The Franciscan Complex forms the structurally complicated, locally
chaotic basement of the Northern and Central California and southwestern
Oregon Coast Ranges. It is an accretionary wedge formed during the Late-
Jurassic-Tertiary subduction along the west coast of North America.
In northern California, the Franciscan is subdivided into three belts, the
Western, Central, and Coastal belts, which show a zonation in age,
metamorphic “grade”, and structural style. Franciscan mélanges are present
in the Central belt, as well as the Diablo Range --a tectonic window within the
structurally overlying Great Valley Group--, and the Nacimiento Block, where
the study area is located.
One of the best exposures of Franciscan mélange, where contact
relationships between blocks and matrix can be observed, crops out along 6
km of seacliffs near San Simeon. Boudinaged blocks of graywacke,
greenstone, chert, and much rarer blueschist and graphite-schist are
ix
dispersed in the shale matrix. Block sizes range from 10 cm to 15 m. The
discovery of interlayered blueschist and graphite-schists, and the presence of
lawsonite in some graphite-schists demonstrates that these two lithologies
were metamorphosed together. Graphite-schist blocks in the Franciscan have
not been reported prior to this study.
Two main mineral assemblages were recognized among the studied
34 mafic blueschist blocks: 1) Lws + Na-amp + Pmp + Phe + Ttn + Chl
recrystallized at ~5 kbar and 200-250°C, and 2) Lws + Na-amp + Pmp + Phe
+ Ttn + Ep + Chl, which recrystallized under slightly higher temperatures but
similar pressures (300-350°C, at 5 kbar). A pre-blueschist facies metamorphic
event under greenschist facies conditions is recorded by calcic cores
overprinted by Na-amp rims in about half of the blocks. Sodic amphibole rims
with a higher Fe3+ content probably developed due to the breakdown of
epidote during a decrease in T. These mafic blueschists followed a
counterclockwise P-T path.
Remnants of “actinolitic rinds”, which are reaction zones formed when
the blocks were in contact with serpentine, were found associated with nine of
the studied mafic blueschist blocks. Such rinds were thought to be unique to
better-studied Franciscan high-T blocks.
Graphite-schist blocks (30 studied) contain Qtz + Phen + Ab + Gr, and
are of two types. One type has relict sedimentary textures with a weak
foliation defined by graphite and pressure solution seams. The other type has
a compositional layering with layers containing well recrystallized quartz. Nine
of these blocks also contain lawsonite within the more graphitic layers.
The geochemistry of the blueschist blocks indicates that they were
derived from the MORB-like oceanic crust, and seamounts underplated during the initiation of subduction. The mainly mafic protolith contained a
small volume of interlayered sediment, as indicated by the presence of
associated metasedimentary graphite-schists. Blueschist facies conditions
were attained at the bottom of the overriding plate during the initial states of
Franciscan subduction (150-155 Ma). Mafic material continued to be
underplated, and low-T dynamic blueschist-facies metamorphism continued
to form until at least ~137 Ma.
A model is proposed in which exhumation was facilitated by normal
faulting near the surface driven by gravitational collapse driven by decrease in
subduction plate dip around 80 Ma that is known to have caused the
Laramide orogeny. During this time, blueschist and graphite-schist blocks
were plucked from the bottom of the hanging wall, incorporated into the shaleand
water-rich shear zone at the plate interface, and exhumed during the
upward flow of mélange driven by the movement of the downgoing plate.
Blocks were extended and boudinaged, and metasomatically altered in the
cataclastic zones that developed along necks and margins of the blocks
during upwelling and the dewatering that led to final compaction near the
surface. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/ETD-UT-2010-05-1518 |
| Date | 20 October 2010 |
| Creators | Ukar, Estibalitz, 1980- |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Type | thesis |
| Format | application/pdf |
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