New Mexico Geological Society Annual Spring Meeting — Abstracts

New detailed mapping (1:6000) of Proterozoic rocks SW of the town of Placitas, New Mexico, has revealed a complex deformational history. Paleoproterozoic deformation ill this area is characterized by a bedding parallel cleavage s^1/0. This fabric is isoclinally folded and transposed resulting in a north-striking, variably east-dipping tectonite fabric s^1/2. Metamorphic grade during this deformation is difficult to constrain due to later metamorphism but most likely occurred at upper greenschist to lower amphibolite grade. This is based on hbl+epi+/-chl+/-gar+/-bt mineral assemblages in mafic rocks collected 2 km from the 1.44 Ga. Sandia granite.

Mesoproterozoic deformation associated with the emplacement of the Sandia granite overprints early fabrics. S³ is the axial plane cleavage of open to tight, moderately NE plunging folds. These folds formed during deformation and intrusion of the Sandia granite. S³ consists of a differentiated cleavage defined by K-feldspar and sillimanite rich segregations in the highest grade rocks adjacent to the pluton. S³ fabrics are also related to the development of transtensional shear bands. Open buckle folds of F³ generation have axial planes which grade along strike into shear bands containing melts. It is not clear if shear bands formed on S³ foliations or if buckle folds form at the termination of shear bands.

Contact metamorphic porp'hyroblasts have microstructures indicating reactivation of s^1/2 during plutonism. S^1/2 matrix foliations developed to stage 5 and 6 of Bell and Rudenach, wrap around andalusite porphyroblasts, which overgrow stage 2 crenulations. S^1/2 is also steepened at the margin of the pluton and has been largely recrystallized to folia of sill+bt+/-k-spar. These features suggest that new movements occurred across old fabrics during pluton emplacement.

Pegmatite dikes can be subdivided into two geometric groups. The first are anisotropy-controlled dikes intruded concordantly with s^1/2. These dikes locally crosscut F³ folds and are subsequently folded, allowing estimates of up to 50% longitudinal strain. This suggests that F³ folds tightened during dike emplacement. The second set of dikes strike east-west and dip steeply south and are not strongly shortened. This suggests that dikes of this orientation are intruded perpendicular to a shallowly dipping, NE trending, incremental stretching direction.

Tertiary normal faults associated with the Rio Grande Rift modify contacts between the Sandia granite and country rocks. North-striking faults repeat the granite and its contact aureole twice in the northern portion of the mapped area. These faults frequently juxtapose Proterozoic rocks and alluvial terraces. Valleys are filled with thick accumulations of sediment which have been incised and show evidence of headword erosion. This suggests that latest movement and uplift on faults may be Holocene in age.