9.2: Ancient Seas Form the Oldest Rocks
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)What We Know From the Oldest Rocks
The oldest rocks exposed in the Sierra Nevada are no older than about 540 million years (Cambrian). Older rocks related to the formation of the Sierra Nevada as we know it are recorded in the White and Inyo Mountains to the east (see Basin and Range).
Geologists recognize two groups of ancient rocks: one Paleozoic (older than 250 Ma) and one Mesozoic (older than 65 Ma).
Sediments Accumulate in Ancient Seas
During the Paleozoic Era, more than 400 million years ago, the western margin of North America was a passive margin that ran from present day southeastern Idaho, through central Nevada, and into Southern California. Offshore, where what is now the Sierra Nevada was under water, thick layers of sediments accumulated: 1 km (0.6 mi) in the south to 10 m (6 mi) in the north indicating that the ocean basin was deeper to the south.
For more than 500 million years, these sediments accumulated, creating a record of what the environment was like during that time. Most of these sediments are marine in origin, forming in a shallow sea. The types of rocks and the few fossils that remain in these rocks today indicate that the eastern side of the sierra was closer to shore.
Seaward, beyond the continent, were "island arcs" (not unlike the Aleutian island arc that exists today). These arcs were later accreted onto the the North American contented during subduction.
Accreted Terranes
(Reference the Klamaths chapter)
How these ancient seafloor sediments wound up metamorphosed and accreted onto the western margin of North America continues to be an area of research for geologists worldwide. Most geologists agree that
These sequential accretions are known as terranes or exotic terranes because they are pieces of land that came from elsewhere. They are each bounded by faults, sometimes called a suture zone, and have a distinct geologic history.
Sediments stopped being deposited around ~XXX Ma. During this time, subduction of the Farallon plate beneath the North American plate initiated, slowly shoving the oceanic crust beneath the continental crust. This early orogenic event is known as the Antler Orogeny.
Some of the oceanic crust, rather than being subducted, was instead obducted up onto the continent where it has been preserved for geologists to study. This process of obduction is further described in the Klamaths and Coast Ranges chapters.
The Sonoma Orogeny brought additional terranes to the North American continent including the Kings-Kaweah Terrane.
Metamorphic Belts of the Sierra Nevada
Metamorphic belts, such as those found in the Sierra Nevada Mountains, are the result of complex tectonic processes occurring over millions of years. The formation of these belts begins with the deposition of sediments in marine or terrestrial environments, which gradually accumulate and undergo lithification to become sedimentary rocks like sandstone, shale, or limestone.
As sedimentary rocks are buried deeper into the Earth's crust during subduction or collision, they experience increasing heat and pressure. These conditions trigger metamorphic processes, causing minerals within the rocks to recrystallize and reorganize without melting entirely. The degree of metamorphism depends on temperature and pressure, with higher conditions leading to more intense changes.
During metamorphism, minerals within the original sedimentary rocks undergo chemical reactions, transforming into new minerals with different crystal structures and compositions. This results in the formation of metamorphic rocks like schist, gneiss, and marble, which exhibit distinct textures and mineral compositions compared to their parent sedimentary rocks.
Along with metamorphic changes, rocks within metamorphic belts undergo deformation due to tectonic forces, leading to the development of features like foliation, cleavage, folding, and faulting. Over time, geological processes such as erosion, uplift, and crustal extension expose metamorphic rocks at the Earth's surface, allowing geologists to study and interpret the geological history recorded within metamorphic belts. These belts serve as archives of Earth's tectonic evolution, offering valuable insights into the dynamic processes that have shaped the planet's crust over geological time scales.
The metamorphic rocks of the Sierra Nevada are organized into distinct belts, each representing a different tectonic environment and geological history. These belts are characterized by specific types of metamorphic rocks and are integral to unraveling the complex geological processes that have shaped the region over millions of years. Some of the prominent metamorphic belts in the Sierra Nevada include the Shoo Fly Complex, the Calaveras Complex, Greenstone Belt, and Nevada-Placer-Upland Belt and the Foothills Terrane.
Shoo Fly Complex
Located on the eastern side of the Sierra Nevada metamorphic belt, the Shoo Fly Complex represents a crucial component of the region's geological evolution. Comprised of diverse lithologies including metamorphic rocks, igneous intrusions, and sedimentary formations, the Shoo Fly Complex offers valuable insights into the complex tectonic processes that have shaped the Sierra Nevada Mountains. Geologists have identified evidence of high-grade metamorphism within this complex, indicative of intense geological activity during its formation. Moreover, the presence of granitic plutons within the Shoo Fly Complex underscores its association with magmatic processes, highlighting the dynamic nature of tectonic interactions in the region.
Calaveras Complex
The Calaveras Complex, also known as the Slate Belt, is located along the western foothills of the Sierra Nevada. It is primarily composed of low-grade metamorphic rocks such as slate, phyllite, and schist, which originated from the metamorphism of ancient marine sediments. These rocks exhibit foliation, a parallel alignment of minerals, resulting from the directed pressure during metamorphism. The Slate Belt represents an early phase of metamorphism in the Sierra Nevada's geological history and provides insights into the ancient marine environments that once existed in the region.
Figure \(\PageIndex{2B}\): Terrane map of the western Sierra Nevada Foothills metamorphic belt with location of gold districts by Arizona Geological Society licensed under public domain.
Greenstone Belt
The Greenstone Belt, situated to the east of the Slate Belt, is characterized by the presence of metavolcanic and metasedimentary rocks that have undergone moderate to high-grade metamorphism. These rocks include greenstone, amphibolite, and marble, and they originated from volcanic and sedimentary protoliths that were subjected to intense heat and pressure. The Greenstone Belt represents a transitional zone between the low-grade metamorphism of the Slate Belt and the high-grade metamorphism of the adjacent belts to the east.
Nevada-Placer-Upland Belt
The Nevada-Placer-Upland (NPU) Belt, located in the eastern Sierra Nevada, is dominated by high-grade metamorphic rocks such as gneiss, schist, and migmatite. These rocks experienced extreme temperatures and pressures during metamorphism, resulting in the recrystallization of minerals and the development of distinctive banding and foliation. The NPU Belt represents the deepest levels of crustal metamorphism in the Sierra Nevada and provides valuable insights into the processes of continental collision and mountain building.
Foothills Terrane
Flanking the western foothills of the Sierra Nevada Mountains, the Foothills Terrane represents a distinctive geological entity within the region. Composed primarily of sedimentary deposits and metamorphic rocks, the Foothills Terrane offers valuable insights into the geological processes that have shaped the western margin of the Sierra Nevada belt. Geological studies have revealed evidence of ancient sedimentary basins within this terrane, providing clues about past depositional environments and tectonic settings. Moreover, the presence of metamorphic rocks within the Foothills Terrane underscores its complex geological history, highlighting the interplay between tectonic forces and geological processes in shaping the landscape of the Sierra Nevada Mountains.
Each of these metamorphic belts represent integral components of the geological mosaic that defines the Sierra Nevada Mountains. Through their diverse lithologies, geological structures, and tectonic histories, these terranes offer valuable insights into the dynamic processes that have shaped one of the most iconic mountain ranges in the world.
Metamorphic Rocks and Gold Deposits
The metamorphic rocks of the Sierra Nevada are closely associated with the formation of gold deposits, which have played a significant role in the region's economic history. Gold deposits in the Sierra Nevada primarily occur in quartz veins that traverse metamorphic rocks, particularly in areas where hydrothermal fluids have interacted with pre-existing structures. During regional metamorphism, fluids rich in dissolved minerals, including gold, were circulated through fractures and faults in the crust, precipitating gold-bearing quartz veins as they cooled and crystallized.
Understanding the distribution and characteristics of metamorphic rocks in the Sierra Nevada is essential for identifying favorable geological settings for gold mineralization. Regions with specific metamorphic belts, such as the Nevada-Placer-Upland Belt, are particularly prospective for gold deposits due to the presence of suitable host rocks and structural features conducive to mineralization.
Later in this chapter, we will delve deeper into the geological processes responsible for the formation of gold deposits in the Sierra Nevada, including the role of magmatism, hydrothermal activity, and tectonic events. By integrating our knowledge of metamorphic rocks with the study of gold mineralization, we can gain a comprehensive understanding of the geological dynamics that have shaped the Sierra Nevada and influenced its economic significance.
Metamorphic Roof Pendants
Today, the oldest rocks of the Sierra Nevada no longer resemble ancient seafloor sediments. They have been metamorphosed by the intrusion of the Sierra Nevada batholith. Since the intrusion, weathering and erosion have exposed the batholith to the surface, leaving small remnants of the metasedimentary rocks interspersed throughout as "roof pendants". The name comes from the fact that the country rock is literally hanging down from the roof of the magma chamber like a pendant (Figure \(\PageIndex{1}\).
These "roof pendants" record moderate to high amounts of strain, resembling stretched taffy in some locations.
More than 100 of these roof pendants speckle the otherwise intrusive igneous rocks of the Sierra Nevada, concentrated in the central and southern parts of the province. The largest of these roof pendants include the Mt. Tom pendant, the Mt. Morrison pendant (Figure \(\PageIndex{2}\)), and the Saddlebag Lake pendant.
References
Prothero