|
|
See also Colorado Rocks, The Earth At WorkLast modified 10/18/04
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
The 'About Time' icon |
Geologic understanding requires careful attention to time as well as three-dimensional space, so we'd better stop here first. If you're unfamiliar with geologic time terminology and abbreviations, take that detour now.
With over 50 million modern human lifetimes elapsed since the planet formed around 4.5 Ga, geologic or deep time is, to say the least, difficult to grasp. But the effort pays, for with a feel for deep time comes a sense of its great power: Given enough time, almost anything energetically possible can happen—even at very large scales. And so it goes with the bending of seemingly rigid rocks, the cutting of majestic canyons, the raising and erasing of entire mountain ranges, the opening and closing of globe-girdling oceans, and the incessant splitting and regrouping of the dancing continents.
Our usually reliable day-to-day sensibilities tell us that such things can't happen, but they can and do happen because solid rock reveals its malleability only over time scales very long compared to human events—typically in spans of tens of thousands if not millions of years. Given enough time, cold surface rock will yield to flowing water and buried rock will bend or even flow rather than break. The planet's had ample time for all of this, even if it's out of our ken.
 |
Upturned sedimentary strata at the mouth of Boulder Canyon |
Under great confining pressures, or at depths where temperatures reach a significant fraction of their melting points (typically 10-15 km), rocks that are quite brittle at the surface become sufficiently plastic to deform without fracture at rates comparable to the rate at which fingernails grow (~10 mm/yr). Granted, that makes molasses look downright mercurial, but then relative viscosity is the whole idea here. My piano tuner's job security rests on the fact that properly tuned piano wire flows (stretches) at a similar rate and falls out of tune in a matter of months. Over a few Ma, fingernail speed is plenty fast enough to fold great thicknesses of sedimentary rock over the east edge of the relatively brittle Front Range basement block, as in the photo at right. Over 50 Ma, an entire mountain range like the Ancestral Rockies can rise up and vanish.
The pink feldspar bands in the severely deformed
metamorphic rock at right (a mylonite from the Homestake Shear Zone
in the northern Sawatch Range) show the kind of internal folding you'd expect
from warm taffy, but this rock didn't actually melt. The wavy fabric indicates
solid flow at mid-crustal temperatures of 400-500°C.
To recast barely conceivable geologic time intervals in a more familiar format, they'll also appear from time to time as military hours in a single 24-hour day (and all-nighter) of creation beginning at the planet's formation and ending with the here and now.
With the formation of the earth pegged at 00:00:00 h (midnight), our work day finishes at 24:00:00 h, just as we pull up to the present. We'll ignore seconds until we get close to 24:00 h. Each hour corresponds to 0.875 billion years (Ga), each minute to 3.125 million years (Ma) and each second to 52.1 thousand years (Ka). Conversely, 1 Ga takes 4 hours, 20 minutes out of the day; 1 Ma, 19.2 seconds; and 1 Ka, 19.2 milliseconds—literally the blink of an eye.
After an introductory section, Colorado's geologic history will unfold in chronological order below. As we come up on major divisions along the geologic timeline, a brief "About Time" header will summarize what event or fossil record breakpoint ushered in the upcoming division and show how it's subdivided. The table below collects the links to these headers.
Links to "About Time" Headers |
||
| Geologic Interval | Years Before Present | Hours Since Earth Formed |
| Precambrian Time | 4.5 Ga - 543 Ma | 00:00 - 21:06 |
| Proterozoic Eon | 1.6 Ga - 543 Ma | 15:28 - 21:06 |
| Phanerozoic Eon | 543 - 0 Ma | 21:06 - 24:00 |
| Paleozoic Era | 543-248 Ma | 21:06 - 22:41 |
| Mesozoic Era | 248-65 Ma | 22:41 - 23:39 |
| Cenozoic Era | 65-0 Ma | 23:39 - 24:00 |
| Pleistocene Epoch | 1.8 Ma - 10 Ka | 21:06 - 23:59:59.81 |
| Holocene Epoch | 10-0 Ka | 23:59:59.81 - 24:00:00 |
Note: Ga stands for a billion years; Ma, a million; Ka, a thousand; and 0, the present. |
||
Notice how even deep time can fly in proper perspective: The Precambrian was admittedly a slow start, taking until mid-evening to unfold on our day of creation, but then life as we know it appeared only in the last 3 hours. The entire Mesozoic Era (295 Ma long) shot by in under an hour, the dinosaurs departed just 20 minutes ago (thank goodness), and we've been comfy and relatively ice-free for only a mere 0.2 seconds. A healthy human female can expect to live just under 2 milliseconds in a developed country.
If you need more help with geologic time or terminology, check out these resources.
"I would have written a shorter letter if I'd had more time." — Benjamin Franklin
"It requires a very unusual mind to undertake the analysis of the obvious." — Alfred North Whitehead
"Science is a way of trying not to fool yourself." — Richard Feynman
"The universe is not only queerer than we suppose, it's queerer than we can suppose." — J. B. S. Haldane
"Civilization exists only with geologic consent subject to change without notice." — Will Durant
 |
Mount Sopris |
Colorado's geologic history is as complex as it is fascinating. The geology is in turn inextricably entwined with Colorado's human history. That's true everywhere, of course, but it goes double here. Colorado's generous endowments of accessible mineral wealth and fertile farmland were not inevitable birthrights. Things could have turned out quite differently.
This overview ventures a "to the best of our knowledge" summary of Colorado geologic evolution current as of late 2004. Colorado's story still includes many gaps and controversies, often around events and structures shrouded in deep time, deep earth or both. I've attempted to point out the most significant unknowns and disagreements, but often, rightly or wrongly, I've simply taken sides.
If you take nothing else away from this humble attempt, please consider this: Of Colorado's 55 Fourteeners, all but 2 (Longs Peak and Pikes Peak) lie either along the Colorado Mineral Belt or on the shoulders of the Rio Grande Rift. Most of them cluster around the intersection of these two profound lineaments, both which almost certainly cut the full thickness of the lithosphere. This telling elevation distribution reflects a powerful synergy between truly ancient plate processes driven by the cooling of the earth in the presence of gravity and ongoing mantle processes driven by Lord knows what.
Colorado spans three major North American geomorphic provinces roughly organized in distinct north-south bands:
|
The Rocky Mountains in the center, imposing by any standard; |
|
 |
Maroon Bells, Elk Range |
||
|
Tablelands to the west, including portions of the Colorado Plateau; and |
|
|
Echo Park, Dinosaur NP |
||
|
High Plains, ramping down to the east toward the Mississippi Valley. |
|
|
East of Kiowa |
This geologic overview focuses primarily on the Colorado Rockies and the Colorado Plateau, but adjoining portions of Wyoming, Utah and the High Plains as far east as the Black Hills of South Dakota also have important and related stories to tell. Throughout this article, I'll refer to Colorado and these adjoining areas as our region of interest, which closely coincides both with the area shown in the NASA photo at Bearings just below and also with the area affected by Laramide uplift.
 |
Western U.S. tectonic upland |
Colorado's also part of an immense, heterogeneous and longstanding tectonic upland covering the entire western third of the United States and ultimately can only be understood in that context. From the Middle Proterozoic on, much of Colorado's story also applies, with some notable exceptions, throughout the Southern Rockies* and the Colorado Plateau. Events in the Basin and Range and along the West Coast have also influenced Colorado's physical development, particularly from ~80 Ma on.
 |
Laramide Orogen from space, NASA |
Let's pause now to get oriented within and around our region of interest. If you haven't done so already, click on the true-color ^NASA Visible Earth satellite photo at right to open a larger version in a new browser window. Find Four Corners, where four state lines meet at right angles. Colorado holds down the northeast quadrant; New Mexico, Arizona and Utah run clockwise from there.
Snow caps highlight the highest of the green forested Rocky Mountain uplifts, all of which first rose in the region-defining Latest Cretaceous through Early Tertiary Laramide orogeny. The uplifts are highest and most densely packed in western Colorado, but they clearly extend well to the north and south. Important outliers include the Black Hills uplift of South Dakota (clipped in the NE corner) and the Uinta Mountains and the Wind River Range in the NW quadrant of the photo. The largest Laramide uplift, the Front Range, dominates the north central portion of Colorado. The highest Laramide uplift, the Sawatch Range, stands on the west shoulder of the Rio Grande rift, the narrow north-south gash between the two largest snow packs near the center of the photo. Further south, the rift is visible as the broad almond-shaped San Luis Valley of south-central Colorado. The narrower Rio Grande Valley, heading directly south from the San Luis Valley, is also part of the rift. (Keep your eye on the Rio Grande rift as we come up on post-Laramide events below. It's a key player in the development of the current landscape.)
In the southwest quadrant of the photo, exposed iron-stained sedimentary rocks nicely mark the Colorado Plateau (CP) in salmon pink. The CP saw action in the Laramide Orogeny but gained most of its elevation via post-Laramide epierogenic uplift. Four Corners lies near the east central margin of the Colorado Plateau.
You'll encounter many similar satellite photos and physiographic maps throughout this site because, in many important ways, the land speaks for itself. Anyone interested in learning to read the landforms will some find very powerful interactive tools at NASA's ^Visible Earth and ^Earth Observatory sites and at UNAVCO's ^Jules Verne Voyager site.
To a large extent, the Colorado Rockies and the Colorado Plateau have evolved together since 1.8 Ga or so, but since the onset of the Laramide Orogeny at around 72 Ma, the rigid, block-like Colorado Plateau has been something of a loose cannon. To complicate matters further, shared events have played out rather unevenly in the two provinces with somewhat disparate timing.
 |
Colorado in context |
The dominant earth processes shaping our region of interest provide a convenient framework for both discussion and understanding. Each set of events has overprinted preceding events in its own way, sometimes preserving and sometimes destroying traces of what came before. Disentangling these layers of process in both space and time makes for a fine puzzle. Geoscientists have gathered many of the pieces on the table and have even fit a good many of them together, but the geologic record is still full of holes, and more pieces have been lost to burial and erosion than anyone would like.
Plate tectonic processes have shaped Colorado since their inception at ~2.0 Ga, but Colorado hasn't been within 1,000 km of a credible plate margin for at least 1.4 Ga. The intracontinental deformations so beautifully in evidence now in Colorado reflect both far-field stresses generated at remote plate boundaries and cryptic upper mantle processes not directly related to plate interactions. One recent, strongly overprinting intraplate process to keep an eye on as we proceed is the Rio Grande rift, which has been busy rearranging the central and south central Colorado landscape in big ways since at least 28 Ma.
Another important influence to track throughout this overview is climate. It's importance is easy to overlook when you're focused on the rocks and how they've moved. Many earth processes play out at depth, well beyond the reach of the atmosphere and hydrosphere. But for processes like weathering, erosion, sediment transport, deposition, diagenesis, isostatic rebound and basin subsidence, the rubber meets the road at the surface, where the atmosphere, the hydrosphere and the land all interact strongly to shape both land and climate in a never-ending dance. Streams cut Colorado's deepest canyons in only the last 10 Ma. The relative contributions of wetter climate and uplift remain topics of contentious debate among geoscientists.
Defining Earth Processes in Colorado's Geologic History |
||
| Time |
Process |
Noteworthy Results |
|
1.78-1.65 Ga |
The southern margin of Laurentia, the nascent North American continent, accreted Colorado's oldest Precambrian basement rocks. |
|
|
1.45-1.35 Ga |
This mysterious and protracted event added copious 1.4 Ga granites, northeast-trending ductile shear zones and differential uplifts to the Precambrian basement throughout the Southwest. It also left behind basement-cutting rift faults that subsequent orogenies (mountain-building events) would reuse for their own purposes. |
|
|
1.2-1.1 Ga |
The voluminous Pikes Peak Batholith intruded the basement of the southern Front Range and the adjacent High Plains. |
|
| 1.4-0.6 Ga | Late Proterozoic continental rifting | Continental rifting related primarily to the Berthoud Orogeny and to the later breakup of the supercontinent Rodinia created enduring basement faults throughout the western 2/3 of the US. The compressive Ancestral Rocky Mountain and Laramide orogenies would later reactivate these faults in reverse. |
| 1.1 Ga - 510 Ma | Great Unconformity | Erosion planed off the differentially uplifted and rifted Precambrian basement during this immense gap in Colorado's rock record. With few exceptions, all pre-existing sedimentary cover was lost, and no new cover accumulated during this time. |
| 510-300 Ma | Early Paleozoic marine sedimentation | The planed-off Precambrian basement accumulated a 2-3 km-thick blanket of flat-lying tropical marine sediments. |
|
300-248 Ma |
The Ancestral Rockies rose along reactivated Proterozoic rift faults as two large island mountain ranges that were eventually buried in their own debris. |
|
| 248-72 Ma | Mesozoic sedimentation | Terrigenous sediments accumulated widely over Colorado through earliest Cretaceous time. Marine sediments blanketed the state from then on, right up to the Laramide orogeny. |
|
72-40 Ma |
Basement-cored uplifts rose again along reactivated Proterozoic rifts, this time in greater numbers as the current Rocky Mountains, the Colorado Plateau and, to a lesser extent, the High Plains. Associated volcanism left behind dozens of Laramide intrusions strongly clustered within the Colorado Mineral Belt (CMB) lineament. CMB mineralization had begun. |
|
| 36-5 Ma | Post-Laramide magmatism | Thick, widespread volcanic piles reorganized much of Colorado's topography and drainage patterns while intrusions peppered the Colorado Mineral Belt from below to complete its mineralization. |
|
28-0 Ma |
The Rockies and the Colorado Plateau rose to their present elevations with the onset of regional epierogenic uplift and extension, including the arrival of the Rio Grand rift. |
|
|
10-0 Ma |
Reinvigorated streams removed mass and incised channels to sculpt the uplands of the Rockies and the Colorado Plateau and eventually the range-front basins as well. |
|
|
1.8 Ma - 10 Ka |
Glaciers and their outwash streams deeply eroded the higher Laramide uplifts into their present dramatic topography but largely spared the topographically lower Colorado Plateau. |
|
| 10-0 Ka | Holocene uplift and erosion | Erosion- and mantle-driven regional uplift and stream erosion continue at slower paces; overall, the Rockies maintain their elevations in the face of erosion. |
The rocks and structures now exposed so beautifully at the surface in Colorado are important traces of all these processes, to be both admired and understood. (Far from diminishing the beauty, the understanding only enhances it, at least to my mind and eye.) Other less accessible but equally important clues come from geophysical investigations (tomography, magnetic and gravity surveys), geomorphic analyses (elevation models, stream courses and profiles), and mine, tunnel and borehole logs, among many other data sets.
Several recurring themes become apparent as one studies Colorado's evolution and current topography.
The interior section of the continent now graced by the Rockies and the Colorado Plateau has been a seat of unrest for nearly 1.8 Ga. Far-field plate tectonic stresses have reshaped the landscape here on and off since the Colorado Province mobile belt joined the continent in Early Proterozoic time; upper mantle stresses have been in on the act since at least 28 Ma.
 |
Four Corners from space, NASA |
Basement rocks and faults loom large here, controlling topography, sedimentation and even subsequent tectonics. Since at least the time of the Ancestral Rockies, reactivated basement-penetrating reverse and thrust faults of Proterozoic rifting ancestry have dominated the mountain-building style throughout the Rockies and the Colorado Plateau. These faults have typically uplifted large blocks cored with hard Precambrian crystalline rocks and have folded and occasionally even broken the overlying sedimentary cover. Thus, flat-topped highlands seldom showing more than 15° of tilt came to be the dominant large-scale landforms, not just in the tablelands of the Colorado Plateau, but even in the highest parts of the Rockies. Faulted sedimentary monoclines showing substantial and opposing dips typically flanked the uplifts.
Antecedent streams armed with steep gradients and generous fluxes of water and sediment entrenched their courses at a time (in the mid- to late Tertiary) when the uplands generally showed little relief, and the streams were free to meander. Later, as uplift and climate synergized throughout the region, the upland streams had no choice but to incise deep, narrow canyons through rather than around buried uplifts of resistant basement and sedimentary cover throughout Colorado and Wyoming. Colorado's most spectacular antecedent canyons — e.g., the Black Canyon of the Gunnison, Glenwood Canyon and Royal Gorge — have become world-famous scenic attractions, but lesser antecedent canyons riddle the Rockies and plateau country.
Mid- to Late Tertiary magmatism profoundly altered the landscape throughout the Rockies and the Colorado Plateau, but most of the direct evidence (the volcanic rock) has been lost to erosion, particularly in the Rockies. The indirect effects of this intense period of magmatism are still visible today in places like the Florissant Fossil Beds National Monument — if you know where to look. Associated intrusions added great wealth to the Colorado Mineral Belt and left us jewels like Mt. Sopris.
These recurring threads are worth keeping in mind as we now launch into the particulars of Colorado geology.
Most sections in this article close with a "Map Units" subsection describing how to find pertinent bedrock (surface rock) units on the Geologic Highway Map of Colorado.
To geoscientists, the "big picture" is a highly-prized and often hard-won 4D understanding of a landscape and its past at local to global scales. Here, I sketch the big picture in Colorado, as best I can put it together, from past to present. Subsequent sections will flesh out the details, also in chronological order.
 |
Modern Banda Sea |
The land we now call Colorado was first assembled in the Early Proterozoic between 1.78 Ga and 1.65 Ga (14:30-15:12 h) from a hodgepodge of island arcs, backarc basins and more mature continental fragments docking or developing against a backstop provided by the Archean Wyoming Province at the southern margin of the nascent North American continent. The mobile belt added to the continent during this time is known as the Colorado Province. Despite a long-standing intracontinental location, it's been unstable ever since.
The assembly of the Colorado Province resembled in some respects the Early Proterozoic assembly of northeast Australia, which has changed little since then and therefore has a history much easier to unravel than Colorado's oft-overprinted story. In other important respects, the gathering Colorado Province must have resembled the swarm of oceanic and continental elements now coalescing feverishly (by geologic standards) in the closing ^Banda Sea of Eastern Indonesia above.
 |
Buffalo Mountain |
Around 1.7 Ga, in the midst of this accretional frenzy, large quantities of granite and associated pegmatites intruded the middle to upper crust throughout the region, as at Buffalo Mountain (right) and ^Rocky Mountain National Park. Colorado intrusive rocks with radiometric dates in the 1.660-1.790 Ga range are sometimes referred to collectively as the Routt Plutonic Suite.
 |
Mount Evans from Denver |
A large number of granitic intrusions, ductile shear zones, differential basement uplifts and rifts peppered the Colorado Province, along with the rest of the continent, in the Berthoud orogeny at 1.45-1.35 Ga (16:16-16:48 h). Colorado's many intrusive rocks with radiometric dates in the 1.400-1.450 Ga range are sometimes referred to collectively as the Berthoud Plutonic Suite.
At 1.2-1.1 Ga (17:36-18:08 h), granite widely intruded the continent once again in a rather mysterious event often called the Grenville orogeny, but this time, Colorado took only one known hit — albeit a big one — in the Pike's Peak area.
The Berthoud and Grenville orogenies appear to have occurred in response to convergent plate interactions playing out far to the south. The many granitic intrusions at 1.4 Ga and the one massive batholith (very large intrusion) at 1.1 Ga added considerably to and effectively completed the Colorado basement as we know it today.
 |
Afar Region Rifting |
Over the long span of Proterozoic time, several major intracontinental rifting events repeatedly tried to pull the western two-thirds of the US apart, Colorado included, just as the East African Rift splits northeast Africa today. Some of the rifts may have originated during the Berthoud and Grenville orogenies, while others clearly developed in Late Proterozoic time at 0.9-0.6 Ga (19:12-20:48 h) around the breakup of Rodinia, the planet's earliest known supercontinent.
Although an ocean-forming continental break-up never ensued in or around Colorado, the recurrent rifting left behind a rhombic network of deep-seated intersecting normal (extensional) basement faults. The E-W and NE-SW extensional stresses driving the rifting produced primarily north- and northwest-trending normal faults and pull-apart basins. (The particularly deep west-trending Uinta rift basin was a notable exception.) Many of the rift faults probably cut the entire lithosphere.
[map coming] |
Rift map from Marshak |
During the Phanerozoic Ancestral Rocky Mountain and Laramide orogenies to follow, regional crustal shortening would exploit the old rift faults by reactivating them in reverse — hence the broad north- and northwest-trending tectonic grains still obvious in any shaded relief map of the modern Rockies and the Colorado Plateau. During the Laramide, two surviving rift basins filled with Precambrian sediments (now the Uinta and Uncompahgre formations) were squeezed and inverted to form the Uinta uplift and the southern (San Juan) portion of the Uncompahgre uplift as well.
Around 28 Ma, rifting returned to Colorado with the arrival of the Rio Grande rift.
Between 1.1 Ga and 510 Ma (18:08-21:17
h), erosion reigned supreme throughout Colorado. With
rare but notable exceptions, all Precambrian sediments were removed, along with ~10 km of
basement. This enigmatic period is marked by a 0.6-1.1 Ga-long gap in the
Colorado rock column known as the Great
Unconformity. Colorado spent most of the ensuing Early
Paleozoic underwater, accumulating great thicknesses of tropical marine sediments
(primarily sandstones, limestones and shales) laid down flat on the planed-off
Late Precambrian basement surface.
 |
Maroon Bells |
Around 300 Ma (22:24 h), the Ancestral Rocky Mountains rose quickly to moderate heights (probably no more than 10,000' above sea level) along reactivated Proterozoic rifts, apparently in response to a continent-continent collision between North America's predecessor Laurentia and the supercontinent Gondwana far to the south. The two largest island mountain ranges developed in Colorado—Frontrangia on the east and Uncompahgria on the west. By 250 Ma (22:40 h), just 50 Ma later, both ranges had completely succumbed to erosion, their debris piled high across half the state.
The mostly nonmarine Late Paleozoic to Early Mesozoic sediments shed off these highlands accumulated to great depths in large range-front basins—in the Denver basin to the east, in the Paradox basin to the west and in the Maroon Basin in between. We have the Ancestral Rockies to thank for the Maroon Bells (above) that now grace the Elk Range.
|
Four Corners, NASA |
Around 72 Ma (23:37 h), the Laramide Orogeny began to buckle up basement from southern Montana to central New Mexico—and to a lesser extent in the High Plains and the Colorado Plateau—along reactivated, basement-penetrating Proterozoic rift faults. The regional deformation apparently occurred in response to the onset of accelerated, low-angle (flat-slab) subduction of the young Farallon plate beneath the western margin of North America. The resulting basement-cored faulted uplifts and their flanking sedimentary monoclines continue to define the broad features of Rocky Mountain and Colorado Plateau topography. The ever stalwart Colorado Plateau moved ~100 km north as a rigid unit during the Laramide but saw less uplift and internal deformation than the neighboring Rockies.
As always, the Laramide uplifts began to erode as soon as they began to rise. By Oligocene time, their debris had filled the structural basins surrounding the uplifts to overflowing. Today, the stream-dissected syntectonic strata of the upper Denver Basin expose a clear record of the progressive unroofing of the rising Front Range block — a process repeated throughout the Laramide orogen.
The Laramide died out around 40 Ma (23:47 h), first in the north and finally in the south. Along the way, Laramide magmatism got the mineralization of the Colorado Mineral Belt off to a good start.
 |
West Elk Range |
Between 36 Ma and 5 Ma (23:48-23:58 h), intense regional magmatism boiled up through Colorado, leaving the Eocene erosional surface deeply buried under volcanic flows and ejecta and the subsurface shot through with intrusions that would elevate and metamorphose pre-existing sediments and inject mineral wealth all along the Colorado Mineral Belt. The causes of this fiery outburst remain obscure. Over most of the state, the far-flung Tertiary volcanics have been long lost to erosion, but the San Juan Mountains and the West Elk Mountains (right) are major exceptions. Nevertheless, the topographic legacy of the volcanic era lives on in the head-scratching courses of some of Colorado's largest antecedent streams. Before their removal by erosion, volcanic accumulations helped to reposition streams directly over several then-buried Laramide uplifts. The Black Canyon of the Gunnison is but one of the results.
 |
Front Range Pediment |
Starting around 28 Ma (23:51 h), in the Late Oligocene, epierogenic uplift probably related at least in part to the approach of the Rio Grand rift domed up the entire region to present average elevations with little tilting. This uplift added over 1.5 km (5,000') to the Colorado skyline. The greatest elevation gains centered on the intersection, near Leadville, of two of Colorado's most profound structural lineaments—the Colorado Mineral Belt and the Rio Grand rift.
Even with concomitant erosion, the nearby Colorado Plateau managed a net 6,000' elevation gain—much of it in the last 5 Ma alone. During the this time, the cohesive Colorado Plateau once again moved as a unit relative to the Rockies, this time to the northwest to accommodate the opening of the Rio Grand rift.
Today, with the help of erosion-driven isostatic rebound, mantle-driven regional uplift continues, albeit at a much slower pace, maintaining the height of the Rockies and the Colorado Plateau in the face of ongoing erosion.
 |
Painted Wall |
Exhumation of the Laramide Rockies and the Colorado Plateau began in the mid-Tertiary and kicked into high gear in the Late Miocene around 10 Ma (23:57 h). Stream incision was swift and deep, and intermontane basins were cleared of their sediments, thanks to continuing regional uplift and a wet Late Tertiary climate. Antecedent streams setting their courses on the low-relief Eocene erosional surface eventually found themselves cutting through buried uplifts that would become the ridges and even ranges of the current Colorado landscape. The range-front basins were uncovered and incised as well.
 |
Gore Range |
By Pliocene time, the Rockies and the Colorado Plateau had been thoroughly dissected by stream erosion but continued to stand high between the deep canyons. The devastating glaciations that would come next, primarily in Pleistocene time, around 1.8 Ma (23:59:25 h), took the project of exhuming the Rockies to new highs and new lows. Nearly everywhere above 8,000', the glaciers gouged out cirques, canyons and U-shaped valleys, dammed lakes, diverted streams, moved house-size boulders like so much furniture, and dispatched massive floods of outwash water and sediment to erode again at lower elevations. The less lofty Colorado Plateau largely escaped the ice.
 |
Maroon Bells |
When the last of the glaciations finally melted away at the close of the Tertiary around 10 Ka (23:59:59.81 h), Colorado looked much as it does today—and quite likely more beautiful than ever before. Purple mountain majesty indeed.
The sections to follow will attempt to flesh out the details of Colorado's most formative processes in chronological order.
The final condensation of planet Earth from the solar nebula at 4.5 Ga (00:00
hours on our metaphoric "day" of creation) kicked off the Hadean Eon (4.5-3.8
Ga, 00:00-03:44 h).
When the intense extraterrestrial bombardment defining the Hadean Eon fell off, the Archean
Eon (3.8-2.5 Ga, 03:44-10:40 h) of initial continent building began. The
long Proterozoic
Eon (2.5 Ga - 543 Ma, 10:40-21:06 h) followed. The current Phanerozoic
Eon (543-0 Ma, 21:06-24:00 h) began when complex life forms built on current
(not Edicarian) plans
first appeared explosively in the fossil record at the beginning of the Cambrian
Period (543-490 Ma, 21:06-21:23 h), which kicked off the Paleozoic
Era (543-248 Ma, 21:06-22:41 h) and ended Precambrian Time (4.5 Ga - 543
Ma, 00:00-21:06 h).
The Precambrian includes the Proterozoic, Archean and Hadean Eons, but most Precambrian rocks exposed in Colorado and elsewhere in the region date from the Proterozoic. At the surface of the earth today, Archean rocks are fairly uncommon and Hadean rocks are very rare.
Only a tiny sliver of Archean rock crops out in Colorado's northwest corner. But just across the Wyoming and Utah state lines, the stiff southern margin of a Late Archean craton (ancient continental core) known as the Wyoming Province served as a backstop for the Early Proterozoic assembly of the Precambrian basement now underpinning the entire southwestern United States, Colorado included.
 |
Structure of the Earth, source USGS |
Our knowledge of the earth in the Archean Eon (3.8-2.5 Ga, 03:44-10:40) is necessarily dim and incomplete, but parts of the puzzle are coming together, and they don't look all that familiar. As the Archean Eon opened, the earth was still piping hot if not frankly molten following its condensation from the solar nebula around 4.5 Ga and a merciless pummeling by stray solar system debris during the Hadean Eon (4.5-3.8 Ga, 00:00-03:40). Moreover, radioactive uranium, thorium and potassium isotopes concentrated primarily in the crust and upper mantle generated radiogenic heat at 2-3 times the current rate. Throughout the Archean, crust and mantle were both much hotter than today's, and earth processes operating then differed considerably from those at work in the last 2.0 Ga.
When and how the earth developed its current internal crust/mantle/core structure remains controversial, but evidence now points to rapid and thorough chemical differentiation of a largely molten earth very near the time of its formation, probably by ~4.4 Ga. This process released more heat, particularly with the fall of iron to form the planet's core. The important discontinuity between the upper and lower mantle at 650 km below the surface probably also developed at this time, although at a shallower depth in the hotter early mantle. The "650" has served as a fundamental barrier to material transport, both up and down, ever since.
Hydrated low-density sialic (silicon- and aluminum-rich) crustal residues floating on a global ocean of dense hydrated mafic (magnesium- and iron-rich) magma probably developed near the time of formation at 4.5 Ga and may have persisted well into the Archean. Buoyant patches of sialic crust eventually coalesced to form a number of large, durable, unsinkable rafts that eventually became the cratons (stable cores) of our modern continents—e.g., the Wyoming Province and the Canadian shield. Through fractionation and underplating of subjacent mantle, these early Archean cratons developed thick (>100 km) hulls and very deep (>250 km) keels of cool, strong, buoyant and refractory lithospheric mantle that to this day protect them from disruption by heat and currents in the deeper upper mantle.
Most of the planet's supply of sialic continental crust formed in the hot early Archean via high-temperature processes never repeated since. Much of that crust was recycled into the upper mantle prior to the advent of plate tectonics at ~2.0 Ga. The remaining Archean cratons have been tied up in continental cores ever since. Continental crust formed since that time, primarily at post-Archean collisional and convergent plate boundaries, has been much more mantle-like (mafic) than the surviving Archean cratons.
 |
LITHOPROBE map of North American basement rocks |
At the close of the Archean Eon, a nearby 3.2-2.1 Ga craton called the Wyoming Province stood at the southwest margin of Laurentia, a northern supercontinent that would eventually be trimmed down to North America. To this day, the Cheyenne Belt lineament of southern Wyoming and northeast Utah marks the southern margin of the Wyoming Province.
The stalwart Wyoming craton played a pivotal role in the Early Proterozoic assemblage of Colorado's Precambrian basement by serving as a backstop for a long series of juvenile Early Proterozoic island arcs and more mature terranes rafted in from the south along a 1,300 km-wide, east northeast-trending convergent plate margin active around 1.78-1.65 Ga.
In the ^LITHOPROBE map of North American basement rocks at right, the Wyoming Province appears in medium blue. The broad band of very light gray 1,800-1,600 Ma basement to its south will become the Colorado Province, which in turn will become the basement for the southwestern United States, including Colorado.
Only one small fault-bound sliver of the Wyoming Province manages to nip across the Utah state line into the northwest corner of Colorado—the 2.3 Ga Red Creek quartzite at the northern margin of the Laramide Uinta uplift north of CO318 near Browns Park.
The Red Creek quartzite is the closest any Colorado surface rock gets to an Archean age. The rest are all at least 500 Ma younger, including the nearby Uinta Mountain Group, a thick mass of unrelated 1.7-0.9 Ga Precambrian sediments preserved in an inverted rift basin.
Unfortunately, the Red Creek Quartzite is too small to show on the 1:1,000,000 Geologic Highway Map of Colorado, but on Ogden Tweeto's 1:500,000 Geologic Map of Colorado, it appears as a tiny purple wedge marked "Wr" against the Utah state line ~10 km (6 miles) south of the northwest corner of Colorado.
|
Colorado in context |
Nowhere on the planet do Archean rocks record clear evidence of the plate tectonic processes dominating earth dynamics today—no large thrusts, no overturned sedimentary piles, no large-scale rifting or collisions, no subduction. Instead, Archean cratons like the Wyoming Province tend to consist of large domal bodies of primitive granite intruding folded greenstone (metavolcanic) belts and covered with odd volcanic and sedimentary supracrustal rock sequences like the Red Creek quartzite. The planetary processes underlying this pattern remain largely unknown, but this much is clear—they haven't been repeated since anywhere on the planet.
Exactly when and how water-filled oceans formed is also unknown, but they were clearly present by the time plate tectonics began to operate with the onset of subduction in the Earliest Proterozoic, around 2.0 Ga (14:24 h). In fact, you must first understand the oceans to understand the land, or at least the basement, in and around Colorado.
Since its Early Proterozoic assembly via subduction, Colorado has been far removed from active plate boundaries, but far-field effects from subsequent plate interactions to the west, south and east continued to play an important role in Colorado's evolution, both at the surface and at depth.
I won't digress to introduce plate tectonics here, but you'll find more than you need to know to understand Colorado's tectonic past and present at The Earth at Work on this site. Many of the explanatory links in this article lead there.
The Proterozoic Eon followed the Archean Eon (3.8-2.5
Ga, 03:44-10:40 h). Many
pivotal geologic and evolutionary biological developments of Precambrian Time
(4.5 Ga - 543 Ma, 00:00-21:06 h) occurred in the Proterozoic, including the initial build-up of
oxygen in the atmosphere and the global oxidation and precipitation of iron from
the oceans.
 |
Red Canyon, Colorado National Monument |
The oldest rocks exposed in Colorado are 1.78-1.65 Ga (14:30-15:12 h) granites and metamorphic rocks of Early Proterozoic age. These basement rocks crop out throughout the Rockies, often on Fourteener summits, as at Mount Evans, and also in the deeper canyons of the Colorado Plateau, as at the bottom of Red Canyon (right). These rocks accreted to the Archean North American continent in a long series of subduction zone collisions known as the Colorado Orogeny.
The name Colorado Province has been given to the large Early Proterozoic mobile belt added to the continent in this protracted event. The Colorado Province joined the Archean Wyoming Province along the oldest of Colorado's Early Proterozoic sutures, an enduring east northeast-trending defect in the lithosphere known as the Cheyenne Belt.
 |
Cheyenne Belt, Southern Wyoming |
Around 1.78 Ga, a south-dipping subduction zone developed off the southern margin of the Wyoming Province and built an offshore magmatic arc now known as the Green Mountain arc. Shortly thereafter, this juvenile arc collided with the Wyoming Province, driving intervening 2.1 Ga oceanic lithosphere to the north, beneath the southern edge of the Wyoming craton. The resulting suture is still discernable in satellite images as an east northeast-trending lineament across southern Wyoming and northeast Utah known as the Cheyenne Belt (CB). (The CB barely nips the northwest corner of Colorado.) Just south of the CB, highly metamorphosed fragments of the Green Mountain arc still crop out in a band across northern Colorado known as the Green Mountain Block.
[diagram coming] |
from GSA Today |
A remnant of the oceanic plate subducted in the Wyoming-Green Mountain collision may be preserved in the lithospheric mantle along the suture. In tomographic (cross-sectional) images, the CB appears distinctly as a steeply south-dipping discontinuity of lithospheric scale accompanied by marked localized crustal thickening consisting entirely of a prominent downward bulge of the Moho to ~60 km. The crust here is some of the thickest in North America and well outside normal variation around the global mean. Isostasy would normally predict a large topographic swell over a crustal thickening of this magnitude. The absence of the expected mountain range means that the bulge (a "mountainless root" if you will) must overlie an unusually dense body—in this case, most likely eclogite replacing oceanic crust trapped in the lithospheric mantle below the basalt-to-eclogite transition (now at ~60 km) during the Wyoming-Green Mountain collision.
A number of other curious contrasts have been noted across the CB. To its south, mantle xenoliths and tomographic imaging reveal hydrated minerals, garnet-bearing rocks and tomographic layering in the lithospheric mantle, but none of these features occur north or the CB; nor do any of the 1.4 Ga Berthoud plutons so common to the south. These differences remain unexplained.
Bottom line: The Cheyenne Belt is a significant lithospheric boundary whose full import is still being unraveled.
Subduction zones reappeared many times off the growing southern margin of the nascent Colorado Province as additional arcs and other oceanic and small continental terranes of varying sizes, orientations and origins rafted in over the 150 Ma following the initial Wyoming-Green Mountain collision. Today, these accreted terranes form several broad northeast-trending belts or provinces that share common Early Proterozoic deformation histories but are far larger than any single island arc they might contain.
West of the Green Mountain arc, in Utah, a package of terranes known as the Mojave Province docked onto the Wyoming Province at 1.75-1.70 Ga. In Colorado, the Rawah arc-backarc complex (part of the larger Northern Yavapai Province) came in behind the Green Mountain arc, followed by the Salida-Gunnison arc (part of the larger Southern Yavapai Province). Terranes of the Mazatzal Province docked next at 1.65 Ga.
The Northern and Southern Yavapai Provinces account for nearly all the 1.7 Ga basement rocks found in Colorado. The Mazatzal Province now floors much of New Mexico, but its docking was clearly felt in Colorado, as evidenced by 1.65 Ga deformations recorded in the older Yavapai basement.
The Cheyenne Belt, the 1.78 Ga suture between the Green Mountain arc and the Archean Wyoming Province craton, barely grazes the NW corner of Colorado. Just to its south, the 1.75 Ga Farwell Mountain-Lester Mountain shear zones mark the suture between the Rawah block and Green Mountain arc. The Homestake shear zone of the northern Sawatch Range may overlie the suture between the Rawah block (part of the Northern Yavapai Province) and the Southern Yavapai Province. The Colorado Mineral Belt roughly follows the same suture, but its association with Early Proterozoic sutures remains controversial. The volcanically active Jemez Lineament of northern New Mexico gives surface expression to the 1.65 Ga suture between the Southern Yavapai and Mazatzal Provinces.
 |
Sunda Arc |
Today, the northbound Australian continent is busily sweeping up a swarm of Eastern Indonesian arcs in the Banda Sea (below) along its northern margin, just as the Wyoming craton gathered up oceanic terranes along its southern margin in the Early Proterozoic. Among the many arcs and continental fragments caught in this ever-tightening Australian-Eurasian vice are the ^Lesser Sunda Islands (right); the largest island shown is Timor.
| |
Modern Banda Sea |
An apparent reversal of subduction polarity attending the docking of the Green Mountain arc also finds a modern analog here: When the leading edge of the Australian craton nosed into the north-dipping subduction zone beneath the island of Timor (part of the ^Banda arc) at ~2.2 Ma (23:59:18 h), subduction ceased there but appears to be starting up again along a new south-dipping subduction zone just north of the arc. The extreme topography and bathymetry of the Banda Sea and nearby New Guinea reflect ongoing collisions with Australia as the Banda Sea closes.
Geologists use the term basement to refer to the igneous and metamorphic rocks between any sedimentary cover that might be present and the Moho, the boundary at the bottom of the crust. Colorado's oldest exposed rocks are hard, resistant granites and metamorphic rocks added to the continent in the protracted Colorado Orogeny lasting from 1.78-1.65 Ga (14:30-15:12 h), and these make up most of the basement in Colorado today. Many of them may be closer to 1.8 Ga in true age, but their radiometric signatures date instead from their alteration in and around the Early Proterozoic suture zones ca. 1.7 Ga. Later orogenies would add voluminous 1.4 Ga and rare 1.1 Ga Middle Proterozoic granites to complete Colorado's basement.
With each incoming Early Proterozoic magmatic arc came a typical package of 1.78-1.70 Ga arc rocks, as shown in the table below.
Arc-Derived Metamorphic Rocks of the Colorado Basement |
|||
| Raw Arc Materials | Original Rock Types | Metamorphic Products | Colorado Basement Example |
|
Volcanic and, to a lesser extent, intrusive igneous rocks, primarily of andesitic composition |
^Andesitic lava, breccia and tuff; gabbro | Gneiss, primarily of quartz and feldspar, sometimes with hornblende | Royal Mountain hornblende gneiss, Black Canyon Gneiss |
|
Aprons of primarily submarine volcaniclastic sediments |
Arkose sandstone and claystone | Mica and pelitic schist | Mount Evans schist |
|
Occasional fringing coral reefs |
Limestone | Marble | ?? |
|
Occasional mature sands |
Quartzose sandstone | Quartzite | Coal Creek Quartzite |
Occasional slivers of oceanic crust |
^Basalt, ribbon chert and turbidites | Gneiss, metachert and metaturbidites | Green Mountain Block |
|
Painted Wall |
With each collision, arc materials and other ocean floor edifices transferred to the building continent, often along with rumpled fragments of surrounding oceanic crust. Overthrusting during the arc-continent collisions buried arc rocks to depths of 11-16 km (7-10 mi) and severely folded and faulted them at high temperatures and pressures. Regional metamorphism accompanying nearly 200 Ma of protracted, recurring arc-continent collisions recrystallized raw arc materials into the suite of metamorphic basement rocks listed above. The dark 1.7 Ga Black Canyon Gneiss in the Painted Wall (right) in the north rim of the Black Canyon of the Gunnison is typical of Colorado's metamorphic basement.
 |
Buffalo Mountain |
Colorado's 1.8-1.7 Ga metamorphic rocks are shot through with slightly younger ~1.7 Ga granites and associated pegmatites, including the 1.72 Ga Boulder Creek batholith and the granites intruding the southern Gore Range at Buffalo Mountain (right). Similar granites occur throughout the southwestern United States. Some probably represent unerupted magma bodies within magmatic arcs or backarc basins. In their rise through the crust, some 1.7 Ga granites near Gunnison entrained 1.8 Ga zircons recycled from older, more mature crust of uncertain provenance. These inherited zircons imply that the Colorado Province accreted both magmatic arcs and older continental crust during its assembly. Where that older continental material originated remains a topic of considerable debate among Precambrian geologists.
In the Rockies, granites and other intrusive rocks with radiometric dates in the 1.660-1.790 Ga range are sometimes referred to collectively as the Routt Plutonic Suite. For convenience, I'll also use that nomenclature.
 |
Coal Creek Quartzite |
Mature light-colored cross-bedded quartz sands accumulated in substantial thicknesses in a number of places in the Colorado Province shortly after 1.7 Ga and now crop out as quartzite in several locations in Colorado and New Mexico. In the central Front Range, these include the Coal Creek quartzite (right) in ^Golden Gate Canyon State Park and a similar quartzite that contributes to nearby El Dorado Canyon's reputation for world-class rock climbing. These quartzites rest on granite of the 1.72 Ga Boulder Creek batholith intruded into even older pelitic (clay-based) metasediments. They imply a source region of elevated continental or felsic arc-generated crust located far overland from the site of deposition. How such crust found its way into the largely oceanic mix of the Colorado Province remains unclear, but the mix of oceanic arcs and continental fragments now forced together in the closing Banda Sea of Eastern Indonesia suggests some possibilities. Deformation of the Coal Creek quartzite in Golden Gate Canyon State Park probably relates to initial motion on the Ralston-Idaho Springs shear zone sometime after 1.7 Ga.
 |
Colorado Mineral Belt (blue) |
Colorado has been far removed from active plate boundaries since at least 1.6 Ga, but far-field stresses from distant plate interactions to the west, south and east continued to find expression in the Colorado Province mobile belt by exploiting persistent weaknesses left over from its assembly.
Never solidly healed, Early Proterozoic basement sutures and other structural defects have been reactivated many times, localizing subsequent deformation and magmatism throughout the Southwest to this day. Prominent examples include ongoing magmatism along Utah's St. George Lineament above the 1.75-1.70 Ga Mojave-Yavapai suture and along northern New Mexico's Jemez Lineament above the 1.65 Ga Yavapai-Mazatzal suture. Extensive Late Proterozoic rifting further cracked up the Colorado basement.
To what extent 1.7 Ga or 1.4 Ga structures have controlled the location of the NE-trending Colorado Mineral Belt remains under ^active investigation, but the important Idaho Springs-Ralston shear zone segment of the mineral belt seems to be a 1.7 Ga structure reactivated in the Berthoud Orogeny at 1.4 Ga and again during the Laramide Orogeny at ~72 Ma.
On the Geologic Highway Map of Colorado, the symbol "Xm" marks the 1.7 Ga metamorphics, while "Xg" marks the 1.7 Ga granites. Both units appear in light gray, the dominant map color throughout the Rockies. The Colorado Mineral Belt is easily found by following the diagonal band of dark maroon "Tki" Laramide intrusions and hot pink "Tmi" mid-Tertiary intrusions from Four Corners to Boulder.
Granites of Middle Proterozoic age (1.45-1.0 Ga, 16:16-18:40 h) intruding the Early Proterozoic basement completed the Colorado Province's Precambrian basement. NE-trending ductile shear zones and rifts ripped the basement as these granites were emplaced, while differential uplifts jostled basement blocks up and down across the state. Interestingly, none of these disturbances affected the Wyoming craton north of the Cheyenne Belt.
|
Mount Evans from Denver |
Colorado was well within the continental interior by 1.45 Ga (16:16 h), when a thus far unidentified convergent plate interaction of immense scale some 1,000 km to the south created ductile shear zones and emplaced numerous mid-crustal plutons (subsurface magma bodies) in a band stretching across the then closely apposed North American and Baltica (Northern European) plates. Intracontinental rifting accompanied these events. Far-field stresses related to transpression (combined transform and convergent plate motions) at the distant plate boundary also produced differential uplift of basement blocks throughout Colorado. This protracted event lasted 100 Ma, ending around 1.35 Ga (16:48 h).
Geoscientists now variably refer to this enigmatic episode as the Berthoud Orogeny, or simply as the 1.4 Ga event, the former after granite and shear zone exposures near Berthoud Pass on US40 in Colorado's Front Range. Indeed, in the Rockies and elsewhere in North America, intrusive rocks with radiometric dates in the 1.400-1.450 Ga range are often referred to collectively as the Berthoud Plutonic Suite. Berthoud granites are now known to dominate the basement across the midcontinental US and make up 20-30% of the basement throughout the Southwest as well. For convenience, I'll also use the Berthoud tag.
In the literature, Berthoud granites are also widely referred to as "A-type" and "anorogenic" granites because they're typically found without evidence of local mountain building. Such names are misleading, however. Berthoud granites are commonly found with evidence of significant regional tectonic stresses probably related to far-field plate boundary influences; in some areas, the Rockies included, these stresses have produced significant mountain belts and rifts.
Of the many Berthoud granites scattered through the crystalline Rockies, these stand out by virtue of size or beauty:
Some 1.4 Ga Granites in Colorado |
||||
| Granite | Age (Ga) | Location | Photo | Photo Notes |
1.422 |
North central Front Range |
|
Horseshoe Park from Trail Ridge in ^Rocky Mountain National Park. The domes adorning the Silver Plume walls of the park are common granite landforms. |
|
1.44 |
Central Front Range |
|
Mount Evans as seen from Denver. |
|
1.40 |
Northern Sawatch Range | n/a |
The St. Kevin granites rose along the ^Homestake shear zone, which shows signs of movement at both ~1.7 and ~1.4 Ga. |
|
The Mount Evans granite apparently rose along the Idaho Springs-Ralston shear zone, which appears to have been moving at the time but probably originated around 1.7 Ga. (Today, this shear zone, a segment of the Colorado Mineral Belt, is home to many of Colorado's richest mining districts.)
Later events exploited weaknesses left by the 1.4 Ga shear zones, as we'll see. To what extent 1.7 Ga structures located the 1.4 Ga granites and shear zones remains unclear, but that appears to have been the case at the Idaho Springs-Ralston and ^Homestake shear zones.
 |
Coarse pink 1.1 Ga Pikes Peak granite with green lichen near the summit of Devils Head, Rampart Range |
Another distant plate convergence far to the south at 1.2-1.1 Ga led to the emplacement of the 1.1 Ga Pikes Peak batholith (right), a huge granite body underpinning the southern Front Range and adjoining High Plains around Colorado Springs. They're common elsewhere in the southwestern US, but plutons of this age are rare in Colorado. (The ^Homestake shear zone and Mount Evans host two small occurrences, but the latter is probably a side lobe off the Pikes Peak batholith).
 |
Pikes Peak looms to the south from Garden of the Gods |
Little is known of the plate interaction triggering this intrusion, but the large Grenville Province of Texas and northern Mexico docked against the North American continent along the southern margin of the Mazatzal Province segment of the Colorado Province around this time. Some authors now refer to this event as the Grenville Orogeny, and for want of a better name, so will I.
Differential uplift of basement blocks accompanying the 1.45-1.1 Ga deformations brought rocks of widely varying crustal depths into juxtaposition along deep-rooted faults that may cut the entire lithosphere in places. These faults sometimes followed Early Proterozoic sutures and shear zones. Most of the basement rocks now exposed at the surface in Colorado appear to have formed at mid-crustal levels. Their mineral assemblages tell of time spent at depths of ~10 km or more prior to their arrival at the surface.
On the Geologic Highway Map of Colorado, 1.45-1.0 Ga granites carry the symbol "Yg" and appear in dark gray.
|
Four Corners from space, NASA |
The most visible legacies remaining from the black hole of time spanned by the Great Unconformity are the north- and northwest-trending tectonic grains of the Southern Rockies and the Colorado Plateau. This texture, clearly visible in any shaded relief or geologic map, follows a network of lithosphere-scale (extensional, normal) rift faults that accumulated throughout the western and midwestern United States from the Berthoud Orogeny at 1.4 Ga through the protracted breakup of the global supercontinent Rodinia between 0.9 and 0.6 Ga.
[diagram coming] |
Murphy et al., 2000, Fig.3 |
Western North America gained a Pacific coast when East Gondwana, a lesser supercontinent, rifted away from Rodinia's southwest margin sometime around 0.7 Ga. This long passive margin most closely approached Colorado in central Utah and remained stable through the Early Paleozoic. Inland, failed rifts would permanently scar the North American basement as far east as the midwestern United States.
After Late Proterozoic rifting subsided around or before 600 Ma, the region fell quiet for ~300 Ma. Its many dormant rift faults would later be reactivated in reverse (i.e., in compression rather than in tension) during Colorado's two most definitive Phanerozoic deformations—the Middle Pennsylvanian Ancestral Rocky Mountain Orogeny and the Latest Cretaceous to Eocene Laramide Orogeny. To this day, Late Proterozoic rift faults still influence the broad features of the Rocky Mountain and Colorado Plateau landscapes.
|
Afar Rifts |
At least twice during the Proterozoic, major rifting events driven by still poorly-understood mantle processes tried to pull apart the western two-thirds of the current North American continent—and Colorado along with it. (Large continents seem to draw such fire, as East Africa does today.) There were some close calls—notably, in the Uinta basin, the Midcontinent Rift and the Reelfoot Rift (the last the source of the ~M8 New Madrid, Missouri earthquakes of 1811-1812). Somehow, the western and midwestern United States managed to hold together against repeated rifting attempts, but the basement sustained severe damage in the process, perhaps through the full thickness of the lithosphere in places.
[diagram coming] |
Marshak et al., 2001, Fig. 2 |
The timing and number of Proterozoic rifting events remain a matter of debate. So far, four suspects have been proposed—a NE-SW extension at 1.5-1.3 Ga surrounding the Berthoud Orogeny, an E-W to NW-SE extension at 1.3-1.0 Ga coeval with the Grenville Orogeny and the opening of the Midcontinent Rift, an E-W to NW-SE extensions at 0.9-0.7 Ga attending the early breakup of Rodinia and again at ~0.7 Ga during East Gondwana's breakaway from Rodinia.
Whatever the timing, these deep-seated rifts left a rhombic network of intersecting north- and northwest-trending basement-penetrating normal (extensional) faults that would reactivate in reverse during the Ancestral Rocky Mountain and the Laramide deformations that have so strongly shaped modern Colorado.
 |
Afar tectonics, source USGS |
Current rift environments at various stages of development include
the Red Sea Rift, the Gulf of Aden and the Great Rift of East Africa (rig
Under
construction 
