Download Determining Slip Sense along the Sur

Determining Slip Sense along the Sur-Nacimiento Fault through the use of Detrital
Zircon Geochronology on the Nacimiento Block at Cerro Alto, California
A Senior Project
presented to
the Faculty of the Natural Resources Management and Environmental Science
California Polytechnic State University, San Luis Obispo
In Partial Fulfillment
of the Requirements for the Degree
Bachelor of Science, Earth Science
by
Karen Decker
March, 2012
© 2012 Karen Decker
Senior Project Advisor:
Dr. Scott Johnston
Department Head:
Dr. Douglas D. Piirto
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Abstract
The Cretaceous geology of California represents an archetypical
convergent margin assemblage. Studying the faulting patterns of the convergent margin
will indicate the way in which the margin deformed as subduction occurred. The SurNacimiento Fault in California is located to the west of the San Andreas Fault. Slip along
the Sur-Nacimiento Fault occurred in the late Cretaceous during the time that subduction
was occurring. The Sur-Nacimento Fault caused the juxtaposition of the subduction
complex and magmatic arc of the convergent margin assemblage. This is unusual
because the fault removed the forearc basin that is normally found in a convergent margin
assemblage. Through the use of detrital zircon geochronology samples of rock are dated
to determine where the sediment was when it was formed. This will aid in explaining
how the Sur-Nacimiento Fault moved when deformation was happening. Detrital zircons
were extracted from quartzite clasts found in conglomerates and were analyzed through
the use of laser ablation inductively coupled plasma mass spectrometry to determine the
age of individual zircon grains. The results of this process show that there are age peaks
around 1.1 Ga, 1.4 Ga, and 1.8 Ga and indicates that these forearc sediments that were
derived from the North American miogeocline. Although further analyses are necessary,
this work is consistent with the interpretation that the Sur-Nacimiento Fault was a thrust
fault that developed as the California Mesozoic forearc was underthrust below the
developing arc.
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Acknowledgements
First and foremost, I would like to thank Brittany Brookshire for being my partner
in this long, crazy journey. Her constant support, encouragement, and all around positive
spirit made the process more than bearable, she made it fun.
Scott Johnston has been an incredible advisor for this project. His enthusiasm and
zeal for all things geology has been contagious and made this project exciting. I would
like to thank him for this opportunity to not only further my knowledge of California
geology, but for the opportunity to learn more about the processes behind the research
papers I’ve learned to love. Scott has also been an incredible teacher in the classroom
and the field.
I’d like to also thank Dr. Tony Garcia for being my first college geology professor
and the reason I started in on the Geology minor. If it weren’t for his enthusiasm for an
introductory geology class, I might never have started this journey. I’ve had the
opportunity to partake in his upper level classes and he never failed to amaze me with his
knowledge and excitement.
Dr. Lynn Moody has been my academic advisor for my entire college career and I
don’t think I’d have made it to this point without her guidance. Her understanding and
willingness to help has been crucial to my higher education.
I’d like to thank my fiancé, Dylan, for his unending support, even when the stress
drove me a little nuts. Mostly, I’d just like to thank him for putting up with me and my
tears.
I’d like to thank my grandparents, Charles and Betty Stolte, for their financial and
emotion support. They never gave up on me, so I promised not to give up on myself.
And finally, I’d like to thank my family, Todd, Penny, Wes, and Wade, for
encouraging me to never back down. I’d like to thank my parents for enabling me to go
to my (expensive) dream school and for being proud of me even when I had a hard time
being proud of myself.
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Table of Contents
Page
Abstract……………………………………………………………….iii
Acknowledgements…………………………………………………...iv
List of figures…………………………………………………………vi
Introduction…………………………………………………………..1
Geologic background………………………………………………....1
Methods and materials………………………………………………...6
Results………………………………………………………………...8
Discussion…………………………………………………………….8
References……………………………………………………………11
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List of Figures
Page
1.
Regional map of California……………………………….4
2. Local geologic map………………………………………..5
3. Methods of obtaining detrital zircons……………………...7
4. Diagram of laser ablation ICMPS…………………………8
5. Relative probability chart of all samples…………………..10
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Introduction
Convergence along California occurred during the Cretaceous and the terrane
assemblage formed represents a classic example of the convergent margin tectonic
setting. The assemblage created is preserved in California today and is useful for
determining the way that similar convergent margins have formed and evolved.
Subduction creates three main formations that are typical of convergent margins. The
accretionary prism is created closest to the convergent margin and a magmatic arc is
formed farther away from the margin. Between these formations is the forearc basin,
where sediments eroded off of the magmatic arc collect. The accretionary prism is
typically composed of mélange. In California the accretionary prism is represented by
the Franciscan Complex. The forearc basin is represented in California as the Great
Valley Group. The magmatic arc is a zone of upwelling magma that creates magmatic
plutons. The Sierra Nevadas are representative of the magmatic arc in California.
Using detrital zircon geochronology to determine the age of the sediments will aid in
determining the sense of slip of the Sur-Nacimeinto Fault. The Sur-Nacimiento Fault
formed during subduction along the California coast and juxtaposes the magmatic arc
with the accretionary prism. Determining the sense of slip along this fault will give
valuable information as to how the California convergent margin deformed as subduction
was occurring. The Sur-Nacimiento Fault is likely a thrust fault formed during
subduction that underthrust the Mesozoic forearc below the developing arc.
Geologic background
The California convergent margin is representative of a Cretaceous convergent
margin assemblage. The Franciscan Complex, Great Valley Group, and Sierra Nevada
batholith characterize the California convergent margin northeast of the San Andreas
Fault. This assemblage is representative of the accretionary prism, forearc basin, and
magmatic arc, respectively (Wright, Wyld, 2007). The Franciscan Complex formed as an
accretionary prism as the Farallon Plate subducted beneath the west coast of North
America during the Cretaceous. Subducted material was heated as it was subducted,
released water entrained in minerals structure, and wet melting in the mantle caused the
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formation of magma. This melted material subsequently rose to the surface and formed
the Sierra Nevada batholith. The sediment of the Great Valley Group, between the
Franciscan to the west and the Sierra Nevadas to the east, formed due to erosion of the
Sierra Nevadas.
The Franciscan Complex is represented in the area of the Sur-Nacimiento Fault as
the Franciscan mélange. This mélange consists of greywacke, argillite, chert, greenstone,
serpentinite, and some limestone, among other material. The Great Valley Group is
composed of clasts formed from the granitic plutons of the Sierra Nevada batholith as
well as clasts from the metamorphic foothills. In some places the Great Valley Group
shows evidence of turbidites, as it overlies the Coast Range ophiolite. The Sierra
Nevadas are composed of granitic plutons formed during subduction in the Cretaceous
(Fig. 1). The Sur-Nacimiento Fault is located to the east of the San Andreas Fault in
California. The Sur-Nacimiento Fault separates the Salinian block to the east and the
Nacimiento block to the west. The Salinian block consists of rock type that is consistent
with igneous and metamorphic rocks of the Sierra Nevada batholith (Dickinson, 2005).
The rock type that forms the Nacimiento block to the west of the Sur-Nacimiento Fault is
consistent with the mélange that forms the Franciscan Complex (Dickinson, 1983, 2005).
The juxtaposition of the Salinian block and the Nacimiento block means that the SurNacimiento Fault moved in such a way that the Great Valley Group has almost been
completely removed. Slip along the Sur-Nacimiento fault occurred during the late
Cretaceous and has been reactivated during Cenozoic deformation, so determining the
sense of slip during the Cretaceous has been difficult. Slip along the Sur-Nacimiento
fault occurred during the late Cretaceous.
Determining where the Great Valley Group was formed, through the use of
detrital zircon geochronology, will help determine which of the three predominant
models best explains the slip along the Sur-Nacimiento Fault.
The first model studied suggested that the Sur-Nacimiento Fault was a sinistral
strike slip fault. Slip along the fault would have occurred in the Cretaceous prior to any
slip along the San Andreas Fault. Samples supporting this theory would show age
signatures similar to that of rocks with miogeoclinal origins. Sinistral slip along the SurNacimiento Fault would have brought the Nacimiento block south of where it lies today.
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Then dextral slip along the San Andreas Fault would have brought the Salinian
block and the Nacimiento block back to their present day location after faulting had
halted along the Sur-Nacimiento Fault (Dickinson, 1983).
The second model suggests that the Sur-Nacimiento Fault was a dextral
strike slip fault with origins in Mexico in the Oaxaca region. Samples supporting this
theory would show a significant Grenville age peak on a relative probability chart with
the peak just before the 1 Ga mark (Wright, 2007).
The third model supports the theory that the Sur-Nacimiento Fault was a thrust
fault that thrust the Sierra Nevadan batholith over the Franciscan complex, thereby
removing the Great Valley Group. This would have occurred near the present-day
location of San Diego, California. The samples would show miogeoclinal origins and the
relative probability chart would show Grenville peaks just above 1 Ga and other peaks
around 1.8 Ga. After thrusting along the Sur-Nacimiento Fault, the Nacimiento block
and the Salinian block would have been transported dextrally to their present-day location
by the San Andreas Fault (Hall, 1991).
In order to determine the slip sense along the fault, samples of rock were collected
from the Nacimiento block. The samples were processed to extract detrital zircons. In
order to get accurate and abundant zircons from the rock, these samples were taken from
a small section that was consistent with sediment from the Great Valley Group, though
found on the Nacimiento block of the Sur-Nacimiento Fault, and consisted of quartzite
clasts and sandstones. Samples also had to be collected from a formation of rocks that
would have formed before slip along the fault. The formation containing the samples is
called the Toro Formation near Atascadero, California (Fig. 2). The Toro Formation is
the lower part of the Great Valley Group stratigraphically and has yielded fossils from the
early Jurassic and the late Cretaceous (Seiders, 1983). This means that it was formed
before slip along the Sur-Nacimiento Fault, so dating these rocks will give a better
determination of where they were deposited prior to slip. The Toro Formation consists of
sandstone and mudstone with lenses of pebble to cobble conglomerate with clasts of
chert, quartzite, mudstone, and sandstone (Seiders, 1983). It also rests on a thin layer of
radiolarian chert that dates to the Tithonian and this layer of chert lies on top of the Coast
Range ophiolite (Seiders, 1983).
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Figure 1. Regional map of California depictingthe rock types of a typical subduction complex. Franciscan mélange
represents the accretioanry prism, the Great Valley Group represents the forearc basin, and the Sierra Nevada
Batholith represents the magmatic arc.
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Figure 2. Local geologic map of the area between Morro Bay, California and Atascadero, California where rock
samples were collected.
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Methods and Materials
Samples of rock were collected from the Toro Formation near Atascadero,
California through the use of rock hammers and sledge hammers. Each sample was
noted and photographed. The three samples were quartzite clasts collected from
conglomerates. These were chosen because of the abundance of zircons and reliability of
zircons collected. In order to extract the detrital zircons from the rocks, the rocks must be
crushed to sand size, or smaller, particles. Before the crushing process could occur, the
area where the crushing was to be performed was cleaned thoroughly as well as the
materials themselves. This was done to prevent contamination from other stray zircons.
The actual crushing of the rocks was performed through the use of a stainless steel mortar
and pestle (Fig. 3). During the crushing process the rocks were put through a 300-micron
sieve to separate the smaller particles. These samples were labeled in beakers and stored
in a clean area. Sample J11212A2-1 yielded approximately 150 ml, sample J11212A2-2
yielded approximately 150 ml, and sample J11212A2-4 yielded approximately 150 ml.
These crushed samples contained more sand sized rock particles than zircons, so
they had to be further processed to concentrate the zircons. First, the samples had to be
panned to rid the samples of very small dust-sized particles as well as to remove a lot of
the extraneous material (Fig. 3). This was done through the use of gold pans and water.
Because zircons have a higher density than the rock they are contained in, the zircons
sink and excess minerals are removed with the wastewater.
This still yielded fairly large samples, however. Because these samples still
contained a large portion of other minerals, further reduction was necessary. This was
done through the use of heavy liquids (Fig. 3). Lithium metatungstate (LMT) was used
because it is has a higher density than quartz and feldspar. However, because the zircons
are so dense, they sink through the LMT while the other rock material floats toward the
top. This makes it very easy to decant the unnecessary material and retain the important
zircons.
Other particles in the rock could have a density similar to the zircons, such as
magnetic minerals so the next step in the process is to remove the magnetic material. The
small samples containing zircons and magnetic material were processed through an LB-1
Barrier Frantz electromagnet to separate the magnetic material from the zircons. Finally
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the samples contained more zircons than other materials and were ready to be mounted.
The samples were mounted in epoxy in order to put them through the laser ablation.
Before the samples could be put in the laser ablation, they had to be polished to expose
the zircons more fully to remove chances of lead loss contamination. The samples were
then mounted in the laser ablation inductively coupled plasma mass spectrometry
(ICPMS) and were shot with a laser to measure the levels of 206/207-lead in relation to
238-uranium for the purposes of dating the samples (Fig. 4).
Figure 3. Methods of obtaining detrital zircons. A) Stainless steel mortar and pestle for grinding rocks. B) Panning
materials to separate unnecessary rock material. C) Lithium Metatungstate used for heavy liquid separation.
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Figure 4. Representative diagram of the laser ablation inductively coupled plasma mass
spectrometer (ICPMS).
Results
The sampled quartzite clasts were collected from conglomerates found along a
stream bed. The conglomerates were about 1 meter in size, but the clasts themselves
were approximately 10 cm in size. The conglomerates were matrix supported
conglomerate. The results of the laser ablation ICPMS were put into relative probability
charts to better show the age signatures of the arenite quartzite clasts collected. The
clasts Sample J11212A2-1 showed a significant peak at 1.8 Ga and less significant peaks
around 2.8 Ga, 3.0 Ga, and 3.8 Ga. Sample J11212A2-2 yielded an age signature with
peaks at approximately 1.1-1.5 Ga and 1.8 Ga. Sample J11212A2-4 resulted in a relative
probability chart that showed age signature peaks around 1.1 Ga, 1.4 Ga, and 1.8 Ga.
Discussion
The three models suggest that the Great Valley Group and the Franciscan
complex formed at a certain location that can be identified through the age signature of
the rocks sampled. If the rocks were derived Mexico, the Grenville peak in the relative
probability chart would have been slightly under 1.0 Ga, though it is shown that the
Grenville peak for these rocks is slightly older than 1.0 Ga (Fig. 5). This inconsistency
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reveals that the rocks were not likely to have been transported from that region. If the
rocks were eugeoclinal, there would be a peak around 1.8-2.0 Ga shown on the relative
probability chart. These samples, however, do not show a peak at that mark and so we
can discount the eugeoclinal theory of massive sinistral slip along the Sur-Nacimiento
Fault (Fig. 5). By comparing the relative probability chart derived from the samples
taken near Atascadero, CA and other relative probability charts taken from previous
research, it is evident that the samples yielded dates similar to miogeoclinal origins.
Miogeoclinal origins suggest that there would be a Grenville peak slightly above 1.0 Ga,
which is what is seen in the relative probability chart taken from the samples. Another
indication of miogeoclinal origins is a peak in the relative probability chart just before 2.0
Ga at around 1.6-1.8 Ga. This peak is shown in the relative probability chart taken from
the samples collected (Fig. 5). The miogeoclinal theory suggests that the Sur-Nacimiento
Fault was once a thrust fault that was located in the present day location near San Diego,
California. Thrusting would have occurred during the late Cretaceous and ended before
slip along the San Andreas Fault occurred. This new juxtaposition of Franciscan mélange
against Sierra Nevadan batholith was then carried north through dextral slip along the
San Andreas Fault to its present day location near Big Sur, California.
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Figure 5. Relative probability chart showing the age signatures of the quartzite samples collected, as well as the sandstone
collected from the same area. Locations for the distinct age signature peaks are indicated.
References
Barbeau, D.L., 2005. U-Pb detrital-zircon geochronology of northern Salinian basement
and cover rocks. Accessed April 17, 2011.
http://gsabulletin.gsapubs.org/content/117/3-4/466.full.
Dickinson, W.R., 1983. Cretaceous Sinistral Strike Slip Along Nacimiento Fault in
Coastal California: The American Association of Petroleum Geologists Bulletin,
v. 67, No. 4, p. 624-645.
Dickinson, W.R., Ducea, M., Rosenberg, L.I., Greene, H.G., Graham, S.A., Clark, J.C.,
Weber, G.E., Kidder, S., Ernst, W.G., and Brabb, E.E., 2005. Net dextral slip,
Neogene San Gregorio-Hosgri fault zone, coastal California: Geologic evidence
and tectonic implications: Geological Society of America Special Paper 391, p.
1-43.
Hall, Clarence A., 1991. Geology of the Point Sur-Lopez Point region, Coast Ranges,
California: A part of the Southern California allochthon. Geological Society of
America, Special Paper 266, p. 1-31.
Jacobson, Carl E., 2011. Late Cretaceous-early Cenozoic tectonic evolution of the
southern California margin inferred from provenance of trench and forarc
sediments. Accessed January 27, 2011. http://gsabulletin.gsapubs.org
Seiders, Victor M., 1983. Correlation and provenance of upper Mesozoic chert-rich
conglomerate of California: Geological Society of America Bulletin, v. 94, p.
875-888.
Wright, James E. and Wyld, Sandra J., 2007. Alternative tectonic model for Late Jurassic
through Early Cretaceous evolution of the Great Valley Group, California.
Geologic Society of America, Special Paper 419, p. 81-93.
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