Earth and Planetary Sciences ETDs

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The Laron Mountains are & westward-tilted fault block and an important western bounding structure of the Rio Grande depression in the Albuquerque-Belen basin. The main massif is a complex of both igneous and metamorphic Precambrian rocks. It consists of a thick sequence of steeply east-dipping quartzite & and schists which are predominantly a product of low- to medium- grade regional Precambrian metamorphism. Granitic gneiss is ptygmatically folded with abundant schist remnant in a large area on the eastern flank of the mountains and represents an ultrametamorphic product of anatexis. All metamorphic rocks have been invaded by two later Precambrian granites. The second of these intrusions has produced much migmatite in the northern part of the mountains.

The mountains are bounded by a roughly triangular-shaped system of faults or fault zones. On the west and southwest is the Ladron fault, and on the east and southeast the Cerro Colorado fault zone. A buried fault zone on the north serves to complete the triangle.

The Ladron fault is hiqh-angle normal along which down-faulted Paleozoic limestones form an impressive north-trending hogback. The Cerro Colorado fault zone is a complex of north-northeast trending normal faults along which Cenozoic rocks have been downfaulted. Occasional wedges of Mesozoic and Paleozoic sediments are found caught as slivers between adjacent faults in this zone. The Northern fault zone is almost completely buried beneath pediment gravel and is presumed to be composed of high-angle normal faults trending northwesterly.

Two unusual structural features are found on the eastern flank of the range. The first of these, the Jeter fault, appears to be a low­angle fault above which thin plate of Cenozoic sediments, including the Tertiary Baca, Popotosa, and Santa Fe Formations, have been thrust westward over Precambrian gneiss.

The second unusual feature, and perhaps an even more important clue to the geomorphic history of the area, is the occurrence of a thin plate of highly disturbed Paleozoic limestone which rests in low-angle discordance across the Precambrian gneiss, the Tertiary Popotosa Formation, and the Jeter fault. Foraminifera thin sections helped determine this limestone to be part of the Atrasado member of the Madera Limestone.

The origin of this plate is debatable, but appears moat likely to have formed as a gravity slide from Pennsylvanian limestones that at one time capped the rising Ladron uplift in much the same manner as the present Sandia and Loa Pinos uplifts are capped. This "klippe" was preserved under Santa Fe fanglomerate until exhumed by Recent erosion.

Mineralization ln the Ladrons has generally been confined to the major bounding fault zones and the Jeter fault. Copper and uranium are concentrated along a 10- to 15- foot sheared, reddish- brown and black, gouge zone of the Jeter fault. The zone contains fragments of Paleozoic, Mesozoic, and Cenozoic sediments which have been dragged into the zone as the

Degree Name

Earth and Planetary Sciences

Level of Degree


Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Vincent Cooper Kelley

Second Committee Member

J. Paul Fitzsimmons

Third Committee Member

Wolfgang Eugene Elston



Document Type


Included in

Geology Commons