The Rio Grande Rift
After compression built the Southern Rocky Mountains, Neogene extension split the range along its axis, opening the 1,000-kilometre-long Rio Grande Rift. The rift has been active ever since, creating dramatic mountains, exposing mineral riches, and burying the landscape with volcanic rocks used by the region’s native inhabitants to build their settlements.
Although continental rifts are familiar components of plate tectonics, why one rift segment exhibits very different characteristics than another remains puzzling. The Rio Grande Rift, thanks to its convenient location and three subregions displaying distinctive characteristics, has long served as a natural laboratory to study the roles that a variety of diverse factors – including the regional stress field, asthenospheric convection patterns, crustal heterogeneities, and variations in lithospheric thickness – play in rift formation and evolution.
The Rio Grande Rift trends north–south from northern Colorado to southern New Mexico. The Colorado portion, which runs through the heart of the Southern Rockies, constitutes the rift’s northern subregion. It consists of a single, dominant half graben, with accommodation zones that transfer extension east or west from one en echelon rift segment to another. An active normal fault bounds one side of each rift valley, and rift-floor volcanism is minimal. But the rift character changes significantly south of the Colorado border. In this central subregion, from Taos to Albuquerque, New Mexico, the Colorado Plateau forms the rift’s western border. The central rift is crossed by a north-east–south-west-trending zone of voluminous, bimodal magmatism known as the Jemez Lineament.
Here the strain is the highest of the entire rift, and strike-slip faults dominate over normal faults. In the southern subregion, south of Albuquerque, the primacy of north–south-trending normal faults returns, and valley-floor volcanism diminishes. But here, multiple half grabens parallel one another. South of Las Cruces, New Mexico, the Rio Grande Rift is subsumed into the broader Basin and Range province, which exhibits Neogene east–west extension from California eastward to western Texas.
Total extension diminishes from 50% in the Rio Grande Rift’s southern subregion to less than 8–12% in the north. This variation has been used as evidence for northward rift propagation. But recent low temperature thermochronologic data indicates that faulting began simultaneously at about 25 million years ago along the rift’s entire length. One potential recipe to produce simultaneous rift onset and northward-diminishing strain is the clockwise rotation of a rigid Colorado Plateau block. But a recent geodetic study reveals that the Colorado Plateau doesn’t behave rigidly, indicating that further research is needed to tease out why the Rio Grande Rift formed. Several recent studies agree that the concentration of volcanism in the central rift is caused by small-scale mantle convection triggered by a step in lithospheric thickness at the south-eastern edge of the Colorado Plateau, a step inherited from a Proterozoic suture zone.
Mineral Bounty on the Roof of the Rockies
The 7.5-hour road trip from Denver to Albuquerque provides a tour of the Rio Grande Rift’s scenic and historic diversity, from the northern rift’s mining legacy to that of native cultures farther south. From Denver, Interstate Highway 70 climbs west into the Southern Rockies. The highway burrows under the Continental Divide, the drainage divide separating Atlantic-flowing rivers from Pacific-flowing ones, in the Eisenhower Tunnel. From the tunnel’s west portal, you enjoy a sweeping view down to the Blue River Valley, the northernmost and narrowest Rio Grande Rift half graben, and the fault-bounded Gore and Tenmile ranges that soar 1,200 metres above its floor.
The highway then descends to Dillon, a ski hub situated on the Blue River, which flows north along the rift to join the Pacific-bound Colorado River. An accommodation zone that transfers extension westward to the Upper Arkansas Valley – the next en echelon rift valley to the south – lies just south of Dillon. To continue your Rio Grande Rift tour, cross this accommodation zone via Colorado Highway 91, which climbs back up to the Continental Divide at Climax, the world’s largest molybdenum mine. The Climax ore formed 33–24 million years ago during the transition from compression to Rio Grande Rift extension.
The 20-kilometre descent from Climax brings you to the historic mining town of Leadville, located at the Arkansas River headwaters. Leadville’s mining district, among the world’s largest lead-zinc-silver deposits, has produced more than $5 billion in ore and is nicknamed ‘the richest 12 miles in the world’. Leadville’s ore formed 39 million years ago when magmatic fluids interacted with Palaeozoic limestones. Subsequent rifting exposed the ores. Prospectors fortuitously struck a rich silver lode here in 1878, the same year the US Congress passed the Bland–Allison Act, which required the US Treasury to buy large quantities of silver dollars. Leadville’s population swelled to 25,000, and the town hosted 100 saloons, a dozen gambling houses and the Tabor Opera House – the largest west of the Mississippi. Wandering through Leadville’s National Historic Landmark District of Victorian buildings offers glimpses of the town’s glory days. You can tour the mining district east of town and view exhibits at Leadville’s National Mining Museum, which traces both local mining history and the broader evolution of mining technology.
Glacial Floods, Wild White-water, and Towering Sand Dunes
Thirty of Colorado’s 54 famed ‘14’ers’ (peaks taller than 14,000 feet, or 4,268 metres) line the Rio Grande Rift, including the five highest peaks in the entire 4,800-kilometre-long Rocky Mountain chain, adding immeasurably to the rift’s scenic splendour. The Sawatch Range, which hosts 15 of those 14’ers, bounds the active normal fault, west of the Upper Arkansas Valley. This mountain rampart, towering 2,000 metres above the valley floor, is visible on your trip south along the rift axis. The Sawatch Range was heavily glaciated during the Pleistocene; several of its east-flowing glaciers protruded into the south-trending rift valley, depositing impressive moraine complexes on the valley floor. Just south of Mount Elbert, the Rockies’ highest peak, the Twin Lakes are cradled by one such complex, and a recessional moraine separates the lakes.
The presence of glaciers and their voluminous outwash gravel on the valley’s western side pushed the Arkansas River hard against the eastern wall, where the river incised several narrow canyons into Precambrian granite. Those canyons make the Arkansas the most popular white-water rafting river in the United States, with more than 175,000 visitors annually. The canyons and the surrounding rift-flank uplands, which are popular for mountain biking, were declared Browns Canyon National Monument in 2015. The cutting of these canyons was accelerated by two glacial lake outburst floods 18,000 years ago. The Clear Creek and Pine Creek glaciers, which headed south of Twin Lakes, once extended across the entire rift valley, blocking the Arkansas River and impounding a 23-kilometre-long, 180-metre-deep lake. Twice the glacial dams failed, sending 21,000 m³/s torrents down the river. The car-size boulders littering the canyons today testify to the power of those floods and add difficulty to the river’s famous white-water runs.
After following the axis of the Rio Grande Rift for 100 kilometres, the Arkansas River takes an abrupt eastward turn at Salida, exiting the rift. This is because the Poncha Pass accommodation zone has raised an east–west ridge here. Highway 285 climbs over this ridge and descends south-east into the next en echelon basin, the San Luis Valley, down which the rift’s namesake river, the Rio Grande, flows. The active normal fault and its accompanying mountain range, the Sangre de Cristo, bound the San Luis Valley on the east. The range boasts two clusters of 14’ers, onnected by a ridge as much as 1,500 metres lower in elevation. Prevailing westerly winds blowing across the valley are funnelled in a venturi through the gap between the 14’ers, depositing the copious sand they carry on the range’s west flank to form North America’s largest sand dunes. Great Sand Dunes National Park is open yearround, but a visit in late spring, when meltwater from the snow-capped peaks swells the dune-crossing Medano Creek, is especially scenic.
Pueblos and Tuff
South of the sand dunes, roads traverse both the east and west sides of the valley. Whichever road you choose, you will spot the first substantial volcanic rift features – basalt lava flows and several cinder cones – near the New Mexico state line. They herald your arrival in the rift’s volcanically active central subregion. If travelling the eastern road, Questa is the first significant New Mexico town you pass. Questa hosts a Climax-type molybdenum deposit that was mined from 1916–2014. If you follow the western road, turn east onto U.S. Highway 64 at Tres Piedras, bound for Taos. A worthwhile stop is the highway’s crossing of the Rio Grande River on the Gorge Bridge 16 kilometres west of Taos. This area’s flat, arid landscape gives no hint that the river has cut an impressive, 180-metre-deep gorge into a stack of basalt flows until you reach the rim.
A visit to the Taos Pueblo is a quintessential Rio Grande Rift experience. The five-story adobe buildings consist of mudbrick walls several feet thick veneered by mud plaster. Members of the Taos tribe have lived in the Pueblo since about 1000 CE, making it the oldest, continuously inhabited community in the United States. Accounts by Spanish conquistadors who visited the Pueblo in 1540 CE reveal that it has changed little since that time. The conquistadors imagined the Pueblo to be one of the fabled Seven Cities of Cibola; they were disappointed that it lacked the huge troves of gold the story described.
Bandelier National Monument, 115 kilometres south-west of Taos, reveals geology’s fundamental role in the development of central rift civilisations. The national monument’s Frijoles Canyon is lined with tall cliffs of Bandelier Tuff, which erupted from the Valles Caldera 25 kilometres to the west 1.25 million years ago. Dwellings carved in the soft tuff by Ancestral Puebloan people between about 1150–1550 CE dot the cliffs, as do circular holes drilled to anchor the roof poles of cliff-side pueblos. Caldera-produced obsidian, used for tools and arrowheads, was a major trade good the inhabitants exchanged with cultures across the American Southwest.
The Urban Rift
Santa Fe, New Mexico’s capital, lies 65 kilometres south-east of Bandelier. It is known as one of the world’s great art cities, so much so that it belongs to UNESCO’s Creative Cities Network, which fosters international collaboration between cities that have invested in creativity to drive sustainable urban development. Santa Fe has been inhabited by Tanoan Pueblo people since 900 CE. The Spanish made it the capital of Nuevo Mexico in 1610, making it the oldest capital city in the US. Santa Fe’s mild climate makes it a year-round tourist destination. It is home to many historical buildings including the Cathedral Basilica of Saint Francis of Assisi, built by Archbishop Jean Baptiste Lamy between 1869 and 1886 on the site of an older adobe church. Visiting in September is especially popular because the aspen trees that grow in the flanking Sangre de Cristo Mountains glow orange and gold, adding yet more colours to the city’s already rich palette.
New Mexico’s largest city, Albuquerque, lies 100 km south-west of Santa Fe and serves as the gateway to the rift’s southern subregion. Albuquerque is renowned for its colourful International Balloon Fiesta, the world’s largest gathering of hot air balloons, held each October. The aerial tram ascent of the Sandia Mountains, the fault-controlled mountain rampart that rises 1,600 metres above the city’s eastern neighbourhoods, provides breathtaking views down to the Rio Grande River. The nearby Petroglyph National Monument protects 24,000 petroglyphs that have been carved into the basalt rimrock on Albuquerque’s West Mesa, which marks the Rio Grande Rift–Colorado Plateau transition.
Socorro, 125 kilometres south of Albuquerque and near the rift’s end, is home to the New Mexico Institute of Technology, the New Mexico Geological Survey, and the Socorro Magma Body. This latter is a still-molten sill at 20 kilometres depth that triggers frequent, small seismic tremors due to magma inflation and is a potential source of geothermal energy. You reach the southern end of the Rio Grande Rift 100 kilometres farther south at Las Cruces, New Mexico’s second-largest city. The Organ Mountains, remnants of a 32-million-year-old caldera formed during the transition from compression to rifting, forms the city’s impressive eastern backdrop. It is a fitting culmination to your 1,000-kilometre tour of the impressive scenery and natural resource bounty produced by the tectonic processes that shaped North America’s largest rift.