The Sierra Nevada range is a prominent mountain chain in western North America, stretching for about 650 km along the eastern edge of California. The range is known for its scenic beauty, diverse ecosystems, and rich mineral resources. But how did this range form and what are the forces that shaped its landscape?
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The Origin of the Sierra Nevada Range
The Sierra Nevada range is a result of a complex geological history that spans hundreds of millions of years. The range was originally part of a volcanic arc that formed along the western margin of North America during the Mesozoic era, when the oceanic Farallon plate subducted under the continental North American plate. The subduction zone generated intense heat and pressure that melted the crust and produced large volumes of magma that rose to the surface and erupted as volcanoes. The volcanic arc also accumulated sediments and marine fossils that were scraped off the subducting plate and added to the continental margin.
The volcanic arc was later intruded by a massive body of granitic magma that cooled and solidified underground, forming the Sierra Nevada batholith. The batholith is the core of the Sierra Nevada range, composed of various types of granite and related rocks that are resistant to erosion and weathering. The batholith was exposed at the surface by uplift and erosion of the overlying volcanic and sedimentary rocks.
The Tilting of the Sierra Nevada Range
The uplift and erosion of the Sierra Nevada range was not uniform or gradual. Instead, it was influenced by regional tectonic events that affected the western United States during the Cenozoic era, especially in the last 20 million years. One of these events was the Basin and Range extension, which is a process of crustal stretching and thinning that created a series of parallel mountain ranges and valleys in Nevada and adjacent states. The Basin and Range extension was driven by the changing direction and rate of plate motion between the North American plate and the Pacific plate, which resulted in a shift from subduction to transform faulting along the western margin of North America.
The Basin and Range extension had a significant impact on the Sierra Nevada range, as it created a major fault zone along its eastern boundary, known as the Sierra Nevada frontal fault system. This fault system accommodated the differential movement between the extending Basin and Range province and the relatively stable Sierra Nevada block. The fault system also acted as a hinge that allowed the Sierra Nevada block to tilt westward, creating a steep eastern escarpment and a gentle western slope. The tilting also increased the elevation difference between the crest of the range and the adjacent basins, enhancing erosion and sediment transport.
The tilting of the Sierra Nevada range is not a simple or uniform process either. It varies in magnitude, timing, and location along the length of the range. Some studies suggest that there were two main phases of tilting: an early phase that occurred around 12 million years ago, and a late phase that occurred around 3 million years ago. The early phase was more pronounced in the northern part of the range, while the late phase was more pronounced in the southern part of the range. The tilting also varies in angle, ranging from 5 to 25 degrees depending on local factors such as fault geometry, rock strength, and climate.
The Link Between Tilting and Mantle Upwelling
The tilting of the Sierra Nevada range is not only related to Basin and Range extension, but also to mantle upwelling under both provinces. Mantle upwelling is a process of rising hot material from deep within the Earth’s mantle that can affect surface topography, crustal deformation, volcanism, and seismicity. Mantle upwelling can be caused by various factors such as mantle plumes, slab break-off, slab rollback, or edge-driven convection.
Some studies suggest that there is a close spatial and temporal correlation between mantle upwelling under both provinces and tilting of
the Sierra Nevada range. For example, seismic tomography images show that there is a low-velocity anomaly under both provinces that indicates hot material rising from depth. Geophysical modeling also shows that mantle upwelling can generate stresses that can cause crustal thinning in Basin and Range province and crustal thickening in Sierra Nevada province. These stresses can also contribute to faulting and tilting along their boundary.
Mantle upwelling can also explain some features that are not easily accounted for by Basin and Range extension alone. For instance, mantle upwelling can explain why there is volcanism in both provinces despite their different tectonic settings. Mantle upwelling can also explain why there is uplift in both provinces despite their different crustal thicknesses. Mantle upwelling can also explain why there is more recent uplift in
the southern part of Sierra Nevada range than in northern part.
Conclusion
The tilting of the Sierra Nevada range is a complex and dynamic process that reflects the interaction of various geological forces over time. The tilting is related to Basin and Range extension, which is a process of crustal stretching and thinning that created a major fault zone along the eastern boundary of the range. The tilting is also related to mantle upwelling, which is a process of rising hot material from deep within the Earth’s mantle that affected both provinces. The tilting varies in magnitude, timing, and location along the length of the range, creating a diverse and spectacular landscape.
