The magnificence of the Grand Canyon for most observers lies in the awesome panoramas of multihued sedimentary rocks eroded into countless cliffs, slopes, ramparts, and spires. Morning or evening light and shadow playing over the landscape, and a sprinkling of cloud cover, only serve to enhance this display. However, not all of the canyon’s exquisite geology, mystique, and beauty is derived from the deposition and lithification of sediment. The crystalline basement rock of the Colorado Plateau, which is neatly sliced open along the ominously dark inner gorge of the Grand Canyon (Figure 1), is composed of metamorphic rock laced by igneous intrusions formed approximately 1,700 million years ago. The rocks of the Grand Canyon region are also deformed by literally hundreds of large- and small-scale geologic structures (Figure 1). Some features indicate that the rocks have undergone bending, while other features formed by breakage; the features were often generated by a combination of extensional, compressional, and/or shearing forces operating within the earth’s crust. Identifying and mapping the crystalline basement rocks and geologic structures exposed in the Grand Canyon region helps geologists to develop a detailed history of the plate tectonic interactions that have shaped western North America over nearly two billion years. The presence of certain geologic structures also significantly influences the nature of volcanism and processes of weathering and differential erosion, and thus, they have played a role in determining the exposure patterns of rocks across the landscape today.
Figure 1. A geologic map of the eastern Grand Canyon area indicating the general outcrop locations of the Paleoproterozoic crystalline basement, Meso- and Neoproterozoic sedimentary rocks of the Grand Canyon Supergroup, and sedimentary rocks of the Paleozoic sequence (modified from Karlstrom et al., 2012).
The Middle Proterozoic Crystalline Basement
(Sydney Neace and Ken Bevis)
The crystalline basement rocks of the Grand Canyon have been variously referred to as the Grand Canyon Metamorphic Suite and Zoroaster Granite or “Precambrian crystalline rocks,” although most recently, Karlstrom et al. (2012) denotes them as the Granite Gorge Metamorphic Suite and Zoroaster Plutonic Complex. The metamorphic rocks formed from the sandstones and mudrocks, volcaniclastic material, and volcanic rocks accumulated within elongate basins formed between successive volcanic arcs and the continental mainland as the island arcs collided with the proto-North American craton. The volcanic arcs traveled northwest to merge with the continental edge, which had only extended to the current location of Utah and southern Wyoming. These collisions folded the basin sediments accordion-style, forced them to a great depth where they underwent metamorphism, and sutured them to the continent to become part of its basement. Co-generational igneous rocks were formed as the deepest of the subducted material melted into magma and proceeded to rise buoyantly to the surface, forcing its way into the fractures and foliations of the overlying metamorphic rocks as it rose. This process is best inferred from the observed ribbon-like bodies of intrusive, light-colored, felsic (granitic) rocks of the Zoroaster Granite imbedded within the darker-colored, vertically-layered metamorphic rocks of the Vishnu Schist. The solidified rocks were slowly uplifted, exhumed at the surface, and removed by erosion in orogenic events caused by the collisions and then aided by isostatic uplift (Blakey and Ranney, 2008). The final exhumation from depths averaging 33,000 feet below the surface occurred between 1.3 and 1.25 billion years ago, determined by the cooling age of the feldspars within the granitic intrusions (Timmons et al., 2012), as much as 500 million years after their formation.
Within the Grand Canyon, the Colorado River has cut the inner gorge; a deep, narrow defile eroded through resistant, Late Paleoproterozoic crystalline basement of the continental craton. Between river mile 77 and 118 in the eastern Grand Canyon (Figure 1), the walls of Upper Granite Gorge spectacularly expose metamorphic rocks of the Granite Gorge Metamorphic Suite, intruded by igneous rocks of the Zoroaster Plutonic Complex (Karlstrom et al., 2012). In general terms, a metamorphic rock is formed by heat and pressure that changes the original rock in some way; this occurs without melting of the preexisting rock. The high pressures and temperatures exerted on the rocks will cause their mineral constituents to recrystallize; undergoing physical changes in size, shape, and orientation, and/or chemical alterations into new minerals. On the other hand, an igneous rock forms by cooling and crystallization of solid minerals from a body of magma (molten rock) that was generated when pressure/temperature conditions cause melting of older rock. Basement rock is the deepest, and usually oldest layer of rock that forms the crust of the Earth; crystalline metamorphic and igneous rocks of the Granite Gorge Metamorphic Suite and Zoroaster Plutonic Complex comprise the foundation for the sequence of rocks exposed in the Grand Canyon (Figure 2) and although most often buried at depth, they occur throughout much of southwestern North America. Approximately 1.8 billion years ago, at depths of nearly 25 km (15 miles) below Earth’s surface, the basement rocks of the Grand Canyon where formed through multiple stages of metamorphism and igneous intrusion. The heat and pressure changed the original sedimentary rock and earliest igneous intrusives into the amphibolites, schists, and gneisses of the Granite Gorge Metamorphic Suite, while later intrusions are preserved as the Zoroaster Plutonic Complex.
Figure 2. A schematic diagram presenting the suite of sedimentary, metamorphic, and igneous rocks exposed by the downcutting of the Colorado River in Grand Canyon National Park.
In the inner gorge of the Grand Canyon, one can observe the steep walls of igneous and metamorphic rocks that have been incised over many thousands of years by the Colorado River (Figure 1). Grand Canyon’s Middle Proterozoic crystalline basement was gradually brought to the surface on the Colorado Plateau through several stages of deformation, uplift, and erosion that spanned more than a billion years (Karlstrom et al., 2003). Basement rocks are now exposed within the depths of the Grand Canyon’s inner gorge due to exhumation during a complex series of geologic and tectonic events that continue even today. These rocks give us a rare view of the birth of the southwestern portion of the North American craton, and are rarely seen elsewhere. The crystalline basement is comprised of four general rock units; from youngest to oldest, they include the Zoroaster granite, Vishnu schist, Rama schist, and Brahma schist, although the Vishnu dominates and the suite of rocks is often referred to as Vishnu basement (Karlstrom et al., 2012). These basement rocks tend to weather to a blotchy, dark coffee to steel gray color, and form only the lowermost portion of the thousands of feet of rock that span the walls and ramparts of the Grand Canyon (Figure 1 and Figure 2).
The Vishnu basement of the Grand Canyon region was likely generated between about 1.84 and 1.62 billion years ago as Laurentia, the proto-North American continent, collided with several island arcs that were tectonically rafted northward toward its southern margin (Blakey and Ranney, 2008; Karlstrom et al., 2012). Initially, parallel chains of volcanic islands were constructed above active subduction zones that formed along oceanic to oceanic plate boundaries (Figure 3a). Volcanic rock, volcaniclastic sediment, and other sandstones and shales were accumulated as the islands grew through volcanic processes and from the deposition of material eroded from the flanks of the rising volcanic islands and was later buried, lithified, folded, squeezed and deformed within the crust (Blakey and Ranney, 2008). Arc-continent collision eventually ensued as these volcanic island arcs were successively sutured to each other and to the Wyoming province of Laurentia to form the Mojave, Yavapai, and Mazatzal provinces (Figure 3b). Multiple collision events produced an extensive orogenic belt called the Vishnu Mountains (Karlstrom et al., 2012), while subduction of oceanic plate also generated magma, intruded as many overlapping plutons into the core of the growing collisional mountain belt. The earlier formed volcanic and sedimentary rock was intruded and metamorphosed from the heat and pressure associated with subduction (Blakey and Ranney, 2008). Magmatic intrusions and concurrent metamorphism of the volcanic and sedimentary rocks formed the Vishnu basement rock that we see today (Karlstrom et al., 2012). Crystalline basement rocks of the Grand Canyon preserve evidence of their origins which strongly suggests that they were generated by igneous and metamorphic processes deep within the core of the rising Vishnu Mountains. Mineral constituents, alignments of crystal grains, features of ductile flow and partial melting, and other characteristics indicate pressure and temperature conditions postulated to occur at depths of about 25 km (15 miles) (Karlstrom et al., 2012). The isotopic composition of other minerals such as mica can be used to indicate the thermochronology, or cooling history of the Vishnu basement as it was gradually unroofed and brought to the Earth’s surface by erosion of the Vishnu mountain belt (Karlstrom et al., 2012).
Figure 3. A plate tectonic model to describe the formation of the crystalline basement rocks of the Grand Canyon region (modified from Karlstrom et al., 2012).
Exhumation and surface exposure of the Late Paleoproterozoic crystalline basement rocks was complete by roughly 1.2 billion years ago when gradual erosion of the 1.8-1.6-billion-year old Vishnu mountain belt beveled the region into a broad peneplain (Blakey and Ranney, 2008; Karlstrom et al., 2012). After being exposed at the surface, the Vishnu basement was buried by deposition of the Meso- to Neoproterozoic sedimentary rocks associated with the Grand Canyon Supergroup over the next 500 million years (Karlstrom et al., 2012). Subsequently, the entire region was deformed by extensional tectonics and uplifted, fractured into large tilted structural blocks, then later eroded, and once more peneplained by complex tectonic activity (Karlstrom et al., 2012). Between 525 and approximately 70 million years ago, the Vishnu basement, preserving wedges of Supergroup rocks, was reburied by a thick sequence of Phanerozoic strata. Beginning in the Late Cretaceous and still ongoing today, several periods of orogenic activity uplifted and eroded the region again (Karlstrom et al., 2012). Exhumation was related to compressional (Laramide Orogeny) and then extensional (Basin and Range) tectonics, and combined with more recent carving of the Grand Canyon by the Colorado River starting around 6 million years ago. Incision of the Colorado and canyon formation was brought on by uplift of the Colorado Plateau to the northeast and base level lowering in the Basin and Range to the southwest; and today, the crystalline basement rocks in the Grand Canyon have once more resurfaced (Blakey and Ranney, 2008).
In the Grand Canyon, crystalline basement rocks are visible in the Upper Granite Gorge from mile 78 to 120, in the Middle Granite Gorge from miles 127 to 137 and the Lower Granite Gorge from miles 217 to 261 (Karlstrom et al., 2003). An isolated outcrop of exposed basement rock also occurs in the vicinity of the Hurricane Fault at miles 190. These rocks were referred to as the ‘dreaded rocks’ by John Wesley Powell because of their complex geologic history, and the fact that the crystalline basement is almost always found in the narrowest and swiftest parts of the Colorado River’s path (Karlstrom et al., 2012).
The oldest of the crystalline basement rocks in the Grand Canyon, the Elves Chasm Gneiss (1840 million years old) is only exposed at the western end of Upper Granite gorge in Blacktail Canyon, as well as several places in the Middle Granite Gorge (Karlstrom et al., 2003). This rock unit is thought to be the basement rock to the Vishnu sediments prior to arc-continent collision, and it is separable from both the Vishnu sediments, and the Zoroaster units. The contact between the Elves Chasm and the Vishnu sediments is gradational, and is marked by a high Ca/K ratio within the various igneous minerals (Karlstrom et al., 2003). This gneiss is primarily composed of layered hornblende, plagioclase, and quartz (Babcock, 1990). Karlstrom et al. (2012) believe that the Elves Chasm unit forms the basement of the Mojave province and interpret it to be an older arc terrane sutured to Laurentia on which younger Vishnu sediments accumulated (Figure 3a). Detrital grains in Vishnu sediments date from 1.75 to 3.3 billion years old, suggesting that erosion of Mojave age rocks, as well as older tectonic blocks, provided the source materials to marine basins where Vishnu sediments were deposited (Karlstrom et al., 2012).
Much of the Paleoproterozoic rocks exposed in Upper Granite Gorge form a separate and younger tectonic block called the Yavapai province (Figure 3a). The Grand Canyon Metamorphic Suite is composed of the Vishnu, Rama, and Brahma Schists, which make up about half of the total basement rock exposed in the canyon (Karlstrom et al., 2003). The three schist units can be separated by their mineralogical differences, as well as their ages. The Brahma Schist (1750 million years old) and Rama Schist (1741 million years old) contain features indicating a volcanic origin (Blake and Raney, 2008). The Brahma Schist is considered to be a metamorphosed basalt, containing small amounts of hornblende, and biotite, with relict pillow structures which are associated with lava that erupted under water and cooled rapidly (Karlstrom et al., 2003). Remnant volcanic breccia and lapilli are also observed indicating explosive contact between lava/magma and surface or groundwater (Karlstrom et al, 2012). The Rama Schist is probably an altered rhyolitic lava or pyroclastic deposit of felsic to intermediate volcanic origin (Karlstrom et al., 2003).
The Brahma and Rama Schists are interlayered with and overlain by Vishnu Schist (1750-1740 million years old), believed to be an altered sandstone and shale deposited in various marine environments (Babcock, Brown, and Clark, 1974; Blakey and Ranney, 2008). This unit contains thick layers of quartz-mica schist, most likely derived from fine sandy sediment containing quartz, clay, and volcanic rock fragments deposited offshore along the outer margins of the islands thought to comprise island-arc chains (Karlstrom et al., 2003). In the sediment, there is a lack of conglomerates and coarse-grained sediments; this combined with relict graded bedding suggests a low-energy oceanic system (Karlstrom et al.,2003 and 2012). The Vishnu Schist’s compositional characteristics indicate an origin as marine graywacke turbidites deposited in oceanic trenches surrounding the island arcs as they were tectonically rafted toward Laurentia (Karlstrom et al., 2012).
Metamorphic rocks of the Grand Canyon Metamorphic Suite were intruded by younger igneous rocks of the Zoroaster Plutonic Complex during three pulses of magmatism (Blakey and Ranney, 2008; Karlstrom et al., 2012). Together, these igneous intrusives form the other half of the Vishnu basement, often crosscutting older metasedimentary and metavolcanic rocks as light-colored, tabular intrusions (Karlstrom et al., 2003 and 2012). Undifferentiated, these rocks are commonly called the Zoroaster Granite by most geologists; however, it should be noted that although the Zoroaster Granite sounds like a singular large intrusion, there are actually is a wide variety of smaller plutons and dike swarms that make up the Zoroaster unit (Karlstrom et al., 2003). Presently, the Zoroaster Plutonic Complex is a general name given to the over twenty distinct, and sometimes overlapping larger intrusive igneous bodies (called plutons), as well as the more diffuse, ribbon-like networks of dikes and sills that were emplaced over about an 80 million year period (Babcock, 1990; Karlstrom et al., 2012). In the Grand Canyon, the plutons are normally divided into three main plutonic super units; the Surprise Canyon unit, the Phantom Canyon Unit and the Ruby Creek unit, although they can contain plutonic outliers, small intrusive bodies of differing age and/or chemistry (Babcock, 1990). The units are subdivided by their mineralogical makeup, and they have tend to have different levels of foliation within the rock; thus dividing the individual plutons into specific intrusive events (Babcock, 1990).
The first major intrusive event is dated between 1740 and 1713 million years old. In the early stages of the metamorphism of the Vishnu sediments, these plutons were probably introduced by melting of the metamorphic rock and are identified by ubiquitous foliation and the presence of amphibolite dikes and sills (Babcock, Brown, and Clark, 1974). Karlstrom et al. (2012) suggest that the metamorphism and intrusives formed concurrently during island arc subduction (within the same magmatic arc system), although they further indicate that the Mojave and Yavapai provinces probably represent two distinct Paleoproterozoic volcanic island arcs. A lack of contact metamorphism of the surrounding country rock suggests shallow emplacement in the volcanic arc, and their relatively intense metamorphism corroborates their early formation (Figure 4). A younger suite of granitic rocks in the crystalline basement taking the form of more dispersed dike swarms dates between 1697 and 1660 million years old, their subsequent emplacement is thought to be associated with initial island arc to island arc collision, followed by arc-continent collision (Figure 3b) (Karlstrom et al., 2012). Emplacement of granitic magma and related pegmatitic fluids is believed to have occurred as magma migrated through fracture systems in the country rock. The common presence of features caused by folding, stretching, and shear indicate swarm emplacement during mountain building and crustal thickening related to compression during arc-arc and arc-continent collision (Karlstrom et al, 2003 and 2012). Suturing of successive island arcs in the proto-North American southwest culminated with a weaker pulse of deformation and magmatism around 1660 million years ago thought to be associated with the accretion of the Mazatzal province onto the growing southern edge of Laurentia. A final phase of magmatic activity is recorded in the Zoroaster Plutonic Complex, an as yet unexplained brief pulse of magmatism that formed the 1.35-billion-year old Quartermaster Pluton which cross-cuts older nearby plutons and is generally unaffected by metamorphism.
Figure 4. An intensely folded granitic dike intruding Vishnu Schist in lower Monument Creek.
The sedimentary and volcanic rocks comprising the Brahma, Rama, and Vishnu Schists were subjected to temperatures between 500 and 800°C, and average pressures of 6 to 7 kilobars (Babcock, 1990; Karlstrom et al., 2012). Different assemblages and compositions of metamorphic minerals were used to infer temperature and pressure conditions during subduction and collision; in general metamorphic rocks associated with later intrusion of granitic and pegmatitic dike swarms contain higher grade metamorphic minerals (to 750 or 800 °C), relative to the earlier phase of magmatism which achieved background temperatures closer to 500 °C. One of the most interesting features of the Vishnu basement rock in the Grand Canyon is the presence of deformational features in the rock layers. Around 1.71-1.68 billion years ago, severe deformation took place when the rock layers were folded accordion-style and underwent ductile, taffy-like flow while the island arc collided with Laurentia (Blakey and Ranney, 2008). By 1.65 billion years ago, the rock had completely deformed and began to cool down (Karlstrom et al., 2003). In some places where the rock was under intense pressure and heat, it was transformed in migmatites, and the more felsic minerals within it were completely melted and turned into pegmatite dikes and other granitic intrusives, themselves later deformed by compression (Blakey and Ranney, 2008). Deformation in the metamorphic rock is best exhibited by NE-SW oriented, near vertical foliation in the schist and gneiss which often displays very tight folding (Figure 5), indicating a dominant NW-SE compressional tectonic regime during mountain building (Karlstrom et al., 2003). The vertically foliated rocks record the rotation of the original bedding and platy (mica) minerals to a near vertical NE-SW orientation, while minerals were elongated parallel to ductile flow during metamorphism, forming a distinctive northeasterly penetrative lineation within the schists. These features help paint a picture of the complex and intense deformation that has occurred since the rock’s original formation (Karlstrom et al., 2003). Karlstrom et al. (2003 and 2012) indicate that this metamorphism likely corresponded to a NE-SW oriented orogenic belt and occurred in several phases initiated by arc-to-arc collisions and finalized by arc-continent collision. A reorientation of the earlier formed dominant foliation can occur where resistant blocks of crustal material slid past one another along weak zones of intensive deformation called shear zones. These large scale features may indicate sites of ancient collision and suture between arc microplates and seem to be centered on the Crystal Rapids shear zone at mile 98, a likely candidate in the Grand Canyon for the suturing of the Mojave and Yavapai provinces (Karlstrom et al, 2012).
Figure 5. A tight “S” fold in the Vishnu Schist from lower Monument Creek; the “S” is lying on its side, faulted on the left edge of the photo, and partially buried by the gravelly stream bed.
Colorado River erosion has recently formed the Grand Canyon, unveiling some of the oldest rocks on earth within its deepest tracts. Walls of the Upper, Middle, and Lower Granite Gorges expose crystalline metamorphic and igneous rocks of the Grand Canyon Metamorphic Suite and Zoroaster Plutonic Complex, collectively known as the Vishnu basement. The geologic story being developed from study of the Vishnu basement begins nearly two billion years ago in Middle Proterozoic time. Geologists believe that between about 1.8 and 1.6 billion years ago several volcanic island arcs collided with the southern leading edge of the proto-North American plate. Crystalline basement rocks exposed in the Grand Canyon record formation of volcanic island chains off the continent’s southeast margin, northwest tectonic movement and collision of these island arc microplates to the protocontinent of Laurentia, and the igneous and metamorphic processes associated with collisional mountain building. Rocks comprising pieces of the proto-North American continent stretch from southern California into Wyoming and form Wyomingland, while the younger island arcs have the same SW-NE trend and form the Mojave, Yavapai, and Mazatzal crustal provinces (Figure 6). Uplift and erosion accompanied by offshore sedimentation during growth of the orogenic belt formed by successive arc collisions were co-mingled with volcanism. An estimated 40,000 foot thick wedge of interbedded lava flows, pyroclastic material, mud, and sand was deposited into the adjacent marine basin formed by oceanic trenches associated with initial island arc and later arc-continent subduction. Subduction processes compressed and grafted the package of volcanics and immature marine sediments in the basin onto the mainland, dragging them some 25 km (15 miles) under the surface. Heat and pressure baked the sediments into the metamorphic rock of the Granite Gorge Metamorphic Suite; forming the 1.75- to 1.66-billion-year old metasedimentary Vishnu Schist and the metavolcanic Brahma and Rama Schists. These rock units form the dark, vertically foliated, intensely folded, resistant rock now exposed at the bottom of the canyon in the Inner Gorge.
Figure 6. Crustal provinces of Laurentia and adjacent continents during the middle to late Proterozoic (modified from Karlstrom et al., 2012).
As the islands of successive volcanic arcs overrode their oceanic trenches and collided with each other and ultimately with mainland Laurentia, partial melting of the subducting oceanic crust produced blobs of magma that rose from the subduction zone and intruded the overlying metamorphic rocks of the future Granite Gorge Metamorphic Suite. The intrusion of the granitic plutons occurred in three phases, the granitic rocks of each successive phase exhibiting less intense deformation and metamorphism. These igneous intrusions slowly cooled to form the rocks of the Zoroaster Plutonic Complex; the light-colored bands within the darker Vishnu Schist. The first two phases occurred during the initial period of Vishnu metamorphism, and were probably associated with emplacement of the Mojave, Yavapai, and Mazatzal arcs; an early pulse between 1740 and 1713 million years ago, and then again from 1697 to 1660 million years ago. A third, as yet unexplained pulse of magmatism flared up briefly around 1.4 billion years ago which cross-cuts older plutons and is generally unaltered.
Following emplacement of the Vishnu basement, many millions of years and a complex trail of tectonism was required to bring these rocks up from miles deep to see the light of day. The extended period of uplift and erosion beginning with onset of the Laramide Orogeny roughly 70 million years ago and culminating with carving of the modern Grand Canyon by the Colorado River over the past 6 million years is actually the third time these crystalline basement rocks have been exposed at the earth’s surface. Mountain building processes and isostatic adjustments of the crust also drove exhumation events that beveled the Grand Canyon region to a peneplain approximately 1.25 billion years ago and again 545 million years ago. Thick sequences of sedimentary rock were deposited and then removed in between those times; with the current stack of Phanerozoic rocks currently being pealed from the continent’s exterior surface, perhaps another peneplain lies in the North American craton’s future?
Geologic Structures Exposed by Regional Weathering, Exhumation, and Canyon Cutting
(Sydney Neace and Ken Bevis)
Geologic structures result from stresses within the crust; stress is caused by the application of a force. At all times on Earth, every material is subjected to some type of stress, but when unequally applied, a force causes differential stress and the rocks undergo changes in shape and/or volume known as strain. Strain accumulates in a body of rock over time by repeated differential stresses, eventually resulting in deformation and the development of a specific geologic structure or related structures. A more detailed description of the forces causing deformation can be found in this website’s GEOLOGY BASICS, but in its simplest terms, there are three types of stress that can be applied to a body of rock; compression, tension, and shear. The mode of stress in part controls the style of deformation. Compression is a squeezing force, tension a stretching force, and shear is a scissor-like, twisting or tearing force. Rock behavior during deformation also depends on its relative brittleness or ductility. Brittle rocks are more easily fractured by differential stresses to produce joints and faults; while ductile rocks are more easily bent and will become folded. Rocks at shallow depth within the crust are under less confining pressure and have lower heat content; these rocks are more likely to fracture by faulting when stressed. Rocks deep within the earth are under greater pressure and are much hotter, thus they are likely to bend by folding when stressed. Of course, joints, faults, and folds often occur in conjunction related to multiple differential stresses applied to the same body of rock, and as the zone of deformation expands and the geologic structure grows through adjacent bodies of different rock type.
The erosion that carved the inner walls of the Grand Canyon over the past 5-6 million years was driven primarily by uplift and faulting caused by plate tectonic motions (Karlstrom and Timmons, 2012). At the highest points on the canyon rim, the elevation can be up to 8,000 feet above sea level, while surrounding areas can be anywhere from 2,000-4,000 feet lower. The reason why there has been so much recent uplift of the western Colorado Plateau is a huge source of controversy with geologists, but we do know that without the added energy from tectonic uplift, the canyon would not look the same as it does today. The uplift is thought by Karlstrom and Timmons (2012) to be caused by decoupling of the crust-mantle boundary underlying this portion of the Colorado Plateau; this means that the crust may have separated from the lithosphere, and in that gap, hot asthenosphere and magma generated by decompression melting flowed in, which heated up the overlying crust (Karlstrom and Timmons, 2012). When the crust was heated from below, its density decreased, increasing its buoyancy and allowing it to lift and float higher on the mantle; just like ice floating in water.
The tectonic activity in the Grand Canyon during the past two billion years has created numerous faults and folds in the sedimentary rock layers and crystalline basement. A fault is defined as a fracture that has formed when large slabs of crustal rock slip past one another (Reynolds, Johnson, Morin and Carter, 2013). The blocks can be displaced vertically up and down, slip sideways relative to one another, or move obliquely (exhibit both vertical and lateral motion). As described in the GEOLOGY BASICS section of this website, a fault produced by vertical motion is considered “normal” when one block shifts down and away from the other, and is caused by stretching (tension) or extension of the crust, or it is considered “reverse” when one block shifts up and over the adjacent block, and is caused by compression of the crust. Faults showing lateral motion are called strike-slip faults and are described with respect to the direction of motion of the opposite block relative to an observer standing on one side of the fault. A right lateral fault indicates that the rocks on the opposite side of the fault moved to the right and a left lateral fault indicates the rocks on the opposite side of the fault moved to the left. Folds are geologic structures in which the rock units have been bent, but not completely broken, although folding often produces some fracturing, joints parallel to the fold axis. Folds generally have two sides or limbs bent up or down relative to their center or axis. Anticlines are convex folds (having two upward oriented limbs) in which the oldest rock is exposed in the center. Synclines are concave folds (having two downward oriented limbs) in which the youngest rock lies in the center. Monoclines are simple folds with only one limb, usually developed where they overlie a reverse fault deeper in the crust.
Deciphering the style and timing of faulting in the Grand Canyon region is very complex due to reactivation of preexisting faults as different types again and again. Karlstrom and Timmons (2012) describe the crust in this area as broken glass, which can make it difficult to interpret when the fault originally formed and what stresses produced it, versus when a preexisting fault was reactivated. Each fault is initially caused by a compressional, extensional, and/or shearing type of stress induced by tectonic motions which control the orientation and style expressed by the fault, but later motions can induce stress regimes of the same or a different type. After a period of time, a normal fault originally formed by extension of the crust could move again as a reverse fault caused by compression, a scenario known to have occurred in the Grand Canyon region. Not only have tectonic stresses caused faulting in the Grand Canyon; but they have generated folding too (Karlstrom and Timmons, 2012). Synclinal folds developed in association with normal faulting and crustal extension in the Late Proterozoic; while monoclinal folds were formed in the Paleozoic sedimentary rocks when reverse faulting at depth was induced by compressional tectonics in the Late Mesozoic and Early Cenozoic.
The faults and folds disrupting the rocks of the Grand Canyon region resulted from stresses related to compression, extension, and/or shear exerted on the earth’s crust. Stress regimes which induced strain and eventual deformation of the crustal rocks of southwestern North America were driven by the motions of tectonic plates over the past two billion years. The story of Grand Canyon deformation begins some 1.8 to 1.6 billion years ago when several volcanic island arcs were sutured to the southern edge of Laurentia, the proto-North American plate, during collisional mountain building (Blakey and Ranney, 2008; Karlstrom et al., 2012). An immensely thick package of interbedded lava flows, pyroclastic material, mud, and sand was deposited into adjacent marine basins formed by oceanic trenches associated with initial island arc and later arc-continent subduction, then compressed and grafted onto the mainland continental margin of Wyomingland as the Mojave, Yavapai, and Mazatzal crustal provinces (Figure 6). Volcanics and immature sediments were dragged deep into the earth’s crust and baked and squeezed to form the metamorphic rock of the Granite Gorge Metamorphic Suite, the metavolcanic and metasedimentary rocks of the Brahma and Rama Schists and the Vishnu Schist. Partial melting associated with subduction and suturing produced magma that periodically intruded the overlying metamorphic rocks of the Granite Gorge Metamorphic Suite over about 80 million years. These granitic igneous intrusions cooled and crystallized slowly to form the light-colored ribbons and pods of the Zoroaster Plutonic Complex found within the darker metamorphic rock. Collectively, these igneous and metamorphic rocks form the Middle Proterozoic crystalline Vishnu basement now exposed at the bottom of the canyon in the Upper, Middle, and Lower Granite Gorges. One of the most impressive geologic structures in the Grand Canyon is the metamorphic foliation exhibited by vertical layering in the Vishnu basement (Figure 5). These layers were not stacked vertically; the original horizontally accumulated volcanic and sedimentary material from which they were derived was transformed by complex metamorphic processes, undergoing intense bending and folding much like warm taffy (Karlstrom et al., 2012). In its dough-like state, the rock was squeezed and rotated deep in the crust so that the layers now only appear to be vertically deposited. Multiple periods of uplift and erosion has subsequently exposed the Vishnu basement along the inner gorge of the Grand Canyon for all to see.
Tectonic uplift associated with plate collision and crustal thickening during growth of the ancestral Mojave, Yavapai, and Mazatzal orogenic belts, combined with subsequent isostatic uplift, generated the erosion necessary to strip away the once mighty mountain ranges over the next several hundred million years, beveling the Grand Canyon region to a peneplain approximately 1.25 billion years ago and exposing the metamorphic and igneous rocks at their cores. For the next 500 million years, approximately 13,000 feet of sedimentary rocks were deposited in coastal and shallow marine environments occupying a shallow seaway that extended diagonally southwest across Laurentia and into the Grand Canyon region (Figure 6). Basinal deposition was recorded within the area as the Grand Canyon Supergroup, five varied Mesoproterozoic geologic formations (the Unkar Group) from 1255-1100 million years ago, and four Neoproterozoic geologic formations (the Nankoweap Formation, Chuar Group, and Sixtymile Formation) from about 900-742 million years ago (see SEDIMENTARY ROCK FORMATIONS OF THE GRAND CANYON REGION in GEOLOGY OF THE GRAND CANYON REGION). During Unkar Group deposition (Timmons et al., 2012), an extensive collisional mountain building event along Laurentia’s eastern margin known as the Grenville Orogeny culminated in accretion of the supercontinent of Rodinia between 1.2 and 1.0 billion years ago (Figure 6). This orogenic event likely created back-arc extension that thinned continental crust in the Grand Canyon region, forming large rift basins, but ultimately failing to split the continent. However, extension and thinning of the continental plate probably caused the Grand Canyon region to sink and aided flooding by the aforementioned shallow seaway. The Cardenas Basalt of the Unkar Group, as well as diabase dikes and sills intruding older, underlying Unkar Group rocks, mark outpourings of flood basalt lavas and their subterranean feeder system commonly produced during such rifting. Structurally, this tectonic event was recorded in the Grand Canyon by 1.1 billion-year-old normal faulting within the Unkar Group (Karlstrom and Timmons, 2012).
Large disconformities separate the 900 million-year-old Nankoweap Formation from the Unkar and Chuar Groups, suggesting that southwestern Rodinia was subjected to an extended period of subaerial exposure and erosion during much of the interval between 1,100 and about 800 million years ago. Deposition of the Nankoweap Formation and lower Chuar Group continued in the interior seaway long after completion of Rodinia (Dehler et al., 2012; Karlstrom et al., 2012), but by 750 million years ago, Rodinia began to break up as the large plate fragments of Antarctica and Australia rifted away (Figure 6). Although the Grand Canyon region lay to the east of the rift zone, continental crust was subjected to east-west stretching that produced extensive NW-SE oriented normal faults (Dehler et al., 2012). The most significant of these was the Butte Fault, exposed in the eastern part of the Grand Canyon downriver of Marble Canyon, as shown in the simplified geologic map of Figure 7, although the developmental history of the Butte Fault and its related geologic structures is believed to mirror other major Neoproterozoic faults in the region. Figure 8 shows the Butte Fault near its southern end, where the Colorado River cuts across the nose of a subsidiary graben at Tanner Rapids. Synclinal folding within the upper Chuar Group and Sixtymile Formation, synsedimentary deposits comprising the 740 million year old Sixtymile Formation, as well as their juxtaposition against the Neoproterozoic Butte Fault, indicate formation in conjunction with continued regional extensional faulting (Figure 9a and b). Much of the Supergroup had accumulated prior to initiation of rifting-induced normal faulting, but sediments represented by the upper Chuar Group and Sixtymile Formation continued to accumulate during faulting and were gradually being folded into the syndepositional Chuar Syncline as deposition progressed (Figure 7, Figure 8, and Figure 9a and b).
Figure 7. A simplified geologic map of the Butte Fault system in the eastern Grand Canyon (modified from Timmons et al., 2001).
Figure 8. The Butte Fault and a subsidiary graben offsetting the Cardenas Basalt and Dox Formation is exposed by downcutting of the Colorado River at Tanner Rapids.
Figure 9. Schematic diagrams describing the geologic history of the Butte Fault system from its origins as a normal fault produced by Late Proterozoic extensional tectonics, to its subsequent reactivation as a reverse fault during the Late Cretaceous to Early Teritary Laramide Orogeny, and finally, to its most recent rendition as a normal fault during Late Teritary Basin and Range extension.
Late Proterozoic normal faulting offset crustal blocks by as much as two vertical miles to form parallel, NW-SE oriented, basins and ranges. The ranges were initially capped by Supergroup rocks, while basins preserved Supergroup rocks, tilted backward into large, one-sided grabens, but subsequent erosion from about 740 million to 545 million years ago removed the Grand Canyon Supergroup and more of the underlying crystalline basement rocks from the highlands throughout the Grand Canyon region, leaving only wedge-shaped remnants of Supergroup rocks preserved in the fault-bounded graben structures (Blakey and Ranney, 2008; Karlstrom et al., 2012). These structural wedges are mainly observed in isolated pockets along the main Colorado River corridor and some of its major tributaries (Figure 1), the largest of which is associated with the Butte Fault (Figure 7), the only graben exposed in the Grand Canyon that includes the Nankoweap Formation, the Chuar Group, and the Sixtymile Formation. By roughly 545 million years ago, continued erosion had once again reduced the mountainous terrain to a peneplain lying near sea level and the western North American continent formed a mature passive continental margin.
Although not actually a geologic structure, in the interest of completing our geologic history lesson, it is probably worth noting at this point that the erosion surfaces formed above the Vishnu basement between 1.75 billion and 1.25 billion years ago, and then again above the Supergroup structural wedges between about 740 and 545 million years ago, form a composite surface commonly known as the Great Unconformity, as original described by John Wesley Powell (Karlstrom and Timmons, 2012a). An unconformity is a gap in the geologic record that results at least in part from tectonically induced uplift. Unconformities represent times of erosion in geologic history, compared to times of deposition. There are three types of unconformities identified by geologists; disconformities, angular unconformities, and nonconformities. A nonconformity is the name for an erosion surface separating a layer of sedimentary rock lying directly on metamorphic and igneous basement rock, representing the longest gap in the rock record (Karlstrom and Timmons, 2012a). Angular unconformities occur where an erosion surface separating a layer of sedimentary rock that is lying on deformed layers of sedimentary rock; passage of time has uplifted and deformed the lower layers, but has not completely removed them to expose crystalline basement. A disconformity, the hardest of the three types to identify, occurs when there is erosion between two otherwise flat-lying rock layers, leaving a relatively short gap in time. In the Grand Canyon, unconformities can be found between almost every layer; although certainly the most famous, and most significant unconformity in the canyon is the composite erosion surface formed by the Great Unconformity. At its greatest, this unconformity represents over 1.2 billion years of missing rock; but where it miraculously preserved slivers of the Grand Canyon Supergroup, it helps geologists fill in the 1200 million year gap in the rock record, shrinking it to the composite gaps of 500 million years before and 200 million years after as described earlier.
The Vishnu basement, preserving its wedges of Supergroup rocks, was reburied by a thick sequence of Phanerozoic strata between about 545 million and approximately 70 million years ago (Figure 9c), although much of the Mesozoic sedimentary rocks were later removed by uplift and erosion (Blakey and Ranney, 2008). Today, a thick sequence of Paleozoic sedimentary rocks overlies the Great Unconformity and comprises the vast majority of rocks exposed in the walls of the Grand Canyon (Figures 1 and Figure 2 – see SEDIMENTARY ROCK FORMATIONS OF THE GRAND CANYON REGION in GEOLOGY OF THE GRAND CANYON REGION). These mudstones, sandstones, and limestones totaling between 2,400 and 5,000 feet thick offer evidence of coastal environments, including several significant marine incursions from the west, developed on a passive continental margin setting between about 545 and 250 million years ago (Blakey and Ranney, 2008; Blakey and Middleton, 2012). Rock formations from the Cambrian, Devonian, Mississippian, Pennsylvanian and Permian periods are present, with lesser gaps in the rock record indicating varied periods of marine retreat and subaerial exposure of the passive margin. Several disconformities mark the missing rock record produced as a result of erosion at these times, the most significant of which is the 150 million- year-gap between Cambrian Muav Limestone and overlying Devonian Temple Butte Formation (Blakey and Middleton, 2012).
The Paleozoic sedimentary rock sequence in the Grand Canyon region is generally undeformed (it retains its horizontal layering, as it was originally deposited), readily expressed by its layer-cake appearance from many locations on the North and South Rims (Blakey and Ranney, 2008; Blakey and Middleton, 2012). However, evidence of two fairly dramatic periods of deformation is well known (Karlstrom and Timmons, 2012b). During the Late Cretaceous to Early Tertiary (70 to 40 million years ago), low-angle subduction of an oceanic plate under the western edge of the North American continent resulted in the Laramide Orogeny. This orogenic event reactivated many older Late Proterozoic age extensional faults in the crystalline basement as compressional reverse faults (Karlstrom and Timmons, 2012b), building the Rocky Mountains elsewhere , but gradually uplifting the Colorado Plateau as a more or less, uniform crustal block. The thick sequence of Paleozoic and Mesozoic sedimentary rocks of the Colorado Plateau was deformed in several places by monoclinal folding over buried reverse faults. Most of the monoclines in the Grand Canyon developed from a single, high-angle fault that produced substantial offset within the crystalline basement rock, and were formed from either sitting above a preexisting basement fault, or from built up compressional stress in the crust that was released elsewhere within weaker Paleozoic sedimentary rocks (Huntoon, 1990). Because the Paleozoic strata was deeply buried at the time, it was ductile and underwent bending and folding, rather than creating through-going faults. Perhaps the best example of this paired fault-fold deformation occurs at the Butte Fault (Figure 7), where Paleozoic rocks were lifted and deformed by the East Kaibab Monocline (Figure 9d). Compression and up-to-the-west movement on the Butte Fault explains why the Kaibab Plateau, capped by Kaibab Limestone, lies at an elevation several thousand feet above the Marble Platform, capped by the same rock layer (Figure 10). The average offset in the canyon from monoclines is well over 1000 feet, with a maximum of 2500 feet at the East Kaibab monocline (Huntoon, 2003). Despite the large number of monoclines in the canyon, it should be noted that the crust in the Colorado Plateau was shortened less than 1 percent from this folding (Huntoon, 2003).
Figure 10. The East Kaibab Monocline is readily observed from Desert View on the South Rim where the Laramide Orogeny produced down-to-the-east monoclinal folding over the reactivated Butte Fault.
Still more recently, subduction ceased along the southwest margin of the North American plate as a mid-ocean ridge collided with the subduction zone beginning about 17 million years ago (Blakey and Ranney, 2008). Subduction was gradually replaced by right-lateral shear at a transform boundary, and the San Andreas Fault system, now stretching from the Gulf of California to Cape Mendocino was born. More importantly for the Grand Canyon region, growth of transform shear in what is now southern California generated an extensional tectonic regime eastward in the Basin and Range, causing the collapse of the Colorado Plateau along its western edge by progressive eastward reactivation of basement faults such as the Butte Fault, once again taking on their original extensional characteristics (Figure 9e). Normal faulting related to this Basin and Range extension is gradually chewing into the Grand Canyon region from the west, generating several major N-S oriented faults such as the Hurricane and Toroweap faults of the Shivwits and Uinkaret Plateaus in the northwestern Grand Canyon region.
Erosion has removed most Mesozoic Era sedimentary rocks from the region, although scattered, small remnants can be found, such as Cedar Mesa near Desert View, and Red Butte, south of Tusayan, AZ. Nearby rock outcrops, particularly to the north in the Grand Staircase area, suggest 4,000 to 8,000 feet of Mesozoic sedimentary layers once covered the Grand Canyon region, but were removed by substantial uplift and erosion in the Early to Middle Tertiary. Cenozoic sedimentary rocks are limited to the western Grand Canyon and to stream terraces and travertine deposits found superimposed on older rocks near the Colorado River itself. Lava flows and cinder cones, including spectacular lava cascades down the canyon walls, formed on the Shivwits and Uinkaret Plateaus in the northwestern Grand Canyon region comprise the majority of Cenozoic deposits. Volcanic activity began about six million years ago and has continued to within the last several thousand years. Cenozoic exhumation in the Grand Canyon region corresponded to late stage Laramide Orogeny compression (70-40 million years ago) and an associated tropical climatic regime, followed by Basin and Range extension (beginning 17 million years ago), and combined with more recent carving of the Grand Canyon by the Colorado River starting around 6 million years ago enhance by a regional increase in aridity (Blakey and Ranney, 2008). Incision of the Colorado and canyon formation was brought on by uplift of the Colorado Plateau to the northeast and base level lowering in the Basin and Range to the southwest which has exposed the marvelous geologic structures on presently on display in the area (Figure 9f). All of these factors corroborated to produce the present-day majestic scenery of the Grand Canyon region.