The role of crustal extension in the metamorphism of Proterozoic rocks in northern New Mexico

Abstract

The Proterozoic metamorphic belt of northern and central New Mexico contains rocks of two distinctly different metamorphic grades, locally lying in direct contact. A large region exhibits coexisting kyanite, andalusite, and sillimanite. The triple-point assemblages resulted from peak metamorphic conditions falling near 500°C, 4 kb, and they occur across 75,000 km 2 in 14 separate mountain ranges. Triple-point metamorphic grade was attained after north-verging folding and ductile thrusting but before southverging mylonitic shearing. Prograde and retrograde P-T paths were nearly isobaric.However, two mountain ranges preserve a different metamorphic history. Granitic and migmatitic gneiss in the Santa Fe Range formed at 650° to 750°C. Sillimanite-Kfeldspar gneiss in the northern Taos Range records peak conditions as high as 700° to 800°C, 9 to 11 kbar. In the latter area, high-grade metamorphism was followed by south-verging shearing, southeast-verging folding, and decompression, on a retrograde P-T path that passed through 500°C, 4 kb. Deformation produced a thick mylonitic shear zone that appears in scattered outcrops across the entire mountain range. The shear zone separates gneiss from structurally overlying kyanite-andalusite-sillimanite quartzite. Mineral textures indicate that the shearing occurred during and after the thermal peak in the gneissic rocks, and during the thermal peak in the overlying triplepoint rocks.We suggest that the kyanite-andalusite-sillimanite rocks are separated from the rocks of higher metamorphic grade by a synmetamorphic ductile detachment fault represented by the thick mylonitic shear zone. The detachment separated an actively extending lower crust from an upper crust that was not being thinned. We infer that the detachment extends in the subsurface beneath the entire belt where rocks show kyaniteandalusite-sillimanite metamorphic grade. Lower-crustal extension, with heat carried upward toward the detachment, seems to have caused the triple-point metamorphism.

The Yavapai-Mazatzal crustal boundary in the Southern Rocky Mountains

“…The limited available data seem to reflect a predominance of amphibolite-grade conditions, typical of much of Colorado and New Mexico (e.g., 'Ihreto, 1987;Grambling, 1988;Reed et al, 1993), although anomalous granulite-grade rocks occur in the Wet Mountains ( Fig. 2; Brock and Singewald, 1968;Ttveto, 1987) and northern Thos Range (Grambling et al, 1989;Pedrick, 1995;Pedrick et al, 1998).…”

Ridge-trench interactions and high- T -low- P metamorphism, with particular reference to the Cretaceous evolution of the Japanese Islands

“…the southwestern United States (Grambling 1986;Grambling et al 1989;Grambling & Dallmeyer 1993); the Arunta Inlier in central Australia (Collins & Vernon 1991, 1993Hand et al 1992Hand et al , 1993Vernon 1996)), although sometimes with good reason, such as paths that are clockwise in the hanging wall and anticlockwise in the footwall of an overthrust complex (e.g. southern Alaska (Sisson & Hollister 1988)).…”

An integrated geophysical study of the Estancia Basin, central New Mexico

“…The Proterozoic history of the New Mexico region is very complex and is the subject of much current research and debate (e.g., Bauer and Williams, 1985;Condie and Budding, 1979;Condie, 1986;Grambling et al, 1988Grambling et al, ,1989Williams, 1990). There are a number of complex dipping reflectors in the basement.…”

“…The best-documented early event was thrusting of Precambrian rocks coupled with metamorphism (Grambling et al, 1988(Grambling et al, , 1989. The seismic data show that many reflectors are present in the basement.…”

“…These authors suggested that at least some of the terrane boundaries were north-verging ductile thrusts. However, Grambling et al (1989) demonstrated that extension also played a major role in the Proterozoic evolution of the region north of the Estancia Basin. Geochronologic studies (Dallmeyer et al, 1990) indicate that the extension occurred at about 1,440 Ma and that the thrusting occurred at about 1,350 Ma.…”