Microfolds in Chapter 1: Folds

LARROVERE, MARIANO A.

Atlas of Structural Geology

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2015; p. 13 - 13

FIGURE 1.24 Angular folds (kink bands) in tourmaline-bearing garnet-mica schist. Dark green and pale brown minerals in dark bands are of tourmaline. Small dark green crystals are the curved three-sided (basal) cross sections of the tourmaline. The folds are asymmetric showing larger limbs in the left side. In the hinge zones some prismatic sections of tourmaline are slightly bent but most of them are undeformed. Bent tourmaline crystals can be interpreted as pre- to syn-folding, whereas undeformed tourmaline grew at least partially after folding. Observed abundant basal sections suggest that tourmaline also grew parallel to the fold axis orientation. Section normal to the fold axis and to the foliation. Width of view 22 mm. Plain polarized light. Location: Sierra de Ambato, Catamarca Province, NW Argentina. GPS point (WGS84): S 27°38′35.6″ - W 66°12′39.5″; Rock type: tourmaline-bearing garnet-mica schist; Formation name: Quebrada del Molle Metamorphic Complex; Age (relative): Ordovician; Facies/grade: greenchist facies / garnet zone. FIGURE 1.25 Microfolded porphyroblast of cordierite (in the center of the photo) in gneiss. Subgrains occur within the large elongate deformed cordierite grain. They are recognized optically by low relief boundaries and differential extinction between subgrains. Inside the cordierite crystal is observed a bent inclusion pattern marked by micas (internal foliation) that shows continuity with the folded subhorizontal gneissic foliation S1. The crystal of cordierite is bordered by two ribbons of recrystallized quartz that also are folded. A subvertical crenulation cleavage S2 can be recognized. The cordierite porphyroblast may be classified as pretectonic with respect to S2 since cordierite is folded and internally deformed by the same deformation phase (D2) that generated the crenulation cleavage S2. On the other hand, cordierite porphyroblast is interpreted as post-tectonic with respect to the subhorizontal gneissic foliation since it includes S1. Section normal to the microfold axis and to the foliation. Width of view 13 mm. Cross polarized light. Location: Sierra de Aconquija, Catamarca Province, NW Argentina. GPS point (WGS84): S 27°34′07.6″ - W 66°14′20.6″; Rock type: cordierite-mica gneiss; Formation name: Quebrada del Molle Metamorphic Complex; Age (relative): Ordovician; Facies/grade: greenchist facies / medium grade.