Angelique Northcutt, Garrett Owen, Chris Perdue, Bo Price, Tory Rogers Geology of the Precambrian Sangre De Cristo Range and Picuris Mountains of New Mexico Angelique Northcutt, Garrett Owen, Chris Perdue, Bo Price, Tory Rogers

June 9th through the 19th we will be staying in Taos Ski Valley in New Mexico. Taos is located in the Sangre De Cristo Mountain Range. The Sangre De Cristo and the Picuris Mountains are divided by the Picuris-Pecos fault system. Where are we going?

Background -The Sangre de Cristo fault is a west dipping fault in New Mexico -It forms the border between the Sangre de Cristo Mountains and the San Luis Basin. -The Sangre de Cristo fault extends from poncho Pass, Colorado to near where we’ll be in Taos, NM.

Background -The section we will be focused on extends from the San Pedro Mesa creek south to the intersection with the Embudo Fault at Rio del Rancho which is about 8 km south of Taos. -The Sangre de Cristo is part of the more recent Rio Grande Rift. -It is a normal fault that moves less than 0.2 mm/year.

History -The Basement rocks in this location are Precambrian in age. -The rocks of Colorado and Northern New Mexico are juvenile Volcanic- Plutonic, Ocean Arc rocks that are approximately 1.78 to 1.65 billion years old. -In New Mexico these rocks have been assigned to the Yavapi and Mazatzal provinces. - These Rocks were deformed during 2 major Proterozoic orogenies.

History -First the Colorado Orogeny has a U-Pb date that goes through 1.78-1.75 Ga. -It was a prolonged thermotectonic episode resulting from collision, subduction, and continued convergence. -This occurred along the paleosuture known as the Cheyenne Belt along the Archean Wyoming Province.

History -Second the Berthoud Orogeny has a U-Pb date that started 1.45 Ga and went through 1.40 Ga. -This was a thermo tectonic episode that produced NE trending ductile shear zones and related folds. -The mechanism powering this Orogeny was granitic plutonism. -This Orogeny also formed many high grade metamorphic rocks through generally pervasive metamorphism.

Pre-Cambrian rocks -Isoclinal folding -East-west strike -Southward dips (60-70 degrees)

Folds -2 major anticlines and 2 major synclines -Wave lengths 1 to 2 miles -Doubly plunging (~20-30 degrees)

Folds Pilar Anticline -Widely displaced by the Pilar-Vadito tear fault -Slightly overturned to the north -Axial plane dipping south (60 degrees) Hondo Syncline -Overturned -Axial plane dips south (60-70 degrees) -Eastern segment plunges 30-40W -Western segment plunges 10-20E

Folds Copper Hill Anticline -Axial plane plunges 30-35W -N85W, 50W -Can be traced 9 miles east of Copper Hill Harding Syncline -Structural detail is obscure -Strikes E-W -Plunge ranges 25-17NE

Jointing -Abundant/wide variability in pre-Cambrian rocks -3 predominant sets -N10W to N10E (almost vertical) -N50W to N70W (dipping steeply NE) -N20E to N40E (dipping steeply SE)

Jointing -N-S joints followed almost exclusively by copper ore-bearing quartz veins -Mineralization after granitic intrusion -Harding pegmatite estimated 800myo -Indicates time of jointing

Stretched Pebbles -Conglomerate of the Vadito -Pebbles have average axial ratio of 1:2:3 -Shortest axis perpendicular to foliation -Some wedge-like shaped with apex oriented down the lineation -Pebbles in close contact have greater impacted shape

Three Major Fault Systems Picuris-Pecos fault system Embudo transfer fault Sangre de Cristo fault zone

Picuris-Pecos Fault System 84 km long fault system consisting of five parallel fault zones: Picuris-Pecos -Major crustal boundary juxtaposing two Proterozoic rock sequences: the Hondo Group and the Miranda Granite La Serna -East-down fault separating Miranda Granite and Picuris Formation Miranda -North-striking strike slip fault McGaffey -West-down branching fault splay Rio Grande del Rancho -Kilometer-wide, west-down fault zone - 84 km long - Near its northern end, south of Taos, the Picuris-Pecos fault system consists of five parallel north-striking fault zones. From west to east they are: 1) Picuris-Pecos: major crustal boundary that has experienced enough slip to juxtapose very different Proterozoic rock sequences. West of the fault is the Hondo group, which is a metasedimentary terrane of quartzite and schist. East is the medium-grained orangish Miranda granite. 2) La Serna: east down fault separating the Miranda granite to the west and the Picuris Formation to the east 3) Miranda: The Miranda fault zone cuts the Picuris Formation with numerous north-striking, strike-slip faults 4) McGaffey: East of the Miranda fault is a set of west-down branching fault splays, which make of the McGaffey fault. 5) Rio Grande del Rancho: Kilometer-wide, west down fault zone consisting of a complex family of branching faults

Embudo Fault Zone Sinistral, antithetic transfer zone which forms border between the Española Basin and the San Luis Basin 64 km long fault thought to be part of Jemez lineament Fault consists of two sections based on reversal of throw Strike of N60E Sinistral net slip rate of .15 mm/year -Most important because we will see this fault at field camp - Sinistral, antithetic transfer zone that forms the border between the west-tilted Espanola Basin and the east-tilted San Luis Basin. - 64 km long fault that is thought to be part of the Jemez lineament, which was a regional structural trend that may have been a zone of crustal weakness since the late Precambrian - The fault consists of two sections based on a reversal of throw - In Proterozoic bedrock units, the major strands of the fault are high-angle, brittle deformation zones that contain central areas of intense strain (breccia, fault gouge, closely spaced fractures) flanked by wide zones of fractured rock. The massive sandstone and conglomerate beds contain through-going fractures, but the more ductile interlayered shales are unfractured. - The strike of the Embudo fault is N60E, and it has a sinistral net slip rate of .15 mm/year.

Sangre de Cristo Fault Zone West-dipping normal fault that forms border between the Sangre de Cristo Mountains and the San Luis Basin Beginning of Sangre de Cristo Fault forms the terminus of the Embudo fault The southern area of the fault is divided into five sections. From north to south, the sections are: - San Pedro Mesa - Urraca - Questa The northern three strike north-south - Hondo Strikes N30W - Cañon Strikes N20E -The Sangre de Cristo fault is a west-dipping normal fault that forms the border between the Sangre de Cristo Mountains on the east and the San Luis Basin on the west. - The east-striking Embudo fault system swings northward and merges smoothly with the Sangre de Cristo fault. - The southern area of the fault is divided into five primary sections based on fault-trace complexity. From north to south, these are the San Pedro Mesa, Urraca, Questa, Hondo, and Cañon sections. The northern three strike generally north-south. The Hondo section strikes N30W, and the Cañon section strikes about N20E.

Aerial View of Faults in Taos Region

Pre-Cambrian Formations: The Hondo Group -The Pilar Formation -The Riconada Formation -The Ortega Formation The Vadito Formation

The Ortega Formation Estimated 2,500 ft. thick Gray to very light gray in color Mostly Quartzite Thin beds of sillmanite - kyanite gneiss Bands of schistose with muscovite Tourmaline

Riconada Formation -Richly micaceous foliated rocks -Consists of four distinct beds: Andalusite-biotite Hornfels bed- Muscovite and quartz matrix that contains biotite, nodular masses of quartz and andalusite Staurolite Schist and Gneiss Bed- Soft and light gray in color micaceous matrix speckled with biotite, contains staurolite crystals Staurolite Andalusite

The Riconada Formation Quartzite bed – Grayish white in color, contains glassy- white and translucent quartzite Muscovite-quartz-biotite-garnet Phyllite bed- Muscovite rich phyllite containing garnet crystals and biotite, sheen that ranges from pearly gray to greenish

Pilar Phyllite Formation Estimated 2,300 ft. thick Black to gray-black in color with a gray sheen Contains muscovite flakes Quartz veins Limonite masses

Vadito Formation Occupies the southern 1/3 of the Picuris Range -Named after the village of Vadito Best outcrops come from within a one-mile radius of the Harding Mine -Total thickness: 4,500 feet Comprised of a Lower Conglomerate Member and an Upper Schist Member

Marquenas Formation Total thickness: ~ 2,000 feet Composed of Quartz Conglomerate and Quartzite, Felsites and Meta-andesites Quartz conglomerate outcrops in a quarter-mile wide east-west belt near Picuris Canyon but grow sparser farther east Composed of gray coarse to pebbly Quartzite with fine-grained micaceous quartzite matrix Thickness of the Quartz Conglomerate: 500 - 1,000 feet

Marquenas Formation Felsites occur to the east and west of the canyon but appear to pinch out to the west along the Ortega-Vadito contact Composed of meta-rhyolite that grades into coarser granite representing partial replacement by granite Gray-white to pink-white and has a dense felsitic texture with micas and feldspars present Thickness of the Felsites: 50-100 feet

Marquenas Formation Meta-andesites occur abundantly to the south and east of the Harding mine Greenish-gray to gray-black in color Hornblende occurs in all of these rocks as prisms and give the variable darkness in color to them Rocks show strong pleochroism in thin-section. Represent thin-layered volcanic material of dacitic and andesitic composition

Schist Member Consists of a schist and phyllite composed of quartz-muscovite and a quartz-biotite granulite The schist is similar to the underlying conglomerate quartzite with more densely disseminated flakes of muscovite Phyllite is a lustrous, silvery-gray rock with stubby biotite porphyroblasts Granulite is fine-grained, sandy, and crudely foliated with micaceous surfaces with flakes of biotite Thickness: At least 1,250 feet and no more than 2,500 feet

Amphibolites The amphibolites found in the Vadito Formation are split into two zones based on lithology: -One of the zones occurs in the Lower Conglomerate and one occurs in the Upper Schist These amphibolites show effects of intrusion by granitic and pegmatitic magma Thickness: 750 feet in Lower Conglomerate, and 1,250 feet in Upper Schist

Questions?