26-2 A Field Guide to Deciphering the Origin of a Franciscan Mélange

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

Presenting Author:

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Abstract:

Deciphering the origin of a Franciscan mélange is challenging, although much has been written on the subject. In general, a mélange is a mappable, km-scale, geologic unit consisting of isolated, structurally competent, map-scale blocks (>30m) and smaller fragments – commonly of dissimilar metamorphic grade – enveloped by or integrated with a relatively ductile matrix, consisting of siliciclastic mudstone and/or serpentinite-bearing mudstone, which typically is pervasively sheared (tectonized). Exceptions include siliciclastic sandstone matrix mélange and metamorphosed matrix mélange.

In the ideal case, if every mélange block is observed, everywhere, to be enveloped by unmetamorphosed matrix, then a sedimentary origin is clear; short of that, however, this sedimentary mélange can alternatively represent a secondary, tectonized component, resting on, or integrated with, an older tectonic mélange, as an intraformational, syndeformational deposit. This interpretation is more likely if the tectonized siliciclastic blocks do not contain intraformational clasts.

Furthermore, if concordant deformation of the isolated greenstone blocks is observed (D₁), then this may link them together as a formerly coherent layer, which deformed in situ, prior to the sedimentary mélange. If the mélange contains blocky, serpentinized mantle peridotite (aka, “tectonite”) that forms a trend in accordance with the deformation observed in the greenstone blocks, then a tectonic origin is more likely. The tectonized, overlying sedimentary mélange can locally consist of a mixed, clastic serpentinite- and siliciclastic-matrix, which may contain blueschist clasts that were eroded from uplifted tectonic mélange bearing blueschist enveloped by tectonite.

The nature of the tectonism may be resolved by the block structures and the kinematic indicators observed. Upright folding, related to layer parallel shortening, and folded marginal faulting, related to reverse faulting, reliably indicate contractile deformation. If this structure is overprinted by normal faulting, then an extensional tectonic origin is indicated.

Although coherent, Lower Great Valley Group is found structurally resting on younger, fully evolved, contractile Franciscan tectonic mélange, bearing unmetamorphosed matrix, the creation of a tectonic mélange and the subsequent activity of the Coast Range fault should be treated as two discrete, independent events, unless it can be clearly demonstrated that the structures are coeval and concordant. Similarly, the uplift of the blueschist blocks from great depth should also be treated as a discrete, independent event, unless coeval concordance is clearly demonstrated.

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