Melange

Melange:

A mixture of rock materials, consisting of small to very large blocks in a ground-up matrix

A mixture of rock types jumbled together by tectonic activity

Mixture of different rocks formed by sedimentary or tectonic processes. Characterized by a block-in-matrix structure: more rigid rock types form blocks in a matrix of softer and strongly deformed rock. The blocks can range in size to kilometers. Most mélanges form in subduction zones.

Tectonically jumbled rock, where blocks of varying rock types are entrained in a sheared matrix (block-in-matrix). The entrained blocks can be of the same origins as the matrix, or can be of exotic origin.

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Where in California:

The Franciscan mélange is an artifact of the Franciscan Formation, and the most prominent and well known mélange in California. Most of the northern portion of the Coast Ranges geomorphic province in California is constructed of the Franciscan Formation (or Complex), which formed in the Mezozoic to Cenozoic-era subduction zone, as geologic units were conveyed eastward by the subducting Farallon plate, and shed off the North American Plate, then metamorphosed within the pressure and heat of the subduction zone. These materials were crushed, sheared off, and later uplifted to form the Coast Ranges.

Subduction ceased with the subsuming of the Farallon Plate, and the newly arrived Pacific Plate brought transform/strike slip mechanics to the western edge of the North American plate. This lateral grinding energy has further deformed and uplifted the Franciscan Complex, as well as regions of crushed mélange matrix within the complex, punctuated by small and large resistant rock outcroppings.   

Within the Franciscan mélange there are distinct units dominated by weak material susceptible to forming the ground-up matrix, such as shale or serpentinite dominated melange. These landscapes form the gently rolling hills and agricultural lands of western Marin and Sonoma Counties. Franciscan melange landscapes can also be seen in the softly slumping, undulating, and eroding landscapes of coastal slopes along Highway One in Marin, Sonoma, and Mendocino Counties; and along the north coast of San Francisco at the Golden Gate.

Rocks of the Franciscan mélange can include, graywacke, chert, basalt, serpentinite, greentone; and less commonly high grade metamorphics such as blueshcists, eclogite, amphibolite, and others.