November 7. The Franciscan Mélange

Last month, in the Jurassic, we talked about the onset of a major subduction zone along the western margin of North America. Ultimately, this subduction created the magmatic arc of volcanoes whose roots are exposed as the Sierra Nevada Batholith, the 400-mile-long granite body that forms the Sierra Nevada Mountains today. 

It’s easy to visualize a standard subduction system, and there are plenty of diagrams out there to help imagine what it was like. It’s sometimes harder to remember that such systems are usually anything but the simple cross-sections you commonly see. There are piles and piles of sediment coming off rising mountains as well as off any islands outboard from the main event, plus sometimes small continental fragments, piles of older volcanics on the subducting oceanic crust, and even slices of oceanic crust that get caught up in the whole process.

In California, the subduction system that got started in the Jurassic got complicated during the Cretaceous. 

Diagram showing model for emplacement
of Franciscan Assemblage and deposition
of Great Valley sequence by
Mikesclark, used under Creative Commons license.

There are two thick assemblages of sediment and other rocks west of the Sierra Nevada Mountains, and they are more or less contemporary with the formation of the Sierra Nevada Batholith. West of the magmatic arc created by the volcanoes above the subduction zone, there was a basin. It’s fair to think of this as a lot like the oceanic trench beyond the margin of the continent and the magmatic arc, much like the Peru-Chile Trench along the west side of the Andes in South America today. High mountains give high erosion rates, and rivers and submarine channels fed sediment eroded off the Sierra Nevada magmatic arc into this basin, which today is mostly under the Great Valley of Central California, as well as in the eastern part of the Coast Range. It’s called the Great Valley Sequence, and in places it’s as much as 40,000 feet thick – a huge pile, which more or less represents the erosional debris of the volcanoes in the magmatic arc, as well as a good bit of the deep-seated granite at their roots.

Further west, where oceanic crust was beginning its descent beneath the North American continental plate, sediments were also building up. But there, they were more deep-ocean abyssal plain type sediments, including the siliceous oozes formed by a submarine rain of silica shells of radiolarians, microscopic floating protozoa. Those siliceous oozes ultimately produced chert. Some deep-water shales and limestones were out there as well, and like the chert, they were deposited more or less directly upon oceanic crust.

The zone where the slab of oceanic crust bends to dive beneath a continent can be a zone of intense deformation. The sediments and rocks are being squeezed between the oncoming oceanic crust and the colliding continental crust, so that often everything there may be broken by thrust faults, with slabs and blocks being pushed over other pieces. The deformation in what is now western California even sliced off some parts of the oceanic crust itself – and maybe even parts of the underlying mantle – and pushed them onto the pile. Slices of oceanic crust like that are called ophiolites – a name that means “snake rock,” for the green, platy appearance of the rocks.

The composition of oceanic crust is more or less basaltic, rich in iron and magnesium and low in silica compared to continental granitic rocks. Ophiolites form under conditions of very high pressure, but at temperatures that are not nearly as high as those associated with deep volcanic mountain roots. The resulting metamorphic rocks are unusual – high pressure but low temperature. Serpentine is one common metamorphic rock found in ophiolites, and in western California, another is blueschist – blue because of the presence of the blue mineral glaucophane, a sodium magnesium aluminum silicate that forms in these special conditions of high pressure but low temperature.

Map by Mikesclark used under Creative Commons license.
Map modified from Irwin (1990) showing distribution
of Great Valley Sequence and Franciscan Assemblage 

The package of rocks I’ve been describing is called the Franciscan Assemblage. It lies west of the Great Valley Sequence, but the deformation has brought the two packages into close and complex relationships in places in the California Coast Range. The relationship is everywhere a fault or a fault zone. Some parts of the Franciscan are so broken up that the rock is called a mélange – an extremely chaotic jumble of rocks on a huge scale in some places – with blocks that may be hundreds or thousands of feet long structurally mixed with ground-up rocks right down to sand size or smaller. A real mess.

It gets even more complicated. In some places, the Franciscan rocks have become caught up in fault zones that essentially squeeze them into anomalous positions in the modern landscape. Mount Diablo, east of San Francisco, is a bulge of Franciscan rocks that has risen along relatively modern faults because the overburden was eroded away. Mt. Diablo is sometimes referred to as a diapir, not a diaper – it’s spelled d ia pir, from a Greek word for ‘piercing through’ -  an uplift that is more or less a slow flowing of the rock, as in a salt dome. It’s probably a combination of diapiric flow and faulting. There are some even larger blocks of the Franciscan that may represent a situation like Mt. Diablo.

Both the Great Valley Sequence and the Franciscan Assemblage range in age from late Jurassic into the Cenozoic, but most of both were laid down during the Cretaceous Period. The deformation that scrunched the rocks together and uplifted them began then too, but the present landscape is mostly the result of forces that are much more recent.

—Richard I. Gibson

LINK:
Melange diaprism 

Diagram showing model for emplacement of Franciscan Assemblage and deposition of Great Valley sequence by Mikesclark, used under Creative Commons license. 

Map by Mikesclark used under Creative Commons license. Map modified from Irwin (1990) showing distribution of Great Valley Sequence and Franciscan Assemblage