NEWMEXICOGEOLOGY ISSN 0196-948X Volume 22, No. 3, August 2000 Science and Service Field and microstructural observations from the Capilla Peak area, Manzano Mountains, central New Mexico
by Joseph R. Marcoline, Steven Ralser*, and Laurel B. Goodwin, Department of Earth and Environmental Science,New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 *Corresponding author ([email protected])
In New Mexico, the timing of deformation events is generally constrained to be con- temporaneous with or younger than plu- tonism associated with crustal accretion at 1,750–1,650 Ma and older than the compar- atively undeformed 1,400–1,450 Ma plu- tons. The timing of regional deformation has, in the past, generally been considered to accompany accretion of the continental crust at ca 1.65 Ga (e.g. Karlstrom and Bowring, 1988; Bauer and Williams, 1994), with only minor deformation accompany- ing metamorphism and pluton emplace- ment at ca 1.4 Ga. However, more recent observations have shown that deformation at ca 1.4 Ga is extensive and regional in nature, often completely overprinting the structures associated with deformation at 1.65 Ga (Lanzirotti et al., 1996; Marcoline et al., 1999; Williams et al., 1999; Ralser, 2000). Locally, the timing of individual defor- mation events can be more accurately tied down. In central New Mexico the timing of deformation can be constrained in a num- ber of areas. In the Magdalena Mountains, for instance, Bauer and Williams (1994) demonstrated deformation at ca 1,660 Ma. In the Manzano Mountains (Fig. 1), two phases of deformation are constrained to FIGURE 1— Capilla Peak area, Manzano Mountains, central New Mexico. be younger than the 1,656 ± 10 Ma Monte Largo granodiorite (U/Pb zircon, Bauer et al., 1993), which is strongly deformed. The facies conditions. Kinematic indicators asso- Abstract comparatively undeformed 1,427 ± 10 Ma ciated with the S2 foliation (e.g. S-C surfaces, Proterozoic rocks in the Capilla Peak area porphyroclast systems) dominantly indicate preserve evidence, at all scales, for multiple an east-side-up sense of shear. A minority (ca deformation events. The dominant north- 5%) record a west-side-up sense of shear. Also in this issue northeast-striking, east-dipping foliation (S2), This variation in sense-of-shear indicators is New Mexico Science Fair Bureau award with a downdip mineral lineation, overprints interpreted to suggest a strain history that winners p. 64 an older foliation. In schists and metarhyo- reflects general shear (simple shear plus flat- 2001 NMGS Fall Field Trip call for papers p. 65 lites, S2 is a crenulation cleavage, whereas in tening strain). The Proterozoic rocks exposed Rockhound State Park p. 66 quartzites, S2 is a mylonitic fabric. Two gen- in the Capilla Peak area are interpreted to harles H. Maxwell (1923–2000) p. 72 erations of amphibole are preserved in represent a part of a large, lower amphibolite C amphibolites, an older, core-forming actino- facies, ductile shear zone, which was active Thesis abstracts p. 75 lite and a younger hornblende. The horn- at ca 1.4 Ga. Geographic names p. 85 blende defines the S2 foliation. Book review p. 86 Microstructures in the quartzite mylonites Introduction Service/News p. 87 vary from fine grained and equigranular to Upcoming meetings p. 88 monocrystalline ribbons. All quartzite mylo- The Proterozoic tectonic history of the nites have a strong c-axis crystallographic southwestern United States is complex, Coming soon preferred orientation. The quartzite mylonite with deformation and/or metamorphic Oil and gas activities in 1999 microfabric and the presence of hornblende events occurring at ca 1.65 Ga, 1.4 Ga, and in the amphibolites indicate that deforma- Geologic history, stratigraphy, and paleontology of 1.0 Ga (Karlstrom and Bowring, 1988; SAM Cave, north-central New Mexico tion associated with D2 occurred under upper greenschist to lower amphibolite Grambling et al., 1989; Heizler et al., 1997). Manzano Mountains State Park ter understand the it crops out as a discrete 3-m-wide shear
character and signifi- zone. .
S cance of 1.4 Ga defor- The Sevilleta Metarhyolite (Stark and
T M mation in New Mex- Dapples, 1946), is exposed predominately
Taos A ico. This area strad- on the east side of the range in the Capilla
e I d
n D a lt dles the northern Peak area (Fig. 3). The metarhyolite is pink
r Santa Fe N u G a Albuquerque Sandia A f extent of the Monte to brown, well foliated, and lineated. In granite S s Largo shear zone lower strain areas, the metarhyolite con- Socorro ra (MLSZ; Bauer, 1983; tains 1–2 mm equidimensional quartz and ije NEW MEXICO T Thompson et al., feldspar phenocrysts. Amphibolite layers io
R . 1991), which sepa- (1–20 m wide and 400 m long) occur with- S
Las Cruces T N rates lower amphibo- in the Sevilleta Metarhyolite and are inter-
M 35° lite facies rocks in the preted to be deformed and metamor-
south from upper phosed early mafic dikes (Stark and Dap-
A T greenschist facies ples, 1946). The amphibolites are petrolog- Manzanita I
N rocks to the north ically complex, containing two amphibole
pluton A
Z (Thompson et al., phases, hornblende and actinolite, along
N 1991, 1996). with plagioclase, biotite, quartz, epidote, A
M and minor amounts of titanite and Fe–Ti Ojita pluton Location and oxides. The mineral assemblages in differ- ent amphibolites all indicate formation
. regional geology
S under amphibolite grade conditions Area of The Manzano Moun-
MT (Marcoline et al., 1999). Fig. 3 Capilla Peak tains are an eastward- The stratigraphic relationships between tilted fault block com- these units are unclear, although it is O Monte Largo N posed primarily of shear zone A Z Proterozoic metasedi- N Monte Largo A mentary, metavolcan- M e pluton ic, and plutonic rocks. NEW MEXICO d Manzano The mountain range n Monte Largo a Peak is approximately 70 r Canyon GEOLOGY G km long and 15 km Science and Service wide and defines the ISSN 0196-948X t eastern margin of the l Volume 22, No. 3, August 2000 u ° a Rio Grande rift be- o Priest pluton f 34 30' i tween the Manzanita Editors: Jane C. Love, Nancy S. Gilson, R Abo Pass a Mountains and Abo Bonnie Frey, and Susan Voss tos Cartographers: Kathryn E. Glesener and Leo Gabaldon n Pass (Fig. 2). The o EDITORIAL BOARD M study area is in the James M. Barker, NMBMMR, Chair .
Los Pinos S central Manzano Steve M. Cather, NMBMMR T Thomas Giordano, NMSU pluton M Mountains, located Laurel B. Goodwin, NMIMT 10 mi between Capilla Peak Barry S. Kues, UNM S 0 Larry Crumpler, NMMNHS O and Trigo Canyon IN 10 km P 0 (Figs. 2, 3). Published quarterly by New Mexico Bureau of Mines and S Three major litho- LO 106°30' Mineral Resources logic units are recog- a division of New Mexico Institute of nized in the Capilla Mining and Technology FIGURE 2—Location map, showing simplified geology of the Manzano Peak area (Stark, BOARD OF REGENTS Mountains (modified from Bauer et al., 1993). Ex-Officio 1956; Stark and Gary Johnson, Governor of New Mexico Dapples, 1946; Michael S. Davis, Superintendent of Public Instruction Priest pluton (U/Pb zircon, Bauer et al., Appointed 1993) is interpreted to have intruded late Reiche, 1949; Bauer, 1982): the Blue Springs Randall E. Horn, Pres., 1997–2003, Albuquerque Schist, the Whiterock quartzite, and the Kathryn E. Wavrik, Student Mem., Sec./Treas., syntectonic to the younger phase of defor- 1999–2000, Socorro mation. This interpretation is supported by Sevilleta Metarhyolite. The Blue Springs Ann Murphy Daily, 1999–2004, Santa Fe the presence of a strongly deformed gra- Schist is composed of intensely folded Sidney M. Gutierrez, 1997–2001, Albuquerque Robert E. Taylor, 1997–2003, Silver City nitic dike that has a U/Pb age of 1,438 ± 6 metasiltstones, phyllites, and muscovite- Ma (Ralser et al., 1997). Detailed 40Ar/39Ar chlorite schists (Stark 1956; Bauer 1982). New Mexico Institute of Mining and Technology geochronologic studies in the Capilla Peak Boudinaged quartzites within the Blue President ...... Daniel H. López area, in the central Manzano Mountains Springs Schist are now interpreted as New Mexico Bureau of Mines and Mineral Resources (Fig. 2), also suggest that the dominant transposed sedimentary layers but were Director and State Geologist ...... Peter A. Scholle structural fabric formed during 1.4 Ga tec- initially believed to be saddle reefs (Stark, Subscriptions: Issued quarterly, February, May, August, November; 1956). subscription price $10.00/calendar year. tonism (Marcoline, 1996; Marcoline et al., Editorial Matter: Articles submitted for publication should be in the 1996, 1999). Localized deformation in the The Whiterock quartzite consists of a editor’s hands a minimum of five (5) months before date of pub- series of quartzite mylonites interlayered lication (February, May, August, or November) and should be no Priest pluton is related to the Grenville longer than 20 typewritten, double-spaced pages. All scientific orogeny at ca 1,000 Ma (Heizler et al., with rocks of the Blue Springs Schist and papers will be reviewed by at least two people in the appropriate 1997). Sevilleta Metarhyolite. These quartzite field of study. Address inquiries to Jane C. Love, Editor of New mylonites represent localized high-strain Mexico Geology, New Mexico Bureau of Mines and Mineral In this paper, we summarize structural Resources, Socorro, New Mexico 87801-4796. relationships and deformation conditions zones within a larger deformation zone Published as public domain, therefore reproducible without permis- in the Proterozoic rocks in the Capilla Peak (Marcoline, 1996). In Monte Largo Canyon sion. Source credit requested. area. Our goal is to document the structur- these quartzite mylonites define the MLSZ Circulation: 1,000 al relationships in this area in order to bet- (Bauer, 1983; Thompson et al., 1991), where Printer: University of New Mexico Printing Services
58 NEW MEXICO GEOLOGY August 2000 to 50° and dips varying from 60° to 90° southeast. Asymmetrical chevron-style crenulations, with limbs 1–3 cm in length and with axial planar S2, are locally evi- dent. The metasiltstones are characterized by small-scale disharmonic folding of the compositional layering. The S2 foliation is poorly developed and highly irregular in orientation in the metasiltstones. The folia- tion is axial planar to the disharmonic folds, which may explain the large varia- tion in orientation of S2. Intensely deformed Blue Springs Schist is found in three distinct, narrow zones less than a meter wide, which are charac- terized by a well-developed, planar S1 foli- ation. The most continuous of these high- strain zones is located on the western con- tact between the quartzite mylonite and the Blue Springs Schist (Fig. 3). Two other highly deformed zones are located along strike within the Blue Springs Schist, adja- cent to one of the smaller quartzite mylonite layers. The contacts between the highly deformed zones and the more typi- cal schist are gradational. Away from the highly deformed zones the schist contains abundant quartz-rich layers. Nearer to the deformed zones, the quartz-rich layers become discontinuous and boudinaged. No quartz-rich layers were observed in the high-strain zones. In quartzite mylonites, S2 is a strong mylonitic foliation that strikes approxi- mately 025° and dips from 60° to 80° south- east. S1 is only preserved in lower strain zones as open to isoclinally folded sur- faces. The development of S2 is controlled by mica content in the quartzite mylonites; S2 is best developed in areas with a higher mica content. The Sevilleta Metarhyolites are, in gen- eral, characterized by a strongly developed north-northeast-striking foliation (S2) with a locally well developed downdip lin- FIGURE 3—Geologic map of the Capilla Peak area showing the distribution of units, the main folia- eation (Fig. 3). S1 is only observed in a few tion (S2), and lineation (L2). Poles to foliation (+)and mineral lineations (o) are plotted on a lower locations where S2 is poorly developed. hemisphere equal area projection. The S2 foliation is defined by aligned biotite and lens-shaped quartz and feldspar porphyroclasts. F2 folds in the believed that the Sevilleta Metarhyolite is ture of this foliation will be described from metarhyolite plunge shallowly to moder- the oldest unit, based on Rb–Sr isochrons west to east (Blue Springs Schist, quartzite ately to the south-southwest and include with an age of 1,700 ± 58 Ma (Bolton, 1976) mylonites, Sevilleta Metarhyolite, amphi- both small 0.5–1-m isoclinal folds and and a U–Pb zircon age of 1,680 Ma bolites) in the following sections. We will 3–5-m open folds. An S2 axial planar folia- (Bowring et al., 1983). describe c-axis crystallographic preferred tion is only locally observed in the folded orientations observed in the quartzite areas. Structure mylonites in the final part of this section. S2 in the amphibolite layers ranges from weakly to strongly developed and is typi- The dominant structure in the central Macrostructural relationships Manzano Mountains is a pervasive, north- cally parallel to the foliation in the sur- S exhibits significant variation in charac- east-striking, steeply southeast-dipping 2 rounding metarhyolite. One amphibolite ter and orientation in the Blue Springs foliation (S ; Fig. 3). In the schistose units, layer contains distinctive less deformed 2 Schist. In the phyllites and quartz-rich this foliation is a crenulation cleavage, cut- ‘pods’ as large as 10 cm in diameter. These zones of the muscovite-chlorite schists, S ting a well-defined earlier foliation (S ). In 2 pods have length to width ratios ranging 1 is a well-developed planar foliation, strik- from 1:1 to 2:1 and contain 1–3-mm, the non-schistose units, S is the main foli- 2 ing approximately 030° and dipping from unaligned, green to black amphibole crys- ation. As this S foliation is interpreted to 2 60° to 80° southeast. However, in quartz- tals. A strong foliation wraps around the have formed during tectonism at 1.4 Ga poor zones of the muscovite-chlorite pods. The pods are sigmoidal in shape and (Marcoline et al., 1999; Ralser, 2000), it will schist, the S2 orientation is variable. S2 asymmetric with respect to the main folia- be described in detail in this paper. The anastamoses around the numerous folded tion, consistently recording east-side-up macrostructural and microstructural na- quartz layers, with strikes ranging from 0° shearing. Several 0.5–1-m-scale asymmet-
August 2000 NEW MEXICO GEOLOGY 59 al., 1999). They pre- serve older actinolite and younger horn- blende, plagioclase, biotite, quartz, and epidote. The older anhedral actinolite is crosscut and over- grown by the folia- tion-forming euhe- dral hornblende (Fig. 5). Actinolite contains inclusion trails that record an older folia- tion parallel to the crystallographic cleavage planes and nearly perpendicular to the well-defined hornblende foliation. FIGURE 5—Photomicrograph showing two amphibole phases. Actinolite Hornblende both (a) forms cores and has inclusion trails nearly perpendicular to the folia- rims the actinolite tion-forming, green euhedral hornblendes (h). Width of photo is 4.5 mm; S is horizontal. and defines the S2 2 foliation. Hornblende rims and foliation- preferred orientation at a 45° angle to S . 2 forming grains show mutual crosscutting The major quartzite mylonite units can relations. The composition of the horn- be divided into three groups on the basis of blende is consistent with formation at microstructures. The first group is charac- lower amphibolite facies conditions terized by 0.1–0.25-mm, recrystallized (Marcoline, 1996; Marcoline et al., 1999). quartz grains with a strong to moderate grain-shape preferred orientation that Quartz c-axis crystallographic preferred defines a foliation at an angle between 45° orientations and 60° from S2 (samples ML 11-3, ML 7-15, and ML 6-2; Fig. 4A). The second group of In general, c-axis crystallographic pre- quartzite mylonites is characterized by ferred orientations (CPOs) develop due to fine-grained, 0.1–0.25-mm, recrystallized a reorientation of crystal axes as a result of quartz grains with 1–3-mm-long mono- dislocation slip (e.g. Hobbs et al., 1976; Van Houtte and Wagner, 1985; Williams et al., FIGURE 4—Photomicrographs showing the crystalline quartz ribbons (samples ML 7-1 variation in microstructures developed in the and ML 7-3; Fig. 4B). The third group is 1994). Dislocation slip occurs when the quartzite mylonites. Three groups can be recog- characterized by 3–20+ mm-long mono- shear stress resolved on the slip plane in a nized based on microstructures. A) Group 1 is crystalline quartz ribbons (samples ML 1- particular slip direction reaches a critical characterized by equant recrystallized quartz 2, ML5-10; Fig. 4C). The ribbons exhibit value, the critical resolved shear stress grains 0.1–0.25 mm in size. Note the recrystal- undulose extinction with minor subgrain (Hobbs, 1985; Van Houtte and Wagner, lized relic grain, and the grain-shape preferred formation. S-C fabrics are locally present 1985). As with microstructural studies, orientation trending from bottom left to top in the third group of quartzite mylonites. CPO studies cannot provide unique con- right. B) Group 2 is characterized by a bimodal straints on the deformation conditions, grain-size distribution with large elongate In all mylonite samples, micas are included grains parallel to the main foliation and fine within the recrystallized quartz grains as because both intrinsic factors (e.g. temper- equigranular crystals. C) Group 3 is character- well as between grains, indicating a signif- ature, strain rate) and extrinsic factors (e.g. ized by 1–20-mm, slightly undulose monocrys- icant degree of grain boundary mobility strain history) can influence the develop- talline quartz ribbons. All photomicrographs and suggesting deformation at a high tem- ment of CPOs (e.g. Hobbs, 1985). normal to the foliation and parallel to the lin- perature or slow strain rate (e.g. Lister and Increasing temperature has the same effect eation; S2 is horizontal; width of photos—A and Dornsiepen, 1982). as decreasing strain rate. Slip systems that B, 0.5 mm; C, 3 mm. The metarhyolites are characterized by have a high critically resolved shear stress 1–2-mm quartz and feldspar porphyro- (CRSS) at low temperatures or fast strain ric sigmoid-shaped structures within the clasts in a fine-grained (0.1–0.25 mm) foliat- rates (e.g. prism slip in quartz) have a amphibolite layers are also observed. ed quartz and feldspar matrix. Although much lower CRSS at higher temperatures These structures are similar to the pods but the majority of the feldspar porphyroclasts or slower strain rates. Such changes in the contain an internal foliation parallel to, but show orthorhombic symmetry with respect operating slip systems are reflected in the weaker than, the surrounding foliation. to the foliation (φ-type, see Passchier and CPOs that develop (e.g. Tullis et al., 1973; Trouw, 1996), scattered asymmetric ∂-type Lister and Hobbs, 1980; Schmid and Casey, Microstructural relationships porphyroclasts are also present. Asym- 1986). The strain history also has a pro- The character of the Blue Springs Schist is metric strain shadows, defined by recrys- found effect on the CPOs that develop; with as variable microscopically as it is at the tallized quartz, are developed around a coaxial strain history, symmetric CPOs outcrop scale. In general, the S2 crenulation feldspar phenocrysts. Although these por- develop, whereas with a noncoaxial strain cleavage is best developed adjacent to the phyroclasts show evidence for both west- history, asymmetric CPOs develop (e.g. quartzite mylonites. With better S2 develop- and east-side-up sense of shear, the latter Lister and Hobbs, 1980). The sense of asym- ment, the cleavage-plane spacing decreas- group is dominant. metry can be used to determine the sense of es. In zones with a large amount of quartz, The amphibolites exhibit complex micro- shear (e.g. Schmid and Casey, 1986). recrystallized grains preserve a grain-shape structures and are dealt with in more detail Two hundred c-axis orientations were elsewhere (Marcoline, 1996; Marcoline et
60 NEW MEXICO GEOLOGY August 2000 developed, planar foliation) are localized in the quartzite of the Blue Springs Schist, near the contact between the Blue Springs Schist and the quartzite mylonites, and in the Sevilleta Metarhyolite. The Monte Largo shear zone (MLSZ) has previously been interpreted as a narrow (3–5 m in width) shear zone characterized by quartzite mylonites (Bauer, 1983; Thompson et al., 1991). This interpretation is based on studies along the southern extent of the MLSZ in Monte Largo Can- yon. Our observations along the northern extent of the MLSZ in the Capilla Peak area indicate that deformation associated with the shear zone is more widespread. This increase in width of the deformed zone is related to the number of quartzite layers present, which is greater in the Capilla Peak area (Fig. 3). Deformation associated with the MLSZ is spread over the zone encompassed by the quartzite mylonites. Deformation under amphibolite facies conditions is indicated by both mineral assemblages and microstructures. The hornblende, which defines the S2 foliation in the amphibolites and accompanying phases, suggests formation under amphi- bolite facies conditions (c.f. Spear, 1981; see Marcoline et al., 1999 for a detailed discus- sion). These hornblendes yield 40Ar/39Ar ages of ca 1,400 Ma, indicating that horn- blende closed to Ar diffusion and was, therefore, at temperatures around the greenschist/amphibolite facies boundary (~450°–550° C) at ca 1,400 Ma. Other fea- tures related to D2 deformation, in particu- lar microstructures and c-axis CPOs, are consistent with lower amphibolite facies deformation. The microstructures in the quartzite mylonites range from fine grained and equigranular to large mono- crystalline quartz ribbons. Some areas (e.g. FIGURE 6—Geologic map of the Capilla Peak area showing location of quartzite mylonites where c- sample ML 11-2) show coexisting zones of axes have been measured and showing equal area, lower hemisphere plots of 200 quartz grains. monocrystalline quartz ribbons and fine- Contours represent multiples of uniform distribution. grained equigranular microstructure. Such microstructures are consistent with defor- measured from thin sections cut perpen- three samples are relatively symmetric, in mation at upper greenschist to lower dicular to the lineation and foliation from both distribution and size, about the pole amphibolite facies conditions (Simpson, each of seven representative quartzite to the foliation. These plots are different 1985; Hanmer and Passchier, 1991). Be- mylonite samples from the Capilla Peak from the first group of plots in that the cause the pressure and temperature at area (Fig. 6). Data were then plotted on maxima are located preferentially in the which deformation took place could not equal area, lower hemisphere projections center and in the outer top and bottom have varied significantly within the small and rotated so that the trace of the foliation portions of the plot. The CPO of sample area encompassing the quartzite mylo- is horizontal and the lineation is horizontal ML 1-2 differs from those of the other sam- nites, changes in these deformation condi- and plunging to the right on each plot. ples, in that a relatively symmetric c-axis tions can not be called on to explain the Three different patterns are observed in girdle is approximately normal to the lin- differences in microstructure observed in the quartzite mylonite samples. Incom- eation (Fig. 6). the quartzite mylonites. The observed plete small circle girdles about the pole microstructure variations, therefore, must normal to the foliation are developed in Discussion be the result of variations in either strain, three samples (ML 11-3, ML 7-3, and ML 7- strain rate, composition, and/or water D structures are developed throughout 15; Fig. 6). In each girdle, two maxima are 2 content (e.g. Kronenberg, 1981). The min- the Capilla Peak area and vary from a symmetrically distributed on either side of eralogy, including the presence of hydrous crenulation cleavage (with wavelengths the pole to the foliation; however, the size phases, is similar in all the quartzite from 0.5 mm to 5 cm) developed in the of the maxima on either side of the pole to mylonites, suggesting that neither miner- Blue Springs Schist to strongly foliated and the foliation vary. Samples ML 6-2, ML 7-1, alogy nor water content affect the lineated tectonites developed in the and ML 11-2 have c-axis distributions that microstructure that developed. All the quartzites and metarhyolites. In most range from normal to the foliation to per- quartzite mylonites are strongly deformed pendicular to the lineation, within the foli- places S1 is not visible. Zones of high strain so a steady state microstructure could have ation plane (Fig 6). The maxima in these formed during D2 (characterized by a well- developed. It is, therefore, most likely that
August 2000 NEW MEXICO GEOLOGY 61 CPOs could result from the almost equal activity of basal and prism slip Rhomb Slip (c.f. Ralser et al., 1991). Equal activity of basal and prism slip is consistent Prism Slip with amphibolite-grade conditions. Only sample ML 1-2 shows a crossed- circle girdle normal to the lineation (Type I girdle of Lister, 1977). Both the CPO and the S-C microstructure of this sample indi- cate a large component of noncoaxial deformation. The CPOs in the other three Basal Slip samples (ML 6-2, ML 7-1, ML 11-2), con- sisting of c-axis maxima nearly normal to the lineation, both parallel and perpendic- ular to the foliation, are not as easy to inter- pret. They may reflect incomplete crossed girdles and, therefore, represent deforma- tion under plane strain. The maxima, both normal to the foliation and in the foliation Critically resolved shear stress plane, result from the activity of a combi- nation basal and prism slip. Only weak asymmetries are observed in the CPOs. The majority of the samples (ML Greenschist Amphibolite 11-2, ML 1-2, ML 7-3, ML 7-15, ML 6-2) are Temperature defined by the more populated northeast- southwest girdle, suggesting a weak com- FIGURE 7—Plot of critically resolved shear stress versus temperature (Hobbs, 1985). The shaded area ponent of east-side-up simple shear. In represents the approximate deformation conditions in the Capilla Peak area. sample ML 11-3, the northwest-southeast girdle has a higher population than the the observed microstructures reflect differ- cates that deformation was primarily northeast-southwest girdle and is inter- ences in strain rate rather than the alterna- accommodated by dislocation slip (e.g. preted to indicate a west-side-up sense of tives. Hobbs et al., 1976; Van Houtte and Wagner, shear. This is consistent with the variabili- The presence of only ∂-type porphyro- 1985), with the operating slip systems a ty exhibited by other kinematic indicators. clast systems, as opposed to σ-type por- function of the deformation conditions phyroclast systems, within the metarhyo- (Fig. 7; e.g. Hobbs, 1985). The most com- Conclusions lites is striking. Previous workers have monly reported slip systems in quartz are ∂ The Proterozoic rocks currently exposed in proposed that the dominance of -type basal, prism, and rhomb planes with slip the Capilla Peak area of the Manzano porphyroclast systems indicates that both primarily in the direction; however, Mountains were all part of an upper green- the strain (rotation) rate was high relative evidence of slip in the
62 NEW MEXICO GEOLOGY August 2000 blende (parallel to the S2 foliation) is inter- An outline of structural geology: Wiley, New ies, the Paterson volume: American Geophysical preted to have formed under amphibolite York, 571 pp. Union, pp. 263–286. facies conditions (Marcoline, 1996; Marco- Karlstrom, K. E., and Bowring, S. A., 1988, Early Simpson, C., 1985, Deformation of granitic rocks Proterozoic assembly of tectonostratigraphic ter- across the brittle-ductile transition: Journal of line et al., 1996, 1999). ranes in southwestern North America: Journal of Structural Geology, v. 7, no. 5, pp. 503–511. Kinematic indicators, including micro- Geology, v. 96, pp. 561–576. Simpson, C., and De Paor, D. G., 1993, Strain and scopic folds, S-C surfaces, asymmetric por- Kronenberg, A., 1981, Quartz preferred orientations kinematic analysis in general shear zones: Jour- phyroclast systems, and c-axis CPOs, in within a deformed pebble conglomerate from nal of Structural Geology, v. 15, no.1, pp. 1–20. general, indicate an east-side-up sense of New Hampshire, U.S.A.: Tectonophysics, v. 79, Spear, F. S., 1981, An experimental study of horn- shear. However, rare porphyroclast sys- pp. T7–T15. blende stability and compositional variability in Lanzirotti, A., Bishop, J. L., and Williams, M. L., amphibolite: American Journal of Science, v. 281, tems in the quartzites and the Sevilleta 1996, A more vigorous approach to dating mid- no. 6, pp. 697–734. Metarhyolite record a west-side-up sense crustal processes—U–Pb dating of varied major Stark, J. T., 1956, Geology of the south Manzano of shear. These differing kinematic indica- and accessory metamorphic minerals tied to Mountains, New Mexico: New Mexico Bureau of tors suggest a strain history of progressive, microstructural processes (abs.): Geological Mines and Mineral Resources, Bulletin 34, 49 pp. general, noncoaxial flow (c.f. Simpson and Society of America, Abstracts with programs, Stark, J. T., and Dapples, E. C., 1946, Geology of the De Paor, 1993); i.e. a combination of both 1996 annual meeting, p. A-453. Los Pinos Mountains, New Mexico: Geological Law, R. D., Knipe, R .J., and Dayan, H., 1984, Strain Society of America, Bulletin, v. 57, pp. 1121–1172. simple shear and flattening. path partitioning within thrust sheets—micro- Thompson, A. G., Grambling, J. A., and Dallmeyer, Acknowledgments. This work was sup- structural and petrofabric evidence from the R. 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