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Shale matrix mélange with clasts of sandstone and greenstone on Marshall's Beach, San Francisco, USA

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Franciscan Assemblage or Franciscan Complex is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andrew Lawson, who also named the San Andreas fault that defines the western extent of the assemblage.

The Franciscan Complex is dominated by greywacke sandstones, shales and conglomerates which have experienced low-grade metamorphism.  Other important lithologies include chert, basalt, limestone, serpentinite, and high-pressure, low-temperature metabasites (blueschists and eclogites) and meta-limestones. Fossils like radiolaria are found in chert beds of the Franciscan Complex. These fossils have been used to provide age constraints on the different terranes that constitute the Franciscan. The mining opportunities within the Franciscan is restricted to deposits of cinnabar and limestone.

The outcrops of the formation have a very large range, extending from Douglas County, Oregon to Santa Barbara County, California. Franciscan-like formations may be as far south as Santa Catalina Island. The formation lends its name to the term describing high-pressure regional metamorphic facies, the Franciscan facies series.

Chevron folds in ribbon cherts of the Marin Headlands, California.

DESCRIPTION:

Ribbon chert of the Marin Headlands Terrane, exposed at Marshall's Beach, San Francisco

The units of the Franciscan complex are aligned parallel to the active margin between the North American and Pacific plates [1]. The Franciscan Complex is in contact with the Great Valley Sequence, with a continuous sheet of serpentinite at its base, along its eastern side[2]. The type area of Franciscan rocks in San Francisco consists of metagraywackes, gray claystone and shale, thin bedded ribbon chert with abundant radiolarians, altered submarine pillow basalts (greenstone) and blueschists [3]. Broadly, the Franciscan can be divided into two groups of rocks. Coherent terranes are internally consistent in metamorphic grade and includes folded and faulted clastic sediments, cherts and basalts, ranging from sub-metamorphic to prehnite-pumpellyite or low-temperature blueschist (jadeite-bearing) grades of metamorphism. Mélange terranes are much smaller, found between or within the larger coherent terranes and sometimes contain large blocks of metabasic rocks of higher metamorphic grade (amphibolite, eclogite, and garnet-blueschist)[4]. The mélange zones in the Franciscan usually have a block in matrix appearance with higher grade metamorphic blocks (blueschist, amphibolite, greenschist, eclogite) embedded within the mélange matrix[5]. The matrix material of the mélanges are mudstone or serpentinite. Geologists have argued for either a tectonic or olistostormal origin[6]. In the Northern Coast Ranges, the Franciscan has been divided into the Eastern, Central and Coastal Belts based on metamorphic age and grade, with the rocks younging and the metamorphic grade decreasing to the west[7][8][4] . The Franciscan varies along strike, because individual accreted elements (packets of trench sediment, seamounts, etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of erosion produced the present-day surface distribution of high P/T metamorphism[4][9].

Blue folded metacherts with glaucophane-rich (ashy?) layers.
Shale matrix mélange of the Franciscan Complex at Marshall's Beach, San Francisco, California. Sandstone blocks (light grey) contain white mineral veins. Dark grey shale matrix displays strongly foliated anastomosing scaly fabric. Geologist John Wakabayashi for scale.
Pillow structures preserved in greenschist-facies metamorphosed basalts of the Franciscan Complex, Black Sands Beach, Marin Headlands, California. Field of view is approximately 2 m wide.
Sheared block-in-matrix fabric composed of serpentinite blocks in serpentinite matrix, exposed on Perles Beach, Angel Island, Marin County, California. Pencil for scale. The strong anastomosing foliation is folded, sub-vertical in the lower part of the photo and more gently dipping in the upper part of the photo.


Tight folds in blue metachert with glaucophane-rich (ashy?) layers

GEOLOGICAL HISTORY:

The Franciscan Complex is an assemblage of metamorphosed and deformed rocks, associated with the east dipping subduction zone at the western coast of North America[10]. Although most of the Franciscan is Early/Late Jurassic through Cretaceous in age (150-66 Ma),[11] some Franciscan rocks are as old as early Jurassic (180-190 Ma) age and as young as Miocene (15 Ma)[12] . The different age distribution represents the temporal and spatial variation of mechanism that operated within the subduction zone [9]. Franciscan rocks are thought to have formed prior to the creation of the San Andreas Fault when an ancient deep-sea trench existed along the California continental margin. This trench, the remnants of which are still active in the Cascadia and Cocos subduction zone, resulted from subduction of oceanic crust of the Farallon tectonic plate beneath continental crust of the North American Plate. As oceanic crust descended beneath the continent, marine basalt and sediments were subducted and then underplated to the upper plate[4]. This resulted in widespread deformation with the generation of thrust faults and folding, and caused high pressure-low temperature regional metamorphism [4]. In the Miocene, the Farallon-Pacific spreading center reached the Farallon trench and the relative motion between Pacific-North America caused the initiation of the San Andreas Fault. Transform motion along the San Andreas Fault obscured and displaced the subduction related structures, resulting in overprinting of two generations of structures. [13]




  1. ^ Wassmann, Sara; Stöckhert, Bernhard (2012-09). "Matrix deformation mechanisms in HP-LT tectonic mélanges — Microstructural record of jadeite blueschist from the Franciscan Complex, California". Tectonophysics. 568–569: 135–153. doi:10.1016/j.tecto.2012.01.009. ISSN 0040-1951. {{cite journal}}: Check date values in: |date= (help)
  2. ^ Turner, Francis J. (cop. 1986). Metamorphic petrology : mineralogical, field, and tectonic aspects. Hemisphere publ. ISBN 089116510X. OCLC 468257861. {{cite book}}: Check date values in: |date= (help)
  3. ^ Wahrhaftig, Clyde (1984). A Streetcar to Subduction and Other Plate Tectonic Trips by Public Transport in San Francisco. Washington, D. C.: American Geophysical Union. ISBN 0875902340.
  4. ^ a b c d e Wakabayashi, John (1992-01). "Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California". The Journal of Geology. 100 (1): 19–40. doi:10.1086/629569. ISSN 0022-1376. {{cite journal}}: Check date values in: |date= (help)
  5. ^ HSÜ, K. JINGHWA (1968). "Principles of Mélanges and Their Bearing on the Franciscan-Knoxville Paradox". Geological Society of America Bulletin. 79 (8): 1063. doi:10.1130/0016-7606(1968)79[1063:pomatb]2.0.co;2. ISSN 0016-7606.
  6. ^ Wakabayashi, John (2011-08), "Mélanges of the Franciscan Complex, California: Diverse structural settings, evidence for sedimentary mixing, and their connection to subduction processes", Geological Society of America Special Papers, Geological Society of America, pp. 117–141, ISBN 9780813724805, retrieved 2019-11-03 {{citation}}: Check date values in: |date= (help)
  7. ^ James O. Berkland (2), Loren A. Ray (1972). "What is Franciscan?". AAPG Bulletin. 56. doi:10.1306/819a421a-16c5-11d7-8645000102c1865d. ISSN 0149-1423.{{cite journal}}: CS1 maint: numeric names: authors list (link)
  8. ^ Blake, M.C.; Howell, D.G.; Jones, David Lawrence (1982). "Preliminary tectonostratigraphic terrane map of California". Open-File Report. doi:10.3133/ofr82593. ISSN 2331-1258.
  9. ^ a b Mulcahy, Sean R.; Starnes, Jesslyn K.; Day, Howard W.; Coble, Matthew A.; Vervoort, Jeffrey D. (2018-05). "Early Onset of Franciscan Subduction". Tectonics. 37 (5): 1194–1209. doi:10.1029/2017tc004753. ISSN 0278-7407. {{cite journal}}: Check date values in: |date= (help)
  10. ^ HAMILTON, WARREN (1969). "Mesozoic California and the Underflow of Pacific Mantle". Geological Society of America Bulletin. 80 (12): 2409. doi:10.1130/0016-7606(1969)80[2409:mcatuo]2.0.co;2. ISSN 0016-7606.
  11. ^ Bailey, Irwin and Jones (1964), p. 142-146; Blome and Irwin (1983), p. 77-89..
  12. ^ McLaughlin (1982), p. 595-605.
  13. ^ Wentworth et al. (1984), p. 163-173; Irwin (1990), p. 61-82.
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