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The area surrounding Whiskeytown National Recreation Area is full of mountain ranges and valleys. Unraveling the origin of the Klamath Mountains and associated basins exposes the complex tectonic history surrounding the growth of the western margin of the North American continent. Prior to the early Paleozoic, there was an oceanic basin in the present day location of Whiskeytown National Recreation Area. Here, sediments were deposited from the continental margin to the east and volcanics intruded during extension of the crust. Some of the oldest rocks, of Cambrian age at Whiskeytown reflect this deposition (figure 11).
The Devonian and Mississippian Antler orogeny shoved rocks eastward onto the continental margin along the Roberts Mountains Thrust fault. The thrust carried deep-marine, continental slope and rise deposits over relatively shallow marine carbonate deposits on the margin (Miller et al. 1992). This orogeny was fairly rapid, lasting only about 25 million years. However, it established a compressional regime along the western coast of the United States that lasted until the Mesozoic.
The next tectonic event affecting the Whiskeytown area was the Sonoma orogeny. This orogeny followed the scant deposition of chert- pebble conglomerates, sandstones, and limestones atop the deformed rocks of the Antler orogeny. The Sonoma orogeny involved the thrusting of an allochthonous block containing volcanic sediments atop the continental margin (Silberling and Roberts, 1962; Miller et al. 1992). This occurred during the Late Permian and Early Triassic periods.
The Mesozoic brought a period of expansion to the western margin of the continent. Large batholiths were emplaced along the western margin beneath the magmatic arcs. Entire landmasses, some associated with the western margin and some foreign to the western shore, were sutured onto the western margin from Alaska to California. These sutured landmasses are called displaced terranes (figure 12).
Early Triassic tectonic activity was marked by the accretion of a major Paleozoic island- arc terrane in the northwest Nevada- northern California region. However, Following the Sonoma orogeny a new subduction zone formed. The subduction zone trended north- northwest to south- southeast and dipped eastward beneath the continental margin. Early to Middle Triassic (245- 230 Ma) igneous rocks were emplaced along this subduction zone located east ofWhiskeytown National Recreation Area (Saleeby et al. 1992).
The western continental margin was well- established by the Late Triassic (230- 208 Ma). Along the California and Arizona margin, the Late Triassic- Early Jurassic arc occupied an extensional graben sediment- trap system (Saleeby et al. 1992). Evidence of this setting is preserved in the metavolcanic and metasedimentary rocks found along the western margin from California to Washington, and in the displaced terranes of the Klamath Mountains. They overlie rocks with compositions similar to oceanic crust and upper mantle, commonly found in mid- oceanridge settings.
Compressional tectonics continued into the Middle Jurassic during the Elko orogeny, which was localized in Nevada, east of Whiskeytown. A north- south island arc extended through what is now central Nevada. During this time, movement of the North American plate changed from a westward direction to a north- northeast direction at a rate of about 45 kilometers per million years (Saleeby et al. 1992). This shift caused California to begin moving towards the northern subtropical latitudes. A significant dextral component of strike- slip faulting affected the relative motion along the plate edge in the Blue Mountains area of central Oregon and northwest Washington. Northwest compressional deformation along the southwest margin also caused transtensional spreading in the western Sierra- Klamath mountains region. Additional accreted terranes were sutured to the margin during the Middle Jurassic.
In the Middle Jurassic a major westward- directed thrusting event occurred in the Klamath Mountains although subduction continued to generate eastwarddirected thrusts against the continental margin. Consequently, during this time, the tectonic configuration of the western United States changed to a two- sided tectonic geometry with both eastwarddirected and westward- directed thrusts.
Overall, California continued to migrate into northern subtropical latitudes as submarine and subaerial volcanism erupted in the northern and western Sierra Nevada and in northwest Nevada (Saleeby et al. 1992). The volcanic and depositional patterns in northwest Nevada were controlled by a roughly east- westtrending sinistral oblique- slip- fault system. Thrusting from the west disrupted major volcanism during the Middle Jurassic.
Magmatic activity along the western continental margin continued into the Late Jurassic (162- 144 Ma), and led to the emplacement of clusters of smaller and more scattered plutons (Saleeby et al. 1992). Even so, the magmatic activity was widely distributed across the western United States from the Pacific margin in California to the edge of the Paleozoic craton (old continental crust) in Utah (figure 13) (Suppe, 1985). Accompanying the magmatic activity was the Nevadan orogeny, a major deformation event of regional scale.
The Nevadan orogeny was short- lived and may have been the result of colliding terranes along the western Sierra- Klamath belt (Suppe, 1985; Saleeby et al. 1992). In the northwest, the Nevadan orogeny marks the first time that accreted terranes in Washington and Oregon shared similar structures. In the south, the western Sierra Foothills show a complex pattern of shortening, extension, and sinistral strike- slip faulting as part of the Nevadan orogeny. West of the broad magmatic arc, the ocean basin adjacent to the Klamath Mountains was destroyed by eastward subduction and north- northwest obduction. The Great Valley fore- arc basin floor in California, however, survived Nevadan deformation.
Thin- skinned deformation also characterizes the Cretaceous Sevier orogeny (Stewart, 1980). The Sevier orogenic belt is comprised of the the fold and thrust belt of western Utah and an eastern branch that extends through Utah and southern Nevada into southeastern California. The development of the thrust system is a classic example of “stacked- shingle” geometry wherein the structurally highest and oldest fault systems formed in the west and were then carried in piggy- back fashion to the east on sequentially younger and lower thrusts as the basal decollement propagated eastward (Cowan et al, 1992). In eastern Washington and Montana, thrusting occurred from before 100 Ma to 50 Ma. In southeastern California and southern Nevada, thrusting took place over a period of time from before 200 Ma to about 85 Ma.
During thrusting in the eastern part of the region, a belt of Upper Jurassic to Cretaceous granitic plutons was emplaced along the converging plate boundary from southern California to northern Idaho (red bodies in figure 12). The granitic plutons coalesced to form the continuous Sierra Nevada batholith. The Sierra Nevada batholith intruded the margin of North America after the Late Jurassic Nevadan orogeny and before about 80 Ma. Individual plutons within the batholithic belt vary widely in size and shape and the Sierra Nevada batholith has probably been displaced as a block about 200 km (120 mi) westward in response to Cenozoic extension in the Basin and Range province to the east.
The western United States (Cordillera) divided into two distinct tectonic regimes during this time: an eastern regime and a western regime. The eastern regime consisted of the Sevier, Idaho- Wyoming, and Montana fold and thrust systems. The Sierran batholithic belt(described above), the Great Valley Group and the Franciscan Complex constituted the western regime.
The Great Valley formed an elongate basin west of and parallel to the Sierra Nevada batholith and southeast of the Klamath Mountains in California. Jurassic oceanic crust floored the basin, which was flanked on the west by an outer- arc high that existed in Late Jurassic to Early Cretaceous time (Cowan et al. 1992). Sediments accumulated in the basin from Late Jurassic to Paleogene time. Within the sediments is evidence for a profound change in the tectonic setting of California in response to the Nevadan orogeny. Stratigraphic relationships suggest that the Nevadan orogeny was more than a simple accretionary event. The Nevadan orogeny coincides with the extinction of the Late Jurassic magmatic arc and the inception of a new forearc basin and magmatic arc represented by the Great Valley group and Sierra Nevada batholith, respectively.
The Franciscan Complex also lay west of the Sierra Nevada batholithic belt. The Franciscan Complex is a chaotic assemblage of upper Mesozoic and lower Tertiary clastic sedimentary rocks, basalt, chert, and metamorphic rocks deformed by a number of imbricate thrusts. The timing and association these thrust faults is not fully understood (Cowan et al. 1992). Exposures of the sequence are present in the Coast Ranges east of the San Andreas fault and west of the Great Valley and Klamath Mountains. The Franciscan is considered a subduction complex.
The Sevier orogeny evolved into the Late Cretaceous - Early Tertiary Laramide orogeny. Instead of thinskinned thrusts involving the upper layers of sedimentary rock, the Laramide orogeny involved deep thrusts that carried basement rocks from the subsurface and stacked them atop younger rocks. Both the Sevier and Laramide orogenies involved subduction of the Farallon plate (preceding the Pacific plate) beneath the western margin of North America. The east- dipping, oceanic Farallon plate subducted at a steep angle during the Sevier Orogeny (figure 14).
Near the end of the Cretaceous, the subducting Farallon plate may have changed its angle of dip from steep to relatively flat (Dickinson and Snyder, 1978; Livaccari, 1991; Fillmore, 2000). Subducting at a flatter angle, likely caused the oceanic plate to stop melting thus cutting off the source for volcanic activity and generating tremendous shear stresses between the two slabs. Stresses at the base of the thick continental crust were transmitted upward in the form of compression which thrust great wedges of basement rock skyward forming the Laramide Rocky Mountains.
Following the Laramide orogenic events the Cordilleran deformation transitioned from compression to extension and strike- slip tectonics. This transition occurred in different places at different times and was probably related to impingement of the North American plate on the western margin, development of the San Andreasfault system, and the opening of the Gulf of California beginning in the Oligocene (Bally et al. 1989; Christiansen et al. 1992). The change in plate motions and stress regimes may have been related to the development of the Basin and Range Province at this time (Balley et al. 1989).
As the crust extended around 15 Ma, the surface broke into the basin- and- range topography seen today in western Utah, parts of California, Nevada, Arizona, and the Rio Grande Rift in New Mexico (figure 15). The Basin and Range province of the western United States is an excellent example of horst- and- graben topography wherein fault blocks forming valleys (grabens) have dropped relative to the adjacent uplifted blocks (horsts) forming mountain ranges on steeply dipping normal faults. These fault planes flatten at depth and merge with a regional decollement surface (Wernicke, 1992).
Transform faulting along the subduction zone on the western margin continued to reconfigure the Coast Ranges of California west of Whiskeytown during the Middle Tertiary. The western Santa Monica Mountains moved westward with respect to the Peninsular Ranges and the San Gabriel fault became the main strand of the San Andreas system across the western Transverse Ranges. East of the subduction zone, the Cascades Arc extended southward along the California-Nevada border and by about 20 Ma, the arc was continuous to as far south as the latitude of Las Vegas, some 500 km (310 miles) from the Klamath Mountains (red area on figure 15).
From about 17- 14 Ma, the Western Cascades- Oregon Coast Range- Klamath Mountain block rotated clockwise by as much as 16o (Christiansen et al. 1992). Tremendous outpourings of flood basalts, between about 17 and 14 Ma, accompanied crustal extension east of the Cascade Arc in eastern Washington and eastern Oregon.
Basins in California were filled by sediment as shown by bathyal (deep water)organisms to neritic (shallow water) or nonmarine organisms deposited near the end of the Miocene as the San Andreas fault system was superimposed over part of the continental- margin subduction zone (Atwater, 1970; Christiansen et al. 1992).Central California basins, such as the Sacramento and San Joaquin Valleys, that had persisted as bathyal depocenters (deep water depositional basins) were tectonically inverted in the Pliocene to Pleistocene period of combined compression and oblique tension and began to collect nonmarine sediments.
Volcanism in the Cascade Arc along the western coast of the United States continues to the present day (figure 16). The Quaternary Period is subdivided into two epochs: 1) the Pleistocene Epoch, about 1.6 Ma to 10,000 years before present (B.P.), and 2) the younger Holocene Epoch that extends from 10,000 years B.P. to the present. The Pleistocene Epoch is known as the Ice Age and is marked by multiple episodes of continental and alpine glaciation. Great continental glaciers, thousands of feet thick, advanced and retreated over approximately 100,000- year cycles. Huge volumes of water were stored in the continental glaciers during glacial periods so that global sea level dropped as much as 91 m (300 ft) (Fillmore, 2000).In at least 5 major pulses, local alpine glaciers carved the peaks of the Sierra Nevada and Klamath Mountain ranges. Evidence of this glaciation includes glacial cirques, deposits, and striations. Ice and snow bodies still exist in the Trinity Mountains, Mt. Shasta, and the Lassen Volcano area, west, north, and east of Whiskeytown National Recreation Area, respectively (Basagic, 2006). Glacial deposits were not specified on the map of Whiskeytown National Recreation Area.
The Holocene, is the age of humans and human impact on the global ecosystem is complex. With the retreat of the glaciers and the end of widespread glaciation about 12,000 years ago, the climate has continued to warm and global sea level has risen. Geologically, the physiographic provinces of the western United States have not changed much during the Holocene. Although, volcanoes periodically erupt in the Cascade Mountains and earthquakes signal the continued movement of the San Andreas Fault system.
The peaks of Shasta Bally and surrounding landscape at Whiskeytown National Recreation Area stand as a monument to the expression of deep time, that time that surpasses human understanding and serves as a reminder that Earth is not static but subject to change.
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