• Introduction
• Orogenies - Opportunities for Learning
• Pre-Mesozoic Orogenies of the Pacific Northwest
• Sevier Orogeny
• Laramide Orogeny
• Basin and Range Orogeny
• North Cascades Orogeny
• Cascadia Orogeny
• Driving Through Orogenies
• Wenatchee to Seattle (SR 97 and I-90)
• Stevens Pass (U.S. 2)
• North Cascades Highway (SR 20)
• Glossary Terms

Related Basics Pages: Orogenies
Related Focus Pages: #8--Orogenies in the Pacific Northwest

Introduction

Welcome to Week 7 of Pacific Northwest Geology. The topic of this week's lecture is orogeny. An orogeny is a major cycle of mountain building that affects an entire region of a continent. The region undergoing orogeny is called the orogen. An orogeny commonly builds several mountain ranges. In the build up of a large mountain range the continental crust is subjected to most of the geologic processes we have looked at so far in this course:

• faulting (reverse and thrust faulting)
• folding (anticlines and synclines)
• volcanism (composite cones)
• intrusion (igneous intrusions up to the size of batholiths)
• metamorphism (deep burial and regional metamorphism of large volumes of continental crust and contact metamorphism of crust adjacent to igneous intrusions)
• erosion (as the mountains are uplifted they increasingly become exposed to the forces of erosion)

These processes combine to thicken the continental crust. The average crustal thickness of most of North American is about 15 miles (25 kilometers). Right now, the thickest continental crust in the continental U.S. is the Cascade Range. The distance from the highest peaks to the base of the continental crust beneath them is about 25 miles (40 kilometers). The roots of this orogenic belt reach down into the mantle somewhat like the keel of a boat.

Erosion is always working to reduce areas that rise above sea level back down to sea level. Because the highest elevations above sea level occur in orogenic belts (mountain ranges), those areas are where rates of erosion are highest. Erosion removes sediments from the mountains. Sediments shed from the eroding mountains are deposited in basins between and beyond the ranges. In the long run, volcanoes that were on top of the mountain ranges will be completely eroded away, but evidence of the volcanoes' existence will be preserved in layers of sediment. Two types of sandstone are particularly indicative of orogenies: graywacke, from active volcanic arcs eroding into a nearby trench or marine basin, and arkose from erosion of granite and gneiss that was formed deep in the crust and uplifted into mountains.

Orogenies - Opportunities for Learning

The most recent orogenies provide abundant evidence in the geologic record of the events that occurred to build mountains. The events of older orogenies may be more difficult to piece together but analysis of rocks and structures combined with an understanding of plate tectonics can shed light on even the oldest orogenies.

Most orogenies can be explained in terms of processes that occur at convergent plate boundaries. However, some orogenies seem to take place far inland from convergent plate boundaries. Mountain building can also occur along continental transform plate boundaries.

The Basin and Range and Rocky Mountain landscape regions are examples of orogenies that are not near plate boundaries. The timing and geologic structures of the Basin and Range indicate that the region began to undergo extension and uplift at the same time the San Andreas fault began to form. This timing suggests that mountain building in the Basin and Range is related to the change in the character of the plate boundary along the coast of California--from a convergent to a transform plate boundary. The continued uplift of ranges in the Rocky Mountains suggests that the effects of a convergent plate boundary may reshape the continental crust much farther inland than the volcanic arc.

The Cretaceous North Cascades orogeny is an example of an orogeny that seems to have involved more than simple plate convergence. On the one hand, the North Cascades contain thrust faults, accreted terranes, folds, intrusive rocks and regional metamorphic rocks that would be expected in a plate boundary setting in which the oceanic plate converges directly into the continent. On the other hand, paleomagnetic data indicate that some of the rocks in the North Cascades moved north, possibly as much as 1,000 miles, since the mid-Cretaceous. Such movement occurs at a transform plate boundary, rather than at a convergent plate boundary.

Orogenic styles seen in the Pacific Northwest include volcanic arcs associated with subduction zones (Cascadia orogeny), block mountain ranges offset along steep faults (Laramide and Basin and Range orogenies), and thrust fault belts (Sevier and North Cascades orogenies). Geologists continue to debate the details of these orogenic styles and what each style tells us about how continents form.

Pre-Mesozoic Orogenies of the Pacific Northwest

During Archean time orogenies occurred in the part of the North American craton that is now the Rocky Mountains. Evidence of these long-ago orogenies includes the granites that formed in cores of these ancient mountains, the metamorphosed volcanic rocks that they erupted, and the sedimentary rocks that formed as they eroded.

The North American craton was flat and stable after the Archean mountains were leveled by erosion. During much of the Paleozoic era, the edge of the craton in the Pacific Northwest was a passive continental margin, with no plate boundaries nearby and no mountain building. There were a few exceptions to this passive plate margin during the Paleozoic era and the geologic record indicates some short-lived mountain-building activity at the western edge of the craton. These Paleozoic orogenies seem to have involved temporary subduction zones along the coast of the growing North American continent where terranes accreted and thrust inland. One of these was the Antler Orogeny, which affected parts of Nevada, Utah, and Idaho in the Pennsylvanian period.

The Sevier Orogeny

The Rocky Mountains run from the interior of Alaska and western Canada to Mexico. In parts of Nevada and Utah there are huge thrust faults that stacked up large slabs of rock, being pushed from west to east, during the Cretaceous period, starting about 140 million years ago. Similar styles and ages of thrust faults also occur in the Rocky Mountains of Montana, Canada, and Alaska. The process that created this stacking up and eastward pushing of thrust-fault slabs is called the Sevier orogeny, named after the Sevier River in Utah.

The Sevier orogeny overlaps in time and space with the Laramide orogeny (see next section). The Sevier orogeny may be an expression of how the upper crust of western North America responded to tectonic processes driven by plate interactions at that time, while the Laramide orogeny may be an expression of how the deep crust and underlying lithosphere responded to the same tectonic processes.

Learn the details of the Sevier Orogeny on Focus Page 8.

The Laramide Orogeny

Many individual mountain ranges make up the overall Rocky Mountains. The highest mountain ranges in the Rockies reveal deep crustal metamorphic and plutonic rocks in their cores. These are called "basement cored" mountain ranges. The orogeny that produced these mountain ranges gets its name from the Laramie Mountains in Wyoming, which are an example of such a mountain range. The Laramide ranges show evidence of having been folded, thrust-faulted, and, in some of the big blocks of Laramide rock, reverse-faulted upward. This style of uplift in which deep-crustal rocks are uplifted to form basement-cored mountain ranges is called "thick-skinned" thrusting.

Learn the details of the Laramide Orogeny on Focus Page 8.

The Basin and Range Orogeny

You enter the Basin and Range landscape region if you go south of the Central Oregon Plateau and the Snake River Plain. The region is defined by mountain ranges oriented in a north-south direction separated by desert basins. Crustal extension dominates the tectonics of the region as seen in the normal faults along the base of many of the mountain ranges and the detachment faults and metamorphic core complexes present in some of them.

Learn the details of the Basin and Range Orogeny on Focus Page 8.

The North Cascades Orogeny

There is no generally agreed-upon name for the orogeny of the North Cascades during the Late Cretaceous period, so we will simply call it the North Cascades orogeny. The North Cascades can be divided into three zones, separated from each other by major faults or fault zones. In the east is the Methow Valley, in the center is the North Cascades crystalline core and to the west is the Northwest Cascades.

The rocks of the crystalline core formed deep in the earth's crust, as plutonic and high-grade metamorphic rocks. The rocks of the Methow Valley and Northwest Cascades formed mainly in the upper crust, as volcanic and sedimentary rocks.

As discussed in Focus Page 8, the North Cascades seem to result from a combination of subduction and transform faulting. A simple model would describe the North Cascades orogeny as a subduction-driven orogeny at a convergent plate boundary with a resulting package of rocks that was shifted northward when the convergent plate boundary changed to a transform plate boundary. Reality appears more complicated, including the possibility that subduction occurred beneath the older rocks of the Cascades even as the rocks were being shifted northward along the coast.

The North Cascades provides an unusual exposure of many parts of an orogeny, from its upper crustal levels to its deep crustal interior. Unresolved questions about when and where the rocks formed and how they were assembled into their present configuration continue to attract geologists seeking to explain how the earth makes mountain ranges.

Learn the details of the North Cascades Orogeny on Focus Page 8.

The Cascadia Orogeny

The Cascadia orogeny is occurring today in association with the Cascadia subduction zone. The volcanoes of the Cascades, the unseen magmas that are intruding and solidifying in the crust beneath the volcanoes, and the accretionary complex of the Coast Ranges, including the Olympic Mountains, are all part of the Cascadia orogeny.

Learn the details of the Cascadia Orogeny on Focus Page 8.

Driving Through Orogenies

Making a geologic traverse of the North Cascades Mountains in Washington State allows a good look inside two orogenies: the North Cascades Orogeny which occurred in the Cretaceous and Paleogene periods, and the Cascadia Orogeny, which began in the Neogene period and continues today.

Wenatchee to Seattle (SR 97 and I-90)

Traveling on State Route 97 and Interstate 90 from Wenatchee to Seattle, you traverse sedimentary rocks that formed in the area before the modern Cascade range near Blewett Pass and Cle Elum. Approaching Blewett Pass you also pass through part of an accreted terrane. East of Snoqualmie Pass are volcanic rocks that erupted from volcanoes in the Cascade volcanic arc. At Snoqualmie Pass you enter the Snoqualmie Batholith, which intruded during the Miocene Epoch. In the forested foothills surrounding I-90 before you reach the Puget lowland, you travel through metamorphosed volcanic and sedimentary rocks of one or more accreted terranes.

Stevens Pass (U.S. 2)

In the Stevens Pass traverse you pass through metamorphic rock that used to be seafloor sediment and was buried deeply and metamorphosed at high temperature and pressure (east of Stevens Pass), and a large batholith of Cretaceous age (the Mt. Stuart batholith, at Stevens Pass and west of there). Once you pass Skykomish, headed west on U.S. 2, you drive past Neogene intrusive rocks (including the Index batholith). West of the Index batholith near U.S. 2 are accreted terranes consisting of pieces of the ocean floor and probably one or more island arcs.

North Cascades Highway (SR 20)

The North Cascades Highway takes you through a variety of sedimentary rocks related to orogeny, plutonic rocks, and high-grade regional metamorphic rocks. In the southern Methow Valley you drive through accreted oceanic crust and island arc rock. Continuing north on the highway, you pass through ocean floor sedimentary rock from the nearby Methow Ocean. The sequence of marine sediments from the Methow Ocean is overlain by sedimentary rocks deposited after the area had come above sea level.

These continental sediments were deposited by rivers that flowed and discharged to the coast of North America. This took place during the latter half of the "Age of Dinosaurs." Therefore, it is possible that someday dinosaur fossils will be found in the sedimentary rocks of the Methow Valley, in the rocks that were originally deposited above sea level along the coast of North America as it existed back then.

Heading from the Methow Valley to the North Cascades Mountains, you pass metamorphic rocks in the crystalline core of the range. The rocks were metamorphosed in the middle to late Cretaceous Period, and most batholiths and smaller plutons in the crystalline core intruded during the Late Cretaceous period as well.

West of the town of Marblemount, you are west of the crystalline core in a zone of thrust-faulted, accreted terranes that were added to North America late in the Cretaceous period. In contrast with the rocks in the Cascades crystalline core, accreted terranes west of Marblemount are not metamorphosed at high temperatures and not intruded by batholiths. The accreted terranes west of Marblemount include basalt and mudstone from the ocean floor that was metamorphosed during subduction and accretion, and dirty sandstones shed from active volcanic arcs. These rocks were formed before the North Cascades orogeny, some of them at considerable distance from North America.

In sum, you can encounter the following on any of these drives across the central or north Cascades Mountains:

• "dirty sandstones" made of sediment shed from eroding volcanic mountain ranges
• large igneous intrusions made of granitic rock
• metamorphic rocks of various types, formed at various pressures and temperatures in the crust
• major faults that may have been the result of tectonic plate interactions
• accreted terranes, made of rock that did not originate in the North American continent
• young volcanic rocks of the modern Cascade volcanic arc
  • however, in the North Cascades, the young volcanic rocks are only found at or near the currently active volcanoes, apparently because the North Cascades have been uplifting and eroding so rapidly that the slightly older volcanic cover has been stripped away, revealing the deeper metamorphic and plutonic rocks, which are the source of the name "crystalline"

When you drive through the North Cascades and pay attention to the rock types, structures, and landforms, you are driving through an active orogen. Rocks of the earlier North Cascades orogeny continue to uplift and erode, capped by the volcanoes of the ongoing Cascadia orogeny.

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Glossary terms that appear on this page: orogeny; orogen; reverse fault; thrust fault; anticline; syncline; composite cone; igneous intrusion; batholith; regional metamorphism; contact metamorphism; erosion; arkose; graywacke; volcanic arc; granite; gneiss; convergent plate boundary; transform plate boundary; accreted terrane; craton; passive margin; subduction zone; detachment fault; accretionary complex; sedimentary rocks; volcanic rocks; metamorphic rocks; plutonic rocks; pluton