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Home , Accretion (geology), Arabian Plate, Basin and Range Province, North American Plate, Oceanic trench

Introduction 1

Introduction: the Global Overview

I. and of the II. Overview of Northwest Geology III. Timeline for Northwest Geologic Events

I. Plate Tectonics and Mohorovicic discontinuity (the Moho) occurs at the Geology of the Earth basethe of the floor is where 3 miles there (5 km) is the below greatest level. change The in seismic velocity. The is a dense hot layer of semi-solid rock about 1800-miles (2900 km) thick. It Earth’s Structure contains more iron, magnesium and calcium than the The Earth consists of three main layers: the crust, crust, and it is also hotter and denser than the crust. mantle, and core. The crust, which is the outer layer of The core is twice as dense as the mantle because it the Earth, is relatively thin and ridged. The continental consists of an iron-nickel alloy. It has two parts: a crust tends to be variable in thickness, averaging about 1360-mile-thick (2,200 km) liquid outer core and a 18-miles (30 km) thick; however, under 775-mile-thick (1,250 km) solid inner core. The Earth’s ranges it may be 60-miles (100 km) thick. The is 3- to 6-miles (5 to 10 km) thick and most of core as the Earth rotates. magnetic field is created by spinning of the liquid outer

Cross section showing the solid inner core, the liquid outer core, the mantle, and the crust. The is overlain by the . 2 Idaho Geology

Block diagram with east-west cross section across the the surface along the arcs, it typically has the Japan , Honshu and the Sea of Japan. At this composition of andesite. If the rises near the convergent plate boundary, Honshu is underlain by edge of the , it may form the great granitic . batholiths. is less dense than and can

Earth’s Crust Oceanic Crust lining the ocean basins is made of easily float through it. basalt. The Continental crust is 15- to 43-miles (25 to 70 km) oceanic crust consists of olivine tholeiitic basalt and thick, ranging up to 43-miles (70 km) thick under isa dark,composed fine-grained of plagioclase volcanic feldspar, rock called pyroxenes and mountain ranges and high . One third of the olivine. The oceanic crust is 3.5- to 6-miles (5 to 10 Earth’s crust is continental because the ocean basins have very little continental crust. The upper 6 to 9 below . The oceanic lithosphere is 30- to miles (10 to 15 km) of the crust is brittle and below 60-mileskm) thick (50 and to most 100 ofkm) the thick. ocean No floor ocean is 3 crust miles is (5 older km) that depth the crust is ductile. Continental crust is than 180 Ma. Basalt is derived from mantle material formed by the differentiation of from basaltic granite called that rises by cells or rock through the process. As the basaltic mantle plumes to the Earth’s surface. As the material rises, the pressure drops because of less overlying gradually cools, becomes denser, and eventually sinks material. Pressure-release melting allows a lower- crust flows away from the ridge riding on the mantle, it as much as 435 miles (700 km) into the mantle at the density liquid (basaltic magma) to separate from the subduction zones. As the basaltic crust travels higher density mantle materials. So basalt, which is towards the subduction zone, the basalt absorbs richer in silicates than the parent peridotite, is also less water through the hydration of certain minerals. The dense. The basaltic magma is injected at the divergent newly hydrated silicate minerals have a lower melting plate boundaries (where two plates are pulling apart) temperature than the rest of the minerals. So as the and forms new crust. of basaltic crust is subducted into the mantle, Lithosphere and Asthenosphere. Because the the silicate-rich hydrate minerals melt. Because this is cooler and more ridged than the deep melt is less dense than the surrounding material, it mantle, it behaves like the lower portion of the crust. Consequently, the crust and upper mantle together floats toward the surface. When the magma reaches Introduction 3 form the lithosphere or the tectonic plates. The than 120-miles (200 km) thick or twice the average thickness of the lithospheric plates ranges from 9 miles thickness of the continental lithosphere. have (15 km) for young oceanic lithosphere to more than a lower density than the oceanic crust so that they 120 miles (200 km) for old continental lithosphere. do not sink into the deep mantle. Cratons consist of However, the worldwide average is about 50 miles igneous and metamorphic rocks and range in age from (80 km) in thickness. The lithosphere overlies a hot, one- to four-billion-years old. These ancient crystalline mobile, semi-solid zone in the mantle referred to as the rocks are commonly referred to as the . At asthenosphere. The ridged lithosphere is believed to the central portion of the cratons, basement rocks are generally exposed but may be covered by much Tectonic plates or lithospheric plates are immense, younger sedimentary rocks and around floatirregularly and move shaped on the slabs asthenosphere. of rock consisting of oceanic and continental lithosphere. Plate boundaries can covers the basement, it is called a platform. be accurately mapped from outer space by satellites. the margins. Where a thin layer of sedimentary rocks Theory of Plate Tectonics composed of granitic rocks, which consist of low- densityContinental minerals crust such floats as because quartz andit is feldspar.primarily By Since the 1840s, the geosynclinal theory was used to contrast, oceanic rocks are composed of denser explain the origin of mountain belts until the theory of plate tectonics gradually superseded it during the basaltic rocks. 1960s. Plate tectonics is now the great unifying theory of geology. Many geological and biological events, Cratons previously thought unrelated, can now be explained by The oldest and most stable part of continental plate tectonics. Although many geologists at the time lithosphere is referred to as a . Geologically more opposed the continental drift active rocks such as mountain belts typically surround (1915), there is now a consensus among geologists the cratons. Most cratons are situated in the interiors that the plate tectonic theory is idea valid. of Plate Alfred tectonics Wegener of tectonic plates and have survived many cycles of merging and rifting of and . Cratons amount of corroborating evidence from almost tend to have thick lithospheric roots and may be more everyhas been subdiscipline repeatedly of tested geology, and paleontology, confirmed by physics, a massive

The major lithospheric plates and their movement directions. U.S. Geologic Survey. 4 Idaho Geology

U.S. Geological Survey world map showing shield areas, platform areas and orogen areas.

chemistry, and mathematics. This evidence includes accreted lithospheric fragments, which may be either of isotope dating, -core analyses, deep-ocean continental or oceanic in origin, are called if drilling, paleontology, seismology, paleomagnetism,

andthey appear are of sufficient out-of-place, size theyand haveare referred consistent to as geological exotic heat-flow analysis, geochemistry, petrology, structural orattributes. suspect Where terranes terranes. Exotic do terranes not fit the are local transported geology continents,geology, acoustic-bottom geomorphology, profiling, and the seismic-reflection correlation of by tectonic plates and are accreted or attached to strata,profiling, fold gravity belts, andprofiling, mountain glaciation, ranges. fit Plate of the activities an existing continental landmass. As a result they have also had an enormous effect on the evolution of are geologically unrelated to the adjacent landmass life. along which they accreted. The geology of western is complicated by many exotic terranes Evidence of Plate Tectonic Activity Before consisting of volcanic island arcs, oceanic sediments, and oceanic lithosphere. Pangea Existed Ocean Magnetism tectonic processes since the breakup of supercontinent PangeaWe now somehave an200 excellent million understandingyears ago. Plate of processes plate Magnetometers are capable of sensing rock magnetism

the oceanic crust is less than about 180 million years magnetism comes from the basaltic oceanic basement old.before Oceanic Pangea crust are appearsmore difficult to be recyclingto decipher about because every all becausemore than the a overlyingmile below sediments the ocean are floor. too Most weakly of the 150 million years. Consequently, all evidence of pre- Pangea plate movements must come from the existing The magnetic properties of the oceanic basement continents. There is now ample evidence that plate dependmagnetized on the to affectamount the of magnetometer magnetite, thickness significantly. of the tectonics have existed for most of the Earth’s history magnetized layer, and its depth below the surface. with several cycles of supercontinent formation. For Marine magnetic surveys are concerned only with example, it has been proposed that Pangea was formed from the breakup of parts of a 540-million-year-old direction of horizontal and vertical components. The measurements of the total field, not with magnitude or supercontinent called Rodinia. Along some continental border zones, geologists gammas at the magnetic equator to about 60,000 have recognized remnants of ancient volcanic arcs, gammasmagnitude at ofthe the magnetic total field poles. ranges from about 30,000 oceanic crust, and ocean sediments accreted onto In 1963 Fred Vine and Drummond Matthews the continental margins by plate tectonic processes observed that basalt along the central ridge of the that operated, in some cases, before Pangea. These was magnetized with normal polarity. Introduction 5

“Magnetic stripes” on the ocean floor indicating reversals of the Earth’s magnetic field. The dark stripes represent normal polarity and the white stripes represent reversed polarity. The width of a magnetic stripe depends on the The Earth’s magnetic field has lines of force just like spreading rate and the time interval that the polarity those of a bar magnet. The strength of the magnetic field event represents. An observed magnetic profile for the changes uniformly from the magnetic equator to the ocean floor across the East Pacific Ridge is shown as it magnetic poles. Consequently, a dip needle is parallel to relates to the magnetic stripes. the Earth’s surface only at the magnetic equator; and, the inclination to the surface increases to 90 degrees at the magnetic poles. Seismology or Evidence Through examination of seismograms, seismologists had reverse magnetism, and that even further from the ridge,They also the crustfound had that normal the crust polarity. on the They flank reasoned of the ridge that along a . The first motion is the initial impulse are able to determine the direction of first motion as new magma moves up the axial crest of the ridge, it that generates a compressional wave (P-wave). Three cools below the curie point (580° C.) and the magnetic of the plates. First-motion studies along all the plate different types of define the boundaries moves up and displaces the older crust, which moves laterallyfield at the away time from is recorded. the ridge. New All the magma magma continuously emplaced against another. Transform faults showed horizontal motion,boundaries and confirmed at the axial the relative motion of one plate either reversed or normal polarity, depending on the expected motion for normal faults. The subducting at the ridge during a specific polarity event will have , first motion was the earthquakes. First-motion studies on earthquakes plate is defined by a dipping tabular-shaped zone of stripmagnetic represents field at alternately the time. Since normal the andEarth’s reversed magnetic associated with subducting plates show tensional magnetism.field reverses periodically, each successive anomaly forces in the shallow portion of the subducting slab and On maps, the positive anomalies (normal polarity) compressional forces in the deep portion of the slab. are shown as black stripes and the reverse polarity as 1. Shallow-focus earthquakes are generated near white stripes. The width of a magnetic stripe depends the landward side of the trench. on the spreading rate and the time interval the polarity 2. Intermediate-focus quakes are generated at event represents. Consequently, these linear anomalies a depth of about 125 miles (200 km) under an island can be used to date oceanic basement and determine belt, and about 90 to 125 miles (150-200 km) towards spreading rates. Although the magnetic stripes are the down-dip direction of the plate. generally symmetrical, asymmetrical spreading has 3. Deep-focus earthquakes are generated at 250- been documented. to 370-miles (400 to 600 km) depth and about 250 to 6 Idaho Geology

310 miles (400 to 500 km) to the landward side of the axial trench. rift The seismic evidence indicates a cool brittle plate earthquakes New ocean and floor gravity is emplaced faulting. along The rugged the at depths as great as 435 miles (700 km). The deep topography or the rift is zone,generated an area by ofa combination high heat flow, of extensionweak earthquake foci indicate compressive stress along the plane of the plate and oriented in a dip-slip (down is accreted equally to the two diverging plates, which makefaulting possible and volcanism. the symmetrical In most magneticcases new stripes. ocean floorThe resistance to penetration by the slab in the high- ridge towers some 6560 to 9840 feet (2000 to 3000 m) strengthdip) direction. zone ofAs the the lower leading asthenosphere, edge of the plate compressive finds above the adjacent abyssal plains. The axial rift portion stress results. On the other hand, the upper part of the of the Mid-Atlantic Ridge is characterized by a rift subducting plate is under tension because the slab is valley or central . A rift valley is a large down- sinking under the force of gravity in the low-strength faulted block or graben creating a valley. This rift valley zone of the asthenosphere. is also an enormous feature extending most the length

Iceland represents a portion of the ridge that has built MAJOR FEATURES OF THE OCEAN FLOOR upof the above ridge sea and level. dwarfing The rift the valley Grand straddles Canyon , in size. Oceanic Ridges so Iceland is breaking apart with the moving westward and the moving The mid-ocean ridges have been known to exist since eastward. The Mid-Atlantic Ridge spreads at an average bathymetric charts prepared during the 1950s revealed rate of 1 inch (2.5 cm) per year. their morphology and extent. Since that time scientists Oceanic crust is continuously forming at the ridge have investigated the ridges in great detail with marine and moves away towards the subduction zones. The geophysical surveys (, magnetics, seismic oceanic crust is youngest at the ridge and becomes progressively older towards the subduction zones, all sea drilling, seismology (earth quake data), and visual profiling, gravity, heat flow, seismic profiling), deep- inspection from submersibles. These ocean ridges are on sediments next to the oceanic basaltic crust, and by far the largest and most prominent feature to exist confirmed by the paleomagnetic anomalies, the fossils either on land or sea. They form a continuous mountain Block diagram showing how new ocean floor of basalt range extending 40,000-miles (65,000 km) long and originates at the axial rift of the Mid-Atlantic Ridge. At this up to 200-miles (320 km) wide around the Earth and slow-spreading divergent plate boundary, the plates move occupy up to 25 percent of the Earth’s surface. away from the axis of the ridge in opposite directions. Introduction 7

Block diagram of the and abyssal hills; the cross section shows how seamounts formed at the Seamounts are conical-shaped submarine volcanic oceanic ridges are moved out to the deep ocean and are peaks which are particularly widespread throughout buried by sediments transported by turbidity currents. The resulting topography is a smooth, plain with seamounts. In many cases, particularly in tropical scattered low volcanic hills. regionsthe Pacific or wherebasin. Manythe island islands is constructed are emergent of friable volcanic rock, erosional forces have reduced the island to sea level. These islands are carried on an oceanic radiometric dating of the crust. The thickness of the plate that progressively sinks as it undergoes thermal contraction and moves towards the subduction zone. ages of oceanic crust. These sediments accumulate at sediments in the ocean basins further confirms the a rate of less than 0.0001 inches per year (0.003 mm/ seamounts referred to as or tablemounts, year). At the ridge crests there is very little sediment. indicatingIn the Pacific a former Ocean, position there are above numerous sea level. flat-topped The thickness of the sediments becomes progressively thicker away from the ridge, but only reaches a few hundred meters thick in the abyssal plain. The Deep-Ocean sediment thickness is fairly well known from extensive The deep-ocean trenches represent the greatest depths in the . The , which According to radiometric dating, the oldest oceanic reaches a depth of almost 7 miles (more than 11 km), crust,deep-sea or oldest drilling part and of seismic the ocean reflection basin is profiles. less than 180 is the deepest trench on Earth. Most trenches are million years old. characterized by great length. The - Trench is 3650-miles (5900 km) long and 30- to 75-miles (50 Abyssal Plains to 120 km) wide. In most cases, trenches are the sites where crustal plates are subducted into the mantle. Subduction zones and oceanic trenches occur seaward Abyssal plains are extremely flat, featureless areas of from the continental margins to the abyssal plain, the Atlantic has and very few the ocean floor. Turbidity currents transport sediment which covers and smooths the mountainous areas. subductionof most of the zones Pacific or oceanicRim countries. trenches. By contrast, 8 Idaho Geology

Block diagram showing a cross section across the Mariana Thermal convection cells in the mantle and (or) the Trench and the Mariana . The Mariana Trench is asthenosphere may also play a role in plate movements. the deepest oceanic trench on Earth, reaching a depth of almost 7 miles. Rate of Plate Motion Plate Motion For several decades we have been able to monitor rates of plate movement by using paleomagnetic records Mechanism for Plate Movement of the oceanic crust. Based on our knowledge of the period of each reversal, we can calculate the average rate of plate movement from the width of the magnetic TheTopographically, ridges are the the divergent oceanic boundariesridges are significantly where new crusthigher is (and generated younger) and than the plates the surrounding pull away from sea floor. one a great deal, depending on the plate. The slowest rates another. Basaltic magma rises from the mantle, possibly appearstripe. We to occurhave learnedalong the that plate Ridge movement with a ratesrate of vary 1 aided by large-scale thermal convection and pressure- inch (2.5 cm) per year. The fastest rate appears to be at release melting. As a consequence of the large-scale movement of hot magma adding to the total crust from a rate of 6 inches (15 cm) per year is recorded. the mantle and the associated thermal expansion (hot the in the South where rocks require more space than cool rocks), the plates PLATE BOUNDARIES at the mid-oceanic ridges are topographically higher and tilt away from the ridge. This lifting of the plate Three Types of Plate Boundaries near the ridge axis creates a major gravitational force Divergent boundaries occur where the plates are as the plate moves downslope towards the subduction spreading away from each other and new crust is zone. This force is referred to as the ridge-push force. created by magma pushing up from the mantle. An additional gravitational force acts on the down- Convergent boundaries occur where crust of one plunging lithospheric plate at the subduction zones plate is destroyed by subduction under the opposing called the slab-pull force. Also, the cold relatively plate. Transform boundaries occur where two plates dense plate will tend to sink into the asthenosphere—a slide past one another and there is no net gain or loss of process that pulls the plate down the subduction zone. crust. Introduction 9

Block diagram showing an east-west cross section across and North Island, New Zealand. The base Looking north at block diagram showing the of the is subducting under and the splitting apart North Island, New Zealand. A six-mile- at a . These divergent boundaries thick layer at the base of the plate is a occur where three-rift zones meet at a common low-velocity channel, which allows the junction. This east-west cross section across the plates to easily slide over the underlying Afar shows a rising with mantle. thinning of the overlying crust. 10 Idaho Geology

Divergent Boundaries asthenosphere, it begins to warm and may reach 435 miles (700 km) before it is assimilated in the mantle. The African Plate and the Arabian Plate are breaking The bending of the plate and the subduction process apart in a triple junction, where 3 rift zones meet creates an oceanic trench which, depending on the size at a common junction. The and the Gulf of of the plate and the resulting subduction zone, may be Aden form a rift valley by the splitting of the Arabian thousands of kilometers long and up to 7 miles (11 km) Plate from the African Plate. The third leg of the triple deep. There are three basic types of plate convergence: junction is a new spreading center called the East (1) oceanic-continental convergence, (2) oceanic- African Rift Zone, which appears to be the early stage in oceanic convergence, and (3) continental-continental the breakup of the African continent. convergence. Oceanic-Continental Convergence. In the case The axial rift areas of the mid-oceanic ridges are where continental crust and oceanic crust converge, characterized by hot spring activity that appears to the more dense oceanic plate is subducted into the asthenosphere and the less dense continental rock hot spring was discovered in 1977 on the Galapagos have great significance to the origin of life. The first the sediment carried on the sinking oceanic plate may submersible at a depth of 1.5 miles (2.5 km). Since befloats. scraped When off a byplate the is over-riding subducted, continental as much as plate.half of As 1977,Rift while many using other Alvin, hot springsthe world’s have first been deep-sea discovered the subducting plate with its layer of water-saturated sediments moves down into the asthenosphere, partial and investigated. What most surprised the investigators melting of the sediments and basalt generates magma was the variety of life that flourished in these deep- with an andesitic to granitic composition. The magma, which is less dense than the surrounding host rock, withsea, hot-spring the superheated ecosystems. water. With water up to 380°C, hydrogen sulfide passes throughout the vents along the surface. Along the west coast of Convergent Boundaries andbegins the floating Peru-Chile to a trench,magma the chamber oceanic several Nazca miles Plate from Because the Earth’s size is constant, the generation of is subducted under the western edge of the South new crust at the spreading center requires that crust American Plate . Consequently, the western edge of the is uplifted by subduction move towards each other, one plate may be subducted to create the . Subduction of beneathmust be destroyedanother and at eventuallythe same rate. destroyed. Where twoThe plates subducted plate tends to be about 60-miles (100 km) Block diagram showing oceanic-continental plate thick, cold and ridged. However, as it descends into the convergence in an east-west cross section across the Peru- Chile Trench and the Andes . Introduction 11

Block diagram showing oceanic-oceanic plate convergence the Tethys Sea. As a result of continuous convergence, in a north-south cross section across the , the , which extend 1800 miles (2900 km) Aleutian and the Aleutian Basin. along the India-Tibetan border, exceed 29,000 feet (8,854 m) above sea level—the highest mountain range in the world with an average elevation of 15,000 feet the beneath the American plate is (4600 m). Much of this spectacular elevation rise is another example of oceanic-continental convergence. due to the tip of the India Plate being pushed under

Oceanic-Oceanic Convergence. the leading edge of the Eurasian Plate. The Himalayas oceanic plates converge, one may be subducted under are still rising at the rate of 0.4 inch (1 cm) per year. the other with the associated oceanic Wheretrench twocreated Mount Everest is more than 5.5-miles (9 km) high and along the line of subduction. As the subducted plate would be much higher except for the opposing force of undergoes partial melting in the asthenosphere, a chain erosion. Other mountain ranges created by continental- of volcanoes or a volcanic island arc is formed parallel continental convergence include the Alps, Appalachians to the trench. Examples of oceanic-oceanic convergence and the Ural Mountains. include the volcanic arcs such as the Aleutian, Mariana, and Islands. These island arcs also have well- developed oceanic trenches. Transform Boundaries Continental-Continental Convergence. If two A transform plate boundary is the zone where two plates with continental crust converge, the continental plates slide past each other in a horizontal direction crust of either plate will not subduct beneath the other, without production or destruction of oceanic crust. presumably because continental rocks are too light These transform faults, with individual lengths in to be subducted under lithosphere. Approximately 50 hundreds and even thousands of kilometers, generally million years ago, northward moving India collided trend parallel to the direction of the plate movement. In with and pushed up the Himalayan Mountain propose that the Earth is covered by individual plates; of India started moving north through the Tethys Sea 1965, J. Tuzo Wilson, a Canadian scientist, was first to approximatelyRange and the Tibetan225 million . years When ago, itthe was large well island south transform faults. of the equator. After a journey of 4000 miles (6400 he alsoAlthough was the most first transform to recognize faults the are significance on the ocean of km), India collided with the Eurasian Plate and closed

floors, the zone in is one of 12 Idaho Geology

Block diagram showing continental-continental plate the Gulf of with the South Gorda-Juan de Fuca convergence in a north-south cross section across the Explorerthe few exceptions. Ridge off the It connects Coast of Northernthe East Pacific California. Rise in Indian Ocean, India, the Himalayas, and the Tibetan This 800-mile-long (1300 km) fault has been moving at Plateau. the rate of 2 inches (5 cm) per year for 10 million years. The strike-slip component of the San Andreas indicates the west side of the fault is moving in a northwesterly direction relative to the east side of the fault zone. II. Overview of Northwest Geology Pacific The “Ring of Fire” refers to a horseshoe-shaped, Precambrian almost continuous subduction zones, oceanic trenches The North American Craton or forms and25,000-mile-long volcanic arcs along(40,000 the km) continental circum-Pacific margin belt of theof the ancient basement rocks of the North American continent. Several times during the tectonic cycles of all earthquakes and 452 active stratovolcanoes, of Earth history Laurentia has been part of a larger whichPacific representbasin. This 75 belt percent is characterized of the Earth’s by volcanoes.90 percent continent and existed as a separate continent as it is now. Laurentia grew by assembling island archipelagos and microcontinents so it now consists of numerous isFor the the collision most part, of plates tectonic that plates directly in the cause Pacific the ringare terranes. The core complexes create windows exposing subducting beneath the Pacific Rim countries and it the deep basement rocks in the North American continental margin of southern British Columbia, Cordillera. They represent exposures of the 3.1-2.6 Ga of fire. At the along the de Fuca plate is subducting beneath and colliding Wyoming Province. withWashington, the North Oregon American and northernplate. For California, about 45 million the Juan Rifting in the Western Laurentian years eastward subduction along the west coast of Neoproterozoic Rift Belt in Central Idaho North America has sustained the Cascade volcanic arc. Continental separation occurred along a 1550-mile- Therefore, the Northwest has an important and typical long (2500-km) Siberian-Laurentia rift zone in early Mesoproterozoic. Sears and Price suggest a stratovolcanoes of the , and earthquakes. segment of the ring of fire, complete with a volcanic arc, Introduction 13

Block diagram above showing geological features of the Block diagram below shows the initial rifting of with an east-west cross section through supercontinent Rodinia between 700 and 600 Ma. The the southern boundary of Oregon and Idaho. originated in this Western Laurentian Rift. Northwestern state boundaries are shown relative to the position of the intracontinental rift. 14 Idaho Geology

Block diagram showing the subsiding of Sedimentation in Miogeocline from the Neoproterozoic through Deposition of the miogeocline occurred uninterrupted . from the time of the rifting until Late Devonian. These passive margin sediments reached 6-miles (10 km) Neoproterozoic to earliest Cambrian continental thick. This continental margin was characterized rifting, which severed from Laurentia. By by seaward-dipping listric, normal faults producing 685 Ma, diamictites and rift volcanics may have been asymmetric basins and ridges and continuous more or less continuous along the Laurentian side of subsidence. the rift from to . There Thick sequences of limestones and dolomites of have been many proposals to identify the continental early Paleozoic age (530 to 300 Ma) were deposited fragments that were separated from the rifted margin on broad carbonate platforms in the warm at the of North America (Laurentia) during the . western passive margin of North America. Between These possible candidates include Siberia, China, Late Devonian to Early Mississippian, the Antler , and Antarctica; reconstructions are based orogeny created a highland oriented in a north-south on similarities in geology, geochronology, structural direction through central Idaho and . trends, and paleomagnetics. A Cordilleran-wide glacial event in 685 Ma was synchronous with the initiation of the Cordilleran Accreted miogeocline. The west side of this rift became the Allochthonous terranes were typically carried across new passive margin that extended the length of the oceans on tectonic plates 1000s of kilometers. The term Cordillera. “allochthonous” in this context simply means rocks formed someplace other than their present position. Introduction 15

Some terranes have moved more than 5,000 miles been controversial with interpretations ranging from (8,000 km). The terms “” or “docking” refer 165 to 118 Ma, or a span of about 45 million years. to the process of attaching to a continental or island border. “Amalgamation” refers to the combining by volcanic arc terranes may have accreted to the west collision of two or more terranes. A “composite terrane” coastSignificantly, of North all America the major by Paleozoic165 to 160 and Ma, Mesozoic and they did or a “super terrane” is the result of amalgamation. so at approximately the same time. The North America Cordillera extends from the Gulf of at the north to the . It is a Cordilleran Batholiths and the Pluton Belt collage of more than 200 accretionary terranes. Most of The pluton belt, which extends from these terranes were accreted during the Mesozoic and to Alaska, is the largest batholithic system on Earth. . A large portion of the Cordillera is less than During the Mesozoic, this magmatic arc covered the 200 million years old. entire length of the Cordillera. Eastward subduction of Accreted terranes are also referred to as exotic terranes. Most of these terranes originated as volcanic continental caused a magmatic arc that was underlain byoceanic this plutonic lithosphere belt. beneath Some older the Westernplutons were Laurentian intruded Ocean and include volcanic rock, associated marine during the Triassic. The eastward migration of island arcs or submarine mountain ranges in the Pacific Cretaceous volcanism and plutonism might be caused terranes were typically transported on and as part of by a decrease in the subduction angle of the Farallon oceanicsediments, plates and and fragments were accreted of the ocean to the floor. western The sideexotic plate. Partial melting and magma generation of a plate of the North American continent. The volcanic arcs or occurs when the plate moves into the aesthenosphere. island archipelagos formed about 250 Ma and accreted So, if the angle of the subducting plate is reduced, along the western margin extending from British partial melting or magmatism would occur farther to Columbia to southern California—a distance of about the east. 1000 miles 1600 km). These volcanic arcs lasted at Most geologists think a subduction zone once least 120 Ma. existed between the accreted terrane and the In 2015, LaMaskin and colleagues proposed a new continental rocks to the east and that the batholiths tectonic model for accretion of the Blue Mountains Province in which the collision was complete by 160 Ma Block diagram with east-west cross section across the or earliest Late . The age of accretion has long American northwest 160 to 110 Ma. 16 Idaho Geology

Block diagram with east-west cross section across (75-45 Ma) the American northwest 60 Ma. Notice the low-angle At the beginning of the Laramide Orogeny about 75 subduction on the . Ma, the large area east of the Sevier fold-and-thrust

originated from partial melting in or about the subduction zone. The entire chain of Mesozoic granitic Thebelt Laramide(now the StatesOrogeny of Coloradolasted about and 30 Wyoming) million years was a batholiths that extend from Alaska to Mexico, the length duringflat, broad which river the flood Rocky plain Mountain covering foreland the foreland ranges basin. of the western cordillera, are most likely related to were pushed up from lateral compression. The area subduction. affected by the Laramide Orogeny includes the central and southern Rockies and the . As the Cordilleran Thrust Belt foreland ranges were forced upwards through regional uplift, they shed limestone, shale and sandstone. The Cordilleran thrust belt of Cretaceous and early The ranges are elongate with thrust faults producing Tertiary age extends from the Mexico to the Brooks mushroom-shaped basement rocks over deformed and Range in Alaska. This belt is about 100 miles (160 km) overturned more recent sedimentary rocks. wide and 3,000 miles (4,800 km) long. The portion of the Cordilleran thrust belt that occurs in eastern Laramide Orogeny Caused by Flat major systems arranged in an eastward Subduction? overlappingIdaho, northern array. Folding and Wyoming and thrust consists faulting of of five the in this area occurred between 130-60 represents the consensus cause of the Laramide Ma. Each of the major thrust fault systems consists of Orogeny.Slab flattening East-northeast of the subducting directed Farallon compression Plate by the almost-horizontal Farallon slab may have pushed up moved over different times. The entire thrust and fold the Laramide ranges and pushed against the Colorado systemmore than had one a maximum significant displacement thrust fault, ofwhich about may 103 have Plateau. This may have happened because a large miles (165 km) and a total displacement on individual on the Farallon Plate was subducted thrust faults as much as 40 miles (64 km); and total shortening was about 70 percent. Thrust sheets may and pushed up against the Laramide ranges. The be up to 20,000-feet (6100 m) thick and the large folds under western North America and became flattened have amplitudes up to 10,000 feet (3048 m). underneath an area extending from southwest Montana toFarallon western plate Texas. may have floated almost horizontally Introduction 17

Block diagram above with east-west cross section across Block diagram below with east-west cross section across the American northwest 53 Ma. the American northwest 40 Ma. 18 Idaho Geology

Block diagram with east-west cross section across and covered more than half of Idaho. Thickness of the northern Oregon from the to western Challis volcanics is variable and in several places is Idaho showing how the subsurface geology relates to the known to be more than 10,000 feet (3050 m). Fossil landscape. plant species and radiometric age dating indicate an age beginning about 55 Ma from a variety of widely‑separated vents and continued until about 40 The Laramide Orogeny ended at 48 Ma in the Ma. northwest, a little earlier than the southwest. This is the same time that accreted to the northwest CASCADIA SUBDUCTION ZONE the subduction to move west to its present position as coast of Oregon and Washington. This accretion caused the Cascadia subduction zone. Siletzia Accretion Challis Volcanics and Associated Plutons The basalt basement of the coast range of Oregon Magmatism was very active around the Siletzia after the Siletz River volcanics. Most Siletz lavas were accretion as documented by the Kamloops, BC, eruptedand Washington between is 56 referred to 49 Ma. to asSiletzia the “Siletz is an oceanicTerrane” Challis, Idaho and Clarno, Oregon volcanic events. Paleocene-Eocene chain of seamounts and/or an This volcanism is possibly caused by slab removal oceanic plateau; as such, it is a fragment of the Farallon of the Farallon plate from the North American Plate. plate, which was accreted to the continental margin

been caused by the of aethenosphere as following accretion, subduction jumped to the western This flare-up of volcanism may have marginof Oregon of Northand Washington America and about the 53Cascade Ma. Immediately volcanic arc was initiated. By 45 Ma, Cascade arc magmatism was thisthe flatignimbrite slab peeled volcanism away from migrated the base southward. of the North So, initiated in the Siletzia rocks. Siletzia is approximately theAmerican Challis plate and Clarnoby 45 Ma. magmatism After the occurrednorthern because flare-up, 18- to 21-miles (30 to 35 km) thick and is now largely buried beneath younger rocks such as the Columbia the North American lithosphere and was followed by River Basalt. decompressionthe aesthenosphere melting. flowed into the hydrated base of Basement Rocks of the Coast Range. In western

volcanic complexes of submarine and subaerial The Challis volcanic field is the most extensive tholeiiticOregon and and Washington, alkalic the associated basement rockswith submarine are depositedand diverse over of the a very Eocene irregular, volcanic mountainous fields in the surface northwestern United States. The Challis field was Introduction 19

Well developed wave-cut terrace occupied by resort properties. This terrace was developed on poorly consolidated sandstones, which are experiencing significant wave erosion. Near Otter Crest, Oregon.

breccias and marine sediments referred to as Siletzia basin(Wells and and accretion colleagues, to western2014). These North thick America sequences was of completedbasalts were between erupted 50.5 56-49 and Ma 49 in Ma. the These northeast complexes Pacific are thought to represent a large oceanic terrane or large igneous province (LID) accreted to western North Yellowstone about 42 Ma. Since then, western America during the early Eocene. Evidence includes (1) Oregon has been moved about 155 miles (250 km) by the large thickness of 13 to 19 miles (22 to 32 km) with tectonic rotation in a clockwise direction about a pole greatest thickness in the Oregon Coast Range, (2) aerial located near the northeast corner of Oregon. extent of more than 92,640 square miles (240,000 Two Rock Types of Eocene Age in the Coast km3), and (3) rapid eruption and dates of accretion; this Range. For the most part, the Coast Range consists evidence supports a hypothesis that Siletzia originated of two rock types of Eocene age. Basalt of the Siletz offshore as an oceanic plateau. Siletzia and . The portion of forms the basement and is exposed in many areas Siletzia that has not been subducted is approximately ofterrane, the Coast originating Range. The as ocean basalt floor is capped and seamounts, by Eocene 8-12 times the erupted volume of the Columbia River basalts. The Siletzia oceanic plateau could have East-west cross section across the and originated from the Yellowstone hotspot (YHS), which west-central Oregon. At the Cascadia subduction Zone, according to some studies, was located on the Farallon the locked zone along the interface between the Juan de plate between 60 and 50 Ma. After the accretion Fuca plate and the North American plate is shown. of Siletzia, the North American plate overrode the 20 Idaho Geology

through marine sandstones; these sandstones Columbia River Basalt lavas were deposited on the originated as submarine fan complexes, , Coast during the Middle Miocene; these basalts became and near-shore delta sediments deposited in the fore headlands along the Oregon coast at Yaquina Head, arc subsiding basin over the earlier basalt foundation. Depot Bay, Cape Lookout, and Tillamook head. The Some of the sandstone deposits are several thousand northern Oregon coast has some of the best textbook feet thick and include the Tyee Formation and the examples of erosional and depositional shoreline Umpqua Group. The coastal block was subsequently features such as headlands, sea cliffs, sea stacks, wave- uplifted between the Oligocene and Miocene. cut terraces, tombolos, spits, pocket , baymouth The Willamette Valley is a subsided portion of the bars, estuaries, lagoons, and offshore bars. These remarkable features are at least partly formed by the uplift of the coast from the converging Juan de Fuca and DuringCoast Range. the Pleistocene, Portions of the the valley valley received floor were deposits covered North American plates. by flows of Columbia River Basalt during the Miocene. Uplift by Subducting Juan de Fuca Plate. The Cascadia Subduction Zone and the fromCoast ice Range age floods.forms a huge anticlinal fold with the east limb tilting towards the Cascade Range and the west Subduction Earthquake Cycle limb tilted towards the ocean. The fold was caused by Megathrust Earthquakes have occurred along the the east-west compression from the Juan de Fuca and Cascadia Subduction Zone in an earthquake cycle North American plates. Marine-cut terraces dated at with gaps of seismic activity lasting 300 to more than 80,000 and 230,000 years have been uplifted as much 500 years. The cause of this earthquake cycle is that as 1600 feet (490 km) by the subducting Juan de Fuca a portion of the interface between the lower Juan de plate. Fuca subducting slab and the upper North American Basalt Headlands. The Columbia River Basalt plate is locked in the brittle zone of the crust during the interseismic period. The brittle-ductile transition zone Oregon and western Idaho, occurs as headlands along occurs at a depth in the crust of 7 to 11 miles (12 to theGroup, northern which coastflowed of fromOregon. fissure These dikes basalts in eastern are more 18 km), with the depth depending on the temperature, resistant to erosion than the marine sandstones. pressure and composition of the rocks. At lesser depths

East-west cross section across the Cascadia Subduction Zone and the interface between the Juan de Fuca Plate and the North American Plate. This cross section shows the areas where there is gradual uplift and where there is gradual subsidence during the Interseismic period of slow strain accumulation. Introduction 21

East-west cross section across the Cascadia Subduction Zone and the interface between the Juan de Fuca Plate and the North American Plate. This cross section shows the areas where there is rapid uplift and where there is rapid subsidence during the sudden coseismic strain release.

Block diagram showing a cross section across the Coast Range, Willamette Valley, older Western Cascades, High Cascades and the High Lava Plateau of northern Oregon. 22 Idaho Geology

rock is likely to deform by brittle fracture, and at great depths, rock is more likely to deform ductilely. During the interseismic period, strain accumulates for hundreds of years . This accumulating strain causes the northwest coastline to slowly rise between to the two hinge lines (Darienzo and Peterson, 1990). The sudden coseismic strain release by a megathrust rupture causes a high- magnitude earthquake and results in a sudden subsidence of the coastal area (as well as the coast range) so that much of the coastal area is under water. Seaward of the hinge line the seabottom suddenly rises and causes a displacement of water that generates a . No Cross section of a stratovolcano showing most of the vertical movement occurs at either hinge line. different types of volcanic deposits. Most of the volcanoes of the high Cascades are stratovolcanoes. Cascade Range The Cascade Mountains extend from northern California north to British Columbia. These mountains represent two ranges: the older and deeply eroded western Cascades and a more recent high Cascade Range positioned on the eastern side of the older range. The Cascadia Subduction Zone is approximately 50-miles (83 km) offshore. The Juan de Fuca plate is moving at a rate of about 1.6 inches (4 cm) per year. This subduction zone is responsible for the Cascade magmatic arc. The Cascade magmatic arc extends from southern British Columbia south to in Looking south at Mt. Hood, a stratovolcano of the High California—a distance of about 600 miles (970 km). Cascades and the highest point in Oregon.

Western Cascades. The Cascade magmatic arc was generated about 45 million years ago by the Cascadia

active from about 40 million years ago and terminated aboutSubduction 5 million Zone. years The ago. Western However, Cascade the rocksrange rangewas in age from about 42 MA to about 10 Ma with the youngest rocks found along the eastern side of the range. It is possible that some of the tuffaceous material

portion of the John Day Formation. generatedThe High by theCascades. Western About Cascades 8 million now makesyears ago up a the volcanic eruptions of the Cascade magmatic arc Looking west at the High Cascades from Newberry moved eastward to the present site of High Cascades. . is not part of the volcanic arc This move can be explained by a decrease in the angle responsible for the High Cascades. Introduction 23

North-south cross section along the John Day River through Picture Gorge and Sheep Rock. Modified after Thayer, 1977. of the subducting Juan de Fuca plate. There have been approximately six times more volcanic material produced in the western Cascades than the High Cascades because the subducting plate has slowed from a rate of 3 inches (7.5 cm) per year to about 1.6 inches (4 cm) per year at the present. The subducting slab starts melting at 60- to 75-miles (100 to 125 km) depth and feeds a batholith below the Cascade Range. The volcanism that created the High Cascades Cretacreous shale of the Hudspeth Formation in the began about 3 million years ago, even though the Ochoco Mountains. magmatic arc could have been active up to 8 million years ago. The magmatic arc is still very active below the High Cascades. About 5 million years ago the High Cascades graben started dropping down and now has John Day Country a downward displacement of about 2,000 feet (610 Cretaceous Sedimentary Rocks of Central Oregon. m). The are huge discontinuous structures The Cretaceous Conglomerate of Goose Rock was that extend about 300 miles north-south and are about deposited by streams soon after entering the ocean 10- to 20-miles (17 to 33 km) wide. These grabens may about 90 to 110 Ma. Outcrops of this conglomerate be caused by loading or isostacy from the increasing are exposed along the John Day River on Highway 19 volume of volcanic rocks along the High Cascades. about 20 miles (33 km) northwest of Dayville. The During the last 2 million years magma has erupted conglomerate contains white quartzite pebbles eroded from thousands of sites along the Cascade magmatic from Precambrian rock near Elk City, Idaho and pebbles arc with a large part of the range constructed during from the Idaho Batholith. the last million years. In addition to a chain of huge The Gable Creek Conglomerate near Mitchell stratovolcanoes, small volcanic cones and shield is similar in age and origin to the Goose Rock volcanoes have formed. Composition of the lavas conglomerate. Shales and sandstones of the Cretaceous include basalt, basaltic andesite, andesite, dacite and Hudspeth Formation are also exposed near Mitchell, rhyolite. Oregon. These turbidites represent an expansive The Deschutes Formation marine delta and deep-sea fan that existed about 90 to 110 Ma. The Gable Creek and Hudspeth Formations volcanoclastic rocks, which were includes most likely ash-flow generated and are exposed in the Mitchell inlier, a 70-square-mile byash-fall the Cascades tuffs, basalt approximately and rhyolite 7.5 flows, to 4.5 debris Ma. The flows and window surrounded by younger volcanic rocks. These Deschutes Formation is exposed east of the Cascades. formations overlie the Blue Mountains accreted terrane and extend far west under Cenozoic volcanic cover. 24 Idaho Geology

individual fossil soil layers. Each soil layer lasted about 10,000 to 200,000 years before it was covered by a volcanic eruption or climate change. Picture Gorge Basalt. The Picture Gorge Basalt Subgroup (of the Columbia River Basalt Group) was

Picture Gorge forms the cap on Sheep Rock; the basalt protectsdeposited the about soft 16underlying Ma. One Johnof the Day 17 tuffsflows because exposed it in is more resistant to erosion.

Metamorphic Core Complexes Picture Gorge was named after the pictographs painted Cenozoic extension in the Cordilleran of NA has on the walls of the basalt. The Picture Gorge Basalt of the revealed metamorphic core complexes along a north- Columbia River Basalt Group was emplaced about 16 Ma. south belt extending from northwestern Mexico to On the left side the Rattlesnake Group, ash-flow caps British Columbia. In areas of extreme extension, such the deposit. as the Basin and Range Province, many areas (referred to as metamorphic core complexes) have had the upper layers of brittle rock stripped off the underlying ductile rocks along detachment faults. The east-west extension occurs along parallel to subparallel listric normal faults that merge at depth with a . The detachment fault, which is commonly represented by a thick zone or layers of mylonite, occurs along the transition zone separating the ductile metamorphic and plutonic core rocks below from the overlying brittle rocks. The brittle rocks above are moved tens of kilometers to the east or west along detachment faults and the metamorphic and plutonic rocks that had previously been buried by as much as 12 miles (20 km) Sheep Rock, a prominent landmark in central Oregon, are exposed by tectonic exhumation. Finally, a dome was named for sheep that once grazed on its slopes. The of high-grade metamorphic rocks is exposed at the capstone is a remnant of Picture Gorge basalt dated about surface. 16 Ma. The colorful John Day Group of air-fall volcanic ash layers underlies the capstone. Basin and Range The Basin and Range Province is characterized by . Clarno Group volcanics erupted Clarno Group produced by extension that began in the from 54 to 37 Ma to form a chain of andesitic Eocene and continues today. The province has more stratovolcanoes. These volcanoes are associated than 400 subparallel, north-northwest-trending mountain ranges and intervening alluviated basins thousand feet thick near Mitchell, Oregon. Black , spaced roughly equidistant apart. The mountains are westwith numerousof Mitchell, mudflows is a well-known or lahars Clarno totaling volcanic several vent typically 9 to 12 miles (15 to 20 km) across and are formed 44 Ma. separated by valleys of the same width. The Province The John Day tuffs consist John Day Tuffs. forms an irregularly shaped measuring more than 900 miles (1500 km) by 300 to 600 miles (500 ancestral Cascade Mountains deposited in the John to 1000 km) wide, extending from southern Idaho and Dayprimarily basin ofbetween ash flow 39 and and ash 18 fallMa. tuffs The fromPainted the Hills Oregon through Nevada, eastern Colorado, western west of Mitchell represents a record of Oregon’s , New Mexico and . Most of the ranges are composed of 200- to 700-million-year-old soils that developed during wet seasons and yellow sedimentary rock capped, in some cases, by Cenozoic layersOligocene represent forests the and dry floodplains. periods. The Red black bands masses represent volcanic rocks. are manganese nodules. Most of the bands represent Introduction 25

Block diagram showing geologic features in Washington Ma and covers more than 81,060 square miles (210,000 and Idaho. East-west cross section along the Oregon- km2 Washington State boundary. greatest thickness of more than 2.5 miles (4 km) near ) of the Northwest. These flood basalts have their Basin and Range extension may range from 50 originated from a series of north-northwest-trending to 100 percent and possibly as much as 200 percent the center of the Columbia Basin. Most of the flows in certain places. The Basin and Range has a thin continental crust, long history of episodic magmatism, fissures located in eastern Washington, eastern Oregon and regional uplift of 6,560 to 9,843 feet (2000 to 3000 and Western Idaho. Although the eruptive period for m) in the late Cenozoic. Ma—athe flows period spans of 11 only million 1.1 million years, years.93 percent of the flood basalt volume erupted between 16.7 Ma and 15.6 Collapse of the Basin and Range Crust volcanic arc, but found access further west through an east-northeast-trending The basalt flows moved lowland west togap the or Cascade the Columbia The crustal thickness averages about 18 miles (30 km) Trans-Arc Lowland and then spread across the as compared to the 30-miles (50 km) thick . The crust was stretched, thinned and collapsed the Coast Range. After crossing the Coast Range they about one vertical mile. Originally, the crust was northern Willamette Valley and farther west through thickened by thrust plates so the collapse explains why more than 400 miles. the area is now the . The crust lost about flowed out on the continental shelf for a distance of is about 50 percent above average because the crust Nevada-Columbia Basin Magmatic Belt and ishalf so of thin its thatthickness the mantle during is the close collapse. to the surface.The heat Also, flow the Origin of the CRBG because of thermal expansion, the heat keeps the Bimodal (basalt and rhyolite) eruptions occurred in elevation high. the northern Nevada rift system from 16.5 to 15 Ma and at the west end of the Yellowstone hot spot tract; Columbia River Basalt Group enormous eruptions of rhyolite occurred in the area from 16.5 to 15.5 Ma. At about the same time interval The Columbia River Basalt Group (CRBG) is the youngest, smallest and best-preserved continental dikebetween system 16.7 in and southeastern 15 Ma, the Oregon Columbia about River 16.7 flood Ma, basalts erupted from dike swarms first from the Steens flood basalt province known to exist on any continent. then migrated north to the Monument dike swarm at It consists of more than 350 continental tholeiitic flood basalt flows erupted between about 16.7 Ma and 5.5 26 Idaho Geology

Block diagram with east-west cross section across the gravity faults at both the northeast and southwest American northwest 5 Ma. boundaries. Both structural and stratigraphic evidence are consistent with the explanation of the western Plain as a large graben. between 16.6 to 15.6 Ma where most of the basalts by 1 mile (1.7 km) of interbedded volcanic rocks and erupted.15.8 Ma, and finally to the Chief Joseph dike swarm lakebed sediments of Tertiary and QuaternaryThe basin isage. filled

trending intracontinental rift or basin bounded by wasPlate-motion located near studiesthe Oregon-Nevada-Idaho and mantle flow rate tri-state estimates normalThe faultsWestern along Snake the Rivernortheast Plain and is a southwestnorthwest- regionindicate at that 16.5 the Ma. seismically At this time, defined large-scale mantle Basin plume and margins. It measures approximately 42 miles (70 km) Range extension started (17-16 Ma) and at the same wide and 180 miles (300 km) long. The Snake River time three large-scale magmatic events occurred: Plain started widening between 11.7 to 11.0 Ma. The (1) volcanism in the northern Nevada rift at 16.6 Ma, normal faulting that created the basin started about 11 (2) rhyolite volcanism at the southwestern end of Ma and by 9 Ma produced more than 1.2 miles (2 km) the Yellowstone hotspot track (16.5 Ma), and (3) the of offset.

Steens Basalt at about 16.7 Ma in Eastern Snake River Plain and the southeasternfirst flows of the Oregon. Columbia River Basalt Group, the Yellowstone Hotspot

Western Snake River Plain depression trending east-northeast from Twin Falls The western Snake River Plain is much different than toThe Ashton, Eastern Idaho. Snake It Riveris approximately Plain (ESRP) 240-miles is a flat volcanic long the eastern Snake River Plain in terms of structure, (400 km) and 60-miles wide (100 km). The eastern lithology and age. The western plain trends northwest- plain is characterized by extensive volcanic deposits southeast and the eastern plain trends southwest- northeast. Unlike the eastern plain, the western Miocene-Pliocene rhyolite. Although Paleozoic rocks Snake River Plain is a structural basin formed by large bothof undissected north and Quaternary south of the basalt Snake flows River overlying Plain have Introduction 27 similar stratigraphy and structure, there is no evidence that these rocks continue across or underlie the plain. Track of the Yellowstone Hotspot. The prevailing overduring glacials. from When Siberia the Native and followed Americans the enteredemergent hypothesis for origin of the ESRP province and the thick continentalthe shelf for thealong first the time, coastal they zone most southward likely crossed during the last glacial. A migration route southward is the track of the Yellowstone hot spot. This may have through the ice-free corridor between the Laurentide occurredsequence byof time-transgressivemovement of the North rhyolitic American ash flow plate tuffs and Cordilleran ice sheet is no longer considered viable. southwestward over a stationary deep mantle plume. The Yellowstone hotspot left a 435-mile- Hildebrand’s Western Subduction Hypothesis northeastward-trending track (700 km) consisting of time-progressive -forming volcanism. This Robert Hildebrand (2009 and 2013) proposed a silicic volcanism appears to have started about 17 collisional model where the western margin of North Ma in a north-central Nevada, and then progressively America was partially subducted to the west beneath moved in an east-northeast direction across northeast another continent to the west he referred to as the Nevada, southeast Oregon and southern Idaho. The Rubian ribbon continent. In contrast, the consensus time-progressive silicic volcanism is explained by the hypothesis for the development of the Cordilleran west-southwest movement at the rate of about 1 inch orogeny is based on an eastwardly dipping subduction (2 to 3 cm) per year of a 60-mile-thick lithosphere zone. Rubia is a long, narrow, ribbon continent (100 km) (the North American plate) over a stationary consisting of amalgamated exotic terranes that grew thermal mantle plume or hotspot. The rhyolitic rocks through time. Under Hildebrand’s model, all the are progressively younger towards the east starting Cretaceous or older rocks west of the east edge of the with dates of 11.3 Ma at the Bruneau Jarbidge eruptive Sevier fold-thrust belt (Cordilleran terrane) would center to the most recent rocks on the topographically represent the allochthonous Rubian ribbon continent high Yellowstone Plateau erupted as recently as 0.6 Ma. overlying basement rocks of the North American The present position of the hotspot may be 54 miles craton. So instead of progressive accretion of many (90 km) northeast of the oldest . terranes along the west coast of North America, all exotic terranes of Rubia were amalgated before the Cordilleran Orogeny. So, only one collisional event Pleistocene Ice Age occurred when North America partly subducted About 2.7 million years ago, a global cooling and a large beneath Rubian superterrane, rather than a succession expansion of occurred. Evidence for this event of terranes accreting to North America. includes oxygen isotope studies from deep-sea cores, glacial , and glacial marine deposits accumulated from ice-rafted material. or 130,000 to 10,000 years ago), North America was coveredDuring by thetwo Wisconsin separate ice Ice sheets Age (the or continentallast glacial cycle III. Timeline for glaciers Cordilleran ice sheet which peaked about 18,000 years ago; and (2) the Northwest Geologic : (1) theLaurentide Late Wisconsin ice sheet which peaked 18,000 to 20,000 thousand years ago. They apparently Events mergedLate Wisconsin during periodic expansions. South of the great continental glaciers, alpine glaciers were very Ka = Thousand Years Ago effective in creating spectacular scenery in all the high- Ma = Million Years Ago country areas of the . During the Ga = Billion Years Ago Holocene Epoch (0.01 – Present) WisconsinDuring Icepeak Age, glaciations, the Cordilleran eustatic ice (global) sheet covered sea the High Cascades (5 Ma – Present) levelnorthern dropped portion up toof 500Washington, feet (150 Idaho m) as and the Montana. Mt Mazama Eruption and Crater (6.9 Ka) was transferred to ice sheets. The continental shelf is Plain during interglacials such as the present and emergent Holocene lava fields in the eastern Snake River the portion of the continental margin that is flooded 28 Idaho Geology

Pleistocene Epoch (2.6 – 0.01 Ma) John Day Formation in eastern Oregon (36 – 18 Ma) Origins of agriculture (~10 ka) Deposition of Loess (wind-blown silt and sand) Ma) in southern Idaho Ignimbrite flare-up in Basin and Range (45 – 21 21,000 and 12,700 years Eocene Epoch (55.8 – 33.9 Ma) BonnevilleMissoula Floods Flood – (17,50040 or more years floods ago) between Siletzia accreted 55-53 Ma Last Glacial Maximum (22 – 10 Ka) Subduction stepped west 53 Ma First humans migrate through Northwest Extension in Northwest after 53 Ma America (22-17 ka) Cascade Arc started 45 Ma

Yellowstone (2 – 0.60 Ma) Tertiary plutons in central Idaho (49 – 42 Ma) EruptionCatastrophic of Snake eruptions River of Group ash-flow basalt tuffs (1.3 at Ma ChallisWestern volcanics Cascades in (42Idaho – 10 and Ma) Clarno volcanics to present) in Oregon (55 – 40 Ma) Ten Mile Gravel (1.7 Ma) Trans-Challis Fault zone Boise Valley initiated (1.7 Ma) Pleistocene basalt in the western Snake River km thick Plain (2.2 – 0.01 Ma) LaramideWestern Columbia Orogeny Basin (75 – with48 Ma) sediment 10-15 High Cascades (5 Ma – present); most of high in north central OR and mountains (<1 Ma)

Pliocene Epoch (5.3 – 2.6 Ma) Paleocene Epochsouth central (65.5 WA– 55.8 10 – Ma)15 km thick Onset of glacial cycles, polar ice sheets & lowered sea level (2.8 Ma) High Cascades (5 Ma – present) Laramide Orogeny 75 – 48 Ma in SW Hells Canyon (6 – 2 Ma) Cretaceous Laramide Period Orogeny (145.5 75 – 53 – 65.5Ma in Ma)NW Lake Idaho (11 – 2 Ma) Conglomerate of Goose Creek near John Day, Oregon (100-80 Ma) Miocene Epoch (23.0 – 5.3 Ma) Gable Creek Conglomerate near Mitchell, Oregon (100 Ma) Rattlesnake Formation in eastern Oregon (7.1 Hudspeth fan near Mitchell, Oregon Ma) (100 Ma) Lake Idaho (11 – 2 Ma) Idaho Batholith uplifted more than 10 miles Basaltic volcanics and hydrovolcanics in (88 – 78 Ma) western Snake River Plain (9 – 7 Ma) Youngest pluton in Idaho Batholith (57 Ma) Intracratonic Rift – major extension faulting of Oldest pluton in Idaho Batholith (111 Ma) western Snake River Plain (12-11 Ma) Mid-Cretaceous interior seaway and world- Mascall Formation in eastern Oregon (15 Ma) wide rise of sea level (100-80 Ma) Uplift of Coast Range Columbia River Basalt Group (16.6 – 5.5 Ma) belt) 142 – 65 Ma Yellowstone Hot Spot/Heat Anomaly (17 Ma – KlamathSevier Orogeny Terrane (Idaho-Wyoming accreted (160 – fold 150 and Ma) thrust present) Granitic rocks in accreted terranes (165 – 120 Basin and Range Faulting (20 Ma – present) Ma) Grand Canyon (6 or 16 Ma) Large-volume rhyolite eruptions in western Jurassic Period (201.6 – 145.5 Ma) Snake River Plain (12 – 11 Ma) First bird Archaeopteryx Blue Mountains and Klamath Terranes accreted OligoceneWestern Epoch Cascades (33.9 (42 – –23.0 10 Ma) Ma) to North America (160 – 150 Ma) (202 – 142 Ma) Coast Range uplifted

Western Cascades (45 – 10 Ma) Introduction 29

Galice Formation in Klamath Terrane (165 – Cambrian Period (542 – 488 Ma) 155 Ma) Appearance of all modern Phyla Emergence of large complex organisms (555 western North America Ma) AtlanticNugget Sandstone Ocean created dune by field rifting covered of Pangea much of (200 Ma) Neoproterozoic Era (1000 – 542 Ma) Brigham Group (700 – 530) Triassic Period (251.0 – 201.6 Ma) Oldest certain metazoan (multicellular) fossils First dinosaurs (620 Ma) First mammals Snowball Earth (750 – 600 Ma) Sonoma Orogeny (Late Permian to Early Pocatello Formation with evidence of Triassic NeoProterozoic glaciation (750 – 700 Ma)

Permian Period (299 – 251 Ma) Rift about 700 Ma) Supercontinent Pangea formed about 300 Ma RiftRodinia to the split south in half (southern along Western Cordillera) Laurentian occurred Extinction event at end of Permian about 570 Ma Sun Valley Group (mid Pennsylvanian – Early Permian) Mesoproterozoic Era (1600 – 1000 Ma) Supercontinent Rodinia fully assembled Pennsylvanian Epoch (318 – 299 Ma) Sun Valley Group (mid Pennsylvanian – Early Belt rocks (1370 Ma) Permian) Belt-PurcellGranitic plutons Supergroup and mafic deposited rocks intruded (1470 –into Coal swamps 1400 Ma) Oldest reptiles Paleoproterozoic Era (2500 – 1600 Ma) Mississippian Epoch (359 – 318 Ma) First Eukaryotes (cells with nucleous that Copper Basin Formation deposited and now divide by mitosis) (1.6 Ga) exposed near Ketchum Archeon Eon (3850 – 2500 Ma) Priest River Complex in northern Idaho (2.6 Devonian Period (416 – 359 Ma) Ga) Many island arcs off coast of western North Green Creek Complex in south-central Idaho America (2.65 Ga) Antler Orogeny in Idaho and Nevada (Late Devonian to Early Mississippian) rocks) Milligan Formation and the Salmon River Wyoming Province (old Laurentian continental Assemblage (Late Devonian) Ga) StromatolitesFirst plate tectonic – microbial activity mats and offirst blue-green ice age (3.1 Oldest rocks in Oregon (400 Ma) algae (3.55 Ga) Age of fishaccreted to North America 160 Ma. Formation of the Earth (4.57 Ga)

Silurian Period (444 – 416 Ma) First vascular land plants

First seeds and large trees First jawed fish

OrdovicianFirst tetrapod, Period first (488 amphibian – 444 Ma) First land plants