Cenozoic Geology of Northern NM

Flashcards about the Cenozoic Geology of Northern New Mexico

What is the approximate timing of the Laramide Orogeny?

70 Ma - 40 Ma, from the Cretaceous to the Eocene.

What makes the Rocky Mountains different compared to classic mountain ranges like the Alp/Himalayas/Andes etc.?

The Rocky Mountains are an interior plate mountain building event formed by subduction from the west and magmatic uplift over 1,000 miles from the nearest subduction zone. What is unique is that Precambrian basement rock was uplifted and thrust (Lewis Thrust Fault) upon younger Cretaceous strata.

What is the traditional ‘old’ model (used for the past 40+ years) to the explain the Laramide Orogeny and the Rocky Mountains?

The old model asserts that the Farallon plate was continuously subducted beneath the North American plate as a shallow slab beginning 140 Ma. It does not consider intra-oceanic arc terraces between North America and the Farallon Plate.

Briefly describe the general concept of the new ‘Hit & Run’ model for the Laramide Orogeny.

The new model suggests there were two intra-oceanic arc terranes, the MEZ and ANG, which converged, “hit “, the North American Plate around 100-85 Ma. These arc terranes then moved, “ran”, northward along a transform fault 85-55 Ma. This series of events caused deformation of the North American Plate interior and formed the Rocky Mountains before the Farallon plate even began to subduct.

What is the approximate timing and evolution of the Rio Grande Rift, how is this rift related to the Basin & Range Province?

The basin and range formed during post-Laramide orogeny extension and volcanism. The Farallon plate begins subducting beneath North America around 50 Ma and is mostly subducted by 30 Ma. The Farallon plate delaminated and sank into the mantle. The melting of the Farallon plate led to igneous uplift, extension of the North American plate, rifting, volcanism, and formation of a horst and graben system, forming the Basin and Range in New Mexico. The eastern edge of the Basin and Range is the Rio Grande Rift, which is asymmetrical. The Colorado Plateau is the high ground between the Basin and Range and the Rio Grande Rift.

What is the Jemez Lineament? Which volcanic features formed along the Jemez Lineament?

The Jemez Lineament is a geologic alignment of 12 Ma and younger volcanic eruption deposits oriented SW to NE. It contains the Valles Caldera, Keres Group, Cerro del Rio Volcanics, Bandelier Tuff, and post-caldera volcanic domes.

What are ‘grabens’, what types of faults (normal/reverse) result in their formation, and what is one interesting feature about the grabens in the Rio Grande Rift?

The grabens are normal faults that form at the margins of rifts from extensional forces in the crust. Stretching of the crust thins it out and allows for higher heat flow and faulting. An interesting feature of the grabens in the Rio Grande Rift is that they are asymmetrical, tilted half-grabens.

Why are the Albuquerque, Española, and San Luis basins not in a straight line along the rift?

The basins are not a straight line because they were offset to the east as the fault continues to displace over 5 miles. As rifting continues, offers are formed between the basins as they are being filled with eroded sediment.

How are features like the Ortiz Mountains and Cerrillos Hills related to the Laramide Orogeny?

The Ortiz Mountains and Cerrillos Hills were formed at the end of the Laramide orogeny as melted Farallon plate crust rose to the surface, causing episodes of volcanic eruptions. These eruptions formed the volcanic centers of the Ortiz Mountains and Cerrillos Hills.

What type of fault is the Pajarito Fault?

The Pajarito Fault appears to be a normal fault with the hanging wall sliding down the surface of the fault. However, the topography appears inverted with hanging wall strata rising above the correlated strata in the footwall. This may be due to the collapse of a previously uplifted section of crust, which currently forms low-elevation basins in the region, such as the Espanola basin.

What does the term ‘aulacogen’ mean?

An aulacogen is a failed rift which results from continued extension of continental crust.

What type of volcanic rock dominates the Taos Plateau? What is the name of this volcanic unit, and when did these rocks form?

The dominant rocky type in the Taos Plateau is olivine-rich tholeiite basalt. The geologic unit is called Servilleta Basalt (Tertiary). The basalt is dark grey and vesicular. 600 ft of the Serville basalt was deposited during shield volcano eruptions that spanned over 2 million years between 4.8 Ma and 2.8 Ma. The lava spread laterally and crystallized as thin horizontal layers at the surface.

What other types of volcanic rocks are found along the Rio Grande Rift?

Other types of volcanic rocks found along the Rio Grande gorge include mafic basalts, intermediate andesites and dacites, as well as silica-rich rhyolites. Several volcanoes in the vicinity are andesite volcanoes, including Cerro de la Olla, Cerro Montoso, and Cerro de los Taos. Others are dacite domes and lava domes.

What is the stratigraphic name for the sediments for the rift-fill sediments? What particular sedimentary environment do these sediments represent?

In the Rio Grande gorge, the rift fill sediments are called the Santa Fe Group. Over the past 25 million years, most of the alluvium was eroded from nearby mountains, alluvial fans extended from nearby mountains, and transported clay, sand, and gravel into the rift basin. The alluvium suggests an alluvial fan depositional envrinoment.