New Mexico Geological Society Annual Spring Meeting — Abstracts

The Picuris Range contains early Proterozoic supracrustal rocks that, have experienced progressive polyphase deformation at medium grade metamorphic conditions. The ortega Group, which is the dominant lithostratigraphic unit in the range, is a 4+ km thick transgressive sequence of terrigeneous metasedimentary rocks that accumulated in a shallow marine setting.

The Ortega Group is structurally dominated by all scales of second-generation, east-trending, tight to isoclinal, overturned folds (F2). The major fold in the range is a shallowly west-plunging synform with half wavelength of 7 km. Well-preserved original sedimentary structures in quartzites and schists indicate that this structure is a syncline. The 1 km thick, highly competent, basal Ortega Quartzite folded mainly by pure buckling, whereas thinly interlayered schists and quartzites deformed mainly by passive folding mechanisms, resulting in highly disharmonic and noncylindrical fold patterns.

The dominant foliation in the Ortega Group is an east-trending schistosity/spaced cleavage (S2*) which is slightly oblique (10-15° counter clockwise) to the axial surface of F2 folds. This transecting S2* cuts both outcrop scale F2 folds and the map scale Hondo syncline. Measurements of bedding-cleavage vergence relationships on the limbs of the Hondo synclineI typically yield the wrong sense for the major fold structure.

There is debate in the literature concerning the interpretation of the relative timing of a transecting cleavage and fold development. One popular model states that transection occurs when cleavage forms late in the folding history, whereas another states that transection is due to multiple deformations in which a foliation-forming event overprints an earlier set of folds. Although geometrical fabric relationships are ambiguous in the Picuris Range regarding the absolute timing of F2 and S2*, deformation was progressive rather than episodic, suggesting thatI S2* developed either late during folding, or soon afterwards.