Tenmile Canyon and Range—Post-Laramide Creations of the Rio Grande Rift

Tenmile Canyon is an impressive north northeast-trending fault-controlled gash running nearly 3,000' deep through the shared Precambrian core of the north-trending Laramide Gore-Tenmile uplift. The canyon hosts I-70 between Frisco and Wheeler Junction, where CO91 comes in at Copper Mountain. Travelers on that stretch of the interstate can hardly miss it, but only those able to look straight up through the open top of a convertible can fully appreciate its spectacular scale.

The faulted west slope of the Tenmile Range forms the canyon's precipitous east wall; lower peaks of the southern Gore Range, its less sharply defined west wall. In the aerial photo at right, the mouth of Tenmile Canyon opens between Royal Mountain (10,501') on the left and Wichita Mountain (10,855') on the right as seen from the south. The town just beyond the mouth is Frisco. Royal Mountain is the closer of the two guardians of the portal and belongs to the Tenmile Range. Wichita is part of the Gore Range.

Tenmile Canyon marks the geographic boundary between the Gore Range on the northwest and the smaller but somewhat higher Tenmile Range on the southeast. Geologically, both ranges are carved from the same uplifted Precambrian basement block, but their post-Laramide tectonic histories differ substantially. The Gore Range is for the most part a classic Laramide uplift of faulted anticline structure (although it too has its complications), while the Tenmile Range is both a Laramide uplift and a major post-Laramide fault-block mountain of the Basin and Range Province. The Tenmile Canyon and Range owe much of their grandeur to the passing of the Rio Grande Rift at around 20 Ma.

The canyon walls expose 1.7 Ga oceanic metasedimentary and metavolcanic rocks likely of island arc origin. These rocks occur in a highly deformed but distinct stratigraphy, with the oldest and deepest rocks exposed at the outlet. In places, the metamorphic banding patterns seen in the west wall mark relict bedding planes, as in the road cut at I-70 Milepost 199. Pegmatite veins of varying widths cut both walls. Within the canyon, there are no younger rocks to help date the movement of the canyon fault.

The Tenmile Canyon Fault

A substantial vertical offset between the rocks exposed in its east and west walls leaves no doubt that Tenmile Canyon follows a fault, but the timing of its movements remains controversial. Structurally, the 1.7 Ga metasedimentary rocks of the east wall are much higher than those on the west. Slickensides marking the variably exposed fault surface can be seen in the east wall near I-70 Milepost 196. A major canyon fault also explains the improbable course of Tenmile Creek across the common Gore-Tenmile Precambrian core.

South of Tenmile Canyon, the steep west slope of the Tenmile Range is the scarp of the Mosquito Fault, a major north-striking post-Laramide normal fault forming the east shoulder of the Rio Grande Rift. There the Mosquito Fault has accumulated ~30,000' of vertical movement (down to the west) since early Miocene time, ~20Ma. The resulting rift graben (down-dropped fault block) sits between Fremont Pass (CO91, 11,318') and Tennessee Pass (CO 24, 10,404').

Some investigators consider the fault in Tenmile Canyon a northern extension of the normal Mosquito Fault with late Tertiary movement. The canyon's trend and observed vertical offset are both consistent with that theory. Others, however, consider the canyon fault a Laramide structure. Based on what little information I can find on the subject, I've adopted the former stance in this article.

Tenmile Creek

As it tumbles from headwaters near Climax and Fremont Pass (11,318') on CO91, Tenmile Creek follows the Mosquito Fault at least as far as the head of Tenmile Canyon. It follows a fault through the canyon as well, but the identity of the canyon fault remains unclear.

In its present form, the creek hardly looks capable of cutting a defile like Tenmile Canyon. It was no doubt more formidable in the wetter Pliocene climate and the during the subsequent Pleistocene glaciations, but it was not likely to have been a major river at any time in its history. Nor was it likely to have been an antecedent stream across the common Gore-Tenmile Precambrian core, as the Arkansas was across the southern Front Range or the Gunnison across the Gunnison Uplift. Tenmile Creek clearly got lots of help from the canyon fault, which softened up the otherwise highly resistant Precambrian rock along its course.

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Tenmile Range

The Precambrian core of the Tenmile Range is continuous across Tenmile Canyon with the core of the southern Gore Range, but Tenmile isn't just another Laramide faulted anticline. It no doubt rose in Laramide time, but it probably rose more, via normal faulting and mantle-related uplift, with the east shoulder of the Rio Grande Rift in late Tertiary time, more than 20 Ma after the main pulse of Laramide deformation had ended. The Mosquito Fault separating it from the sedimentary strata of the Late Paleozoic Minturn and Maroon Formations along its west flank is not a Laramide thrust or reverse fault, but a post-Laramide normal fault with ~30,000' of vertical movement to its credit. On the east flank of the Tenmile Range, gently folded and tilted Cambrian through Cretaceous sedimentary strata rest unconformably on the Tenmile Precambrian core. Their structural relationships to the core and to one another remain uncertain, but these densely intruded sediments played a pivotal role in the mining history of the Tenmile Range.

Geographically, the Tenmile Range sports ten peaks named, from north to south, Peak 1 (right) through Peak 10. The prosaic names notwithstanding, all ten are worthy summits above 12,600'. Paramount Studios could easily have modeled their famous logo after Peak 1 (12,805'), a near-perfect pyramid in shape—especially when viewed as at right from nearby Buffalo Mountain.

The highest peak at the north end of the range corresponds to Peak 2 but more often goes by the name Tenmile Peak (12,933'). The east slope of Peak 8 (12,987') is home to Breckenridge, a rich 1860s mining camp turned rich ski resort.

At the range's higher south end, just south of Peak 10 (13,633'), are several Fourteeners clustered along the Continental Divide, which arbitrarily separates the southern Tenmile Range from the northern Mosquito Range. Among these giants are Quandary Peak (14,265'), the only Tenmile Fourteener, and Mt. Lincoln (14,286') on the Mosquito side.

Location, Location, Location

As already noted, the Tenmile Range stands high on the east shoulder of the Rio Grande Rift (RGR). No less significantly, it also stands directly astride the Colorado Mineral Belt (CMB). This position near the intersection of the RGR and CMB—by far Colorado's two most important structural and magmatic trends—left the Tenmile Range a sitting duck for intrusive attack from below. In the latest Cretaceous and early Tertiary, it drew Laramide intrusions in prodigious volume and took several notable mid-Tertiary hits as well, the latter probably related to active rifting along the RGR. Vein filling and contact metamorphism attending the numerous intrusions brought great mineral wealth to the entire area, particularly in the heavily intruded sedimentary cover of the southeast flank. In the years following 1859, mining districts like Breckenridge, Climax, Fairplay and Frisco quickly sprang up along the flanks of the Tenmile Range to harvest its bounty of gold, silver, copper, lead, zinc and molybdenum. (By weight, most of pay came in the form of placer gold and molybdenum ore.) The Gore Range was too far off both the CMB and the RGR to get in on much of this action.

In Pliocene and Pleistocene time, the Tenmile Range saw substantially less glaciation than the Gore Range (right) and now has a much less craggy look to show for it. Whether it stood lower than the Gore then, or stood off the prevailing easterly storm tracks of the time, or received more shielding from higher peaks to the east, is unclear, but to this day, the resulting topographic difference is striking, especially from the air. Otherwise, the higher reaches of the Tenmile and Gore Ranges are physically similar, having been exhumed from similar Precambrian rock.

For better or for worse, its location at the intersection of the CMB and the RGR and its smoother topography destined the Tenmile Range to attract much more mining, residential, resort and recreational development than the Gore Range, much of which remains designated wilderness to this day.

Royal Mountain Remembrance

Royal Mountain (10,501') is a very steep but rewarding 1,500' climb from Frisco en route to Peak 1. This crag is a dramatic outcrop of the same 1.7 Ga horneblende gneiss of metasedimentary and metavolcanic origin that makes up much of the east wall of Tenmile Canyon.

The 1.5 mile trail to the top climbs nearly 1,000' in the last half mile. When I first hiked it in a light snowfall one February morning in 2002, the temperature was around 15°F. The dogs ran circles around me up and down the trail while I took the steep section one step at a time, huffing all the way. I had on 4 layers of high-tech outerwear; they were in the same coats they wear in the condo. I had on fancy Sorel boots over wool and polypro socks; they were in their bare feet. Never once did I see either of them slow down to catch their breath or shiver at the cold.

Our Belgian sheepdog Orca (on the right) was 14 years old at the time. She died of old age a year later. We miss her big black and white smile. Imagine the evolutionary engineering behind a creature as hardy and versatile and noble and loving as a dog.

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An Untidy Divide

For no obvious reason, the Continental Divide in central Colorado jumps west from the central Front Range to the northern Mosquito Range along a very high and rather peculiar southwest-trending ridge that precisely follows the Colorado Mineral Belt (CMB). The same ridge also happens to form the northwest wall of South Park.

Laramide intrusions and adjacent altered Cambrian to Cretaceous sedimentary strata overlying the east flank of the southern Tenmile Range make up much of the ridge, with lesser outcrops of crystalline Precambrian rock. Boreas Mountain (13,082', top right) is part of a very large Laramide intrusion straddling this portion of the divide. Also on the divide here are Mt. Guyot (13,370') and Mt. Baldy (13,684'), both prominent Summit County landmarks. Where the ridge joins the Front Range, we have a cluster of Fourteeners, including Torreys Peak (14,267), Grays Peak (14,270'), Mt. Evans (14,264') and Mt. Bierstadt (14,060'). Where it meets the southern Tenmile and northern Mosquito Ranges, we have another tight cluster of Fourteeners, including Quandary Peak (14,265'), Mt. Lincoln (14,286') and Mt. Cameron (14,238') among others.

Something Big

Something big's clearly happening here, but why this ridge stands so high remains unclear, at least to me. Structurally, it's not part of any Laramide uplift, and it's east of the main uplifts associated with the Rio Grande Rift (RGR). The CMB's clearly implicated, but how? CMB-related intrusions have added significantly to the volume of the upper crust along the ridge. Geophysical methods show that unusually hot upper mantle still underlies the CMB, and large magma bodies related to Laramide and post-Laramide CMB intrusions may well still reside in the lower crust here. These hot materials may buoy up the ridge. Alternatively, the rocks that make up the ridge may be particularly resistant, perhaps due at least in part to contact metamorphism. Of course, these factors may well be acting in concert, and others may also play a role in this spectacular if untidy piece of Rocky Mountain topography.

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Tenmile Gallery
Tenmile Canyon
	North end of Tenmile Canyon: The 1st frame is an aerial view looking north toward Frisco and the mouth of Tenmile Canyon between Royal Mountain (10,501', left) and Wichita Mountain (10,855', right). From Frisco, the canyon runs south southwest to Wheeler Junction and Copper Mountain—hence the talk of east and west walls. The 2nd frame shows the sheer east wall of the canyon at its mouth. This wall is in fact the scarp of the canyon fault responsible for the canyon's location. Here at the outlet, the east wall rises about 1,400' above the canyon floor, but further south, the relief reaches almost 3,000'. The 3rd frame shows the canyon's east wall en face in fall color from the frontage road at Officer's Gulch. The 1.7 Ga oceanic metasedimentary and metavolcanic rocks rocks exposed here are structurally higher than those across the canyon.



	South end of Tenmile Canyon: Peak 9 (13,795'), Peak 10 (13,633') and Crystal Peak (13,852') of the southern Tenmile Range ring a large cirque on the distant skyline to the south over the paved Summit County Recreation Trail connecting Frisco and the Copper Mountain (12,441') ski resort (domal peak on the right, 1st frame). From here, the path and I-70 both lead northwest to Vail Pass (10,666'). North-flowing Tenmile Creek enters the deepest part of the canyon just north of here. There's been no glaciation within the depths of the canyon, but the big cirque on Crystal Peak proves that glaciers and glacial outwash were close at hand in the Pleistocene.

	Copper Mountain: Copper Mountain sits at the south end of the canyon. Its ski slopes run down the southernmost tip of the geographic Gore Range, which continues west as far as Vail. The Precambrian core of the Gore Range continues south through the Tenmile Range across the canyon.
	Map check: John Barr stops to consult our well-worn copy of the Colorado Geologic Highway Map—a must for anyone interested in the rocks behind and beneath Colorado's incomparable scenery. Much can be learned from careful study of this map alone, but the excellent Roadside Geology of Colorado is an excellent companion reference, particularly the updated 2002 2nd edition. Andrew Taylor's Guide to the Geology of Colorado is another invaluable tool featuring annotated outcrop photographs to simplify field identifications.

	Optical illusion: The long, smooth downhill ride from Copper Mountain back to Frisco was a blast; hence the grin on my face. In that direction, there could be no mistake—the path had a substantial grade. On the way up, however, the bike path looked like a gentle grade even though it felt like the huff that it truly was. One particularly deceiving uphill stretch along a lake actually looked like a downhill but proved to be a fast downhill coast on the way back down. Next time, I'll believe my quads, not my eyes.
Northern Tenmile Range
	Tenmile Range from Boreas Pass: The 1st frame, a northwest view from Boreas Pass (11,481'), lays out most of the length of the Tenmile Range. Peaks 1-9 are seen here from right to left. Compared to the Gore Range (2nd frame), the rounded summits of the Tenmile Range show only mild glacial sculpting.

	Peak 1: Always striking, Peak 1 (12,805') of the northern Tenmile Range is particularly beautiful in morning light, as seen here from the base of Buffalo Mountain above Silverthorne. This view of Peak 1 from the deck of our little Silverthorne condo was my No. 1 reason for buying there, and I have yet to tire of it. The 2nd frame shows Peak 1, Tenmile Peak (12,933') and a portion of the northern Tenmile Range, here looking south from the southeast face of Buffalo Mountain. Taken from the scenic overlook on westbound I-70 between Silverthorne and Frisco (Exits 205 and 203), the 3rd frame looks southwest at Peak 1 and Tenmile Peak, the latter barely visible to the left of Peak 1's summit. Fall colors brighten the lower slopes south of Frisco (right center), which nestles up against the mountain front beneath the rocky face of Royal Mountain at right center.


	Royal Mountain: The 1st frame is an aerial view looking north toward the mouth of Tenmile Canyon between Royal Mountain (10,501') and Wichita Mountain (10,855'). Royal Mountain is the closer of the two. Now a resort community and commercial center of sorts, the old mining and smelting town of Frisco stands in the foreground. In the 2nd frame, taken from the southeast face of nearby Buffalo Mountain (12,777'), we see Frisco and Royal Mountain from the north. Wichita Mountain is hidden from view here. Above and to its right are Peak 1 (12,805') and Tenmile Peak (12,933') of the Tenmile Range. Royal Mountain sits at the north end of Tenmile Canyon, which continues out of sight to the right in this view. The 3rd frame shows Royal Mountain from the bank of Tenmile Creek at the west end of Frisco, CO. In the 4th frame, also taken from Frisco, Royal Mountain stands at the near end of the tall, steep scarp of the canyon fault. The 5th frame, taken from the frontage road along Dillon Reservoir, shows Royal Mountain in proper pecking order in last light. Snow-dusted, triangular Peak 1 stands at skyline above it. The forested summit along the ridge between Royal Mountain and Peak 1 is Mt. Victoria (11,785'), known for its stunning Summit County views. In the 6th frame, the craggy metavolcanic hornblende gneiss atop Royal Mountain affords a dizzying perch overlooking the southern end of the Gore Range, the north end of Tenmile Canyon The 7th frame shows I-70 with ant-size toy cars whizzing by 1,400' below. The trail up to Royal Mountain affords many fine views of the Continental Divide to the east and southeast. In the 8th frame, the divide to the east is partially enshrouded by an incoming winter storm front.

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References

In addition to the references cited on the home page and in the supporting articles, this article relies on the following sources, in alphabetical order by first author:

Colorado Geology Photojournals

A Tribute to Colorado's Physical Past and Present

Right: Trees and snow mark major Laramide uplifts in green and white while salmon pink marks the Colorado Plateau in this true-color satellite image of Colorado and surrounding states, courtesy NASA, ^Visible Earth

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