UNFOLDING THE TECTONIC HISTORY OF JOSHUA TREE NATIONAL PARK: A STRUCTURAL AND PETROLOGIC ANALYSIS OF THE JOHNNY LANG CANYON REGION Author: David Horrell, USC Department of Earth Sciences ABSTRACT: Joshua Tree National Park, in southeastern California, displays a spectacular ~ 2 billion year geologic history of the evolution of the western North American margin. Evidence lies in deformed and metamorphosed Precambrian sedimentary and plutonic rocks, the construction of a Mesozoic magmatic arc, and recent strike-slip faulting and block rotation. In the Johnny Lang Canyon region of the park, a large anticline of Mesozoic age folds the Precambrian rocks and is closely tied to formation of the Mesozoic magmatic arc. The ~1.75 Ga (age of zircon cores dated by Barth and Wooden) Precambrian rocks (para- and orthogneisses) were highly deformed and metamorphosed around 1.4-1.45 billion years ago (age of zircon rims from Barth and Wooden). During the late Cretaceous (~82-70 Ma) a complex of sheet-shaped to large elliptical plutons were emplaced, which field relationships establish is the same time as the formation of the large anticlinal fold. Thin section analysis of the rocks reveals changing deformation and metamorphic assemblages that follow a retrograde path from high pressure and temperature assemblages of biotite+garnet+kyanite associated with intense deformation to moderate P and high T assemblages of biotite+sillimanite associated with moderate deformation and finally to lower PT static assemblages with chlorite. We link the high PT assemblage to the 1.4 Ga zircon rims, the moderate PT assemblages to Cretaceous pluton emplacement, and low T to post pluton cooling.
Horrell 2 INTRODUCTION: Today the western edge of the United States is an active plate margin. There is an active transform boundary in the south, the San Andreas Fault, and an active convergent boundary to the north, the Cascadia Subduction zone. However, this was not always the case. Besides the transform fault that alters the southwestern edge of the continent now, there have been collisions with other continents and island arcs as well as rifting and arc magmatism during periods of activity. These active periods were interspersed with passive times of depositional environments. Events from all types of tectonic environments are recorded in the rocks found in California. The rocks of Joshua Tree National Park record a number of these events. The terrain of Joshua Tree National Park (fig. 1) is made up of mostly Precambrian igneous and sedimentary rocks that have been highly deformed and metamorphosed to gneisses. They are intruded by much younger, Mesozoic igneous rocks. The intrusions take the form of large blobby plutons underlain by a sheeted sill complex. All of these rocks are tilted slightly Figure 1: Map of Joshua Tree National Park. Johnny Lang Canyon shown in black box
Horrell 3 downward to the east so that crustal depths up to ~20 km can be seen on the west side of the park. Johnny Lang Canyon is located near where the large, upper crustal plutonic intrusions meet the midcrustal, sheeted sills in the sequence. The edge of the circular Palms pluton is at the mouth of the canyon to the north and granitic intrusions from the pluton run through the gneisses of the canyon (fig. 3). Rocks on either side of the canyon dip into the walls indicating the presence of a large, upright fold in the canyon. For my research, I worked with a larger group of Dr. Scott Patterson and Adam Ianno, a graduate student at USC, who were studying the entire area of Joshua Tree National Park. With their help, I looked for evidence of the upright fold while mapping Johnny Lang Canyon. I put together a timeline for the deformational events of the area and related them to activity along the western continental margin. In this paper, I will describe the geology of the region in more detail and give all results of research which led to the construction of the timeline. Data sets used include age data, thin section observations and analysis, and pressure-temperature data. I will give the interpretations of our research and what their significance is along with further research that needs to be done in the area. I found several distinct periods of deformation and metamorphism. The first few were around the ~1.70 orthogneiss expressed as small tight folds. The strongest metamorphic event was dated at ~1.4 Ga (Barth pers comm. 2011) and the rocks reached their peak metamorphic grade at this time with maximum temperatures of ~680 degrees C and pressures of 9.3 kbar (Stowell et al. 2007). These separate events were related to tectonic processes during the formation and growth of the supercontinent Columbia. The final deformational period formed a large, upright fold during the Mesozoic simultaneously to the magmatic intrusion of the Palms and Stubbe plutons (shown in solid pink on the upper left side of fig. 1) around 70-80 Ma (Barth
Horrell 4 et al. 2009). A middle period occurred sometime between 1.4 Ga and 80 Ma and folded the rocks at temperatures and pressures of 670 C and 5.4 kbar (Barth pers comm. 2011). This event is difficult to date but was most likely related to the formation of Rodinia around 1.0 Ga. GEOLOGY: UNIT DESCRIPTIONS: The Johnny Lang Canyon region has several distinct units that were mapped in the field (fig. 3). The main ones were the Proterozoic gneisses of igneous and sedimentary origin, Cretaceous Palms granite, and Proterozoic mafic amphibolites. The orthogneisses are old igneous rocks that have been very deformed. They contain about 50% quartz and the rest is mostly mica and plagioclase. Biotite is the primary form of mica but there is a small amount of muscovite in the rocks as well. The plagioclase is common feldspars including some potassium feldspar. The final mineral is chlorite which only makes up a small fraction of the rock. Small opaque minerals also exist in the orthogneisses. The deformational fabric is mostly seen in the biotite mica which is aligned in horizontally parallel planes. Figure 2: Outcrops on garnet hill. Top photo shows large garnets and lower photo shows long bladed aluminosilicate structures. The paragneisses are deformed and
Horrell 5 metamorphosed, old sedimentary rocks. The main mineral is once again quartz and it makes up anywhere between 40-60% of the rock. They also contain biotite and small amounts of chlorite. In some outcrops, metamorphic minerals such as garnet and aluminosilicates are present (fig. 2). These appear in different quantities and range in size. Garnets range from microscopic to 2-3 cm in diameter. The main aluminosilicate mineral appears to be sillimanite and it is found in clusters which can be as big as 6 cm long by 2 cm wide. The paragneisses tend to be darker in color than the orthogneisses and granites but the deformation fabric is still visible (fig. 2). One location named garnet hill at the mouth of the canyon had the largest metamorphic minerals and was a good type location for metamorphic studies of the paragneisses. The Palms granite is found at the north end of the canyon (fig. 3). The rock is usually a white to light pink color due to the potassium feldspars. They also contain two micas, both biotite and muscovite. Unlike the gneisses though, its composition does not change much between outcrops. The deformation is much weaker in the Palms units but is still visible in the alignment of the micas. The final unit is the mafic amphibolite that intrudes the gneisses as Figure 3: Geologic Map of Johnny Lang Canyon sheets. The main mineral is hornblende which gives the rocks a dark, solid, black color, differing from the other units. The amphiboles also contain plagioclase.
Horrell 6 STRUCTURE: Four periods of deformation were observed in the field, with the later three being expressed as folding. The first three were seen in the paragneiss on garnet hill. The oldest was a very old deformational fabric that was deformed by all subsequent events. The next was expressed as tight, isoclinal folds which folded the oldest fabric but were also deformed by later fabrics. They had a trend and plunge of 005 /20 along the hinge line. The third set was also small but was found to be only deformed by the main fabric in the rocks caused by the latest folding event. The trend and plunge along the hinge line of a fold from this event was 288 /44, a higher angle than the second deformational event. There were also small ductile shear zones about 1-3 cm wide that were associated with both of these earlier folding events and cut the main fabric in two directions. The shear zones were similar to each other except the later was, once again, at a much higher angle. The final period of deformation involved the rocks of the entire canyon in an upright, gently plunging, anticline. On the canyon floor, near the creek bed, one outcrop of the hinge was found. It was closing to the north meaning that it was trending and plunging down in that direction. Here, the anticline had several smaller, parasitic folds on either side of the hinge. Strike and dip measurements from both sides of the Figure 4: Stereonet projection showing poles to foliation of the strike and dip of foliation fabrics in rocks from both sides of the canyon. Fold axis found at 05/356. canyon were plotted on a stereonet as poles to foliation (fig. 4). This means that the dots shown on the diagram
Horrell 7 are the poles of the measurement which are 90 from the original measurement along a straight line in the stereonet. A best fit line was drawn along a great circle through the poles and shows that the foliations were striking in a general east to west direction on both sides of the canyon. The red dot at the top of the stereonet is the pole of the best fit line and represents the fold axis of the anticline. This fold axis was found to be trending 356, almost due north, and plunging very gently at 05. THIN SECTIONS: Multiple samples were taken from the field area and brought back for further analysis. Many of these were cut into billets and shipped off to be made into thin sections for microscopic examination. From observation of these thin sections, several metamorphic mineral assemblages were seen, and relationships between these minerals helped to tell in what order they formed. In paragneiss samples from garnet hill, high amounts of the aluminosilicate sillimanite were found in long bladed shapes which were aligned in the main foliation fabric (fig. 5, a., in upper half of slide above the garnet). Some of these sillimanite minerals contained inclusions of biotite and plagioclase along the long axis of their blade shape. Large garnets also were observed in the sections aligned along the fabric (fig. 5 a,b). These garnets were very deformed and broken up along fractures and cracks. The fractures were often filled with biotite or quartz (fig. 5, b). They were also deformed and looked like porphyroclasts which were elongated and flattened along the foliation fabric so that small tails were formed (fig. 5, a). Inside the garnets were inclusions of quartz, biotite, and small acicular minerals that resembled apatite. Biotite was another prominent mineral that was seen mostly around the edges of the sillimanite and garnet in the paragneisses. There were small amounts of chlorite in the paragneiss samples as well and they were found in association with the biotite.
Horrell 8 Figure 5: Thin section slides from samples. a) JT-449b in ppl shows the broken up garnets with elongated tails. b) JT- 338 in ppl shows the garnets breaking up and being replaced by biotite and quartz. c) JT-1-175 in xpl shows the replacement of biotite with unaligned chlorite. d) DH-009 in xpl shows biotite and quartz aligned along the main deformational fabric in an orthogneiss. In the orthogneisses, the main minerals were quartz, plagioclase, and biotite. The quartz was usually medium sized grains that were aligned along the main fabric of the rock. However the biotite was most useful for identifying the foliation as it was strongly aligned (fig. 5, d). Its single cleavage plane was also in line with the rock fabric. Near the biotite, replacing it was chlorite. This mineral looked greenish in color and was not aligned like the other minerals (fig. 5, c). The small, needle-like blades of chlorite were pointing every direction with no apparent alignment.
Horrell 9 GEOCHRONOLOGY: One of the most useful tools for finding the ages of the rock units was U-Pb zircon dating. Samples are crushed up and zircon minerals are separated out. The mineral zircon is ideal because it is very hard and resistant to weathering reactions. Zircons cool slowly and are often zoned with the different internal layers recording multiple events that occurred during their formation. The ages found for the orthogneiss were measured in the cores of zircons and came out to be about 1.70-1.75 Ga (Barth et al. 2009). The same zircons had rims that gave ages of 1.4-1.45 Ga. The youngest detrital zircons dated in the paragneisses gave ages of around 1.70 Ga (Needy et al. 2009). The age of the Palms granite was also found by U-Pb dating of zircons at around 70-80 Ma (Needy et al. 2009). Argon in hornblende minerals is locked into the crystal and begins its decay process at ~510-550. So using 40 Ar/ 39 Ar ratios in the hornblende gives the age at which the rocks cooled below 500 degrees. The argon ages were found be about 74 Ma (Fleck pers comm. 2011). Likewise, biotite contains argon which cools at ~310-350 C instead of 500 C. Therefore, using the argon ratios in the biotite put the temperature of the rocks around 350 C around 70 Ma (Fleck pers comm, 2011). Another age constraint put on the rocks was the uranium fission tracks in apatite minerals. As uranium decays, particles shoot off of the parent uranium and leave tracks in the surrounding crystal. However, above a certain temperature (90-110 C for apatite), these tracks are annealed and erased. Below ~100 C, the mineral anneals enough for the emission tracks to leave a mark. The length of these particle tracks is measured and an age of cooling through ~110 C can be calculated. For apatites in Joshua Tree National Park near Johnny Lang Canyon,
Horrell 10 an age of 60 Ma was calculated for the cooling temperature of ~100 C (Luke Sabala and Phil Armstrong, pers comm, 2011). DISCUSSION: Using the data obtained from the field, collaboration with others, and literature sources, I was able to construct a pressure-temperature-time-deformation (PTtd) diagram (fig. 7, Table 1) to better summarize the history of the Joshua Tree National Park region. The deposition of the paragneisses had to be the first event in the sequence around 1.7 Ga (Barth et. al 2009). Sedimentary rocks must be younger than the youngest detrital zircon. Next, the orthogneisses had to be emplaced. They came after the sedimentary paragneisses because they intrude through them as plutons. Although ages for these two units are found to be relatively the same, the error can account for this because they are so old. Near this time, the rocks experienced the oldest period of folding and deformation (mentioned above) during the process of burial to great depth. Some time afterwards, a very strong metamorphic and probably deformational event occurred during which the rims of the zircons in the gneisses at 1.4-1.45 Ga (Barth pers comm 2011) continued to grow. To find out the nature of this event, the thin sections had to be analyzed. The highest grade metamorphic mineral assemblage preserved in the paragneiss was at garnet hill and contained high quantities of the aluminosilicate sillimanite. The sillimanite was preserved in Figure 6: JT-388 in xpl shows a blade shaped sillimanite cluster which replaced kyanite between two garnets.
Horrell 11 uncharacteristic, long bladed shapes (fig. 6). The shape was more diagnostic of kyanite which forms at higher pressures than sillimanite. Thus we concluded that the sillimanite was once kyanite because of its bladed shape and retrograde reactions transformed the mineral to its different polymorph. Including the other minerals present in the sample, the mineral assemblage at that time would have been quartz + plagioclase + garnet + biotite + kyanite. Another research site that had samples with the same mineral assemblage found the temperature and pressure of formation to be ~680 C and ~7.5-10.2 kbar respectively (Stowell et al. 2007) This strong event is associated with the third deformation of high angle folding and deformation observed at garnet hill. Another period of metamorphism occurred later but the timing is not as well known as the other events. All we know is that it happened between the 1.4 Ga event and the Mesozoic plutonic intrusions. Although, by using the mineral assemblages observed in the thin section slides, pressures and temperatures were extrapolated. This event was seen in the thin section mineralogy as the transformation from kyanite to sillimanite which must occur along the reaction line of the aluminosilicate phase diagram. Since the deformation was not as intense in this second folding event, it was interpreted that the rocks decreased in pressure to about ~5.4 kbar and a temperature of 670 C to form the sillimanite (Barth pers comm. 2011). This event occurred after the 1.4 Ga event because the blade shape of the kyanite is still present. Had more deformation occurred since that time, the replacement mineral would not have kept the same blade shape. According to field relationships, the third and final stage of folding in the rocks occurred around the same time as the Mesozoic pluton intrusions. U-Pb dating on zircons in the granite put these intrusions at about 70-80 Ma. These granite dikes were deformed by the large upright
Horrell 12 fold across the canyon but not as strongly as the Precambrian rocks they intruded, implying that the pluton emplacement was concurrent with the folding and deformation event. Using thermobarometry data about the granites (Needy et al. 2009), it is thought that the Palms and Stubbe plutons were emplaced at about ~3-4 kbar and ~650 C. The ~74 Ma argon cooling age from hornblende put the rocks at ~500 C (Fleck pers comm 2011). The pressure of 2.5 kbar for this age was extrapolated using a typical geothermal gradient for active arcs. Since plutons were intruding and cooling at the same time, the gradient would be higher than the average 25 degrees C per kilometer, making the pressure less for a higher temperature like 500 C. Similarly, the pressure constraint for the ~70 Ma argon cooling age in biotite data (Fleck pers comm 2011) was extrapolated using the geothermal gradient with a temperature of ~325 C when the biotite cooled. Since the ~60 Ma apatite fission track data of Armstrong and Sabala (pers comm. 2011) had to be locked in at low temperatures of around 100 C, it probably occurred at very low pressures as well. Since this was several million years after the pluton emplacements, the geothermal gradient was closer to the average value putting the pressure at about 1 kbar for a temperature of ~100 C. After the biotite cooled and the last deformational event around 70 Ma, the biotite reacted to form chlorite. This reaction is mostly temperature dependent, occurring between 300-400 C and at almost all pressures (Svensen et al. 2010). In the thin sections, it was observed that the chlorite grew randomly unlike the biotite that preceded it. Therefore it must have grown from the biotite after deformation ceased so that they were not preferentially aligned. This was the final step in the metamorphic history of these rocks followed only by their tilting and exhumation to their current positions.
Horrell 13 Figure 7: Constructed PT path for rocks with ages of each event. Deformational explanations in table below Temperature ( o C) Depth (kbar) Time (years) 0 0.25 1.70 Ga Deformation/Events (not shown in diagram) Paragneiss Deposition and Quartzites - (youngest detrital zircon age) Implacement of Orthogneiss as old plutons (at least 2 folding events) High Grade Metamorphism to Kyanite + Garnet + Plagioclase + (Biotite) assemblage, deformation Kyanite reacts to Sillimanite + Biotite, Strong Cleavage, lineation 1.70-1.75 Ga 680 9.3 1.4-1.45 Ga 670 5.4 1.4 Ga - 80 Ma 600 4 70-80 Ma Stubbe and Palms Pluton Intrusions and large upright folding 500 2.5 74 Ma Average Hornblende cooling age 325 2 70 Ma Average Biotite cooling age 100 1 60 Ma Apatite fission tracks on Plateau 25 0 5-25 Ma Tilting and exhumation due to regional block rotation from San Andreas Fault movement Table 1: Pressure, Temperature, Ages, and Deformation Events corresponding to the PTtd Diagram
Horrell 14 Another goal of my project was to relate the deformational and metamorphic events to likely tectonic causes along this section of the North American margin. During the Proterozoic, the edge of the North American protocontinent was further inland near Nevada and the far eastern edge of California. This would make the area a great depositional environment for sediments during that period in which the protoliths for the paragneisses were deposited. Around 2.1-1.8 Ga, an early supercontinent called Columbia was assembled. After its formation, the large landmass underwent a period of growth by accretion due to subduction related orogenies. The southern half of western North America experienced this growth from 1.8 Ga up until Columbia s breakup around 1.3 Ga (Zhao et al. 2004). The Proterozoic plutons emplaced around 1.7-1.75 Ga were a product of this subduction zone magmatism. Many other crustal rocks in the American southwest, dated from ~1.6-2.0 Ga using Nd istotopic methods, were emplaced and are identified as plutons as well (Bennett and DePaulo 1987). In the arc, the first two periods of deformation, forming the oldest fabric and folding it the first time, occurred. Tectonic forces during the accretion period related to subduction were responsible for metamorphism and deformation at their maxium depth and temperature around 1.4-1.45 Ga during which the younger rims of the zircons grew. The next recorded period of metamorphism, identified by the replacement of kyanite with sillimantite, happened sometime between 1.4 Ga and 80 Ma, thus making it difficult to draw correlations to specific tectonic events. During this large time span, several important tectonic events occurred in the history of the western margin of North America. First, Columbia broke apart through rifting. Then another supercontinent of Rodinia was assembled around ~1.0 Ga in the Neoproterozoic as all the continental fragments collided again. Rodinia rifted apart into fragments as well and later formed the third supercontinent Pangea. However, the western
Horrell 15 margin of present day North America remained passive during its formation and breakup. Therefore, the most likely cause of this set of metamorphism and deformation was the collisions which formed Rodinia. During the Late Paleozoic and Mesozoic, the western margin of North America became active again. Early on, subduction off the coast formed several island arcs. These arcs, collectively known as Sonomia, collided with the western margin and were accreted to the landmass in the Late Proterozoic. In the Mesozoic, the subduction zone moved closer to the coast and formed the continental margin magmatic arc including the Sierra Nevada batholith, and the Peninsular Ranges batholith. Arc magmatism from this subduction zone is also responsible for the Mesozoic plutons found in Joshua Tree National Park such as the Stubbe and Palms plutons, formed around 70-80 Ma (Needy et al. 2009). Compressional forces perpendicular to the subduction zone may be responsible for the upright folding seen in Johnny Lang Canyon. The alignment of this large fold trending 356 makes sense because forces from the subduction would push on the rocks from the west and east, causing the rocks to buckle in the middle and fold in the orientation observed. The final stage in the tectonic history of Joshua Tree National Park was the exhumation and tilting of the units. This process is related to the formation and movement along the San Andreas Fault. At about 25 Ma, a large enough portion of the Farallon plate was subducted that the Pacific plate reached the North American plate and the remnant of the Farallon plate was split into two smaller plates, the Cocos and the Juan de Fuca. The contact with the Pacific plate became the transform boundary we know as the San Andreas Fault with a right lateral sense of movement. Block rotation along the San Andreas uplifted the rocks and tilted them to their current locations today.
Horrell 16 IMPLICATIONS: Our work in Joshua Tree National Park provides a base knowledge of the regional geology. About 2 billion years of history are recorded in the rocks from emplacement to burial and metamorphism and exhumation. Using this information it can be related to the tectonic regime of the entire western margin of North America. The research done here can be used to aid the study of other areas along the western margin that have similar tectonic histories. Likewise, research in other areas of the world that have endured similar tectonic settings can use our findings of what events occurred. There is still further research to be done in the area. In Johnny Lang Canyon specifically, an old fault is suspected through the canyon as well. Although evidence of the fold is very strong in the dip of the beds and the stereonet plot, the layered units do not appear to match up on opposite sides of the canyon. Further analysis of structure and microstructure would help to solve the problem. On the larger scale of the entire Park, not a lot is known about the cause for the transition from blobby plutons to sheeted dikes which formed at the same time in the Cretaceous. More research is currently underway to solve this anomaly. CONCLUSIONS: The main findings of my research were the distinct deformational and metamorphic events expressed by the folds, fabrics, and mineral assemblages in the rocks. Evidence for the first few folding events came from observed folded fabric in the paragneiss of garnet hill. A change in growth of the zircon rims which were dated to ~1.4 Ga (Barth pers comm 2011) marked the next event. At this age, the supercontinent Columbia containing most of the
Horrell 17 protocontinents existed and underwent growth related to subduction processes which folded and metamorphosed the rocks. The rocks reached their peak metamorphic grade at temperatures of ~680 C and pressures of 9.3 kbar (Stowell et al. 2007) containing a kyanite + garnet + biotite + quartz + plagioclase assemblage at this age. The next period is characterized by the replacement reaction of kyanite to sillimanite. This occurred at shallower depths with pressures of 5.4 kbar and temperatures of 670 C (Barth pers comm 2011). Formation of the supercontinent Rodinia around 1.0 Ga is a probable source of the tectonic stresses necessary to uplift the rocks and deform them. The final stage of deformation came in the Mesozoic, forming an upright fold across Johnny Lang Canyon. Field relationships determine that it occurred simultaneously with the emplacement of the Palms and Stubbe plutons around 70-80 Ma (Needy et al. 2009), relating it to subduction that was occurring off the coast in the Cretaceous. To reach their current positions, the rocks were tilted and uplifted by block rotation caused by motion along the San Andreas Fault, starting around 25 Ma. Figure 8: A view of Johnny Lang Canyon looking northwest to the mouth of the canyon
Horrell 18 ACKNOWLEDGEMENTS: I would like to thank Professor Scott Paterson for his guidance and support with my research. Many thanks to Adam Ianno as well for letting me assist with his larger scale project in Joshua Tree National Park. I could not have completed my project without his help and knowledge of the region. Drs. Barth, Armstrong, Sabala, and Fleck provided data from previous and ongoing research which was very valuable to my thesis and was very much appreciated. The Univeristy of Arizona Laserchronology Laboratory allowed us to use their machines for dating zircons to find ages for several of the rock units. Lastly I would like to thank Joshua Tree National Park for permitting us to conduct research and collect samples within the park boundaries.
Horrell 19 REFERENCES: Armstrong, Phill., and Sabala, Luke. Apatite Fission Track and He ages in Joshua Tree National Park. Unpublished Research. Barth, Andrew P., et al. "Assembling and Disassembling California: A Zircon and Monazite Geochronologic Framework for Proterozoic Crustal Evolution in Southern California." The Journal of Geology 117 (2009): 221-239. Print. Barth, Andrew P., Wooden. Zircon Ages in Joshua Tree National Park. Unpublished Research. Bennett, Victoria C., and Donald J. DePaolo. "Proterozoic crustal history of the western United States as determined by neodymium isotopic mapping." GSA Bulletin 99.5 (1987): 674-685. Print. Blakely, Ron. "Geologic History of Western US." Colorado Plateau Stratigraphy and Geology and Global and Regional Paleogeography. Northern Arizona University, Mar. 2011. Web. 2 May 2011. <http://www2.nau.edu/rcb7/text_wus.html>. Fleck. Argon Cooling Ages for Biotite and Hornblende in Joshua Tree National Park. Unpublished Research.
Horrell 20 Needy, Sarah K., et al. "Mesozoic magmatism in an upper- to middle-crustal section through the Cordilleran continental margin arc, eastern Transverse Ranges, California." The Geological Society of America Special Paper 456 (2009): 187-218. Print. Stowell, Howard H., et al. "Mid-Crustal Late Cretaceous metamorphism in the Nason Terrain, Cascades crystalline core, Washington, USA: Implications for tectonic models." 4-D Framework of Continental Crust. Ed. Robert D. Hatcher, et al. 2007. Boulder, CO: Geology Society of America, 2007. 211-232. Print. Geological Society of America Memoir. Svensen, Henrik, et al. "Sandstone dikes in dolerite sills: Evidence for high-pressure gradients and sediment mobilization during solidification of magmatic sheet intrusions in sedimentary basins." Geosphere 6.3 (2010): 211-224. Print. Zhao, Guochun, et al. "A Paleo-Mesoproterozoic supercontinent: assembly, growth, breakup." Earth Science Reviews 67.1-2 (2004): 91-123. Print.