• Introduction
• Unresolved Questions About Orogenies
• Highlights of Pacific Northwest 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. An orogeny commonly builds not just one mountain range, but several. 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 dip 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.

Unresolved Questions About Orogenies

Most orogenies can be explained in terms of processes that occur at convergent plate boundaries plate boundaries. However, some orogenies seem to take place far from convergent plate boundaries. The Basin and Range and Rocky Mountain landscape regions are examples of modern orogenies that are not near plate boundaries. The Cretaceous North Cascades orogeny is an example of an orogeny that seems to have involved more than simple plate convergence.

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 North Cascades orogeny of the Cretaceous Period presents some interesting questions. On the one hand, the North Cascades contain thrust faults, folds, intrusive rocks and regional metamorphic rocks that would be expected in plate boundary setting in which the oceanic plate converges directly into the continent. On the other hand, the paleomagnetic data indicate that the some of the rocks in the North Cascades moved north 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.

Highlights of Pacific Northwest Orogenies

There were Precambrian orogenies that created and shaped the Archean continental basement (deep levels of the crust) in Idaho, Montana and Wyoming. Evidence of these orogenies is seen in the cores of some of the Rocky Mountain ranges, where uplift and erosion have revealed the basement of the crust. We will not go into detail about these ancient orogenies, about which we have sketchy geologic knowledge.

During most of the Paleozoic Era the coastal margin of western North America was a passive margin, with no plate boundary and no orogeny. However, there were some exceptions in the form of relatively short-lived orogenies. The largest and most widespread of these Paleozoic orogenies was the Devonian Antler orogeny, which involved subduction, accretion of one or more island arcs, and widespread folding and thrust faulting. The area affected by the Antler Orogeny extends no farther into the Pacific Northwest than southern Idaho, apparently, so we will not delve into it.

Starting by early Jurassic time, nearly the entire western coastal region of North America became an active margin. In addition, since the Jurassic Period, transform plate boundaries have occasionally occurred along portions of the continental margin of western North America. Plate convergence, with the occasional intervention of transform plate motion, has been the driving force behind the complex history of orogenies in the Pacific Northwest since the Jurassic.

Making a geologic traverse of the North Cascades Mountains in Washington State allows a good look inside two orogenies: the Cretaceous North Cascades Orogeny, and the late Tertiary-Quaternary Cascadia Orogeny that still goes on today.

Wenatchee to Seattle (SR 97 and I-90)

If you take State Route 97 and Interstate 90 from Wenatchee to Seattle, you would traverse sedimentary rocks that formed in the area before the rise of the modern range (Blewett Pass and Cle Elum area), volcanic rocks that erupted more recently from volcanoes in the Cascade volcanic arc (Interstate 90 east of Snoqualmie Pass), a batholith that occurred during the Miocene Epoch (the Snoqualmie Batholith, seen at and east of Snoqualmie Pass), and metamorphosed volcanic and sedimentary rocks of one or more accreted terranes (in the tree-covered lower mountains and hills west of Snoqualmie Pass).

Stevens Pass (U.S. 2)

In the Stevens Pass traverse you would pass through metamorphic rock that used to be seafloor sediments and was buried deeply and metamorphosed at high temperature and pressure (east of Steven Pass), and a large batholith of Cretaceous age (the Mt. Stuart batholith, west of Steven Pass).

North Cascades Highway (SR 20)

The North Cascades Highway takes you through a variety of sedimentary rocks (accreted terranes in the Methow Valley), plutonic rocks, and high-grade regional metamorphic rocks in the crystalline core of the North Cascades. The metamorphic rocks in the crystalline core of the range formed in the middle to late Cretaceous Period, as did most of the batholiths and plutons in the crystalline core. Once you go west of the town of Marblemount, you leave the crystalline core and enter a zone of thrust-faulted, accreted terranes that were added to North America late in the Cretaceous period.

The North Cascades contain evidence of orogeny in the Cretaceous Period, intruded and covered in places by rocks from the younger Cascadian orogeny. Orogenies are still ongoing in the Pacific Northwest. The Cascadian Orogeny continues to form the Coast Ranges and Cascade Range. The ranges of the Basin and Range and the Rocky Mountains continue to rise.

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Glossary terms that appear on this page: orogeny; reverse fault; thrust fault; anticline; syncline; composite cone; igneous intrusion; batholith; regional metamorphism; contact metamorphism; erosion; convergent plate boundary; transform plate boundary; volcanic arc; subduction; active margin; sedimentary rocks; volcanic rocks; accreted terrane; metamorphic rocks; plutonic rocks; pluton