RECOGNITION AND MAPPING OF MELANGES: IMPLICATIONS FOR ENGINEERING PROJECTS

Melanges, geologic bedrock units with blocks in a weak, commonly sheared, matrix have been recognized by geologists as significant components of orogenic belts for decades, yet the engineering community has been slow to appreciate their importance. Serious errors continue to be made because of mischaracterization of melanges. The history of mapping melanges in the Franciscan Complex of California serves as an illustration of how strongly interpretive field geology is, and how drastically different interpretations of the same unit can be. Prevailing theories determine how geologists draw contacts on maps and cross sections between scattered outcrops or between exploratory boreholes. Prior to the late 1960's, geologists portrayed Franciscan geology exposures as ordinary stratified geologic packages; geologists mapped continuous "beds" between outcrops. Following the recognition of melanges in the late 1960's, geologists mapping the same region outlined blocks in a (commonly unseen) melange matrix rather than depicting continuous stratigraphy on the map. Further research showed that the Franciscan was composed of both coherent thrust sheets and melanges, whereas many viewed (and still view) the entire Franciscan Complex as a melange. In many engineering projects, melanges are still treated as stratified units, or occasionally mistaken for soil with boulders.

Whether a melange is of tectonic or sedimentary origin is unimportant for engineering purposes. In contrast, the grade of metamorphism of the matrix can be important, because significant recrystallization of the matrix strengthens it and reduces block-matrix strength contrasts. A melange of engineering significance has a brittle rather than ductily-deformed matrix, metamorphosed under greenschist or lower temperature conditions. For example, in the Sierra Nevada, some melanges have a brittle shale matrix in the north (greenschist or lower metamorphic grade), but this matrix is quartz mica schist (lower amphibolite and higher grade) to the south. Recognition of coherent (non melange) units in contrast to melanges in the field is not trivial; experience with such rocks on the part of the participating geologist(s) and a well conceived exploration plan are critical for adequate characterization.