Upper Granite Gorge, Grand Canyon

Mg Ka map from a garnet crystal, mile 83, Upper Granite Gorge, Grand Canyon (Green - garnet; red - chlorite; orange - biotite; dark blue - muscovite; dark colors - primarily quartz). Note that the garnet contains a slightly curving inclusion trail fabric. This is interpreted to be an older, sub-horizontal deformational fabric, that strikes northwest and dips shallowly to the north throughout much of the Grand Canyon. The external fabric is northeast-striking and steeply dipping, and represents the D2 fabric that defines zones of intense late-stage strain throughout the Canyon. The core of the garnet crystal is Ca-rich. It is interpreted to have grown at temperatures of 550 C and pressures of ca. 6 kbars (20 km depths). This core overgrew a straight S1 matrix fabric during early stages of the folding and transposition of S1 into the vertical S2 orientation now seen outside the crystal. The rims of the garnets are lower in Ca, higher in Fe and grew at temperatures of 550 C and 3 kbars (10 km) in the late stages of the D2 event. Thus, this one crystal may record the decompression of the rocks from 20 to 10 km depths during D2 deformation. Note that the dark quartz-rich domains adjacent to the garnet form asymmetric strain shadows that document a shearing component during D2 shortening. However, this shearing component is incompatible with the apparent curvature of the inclusion trails suggesting that the sample may have an even more complex history.

For more information about this image contact: Michael L. Williams