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GEOREF RECORD

Heterogeneous deformation in a ductile fold-thrust belt; the Proterozoic structural history of the Tusas Mountains, New Mexico

Michael L. Williams
Heterogeneous deformation in a ductile fold-thrust belt; the Proterozoic structural history of the Tusas Mountains, New Mexico
Geological Society of America Bulletin (February 1991) 103 (2): 171-188

Abstract

The presence of widely variable deformational styles in the Proterozoic terranes of northern New Mexico has led to conflicting interpretations of the geologic history of the region. Proterozoic rocks of the Tusas Mountains display a range of deformational styles typical of northern New Mexico and suggest a model for interpreting the heterogeneity. Three broad types of structural domains have been identified: (1) regions characterized by complex polyphase fold patterns, (2) regions dominated by a single intense schistosity, and (3) regions with nearly pristine primary sedimentary and volcanic structures. Domain boundaries range from continuous deformational gradients to localized faults or shear zones.Structural and metamorphic evidence suggests that all domains were deformed in a single regional shortening event. The deformation occurred during prograde metamorphism at upper greenschist- and lower amphibolite-facies conditions. Despite the metamorphic grade, kilometer-thick massive units were shortened by folding and thrusting, producing geometries similar to those of thin-skinned deformational belts. Regions located beneath the ramps of large-scale ductile thrusts preserve primary structures and simple apparent deformational histories because much of the regional strain was partitioned to the overlying faults. Regions structurally above ramps display complex multiple fold patterns with strain histories that progress from subhorizontal shearing to upright folding and shortening. Regions away from ramps have an intense multi-phase schistosity and a deformational history involving progressive simple shearing. The large scale of the domains (as much as 75 km (super 2) ) and the extreme structural heterogeneity from domain to domain may reflect, at least in part, the competence, homogeneity, and extreme thickness (>1 km) of the Ortega orthoquartzite.Spectacularly refolded and multiply sheared regions in the Tusas Mountains reflect complex local strain environments that cannot be correlated from domain to domain. The kinematic histories of these environments are locally important but are misleading on a regional scale. The regional kinematic history was relatively simple, involving a single shortening event. Deformational styles in other Proterozoic terranes in northern New Mexico are directly comparable to those of the Tusas Mountains and can be interpreted in terms of this large-scale heterogeneous shortening model.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 103
Serial Issue: 2
Title: Heterogeneous deformation in a ductile fold-thrust belt; the Proterozoic structural history of the Tusas Mountains, New Mexico
Affiliation: Univ. Mass., Dep. Geol. and Geogr., Amherst, MA, United States
Pages: 171-188
Published: 199102
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 47
Accession Number: 1991-010818
Categories: Structural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. geol. sketch maps
N35°55'00" - N37°00'00", W107°42'00" - W105°34'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 1991
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