN THE ANACONDA METAMORPHIC COMPLEX

The footwall of the Anaconda Complex in SW Montana contains a locally mylonitic package of rocks with a thinned stratigraphic sequence that hosts a confusing array of mesoscopic structures. Recent detailed mapping and geometric analysis reveals a sequence of events something like this: 1) Transposition of layering and isoclinal folding (F1); 2) Boudinage on all scales; 3) Two generations of asymmetric folding (F2 and F3) with axial plane crenulation cleavages. The two generations may have been separated by; 4) Extension on a sub-horizontal a mylonitic detachment; 5) Thrusting that placed Carboniferous rocks over Cretaceous rocks, generating a foliation that may be the same as one of those in (3) above; 6) Gentle folding of the mylonitic detachment and the thrust fault; 7) Formation of a widespread, N-S-striking, vertical fracture cleavage. Timing of specific events is not known, but crosscutting relationships and preliminary radiometric dating by other authors suggest that deformation began sometime after deposition of the late Cretaceous Colorado Group and continued into and perhaps through Eocene times.

Complexity and topography limit the rate at which detailed structural analysis can occur, and we do not yet know how widely the various structural generations can be correlated. Some have been seen over distances of many miles, some only within a few square miles. Problems also lie in determining which generations of structures were produced by crustal extension and which by contraction. It seems reasonable to distinguish extension and contraction events by whether they create older-over-younger or younger-over-older relationships, and whether they thin or thicken stratigraphy. In the case of fold generations, though, controversy exists. The kinds of folds expected to form during extension include sub-horizontal sheath folds, folds formed in high-grade rocks due to back rotation between shear zones, flow between boudins, fault-bend and fault-propagation folds drape folds, and folds formed by coaxial sub-vertical shortening and horizontal extension. Upright folds with steep axial plane foliations like F2 and F3 described above are more likely to have formed during crustal shortening. Given the sequence of structures described here, then, it appears as if contraction and extension in this part of the Rocky Mountains broadly overlapped in time.