6.1: Developing a Geologic History

How Do Geologists Develop a Geologic and Tectonic History?

We can determine a very general and broad history of an area based on what we know about the global distribution of tectonic plates and how they have moved through time. We can track the motion of tectonic plates quantitatively using the magnetic anomalies and fracture zones on the seafloor. The information that we gather from these types of studies has low spatial and temporal resolution but gives us a framework to start with of our understanding of the tectonic development of a specific area, such as California.

To flesh out and tell a more detailed tectonic history of an area we need to look at the rocks. Rocks record information about the environment in which they formed. From sedimentary rocks we can determine if the environment was marine or non-marine, and the depth of water. We can determine flow directions of wind and water transported sediments. Igneous rocks will have a chemistry that is determined by the processes through which that melt formed and subsequently cooled. Metamorphic rocks will hold information about the original parent rock and the pressure and temperature conditions that it experienced during metamorphism. By learning to read the rocks we can determine the environment that existed when that rock formed.

Another important clue that we can glean from rocks is when they formed or when they were metamorphosed (see Geologic Time). If we can determine ages for rocks, we can put them in a sequential order and see how the geologic environments have changed through space and time.

Earth is a tectonically active planet that continues writing its geologic history. Recent tectonic events often overprint or even erase the evidence for earlier events. Additionally, not all periods of geologic time are equally represented or accessible in certain regions leading to gaps in geologic histories. Geologists interrogate the rocks with various laboratory and field techniques in order to unwrap these overprints and interpolate across space and time to tell the geologic history of an area.

Video \(\PageIndex{1}\): A Quantitative Plate Reconstruction of Western North America and the Eastern Pacific Basin [no audio]

This animation shows the tectonic evolution of western North America and the eastern Pacific Ocean over the past 170 million years, from the Jurassic Period to the present. The reconstruction is based on a combination of geological evidence at Earth’s surface and seismic imaging of subducted plates deep within the mantle.

Overview

The video illustrates the western margin of North America forming through a complex sequence of interactions involving multiple oceanic plates, island arcs, and microcontinents. Rather than a single oceanic plate subducting beneath the continent, the region was characterized by a broad offshore system of subduction zones and volcanic arcs. North America (dark blue) gradually moved westward into this system, overriding and accreting these terranes over time.

Key ideas shown in the animation:

• Multiple oceanic plates, including the Farallon plate (green) and later the Kula plate (pink), exist between the Pacific Ocean and North America. Subduction occurs in more than one direction at the same time.
• Terranes comprised of island arcs and microcontinents—Angayucham (red), Intermontane (purple), Insular (dark orange), Western Jurassic (orange), and Guerrero (yellow)—form offshore before being added to the continent.
• North America (dark blue) moves westward and overrides this system

Early configuration (Jurassic, ~170–147 million years ago)

At the beginning of the animation, North America (dark blue) is positioned farther east than it is today. To the west lies a large ocean basin which is subdivided by divergent plate boundaries (red lines), convergent plate boundaries (blue lines with triangular teeth on the overriding plate), transform plate boundaries (green lines).

Subduction is active within this ocean basin. Oceanic plates descend into the mantle beneath island arcs, and subduction occurs in multiple directions, including both westward- and eastward-dipping zones. These processes form a broad archipelago of volcanic arcs and microcontinents, including the Angayucham arcs (red), Insular superterrane (dark orange), Intermontane superterrane (purple), Guerrero superterrane (yellow), and the Western Jurassic belt (orange), located offshore of North America (dark blue). The Farallon plate (green) is already present and subducting beneath parts of this offshore system rather than directly beneath the continent.

Important features at this time:

• North America (dark blue) is not yet at a subduction boundary.
• Offshore regions contain multiple plates and volcanic arcs.
• Subduction zones dip in different directions.
• The Farallon plate (green) subducts beneath offshore terranes rather than the continent.
• The Angayucham arcs (red) and Mezcalera-related systems are separated from North America by ocean basins

Evolution of the offshore arc system (~147–115 million years ago)

As the animation progresses, the offshore plate system becomes more complex. Multiple small plates and island arcs evolve within the eastern Pacific basin. The Farallon plate (green) continues to subduct beneath offshore plates and arcs, including those associated with the Insular superterrane (dark orange), Guerrero superterrane (yellow), and Western Jurassic belt (orange). Subduction zones reorganize, and new plate boundaries form.

At the same time, North America (dark blue) continues to drift westward toward this system of arcs and microcontinents.

Collision and accretion along the continental margin (~115–83 million years ago)

North America (dark blue) begins to interact directly with the offshore archipelago. Island arcs and microcontinents, including the Insular superterrane (dark orange), Intermontane superterrane (purple), Guerrero superterrane (yellow), and Western Jurassic belt (orange), are progressively accreted to the western edge of the continent. The Angayucham terranes (red) are also involved in complex interactions and deformation, particularly in northern regions.

This accretion occurs at different times along the margin, and some terranes are transported long distances before final attachment.

As the continent overrides these subduction zones, the geometry of subduction changes. In some locations, the direction of subduction reverses, and subduction begins beneath the continental margin, similar to modern Andean-type systems. During this time, the Farallon plate (green) increasingly subducts beneath North America.

Continued plate reorganization (~83–55 million years ago)

The Kula plate (pink) forms and begins interacting with the Farallon plate (green) and other plates. Terranes continue to move along the continental margin, including continued translation of the Insular (dark orange) and Intermontane (purple) terranes.

Subduction beneath the continent continues and is associated with widespread magmatism. By this time, much of the previously offshore archipelago has been accreted onto North America (dark blue).

Development of the modern plate boundary (~55 million years ago to present)

In the final portion of the animation, most terranes—including the Insular (dark orange), Intermontane (purple), Guerrero (yellow), Western Jurassic (orange), and Angayucham-related terranes (red)—have been accreted to North America (dark blue). Subduction is concentrated along the continental margin, including the Cascadia region, where remnants of the Farallon plate (green) continue to subduct.

As plate boundaries reorganize, parts of the margin transition from subduction to transform motion, leading to the development of the San Andreas Fault system. The influence of the Kula plate (pink) diminishes as plate configurations evolve.

The present-day configuration of western North America is established during this time.

Download the paper or on ResearchGate and download the plate model

Clennett, E. J., Sigloch, K., Mihalynuk, M. G., Seton, M., Henderson, M. A., Hosseini, K., et al., 2020, A quantitative tomotectonic plate reconstruction of western North America and the eastern Pacific basin. Geochemistry, Geophysics, Geosystems, 20, e2020GC009117. https://doi.org/ 10.1029/2020GC009117.