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TABLE OF CONTENTS
The Schist of Sierra de Salinas, in detail
Originating
as a link between the Cretaceous southern Sierra Nevada and Peninsular Ranges
batholiths, Salinia was juxtaposed
to the east against the accretion-related Franciscan assemblage by ~330 km of
slip along the San Andreas fault system (CA Geo Map). The ages and isotopic
characteristics of the Salinian granitoids indicate an origin for the Salinian
basement as a west-facing Cretaceous arc straddling the cratonic margin
(Mattinson, 1990). This can be
easily visualized in a map of Ca batholithic rocks contoured by initial
strontium isotopes (Sr Map). Magmatism in the Salinian arc began between
100 and 110, and continued until 79 Ma, coincident with the major pulse of
magmatism that generated the other segments of the California arc to the north
and south (Kidder et al, 2003). Click here to see a comparison of
compilations of Cretaceous Salinian and Sierra Nevada U/Pb ages. As in the Sierra
Nevada Batholith, Cretaceous plutonism drifted
eastward in Salinia in the Cretaceous.
The Santa Lucia range south of Monterey contains the largest exposures of basement
rock in the Salinian block (Map and description).
The plutonic rocks of the central block are predominantly
Cretaceous calc-alkaline to calcic tonalites and granodiorites. In composition, appearance and isotopic
characteristics they resemble the eastern side of the California
batholiths. Interpluton contacts appear
mixed and gradational with migmatitic gneisses comprising about half of the
core of the Santa Lucia Range. (Click here to see Central Block Rocks) These
characteristics suggest deep conditions when contrasted with shallow granitic
terranes of the Gabilan Range or central Sierra Nevada which are dominated by
granitic plutons in sharp contact both with one another and with rare gneissic
rocks and screens.
We carried out garnet-biotite and GASP thermobarometery to
better understand the position of these rocks in the crust of the Cretaceous
batholith. This represents the first quantitative thermobarometry in the area.
(Sample Locations and Garnet
Images) Our samples come
from the core of the range, where gneisses make up about half of the rock, we suspect that this area exposes deeper rocks than
areas dominated by granitic rocks near Monterey or the Gabilan range.
Garnets appearing in the central block have homogenized cores,
we paired core compositions with biotite inclusions in quartz and feldspar, and
found temperature conditions in three samples around 900 C. The biotite inclusions have higher Ti than
other biotites in the rock, supporting an origin under peak conditions. Only two of our samples (sillimanite bearing)
contained assemblages appropriate for GASP barometric calculations. Pressures
we obtained of 8 and 10 kbar are nearly twice as deep as previous rough
estimates in the area, but are consistent with the results of gt-il and GRAIL
thermobarometry we also conducted.
These results show that the granitic crust of the Salinian arc
was quite thick, ~35 km. We estimate
that paleo- p-wave velocities would be a bit over 6 km/sec down to 35 km based
on the composition of the central block rocks.
This velocity is indistinguishable from that measured in the current
Sierra Nevada batholith, suggesting that the central block represents a typical
slice of the lowest granitic depths of the Sierran arc. The Sierran arc p-wave velocities, and our
work demonstrate granitic thicknesses 1.5 to 2 times the values of average
“modern arc” calculated in recent reviews, e.g. Christenson and Mooney,
1995. Seismic work reveals a similarly
thick granitic crust in the west central Andean cordillera. Click here to see a comparison of the above
mentioned seismic velocities. We suggest that continental arcs with thick
granitic columns represent an important subgroup of arcs. As these mature arcs contain the largest
masses of new or reworked continental compositions on the earth, they play a
critical role in generating, regenerating and maintaining the crust.
Of particular interest in this rare deep exposure of a mature
continental arc, is the presence of over fifty known partially to completely serpentinized ultramafic bodies. Previous petrologic work indicates that they
are cumulates akin to Alaskan type bodies (Bush, 1981). Our Rb/Sr and Sm/Nd isotopic analysis of
clinopyroxene and hornblende separates from these bodies show continental
signatures, falling from .707 to .715, indistinguishable from Salinian granitic
rocks. While we are still working out
the role of these bodies, we can now eliminate one suggested possibility. Sr isotopes indicate that the bodies are not
related to underthrust Franciscan rocks.
The schist of Sierra de Salinas is a homogeneous metagraywacke
with a fundamentally different origin from the heterogeneous framework gneiss
in the rest of Salinia. It has been
convincingly linked to the Pelona, Orocopia, and Rand schists (POR schists)
of southern California, which represent Cretaceous and Tertiary accretionary
wedge or forearc sediments underthrust after collision with a segment of
anomalously thick ocean plate (see e.g. Grove et al, 2003). The POR schists are found today in a belt
that stretches from Salinia to southwest Arizona along the San Andreas and Garlock
faults, and are believed to underlie a large portion of the Salinia and Mojave
regions. The replacement of the
Mojave-Salinia mafic arc root with continent-derived schists represents a
fundamental crustal reorganization in the late Cretaceous. It coincided with flat slab subduction, collapse
of the Salinian-mojave arc, cessation of magmatism in the Sierra Nevada and
Peninsular Ranges Batholiths, and the Laramide orogeny.
From modern analogs we know that the collision of seamounts
plays a key role in controlling long-term rates of tectonic erosion at arcs
(Clift and Vannucchi, 2004). We know
that even in accretionary margins, the majority of sediments are subducted (Van
Huene and Scholl, 1991). The proportion
that melts in the arc, the proportion that sticks to the lower crust, and the
proportion subducted deep into the mantle are unknown but critical figures in
understanding the growth and stability of the continents. The POR schists represent a large body of
underplated sediments, perhaps about the equivalent of a good sized
accretionary prism, which were tectonically eroded but escaped transfer to the
mantle. Understanding the origin,
structure and metamorphism of the schists can place key constraints on Laramide
schist underplating and crustal evolution in Southern California, and may also
help constrain general issues regarding sediment recycling and tectonic
erosion.
The Schist of Sierra de Salinas, in detail
The schist of Sierra de Salinas is among the oldest of the POR
schists, and is long known to have experienced higher metamorphic temperatures
than documented in the other schists. This is evident qualitatively by the
higher concentration of biotite in the Sierra de Salinas schist and the
widespread presence of partial melts.
Many of the POR schists preserve inverted thermal gradients, and we
found that the schist of Sierra de Salinas is no exception (see map). Muscovite, which is common on the east side
of the schist, is completely absent along the western margin. Garnet-biotite thermometry shows that this
coincides with a down-section temperature decrease from at least 730 C along
the west side of the range to ~560 C on the east side. This inverted thermal gradient occurs over
thickness of ~2.5 km, at an average value of 68 deg/km. Considering that metamorphism and shearing
occurred during a well constrained time window of <8 m.y., simple 1D
conductive thermal modeling constrains heat sources for the inverted gradient
and demonstrates that shear heating was important in generating the inverted gradient
and high temperatures in the schist (see Graham and England, 1976). Initial pressure estimates using the
gt-plag-bt barometer range from 9 to 14 kbar.
The western contact of the schist of Sierra de Salinas and
overlying granitic rocks is of particular interest because most of the other
Southern California schist contacts are overprinted by deformation related to
tertiary extension and exhumation. This
has frustrated efforts to assess sense of shear and interpretation of inverted
thermal gradients in the schists. The
contact between the schist and Salinian granites was originally considered a
Neogene brittle fault, then an intrusive contact, and recently, based on Ar/Ar
work, a brittle fault again. We observed
the contact in a number of places, and most places it is a brittle fault. We did find in one location however a ductile
shear zone with overlying granitic rocks.
In the most highly deformed upper plate rocks,
hornblende is recrystallized to calcic clinopyroxene, and the deformed quartz
diorite has been overprinted by garnet porphyroblasts. The mylonite zone thus involves a prograde
granulite facies overprint.
We are working on the thermobarometry of the mylonite rocks, however we have some preliminary constraints from the
work of Barth et al, 2003. The youngest detrital zircon age in the schist is 77
Ma, and lower and upper plate Ar/Ar ages in the region range from 70 to 76
Ma. Because clinopyroxene-mylonite and
garnet overgrowth did not form below the biotite closure temperature of ~300 C,
we conclude that this deformation occurred prior to ~70 Ma. The mylonite zone thus represents a stage of
late Cretaceous structural juxtaposition of the schists and collapsing arc. Planned thermochronologic and ongoing work on
the sense of shear in the schist and mylonite zone should help clarify the
nature of the contact.
1) The Salinian continental arc was a thick, “mature” arc, with
felsic composition to depths of at least 30 km.
2) The Salinian continental arc felsic thickness was
indistinguishable from the Sierra Nevada Batholith and comparable to the
Western Cordillera of Central South America.
3) Salinian ultramafic rocks were not derived from underthrust
Fransiscan material.
4) The schist of Sierra de Salinas exhibits an inverted thermal
gradient.
5) A granulite facies ductile shear zone is locally preserved
between the schist and upper plate.
6) Shearing occurred between 70 and 76 Ma.
7) Inverted metamorphism in the Schist of Sierra de Salinas
resulted partly from shear heating.
