Provenance Evidence for Major Post–Early Pennsylvanian Dextral Slip on the Picuris-Pecos Fault, Northern New Mexico

Home , Alamitos Formation, Arroyo Penasco Group, Composita, Fenestella (bryozoan), Flechado Formation, Picuris Mountains

Provenance evidence for major post–early Pennsylvanian dextral slip on the Picuris-Pecos fault, northern New Mexico

Steven M. Cather1, Adam S. Read1, Nelia W. Dunbar1, Barry S. Kues2, Karl Krainer3, Spencer G. Lucas4, and Shari A. Kelley1 1New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA 2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA 3Institute of Geology and Paleontology, University of Innsbruck, Innrain 52, Innsbruck A-6020, Austria 4New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, New Mexico 87104, USA

ABSTRACT INTRODUCTION Ancestral Rocky Mountain deformations, dextral slip on the Picuris-Pecos fault potentially The Picuris-Pecos fault is a major strike- The Proterozoic basement of the Southern contributed to crustal shortening in uplifts and slip fault in northern New Mexico (USA) Rocky Mountains in northern New Mexico, basins northwest of the fault (Fig. 1). that exhibits ~37 km of dextral separation of USA, has long been known to be dextrally The Picuris-Pecos fault formed the western Proterozoic lithotypes and structures. The faulted (Montgomery, 1963). Basement-related boundary of the late Paleozoic Taos trough dur- timing of dextral slip has been controversial aeromagnetic patterns have been interpreted to ing the early part of the Ancestral Rocky Moun- due largely to a lack of deﬁ nitive piercing show net dextral offsets of ~55–130 km on sev- tain orogeny. During the Pennsylvanian, terranes points of Phanerozoic age. The Picuris- eral north-striking faults (Chapin, 1983; Cordell of distinctive Proterozoic lithotypes (metasedi- Pecos fault formed the western boundary of and Keller, 1984; Karlstrom and Daniel, 1993; mentary versus dominantly plutonic and meta- the late Paleozoic Taos trough. A distinctive Cather et al., 2006). The Picuris-Pecos fault is plutonic) were exposed in the upthrown western metasedimentary terrane that shed detritus the largest of these faults, with a dextral separa- block (the Uncompahgre uplift) of the Picuris- into the western Taos trough was exposed tion of ~37 km (Montgomery, 1963). Although Pecos fault. The paleolocations of these terranes on the Uncompahgre uplift west of the fault there is agreement that major dextral separa- can be ascertained using the provenance charac- during the early to middle Pennsylvanian. tions of Proterozoic rocks and structures exist teristics of Pennsylvanian strata in the adjoining We use the distribution of metasedimen- in northern New Mexico, the timing of dextral Taos trough, an approach that was pioneered by tary clasts and the age of monazite grains slip is controversial. This controversy derives in Sutherland (1963). We show that proximal Penn- within clasts from conglomeratic strata of part from the lack of deﬁ nitive piercing points sylvanian (Morrowan? to early Desmoinesian) the western Taos trough to determine the in Phanerozoic rocks of the region. As a result, petrofacies are today mismatched from their paleolocation of the southern boundary of dextral slip has variously been inferred to have distinctive source terranes west of the fault. We this metasedimentary terrane during the occurred primarily during the Proterozoic utilize clast composition and monazite geochro- middle Pennsylvanian (Desmoinesian), and (Montgomery, 1963; Yin and Ingersoll, 1997; nology as provenance indicators to show that thereby quantify the subsequent separa- Fankhauser and Erslev, 2004; Wawrzyniec et al., ~40–50 km of dextral slip has occurred on the tion on the fault. The rematching of detri- 2007), mostly during the late Paleozoic Ances- Picuris-Pecos fault since the early Pennsylvanian. tal petrofacies with source terranes in the tral Rocky Mountain orogeny (Baars and Ste- adjacent uplift requires ~40–50 km of dex- venson, 1984; Woodward et al., 1999), mostly BACKGROUND tral separation on the Picuris-Pecos fault during the Late Cretaceous–Eocene Laramide since the early Desmoinesian. This exceeds orogeny (Chapin and Cather, 1981; Chapin, The Picuris-Pecos fault is the best exposed the present ~37 km dextral separation of 1983; Karlstrom and Daniel, 1993; Daniel et al., and most studied of the dextral faults in north- Proterozoic features by the fault, and thus 1995; Bauer and Ralser, 1995; Cather, 1999), or ern New Mexico (Fig. 2). The fault strikes north, implies that an ~3–13 km sinistral separa- during both the Ancestral Rocky Mountain and dips steeply west, and cuts rocks ranging in age tion existed on the fault in the early Des- Laramide orogenies (Cather, 2004; Cather et from Proterozoic to Paleogene. It is exposed moinesian. The ~40–50 km of post–early al., 2006). The possible regional kinematic role for ~80 km along strike, and may extend the Desmoinesian dextral separation on the of the Picuris-Pecos fault during Proterozoic length of the state (~600 km) if probable fault Picuris-Pecos fault is the result of slip that deformations is unclear; regional strain analysis linkages to the north and south are considered accumulated late in the Ancestral Rocky suggests lateral slip on the fault during known (Cather and Harrison, 2002; Cather, 2009). With Mountain deformation and/or during the Proterozoic deformations was probably sinistral ~37 km dextral separation of Proterozoic litho- Laramide orogeny. (Cather et al., 2006). During the Laramide and types and east-west–trending ductile structures,

Geosphere; October 2011; v. 7; no. 5; p. 1175–1193; doi: 10.1130/GES00649.1; 22 ﬁ gures; 1 table; 1 supplemental ﬁ le.

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the Picuris-Pecos fault exhibits the largest mud that fi lls fi ssures and the interstices of brec- of the fault shared similar ca. 1.2–0.4 Ga cool- known separation of any fault in the central and cias in the damage zone of the fault (Erslev et al., ing histories and passed into the purely brittle Southern Rocky Mountains. It exceeds that of 2004; Fankhauser, 2005; Cather et al., 2008). Slip regime (<300–250 °C) during or soon after the the next largest fault (the Wind River thrust) by on the Picuris-Pecos fault during both the Ances- Grenville orogeny (ca. 1.2–0.9 Ga; Sanders et a factor of nearly two. tral Rocky Mountain and Laramide orogenies al., 2006). Modest west-up components of slip The Picuris-Pecos fault has been repeatedly had a west-up component (Sutherland, 1963), but on the Picuris-Pecos fault defi ned the western reactivated. During the Proterozoic, plutonism, regional strain balancing suggests that it also may margin of the Ancestral Rocky Mountain Taos ductile deformation, and peak metamorphism have hosted signifi cant dextral slip during these trough (Casey, 1980) during the early Penn- in northern New Mexico occurred ca. 1.4 Ga deformations (Cather et al., 2006). sylvanian, but topographic relief on this basin (Williams et al., 1999a). The lack of mylonites Several lines of evidence indicate that the margin was low enough that it was lapped over along the Picuris-Pecos fault, however, suggests Picuris-Pecos fault did not accommodate major and buried by marine strata during the middle that no ductile precursor to the fault was active at dip slip. Metasedimentary rocks exposed in the Pennsylvanian (early to middle Desmoines- the time (Cather et al., 2006). Subsequent brittle Picuris and Truchas Mountains are separated ian; see following). West-up components of slip along the Picuris-Pecos fault, however, may by the Picuris-Pecos fault, but show evidence slip also occurred on the Picuris-Pecos fault have occurred during the Grenville orogeny and for similar peak metamorphic conditions near during Laramide deformation, but similar late during Neoproterozoic deformation related to the the Al-silicate triple point (3.5–4.0 kbar, 500– Laramide apatite fi ssion-track cooling ages breakup of Rodinia (Cather et al., 2006). The ear- 550 °C; Grambling, 1979, 1981; Daniel et al., occur at similar elevations on both sides of the liest undisputable evidence for slip on the fault 1995). Thermochronologic data from the south- fault (Kelley and Chapin, 1995, their fi g. 8) and is early in the Ancestral Rocky Mountain orog- ern Sangre de Cristo Mountains indicate that suggest that Laramide differential uplift across eny, as shown by Mississippian marine carbonate rocks at similar modern elevations on both sides the fault was not large. The observed ~37 km of dextral separation on the Picuris-Pecos fault must therefore be largely the result of strike slip.

STUDY AREA

The study area encompasses the central part of the Picuris-Pecos fault adjacent to the Tru- chas uplift (Fig. 3). Outcrops north of the Tru- chas uplift are near roads, but areas to the south are within the Pecos Wilderness and are acces- sible only by foot or on horseback. Within the study area no major faults intersect the Picuris- Pecos fault, resulting in a relatively simple structural geometry. The Picuris-Pecos fault transects and offsets two distinctive terranes of Proterozoic rocks. The southern terrane consists of ca. 1.72–1.44 Ga metaplutonic, plutonic, and subordinate metavolcanic rocks, granitic gneiss and granite being the volumetrically dominant lithotypes (Karlstrom et al., 2004). The northern terrane is dominated by metasedimentary rocks (Hondo Group, ca. 1.69 Ga) and subordinate metavolcanic rocks (Vadito Group, ca. 1.70 Ga). Metasedimentary rocks of the Hondo Group include the Ortega Quartzite and schist, phyllite, and quartzite of the Rinconada, Pilar, and Piedra Lumbre Formations. The Proterozoic terranes are separated by a 200 major south-dipping ductile thrust fault. East of the Picuris-Pecos fault this fault is termed the Pecos thrust; to the west it is termed the Plomo fault (Fig. 3). The ~37 km dextral separation of ductile structures and Proterozoic lithotypes by the Picuris-Pecos fault is the basis for the Figure 1. Map showing trace of the Picuris-Pecos fault (PPf) in relation to Ancestral interpretation by Montgomery (1963) and sub- Rocky Mountains uplifts (gray) and Laramide uplifts (blue). Dextral slip in the Picuris- sequent workers that the Picuris-Pecos fault is a Pecos fault may have contributed to crustal contraction in uplifts and basins northwest major strike-slip fault. The originally continuous of the fault during both orogenies. Abbreviations: Tt—Taos trough, Unc—Uncom- Pecos and Plomo faults deﬁ ne the steep south- pahgre, FR—ancestral Front Range, WRp—White River Plateau, Uu—Uinta uplift, ern limb of a regional synclinorium that formed WRu—Wind River uplift, SWu—Sweetwater uplift. No palinspastic restoration. ca. 1.65–1.4 Ga (Williams et al., 1999a). This

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synclinorium, marked by the Hondo syncline al., 2004). Mississippian strata unconformably Mountain basin that is bounded on the west on the west side of the Picuris-Pecos fault and overlie Proterozoic rocks and exhibit a range of by the Picuris-Pecos fault (Fig. 4). We divide the Brazos Cabin syncline on the east, contains thickness (0–45 m) that reﬂ ects the underlying, Pennsylvanian beds using the nomenclature mostly metasedimentary rocks. The low aero- low-relief erosional paleotopography (Suther- advocated by Sutherland (1963), Sutherland magnetic value of the metasedimentary rocks land, 1963; Baltz and Myers, 1999). Mississip- and Harlow (1973), and Kues (2001), who has been used to evaluate strike-slip offsets in pian rocks in the study area accumulated before apportioned these strata among four formations the subsurface of northern New Mexico (e.g., and during the earliest stages of the Ancestral (Fig. 5). Early to middle Pennsylvanian beds Karlstrom and Daniel, 1993; Cather et al., 2006). Rocky Mountain orogeny, but prior to develop- (Morrowan to middle Desmoinesian) discon- The oldest Paleozoic strata in the study area ment of signiﬁ cant orogenic topography (Arm- formably overlie Mississippian and Protero- are the Mississippian Arroyo Peñasco Group, strong, 1967; Baltz and Myers, 1999). zoic rocks and consist of quartzose sandstone which consists of the Espiritu Santo Formation Pennsylvanian strata in the study area were and conglomerate, mudstone, and limestone. (late Osagean) and the Terrero Formation (Mer- deposited near the paleoequator in the western Sutherland (1963) divided these beds into two amecian and early Chesterian) (Armstrong et part of the Taos trough, an Ancestral Rocky formations based on their content of sandstone

Figure 2. Map of north-central and northwestern New Mexico showing major pre–Rio Grande Rift (Ances- tral Rocky Mountains and/or Laramide) structures. Map location shown in Figure 1. Abbreviations: PPf— Picuris-Pecos fault, Bf—Borrego fault, TPf—Tusas–Picuris fault, Pf—Pajarito fault, GMf—Glorieta Mesa fault, SdCff—Sangre de Cristo frontal faults, TCf—Tijeras-Cañoncito fault, Cf—Chupadera fault, Tf— Tecolote fault, SHf—Sand Hill fault, Nf—Nacimiento fault, Gf—Gallinas fault, SCd—Salado-Cumbres dis- continuity, Aa—Archuleta anticlinorium, Hbm—Hogback monocline, CB—Chama Basin, Zu—Zuni uplift. No palinspastic restoration.

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and limestone. The northern sandstone-rich Cristo Formation (late Pennsylvanian–Early ciated conglomerate, which is almost entirely lithofacies is termed the Flechado Formation, Permian). The contact between these formations composed of clasts of Ortega Quartzite in the and the southern carbonate-dominated beds are is deﬁ ned at the top of the stratigraphically high- western Taos trough. In the central Taos trough, the La Pasada Formation. These units are con- est marine limestone. however, conglomerate is rare and quartzose formably overlain by the middle to late Penn- The quartzose sandstone that is characteris- sandstone compositions may have resulted from sylvanian (middle Desmoinesian to Virgilian) tic of the Flechado and La Pasada Formations feldspar destruction in initially arkosic sands Alamitos Formation, which consists of arkosic was interpreted by Sutherland (1963) and most by tropical weathering or intrastratal dissolu- sandstone and conglomerate, marine limestone, subsequent workers to have been derived from tion (e.g., Chandler, 1988; Soegaard, 1990). In and mudstone. The Alamitos Formation, in turn, the quartz-rich metasedimentary rocks west the eastern Taos trough early to middle Penn- grades upward into arkosic sandstone, conglom- of the Picuris-Pecos fault. This conclusion is sylvanian sandstones are commonly feldspathic erate, and mudstone of the nonmarine Sangre de supported locally by the composition of asso- (Baltz and Myers, 1999). In view of the possible

Figure 3. Simpliﬁ ed geologic map showing the study areas and localities discussed in the text. Map location is shown in Figure 2. TPf— Tusas-Picuris fault, Bf—Borrego fault, Jf—Jicarilla fault.

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Figure 4. Map showing Pennsylvanian basins, uplifts (stippled pattern), and selected structures in the region of northern New Mexico and southern Colorado. Map location is shown in Figures 1 and 2. Abbre- viations: Tt—Taos trough, Rt—Rainsville trough, Ca—Cimarron arch, CCb—Central Colorado basin, Db—Denver Basin, FRu— Front Range uplift, ECb—Eastern Colo- rado basin, WMu—Wet Mountains uplift, Pb—Paradox Basin, Dhb—Dalhart Basin, DZu—Deﬁ ance-Zuni uplift, Pu—Peñasco uplift, Bd—Brazos dome, Au—Amarillo uplift, SJb—San Jose Basin, Lb—Lucero basin, Eb—Estancia Basin, CTb—Cuervo- Tucumcari basin, PDb—Palo Duro Basin, Ju—Joyita uplift. Modiﬁ ed from Baltz and Myers (1999, their Fig. 78). No palinspastic restoration.

South North 290

Wolfcampian Sangre de Cristo Formation Permian

300 Virgilian

Missourian Alamitos Formation Figure 5. Stratigraphic nomenclature for late Desmoinesian Paleozoic strata in the western Taos trough. Stippled pattern indicates units in which 310 sandstone is dominantly quartzarenitic. Mod- Atokan iﬁ ed from Sutherland and Harlow (1973). Age (Ma) La Pasada Flechado

Pennsylvanian Formation Formation

Morrowan

320

Chesterian

? ? ? Mississippian Terrero Formation

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modiﬁ cation of initial sandstone compositions, Rio Chiquito Area east with Proterozoic granite-gneiss on the west herein we limit our provenance analysis to (Fig. 6). Basement rocks on both sides of the proximal conglomeratic successions near the Near the Rio Chiquito, the Picuris-Pecos fault Picuris-Pecos fault are overlain by Pennsylva- Picuris-Pecos fault where pebble- to boulder- dips steeply west and consists of several anas- nian strata; Mississippian beds are absent in this size clasts are assumed to reﬂ ect the lithotypes tomosing fault strands that juxtapose basement area. East of the Picuris-Pecos fault, the strike of the source region. rocks of the ca. 1.69 Ga Ortega Quartzite on the of Pennsylvanian beds is rotated clockwise

36° 5′N

105° 40′W

Figure 6. Geologic map of the Rio Chiquito area (modiﬁ ed from Aby and Timmons, 2005). P-P—Picuris-Pecos. Map location is shown in Figure 3. See text for discussion.

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~70°–90° near the fault, consistent with a dex- lying Proterozoic Ortega Quartzite is low relief cross-beds (facies St of Miall, 1996), and rare tral sense of shear on the fault. and records only a few meters of local paleo- horizontal lamination (facies Sh). Coarse pebbly In this section we employ stratigraphic analy- topography. For the purposes of mapping, we sandstone and conglomerate also commonly dis- sis, paleontology, paleocurrent measurement, placed the contact between the Flechado For- play trough cross-bedding (Gt), and subordinate and petrology to establish the following. (1) The mation and overlying Alamitos Formation at the horizontal bedding (Gh). Some conglomerate Pennsylvanian succession east of the Picuris- top of the uppermost thick (>5 m) quartzarenitic beds appear massive and display normal grading Pecos fault near the Rio Chiquito contains prox- sandstone (unit 92, Fig. 7), a laterally traceable (facies Gcm). Rare pebbly sandstones show a imal, scarp-related braid-delta deposits that fi ne bed. This differs from the original defi nition of matrix-supported fabric with pebbles fl oating in to the east and were derived from metasedimen- these formations (Sutherland, 1963) that placed a coarse, sandy matrix (Gmm). Most conglom- tary rocks from nearby to the west of the fault. the contact at the base of the fi rst arkose (unit erate beds have an erosive lower contact. Clast (2) The closest metasedimentary source rocks 105, Fig. 7). The lowermost arkosic sandstone, imbrication in the Flechado Formation and the for these deposits are in the Picuris Mountains, however, is thin and weathers recessively, and lower, quartzarenitic part of the Alamitos For- now dextrally separated from the Rio Chiquito thus does not provide a useful datum for map- mation indicates that paleofl ow was toward the area by at least 20 km. (3) The Pennsylvanian ping. Moreover, several thin quartzarenite beds southeast (Fig. 10). If the observed clockwise succession west of the Picuris-Pecos fault at the stratigraphically overlie this arkose, indicating defl ection of bedding strike near the Picuris- Rio Chiquito, although in part the same age as interfi ngering of compositionally distinct sand- Pecos fault is the result of vertical-axis rotation beds east of the fault, is much fi ner grained, sug- stones in the stratigraphic transition between the near the fault, then the restored paleocurrent gesting post–early Desmoinesian juxtaposition two formations. Thus, by our mapping criteria direction is approximately eastward. by strike-slip faulting. (4) Dating of monazite the Flechado Formation is 391 m thick, but the We examined 14 thin sections from the grains in quartzite clasts from the Rio Chi quito thickness of the Pennsylvanian section beneath Flechado Formation and 4 from the Alamitos area suggests that these clasts are dextrally sep- the fi rst arkose is 477 m. Formation (Fig. 7). Sandstones throughout the arated from their likely source terranes in the The lower part of the Flechado Formation Flechado Formation are quartzarenites (Fig. 11) Ortega Quartzite by ~40–50 km. east of the Picuris-Pecos fault is dominated by according to the classifi cation of Folk (1974). sandstone, pebbly sandstone, conglomerate, Monocrystalline quartz is dominant; a few poly- Rio Chiquito Area East of and sedimentary breccia (Fig. 8). The mea- crystalline quartz grains were noted. Many quartz the Picuris-Pecos Fault sured section fi nes upward, in both maximum grains exhibit undulose extinction and deforma- East of the Picuris-Pecos fault at Rio Chi- clast size and mudstone content. The Flechado tion lamellae. Feldspar grains are rare. Schistose quito, Pennsylvanian strata are well exposed Formation also becomes markedly fi ner and metamorphic rock fragments are rare and com- and unconformably overlie the Proterozoic thinner toward the east, away from the Picuris- posed of quartz, mica, and minor feldspar. Ortega Quartzite. We measured and sampled a Pecos fault. Outcrops are commonly conglom- Recessive, regolith-covered slopes in the 479-m-thick section of the uppermost Ortega eratic near the fault, and breccia is restricted to upper part of the measured section appear to be Quartzite, Flechado Formation, and lower an area within ~250 m of the fault. Sandstone underlain mostly by drab shale. Where exposed, Alamitos Formation (Fig. 7). The Proterozoic and conglomerate are poorly sorted. Beds are these shales commonly contain marine fossils. Ortega Quartzite is a gray to white, amphibolite generally tabular, and bed thickness typically A thin section from one of these marine shales facies quartzarenite. It commonly exhibits dis- ranges between 0.3 and 3 m. (unit 57, Fig. 7) was examined. It is a gray to dark tinctive bedding and cross-bedding defi ned by Clasts in the Flechado Formation east of the gray, nodular, bioclastic, bioturbated calcareous heavy mineral laminae. The Ortega Quartzite is Picuris-Pecos fault consist almost entirely of siltstone. Unit 57 contains crinoids, brachiopods nearly pure quartz (~98%; Montgomery, 1963), metasedimentary rocks; most (~90%) are Ortega (Orbiculoidea, Derbyia, Linoproductus, cf. with rare muscovite, schistose rock fragments Quartzite. Vein quartz, common in the Ortega Parajuresania, Desmoinesia cf. missouriensis, (quartz, mica, feldspar), hematite, kyanite, sil- Quartzite, composes an estimated 5%–10% of Mesolobus striatus, Hustedia, Anthracospirifer, limanite, andalusite, tourmaline, and zircon. We clasts and is more common in the upper part of Neospirifer cameratus, Composita, Crurithy- also identifi ed and dated monazite grains in the the Flechado Formation. The remainder of the ris, Phricodothyris), gastropods (Pharkidono- Ortega Quartzite and in quartzite clasts in the clasts are schist (some are kyanite bearing) that tus, Euconospira sp., Glabrocingulum? sp.,), Pennsylvanian section (see the following). resemble lithotypes in the upper Ortega Quartz- bryozoans (Penniretepora, Fenestella?, Rhom- The lower 230 m of the Phanerozoic section ite (P.W. Bauer, 2007, personal commun.). bopora?), ostracods, bivalves (Aviculopecten, east of the Picuris-Pecos fault at Rio Chiquito Schist clasts become less abundant to the east, Acanthopecten, Paleolima?, Streblochondria?), were interpreted by Sutherland (1963) to be a away from the Picuris-Pecos fault. No granite or rugose corals, trilobite fragments, agglutinated conglomeratic facies of the Del Padre Member gneiss clasts are present. Maximum clast size of foraminiferans, a small Eotuberitina foramini- of the Mississippian Espiritu Santo Formation. conglomerates is ~20 cm. The breccia bed (unit feran, and a nautiloid (Domatoceras?). Based Armstrong (1967, p. 9), however, showed that 23, Fig. 7) contains clasts of quartzite and schist on the biostratigraphy of Sutherland and Har- these strata are Pennsylvanian, based in part on as much as 2 m in length. Breccia clasts com- low (1973), the brachiopod assemblage in unit fossils from marine shales that interfi nger with monly exhibit a jigsaw fabric where individual 57 is suggestive of an age close to the Atokan- the conglomeratic section. We also collected large clasts have been broken into smaller, Desmoinesian boundary (Table 1, sample A). Pennsylvanian fossils from these shales. semicoherent fragments between which the ori- The Alamitos Formation east of the Picuris- The Pennsylvanian section east of the Picuris- entation of bedding and foliation has not been Pecos fault encompasses the uppermost 84 m Pecos fault at Rio Chiquito consists of an strongly rotated (Fig. 9). of the measured section (Fig. 7) and is poorly upward-fi ning succession of sandstone, pebbly Sedimentary structures in the Flechado For- exposed with thick covered intervals. These sandstone, conglomerate, sedimentary breccia, mation are commonly indistinct, probably covered intervals are probably mostly under- mudstone, and marine limestone. The contact due to bioturbation or diagenetic overprinting. lain by marine shale, as can be demonstrated in between the Flechado Formation and the under- Sandstone shows solitary or grouped trough some cases by tracing them laterally to areas of

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Figure 7. Measured stratigraphic section and sampled intervals near Rio Chiquito. Section measured with jacob staff and compass by Lucas, Krainer, and Cather. Location of measured section shown in Figure 6.

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Alamitos Formation are well exposed. Paleocur- rent measurements from pebble imbrication and cross-bedding in these deposits show derivation of arkosic sand was from the northeast (Fig. 13), even in outcrops near the Picuris-Pecos fault. These paleocurrent data indicate that arkosic sandstone in the Alamitos Formation was not derived from the Uncompahgre uplift west of the Picuris-Pecos fault. This conclusion is supported by the lack of coarsening of the Alamitos Formation toward the Picuris-Pecos fault in the study area. Unlike the Flechado Formation, the mudstone, limestone, and ﬁ ne sandstone of the Alamitos Formation are juxtaposed against the Picuris-Pecos fault without intervening fault scarp deposits. Limestone beds (units 101–104, Fig. 7) beneath the arkose contain diverse marine fossils. These include brachiopod shell and spine fragments, crinoid stems and other echinoderm fragments, smaller foraminiferans (Bradyina magna, Bradyina sp., Calcivertella, Clima- cammina ex gr. moelleri, Climacammina sp., Figure 8. Conglomeratic strata in lower Flechado Formation at measured section ~250 m Endothyra, Eotuberitina, Globivalvulina ex gr. east of the Picuris-Pecos fault near Rio Chiquito. View is to the northwest. bulloides, Glomospira, Syzrania, Tetrataxis), ostracods, bryozoans, fusulinids [Wedekindel- lina euthysepta (Fig. 14), Beedeina aff. B. hay- partial exposure. Sandstone and conglomerate feldspar, dominantly potassium feldspar (Fig. ensis (Fig. 15)], algae and/or problematic algae in the lower part of the unit are quartzose and 12). The feldspars are strongly altered. Rare (Kamaena, Insolenthica, Anthracopelopsis), are similar in detrital composition to those in the chert grains and detrital micas are also present. small gastropods, and very rare Tubiphytes. The underlying Flechado Formation. The top of the Because the Alamitos Formation is poorly fusulinids from units 101 and 103 are of early measured section contains thick marine lime- exposed, we obtained no paleocurrent informa- to early-middle Desmoinesian age (Table 1, stones, and is capped by a 1-m-thick arkosic tion from the unit in the study area. In the valley sample B; cf. Wilde, 2006). sandstone. This arkose contains mostly mono- of the Rio Santa Barbara, ~10 km to the north, crystalline quartz grains and abundant detrital however, ﬂ uvial deposits in the middle part of the Rio Chiquito Area West of the Picuris-Pecos fault Pennsylvanian strata are poorly exposed west of the Picuris-Pecos fault and consist mostly of

Figure 9. Clasts of quartzite and schist in sedimentary breccia, lower Flechado Formation (Fig. 7, unit 23). Note common jigsaw fracturing of clasts that may indicate deposition by rock ava- Figure 10. Paleocurrent directions for lanche. Location is 441435E, 3991111N (North American Datum, Flechado Formation near Rio Chiquito 1927). Compass is 7 cm wide. from pebble imbrications.

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mismatch is likely the result of post–early Des- moinesian juxtaposition by strike-slip faulting (discussed in the following).

Depositional Environments and Provenance of Pennsylvanian Strata in the Rio Chiquito Area Based on textural trends, bedding types, paleontology, and paleocurrent data, we inter- pret the Pennsylvanian succession east of the Picuris-Pecos fault as proximal braid-delta deposits that grade upsection and eastward into marine mudstone and limestone. Quartzarenitic fl uvial deposits of the Flechado Formation and the lower Alamitos Formation were derived from Proterozoic metasedimentary rocks in the adjacent Uncompahgre uplift to the west. Uncompahgre-derived quartzarenitic fl uvial deposits were onlapped and buried by marine strata during the early to middle Desmoinesian. Mostly marine deposition again gave way to fl uvial and deltaic sedimentation during deposi- Figure 11. Flechado Formation quartzarenite from the Rio Chiquito measured tion of the Alamitos Formation. Arkosic fl uvial section (Fig. 7, unit 20). Crossed nicols; width of fi eld is 3.2 mm. deposits in the Alamitos Formation prograded from the north and east, probably mostly from the Cimarron arch, the Sierra Grande uplift, or mudstone with subordinate sandstone, pebbly Isogramma sp., Derbyia sp., Anthracospirifer from sediments that spilled over the Cimarron sandstone, and minor marine limestone. These sp.), crinoid stem fragments, encrusting and arch from the central Colorado basin (Fig. 4). strata are faulted down against, and apparently fenestrate bryozoans, the pectinid bivalve Streb - The fl uvial and marine deposits of the Alamitos overlie, Proterozoic granitic gneiss, but the basal lochondria, a plant stem (Calamites), a trilo- Formation grade upward into the entirely non- contact is not exposed. Sandstone in the lower bite pygidium, and gastropods. Although no marine Sangre de Cristo Formation, which was (southern) part of the Pennsylvanian succes- age-diagnostic fossils were found in the lower, also derived largely from the north (Soegaard sion is quartzarenitic, and is overlain by a fi ne- quartzarenite-bearing beds, we tentatively cor- and Caldwell, 1990). Arkosic fl uvial and grained succession containing arkosic sand- relate these strata to the Flechado Formation marine deposits derived from the north and stone. Because of poor exposure, Pennsylvanian based on quartzose sandstone compositions and east prograded across and buried part or all of strata west of the Picuris-Pecos fault were not lack of abundant limestone. the southeastern Uncompahgre uplift during differentiated into formal units during mapping. Pennsylvanian quartzarenitic strata west the middle to late Desmoinesian, although the Early to middle Desmoinesian marine fossils of the Picuris-Pecos fault at the Rio Chiquito paleogeographic extent of this burial is uncer- occur in the upper arkosic succession (Table 1, are distinctly fi ner grained than those east of tain because most Pennsylvanian deposits in samples C and D) and allow correlation to the the fault. Maximum clast size in pebbly sand- the Santa Fe Range west of the Picuris-Pecos Alamitos Formation. These fossils include bra- stones is 1–2 cm. Despite being apparently more fault have been eroded. chiopods (Antiquatonia hermosana, Echinaria proximal to the source than deposits east of the The Flechado Formation east of the Picuris- knighti, Composita subtilita, Phricodothyris Picuris-Pecos fault (as shown by paleocurrent Pecos fault at Rio Chiquito is remarkably coarse perplexa, Linoproductus sp., Linoproductus data; Fig. 10), the Flechado Formation west grained, more so than any other Pennsylvanian sp., Punctospirifer, Neospirifer cameratus, of the fault is much fi ner grained. This textural succession we have observed in New Mexico.

TABLE 1. FOSSIL LOCALITIES AND AGE DETERMINATIONS FOR PENNSYLVANIAN STRATA IN THE WESTERN TAOS TROUGH, NEW MEXICO Sample Location (UTM, Stratigraphic context Age-diagnostic taxa Age interpretation NAD1927)* 441516E, Mudstone in middle part of Flechado Formation, ~190 m above Brachiopods Desmoinesia cf. Near Atokan-Desmoinesian A 3991193N base of Pennsylvanian section near Rio Chiquito, east of missouriensis, Neospirifer cameratus, boundary Picuris-Pecos fault and Mesolobus striatus 441967E, Limestone in lower part of Alamitos Formation, ~475 m above Fusulinids Wedekindellina euthysepta Early to early-middle B 3991437N base of Pennsylvanian section near Rio Chiquito, east of entiseptata, Beedeina aff. B. hayensis Desmoinesian Picuris-Pecos fault 435531E, Brachiopods Echinaria knighti?, Limestone in Alamitos Formation, west of Picuris-Pecos fault C 3968141N Neospirifer?, and Anthracospirifer?; Desmoinesian near Rio Chiquito pectinid bivalve Streblochondria 440642E, Carbonaceous siltstone in Alamitos Formation, west of Picuris- Brachiopods Antiquatonia hermosana and Early to middle D 3992363N Pecos fault near Rio Chiquito Echinaria knighti Desmoinesian 435531E, Limestone in Alamitos Formation east of Picuris-Pecos fault, Brachiopods Anthracospirifer sp., and E Desmoinesian 3967445N ~0.8 km south of Cave Creek Antiquatonia cf. A. hermosana *Universal Transverse Mercator, North American Datum.

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Figure 13. Paleocurrent directions for Alami- tos Formation near Rio Santa Barbara from pebble imbrication and cross-bedding.

cise knowledge of which part of the Ortega Figure 12. Alamitos Formation arkosic sandstone from the Rio Chiquito mea- supplied the detritus is desirable. The Ortega sured section (Fig. 7, unit 105). Crossed nicols; width of fi eld is 6 mm. Quartzite is best exposed in the Tusas Moun- tains ~70 km northwest of the study area (Fig. 16), where it forms an exposure belt ~30– Pebbly sandstone, conglomerate, and sedimen- consisted of metasedimentary rocks. Today, the 40 km wide that is an expression of the structur- tary breccia constitute ~20% of the Flechado terrane west of the Picuris-Pecos fault consists ally complex synclinorium in which the quartzite Formation at Rio Chiquito. Clasts within the of Proterozoic granite and granitic gneiss. This is preserved. East of the Picuris-Pecos fault, the breccia bed are generally highly fractured, but lithologic mismatch provides strong evidence Ortega Quartzite also forms a structurally com- the fractures generally do not extend into the that major strike slip on the Picuris-Pecos fault plex belt ~40 km wide that is partly obscured by matrix between the clasts (Fig. 9). Such jigsaw has occurred since the early Desmoinesian. The Paleozoic strata. There, the Ortega is preserved fracture fabrics are common in rock-avalanche closest possible source for the Ortega Quartzite in a complex asymmetrical synclinorium that is breccia (Yarnold, 1993; Friedmann, 1997). clasts in the Flechado Formation exposed east of bounded by the steep Pecos thrust on the south Sedimentary breccia suggests a nearby, steep the Picuris-Pecos fault is in the Picuris Moun- and a beveled erosional edge beneath Paleozoic source terrane. tains, now dextrally separated by at least 20 km strata on the north near the Picuris Mountains Textural and facies evidence indicates that from the Rio Chiquito exposures. (Fig. 3; see fi g. 5 of Cather et al., 2006, for cross fl uvial strata of the Flechado Formation east of sections that show the synclinorium geometry). the Picuris-Pecos fault were deposited in prox- Monazite Age Constraints on Provenance of Because the Ortega Quartzite belt east of the imity to their source area. Paleocurrent data and Quartzite Clasts at Rio Chiquito Picuris-Pecos fault is still partly mantled by late the eastward fi ning and thinning of the Flechado Although it is clear that the source of quartz- Paleozoic strata, the areal distribution of the Formation show this source region was to the ite clasts in the Pennsylvanian strata at Rio Ortega in this region is an approximation of the west, and clast composition indicates the source Chiquito is the Ortega Quartzite, a more pre- geometry of the Ortega paleoexposure belt dur-

Figure 15. Fusulinid Beedeina aff. B. hayen- Figure 14. Fusulinid Wedekindellina euthysepta from the Rio Chiquito sis from the Rio Chiquito measured section measured section (Fig. 7, unit 103). Width of ﬁ eld is 5.4 mm. (Fig. 7, unit 103). Width of ﬁ eld is 3.6 mm.

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Figure 16. Simpliﬁ ed geologic map of the Tusas Mountains region showing southward younging of monazite ages on the Ortega Quartzite (Kopera, 2003). Map location is shown in Figure 2. Aeromagnetic lineament corresponds to the southern structural boundary of the Hondo Group metasedimentary terrane (Cather et al., 2006).

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Formation. These yielded only ca. 1.4 Ga ages. In contrast to the dominantly mixed ages in clasts at the Rio Chiquito, the basement rocks in the nearest possible source region, the Picuris Mountains, exhibit nearly all ca. 1.4 Ga monazite ages. Monazite grains from the southern (Wingsted, 1997) and northern (Daniel and Pyle, 2006) parts of the range have yielded only ca. 1.4 Ga ages. We analyzed two Ortega Quartzite samples from the central part of the range near Picuris Peak. One sample yielded only ca. 1.4 Ga ages; the other exhibited a range of ages between ca. 1.1 and 1.7 Ga (samples PPF-35 and PPF-38; the Supplemental File [see footnote 1]). Thus, unlike the southernmost exposures of Ortega Quartzite in the Tusas Figure 17. Histogram of ages of monazite grains in quartzite Mountains that so far have yielded only ca. 1.4 clasts from the Rio Chiquito measured section (Fig. 7). Ga monazite (Kopera, 2003), the Picuris Moun- tains also contain a small portion of ca. 1.7 Ga monazite. Nonetheless, the dominance of ca. ing the Pennsylvanian. In the Picuris Mountains We analyzed monazite grains from seven 1.4 Ga monazite ages in the Picuris Mountains directly west of the Picuris-Pecos fault, only the quartzite clasts in the Flechado Formation and suggests that the southward-younging zona- southernmost 7 km of the Ortega Quartzite belt one quartzite clast from the lower part of the tion of monazite ages documented in the Tusas is currently exposed; the remainder is presum- Alamitos Formation (Fig. 7; analytic techniques Mountains (Kopera, 2003) probably also exists ably faulted beneath the fi ll of the Neogene San are presented in the Supplemental File1). These near the Picuris-Pecos fault. Luis Basin of the Rio Grande Rift, as is sug- clasts were sampled from the lower 430 m of the Several conclusions can be drawn from the gested by an area of low aeromagnetic value that Pennsylvanian section from east of the Picuris- presence of mixed ca. 1.4 and 1.7 Ga ages of extends ~30 km northward from the Plomo fault Pecos fault at Rio Chiquito. Between 10 and 20 monazites in quartzite clasts from Pennsylva- (Cather et al., 2006). individual age determinations on 5–6 monazite nian strata near the Rio Chiquito. (1) The clasts No systematic lateral variation of lithology is grains were carried out for each sample. Mona- were probably not derived from the northern part apparent within the Ortega Quartzite (Soegaard zite grains yielded ages ranging between ca. of the Ortega Quartzite outcrop belt in which ca. and Eriksson, 1985), so conventional petro- 1.2 and 1.8 Ga (Fig. 17; for individual samples, 1.7 Ga ages predominate. (2) The clasts were graphic techniques cannot be used to specify a see the Supplemental File [see footnote 1]) and probably not derived from the southernmost part detrital source within the Ortega outcrop belt. form two distinct modes, one centered ca. 1.4 Ga of Ortega belt in the Picuris Mountains because The age population of detrital zircons in the and the second ca. 1.7 Ga. most monazite grains there are ca. 1.4 Ga. This Ortega Quartzite is also uniform (1.80–1.70 Ga; Monazite grains in the quartzite clasts are conclusion is in need of further testing, however, Jones et al., 2009). Monazite ages within the typically fi ne grained (~10–20 µm in diam- as one of our samples from the Picuris Moun- unit, however, exhibit a systematic southward eter), and many exhibit multiple compositional tains did contain ca. 1.7 Ga monazite grains. younging in the Tusas Mountains (Kopera et al., domains that in some cases correspond to dif- (3) The clasts most likely were derived from the 2002; Kopera, 2003). In the northern part of the ferent age determinations (Fig. 18). Some of the central, mixed-age part of the Ortega belt. The Tusas Mountains, the Ortega Quartzite contains compositional domains are small relative to the zone of mixed-age monazite in the Tusas Moun- ca. 1.7 Ga (1.85–1.65 Ga) monazite that repre- 3–5 µm beam size, so some of the determined tains is ~20–30 km north of the southern edge of sents both pre–1.69 Ga detrital grains and mona- ages may be a combination of older and younger the Ortega belt. If a monazite zonation similar to zite that grew during ca. 1.67 Ga metamorphism. ages. These may account for some data points that documented by Kopera (2003) in the Tusas In the south, monazite ages are entirely ca. 1.4 Ga that plot between the two age-distribution peaks. Mountains existed near the Picuris-Pecos fault, and refl ect recrystallization and growth of new Quartzite clasts from the Pennsylvanian sec- then the most likely source of the quartzite clasts monazite at deeper structural levels to the south tion at Rio Chiquito all contain monazite grains during ca. 1.4 Ga peak metamorphism (Kopera representative of the ca. 1.4 Ga and 1.7 Ga age et al., 2002; Kopera, 2003, his fi g. 6.19). Mixed modes, except 3 clasts sampled from 23 m, ages, commonly with older (ca. 1.7 Ga) cores 68 m, and 100 m above the base of the Flechado and younger (ca. 1.4 Ga) rims, exist in the central Tusas Mountains in a zone ~20–30 km north of the southern structural boundary of the Ortega 1Supplemental File. PDF fi le of Monazite Ana- outcrop belt (Fig. 16). Although our results lytical Techniques; includes Supplemental Figure show that the age zonation of Ortega monazite showing age histograms for monazite grains in is less systematic near the Picuris-Pecos fault quartzite clasts and Ortega Quartzite bedrock, and (see following), we dated monazite grains from Supplemental Table showing location of monazite Figure 18. Backscattered electron image clasts sampled from the measured section in the samples. If you are viewing the PDF of this paper and Th X-ray map of representative or reading it offl ine, please visit http://dx.doi.org/10 Flechado Formation at Rio Chiquito to better .1130/GES00649.S1 or the full-text article on www monazite grain showing age determina- refi ne the provenance of these clasts. .gsapubs.org to view the Supplemental File. tions (Ma). Monazite grain is ~60 μm.

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is the central Ortega belt, now downfaulted into Cave Creek Area Description the Flechado Formation, and the intercalated the Rio Grande Rift near Taos. If this is correct, South of the Truchas uplift, Pennsylvanian limestone and quartzarenitic strata to the east then the quartzite clasts in the Rio Chiquito sec- strata overlie Mississippian strata and are jux- are the La Pasada Formation. Because poor tion east of the Picuris-Pecos fault have been taposed against Proterozoic granitic gneiss exposure prohibits confi dent mapping of the separated from their source regions west of the west of the Picuris-Pecos fault. The fault there lateral gradation between these formations, we fault by ~40–50 km of dextral slip since the early dips steeply west (~80°) and exhibits a linear refer to their clastic components collectively Desmoinesian. (4) Clasts containing mixed ca. trace with segments that branch or anasto- as the quartzarenitic petrofacies. We collected 1.4 Ga and 1.7 Ga monazite ages are present mose (Moench et al., 1988). Exposure qual- paleocurrent data from the quartzarenitic petro- at both the bottom and top of the 430-m-thick ity in this deeply forested area is poor, except facies along Cave Creek ~2 km east of the fault; sampled interval and imply that the detritus for locally in river valleys. We examined exposures these show eastward paleofl ow (Fig. 20). this stratigraphic interval was derived primarily along Cave Creek and in adjacent drainages to Near the Picuris-Pecos fault, the quartzaren- from the central mixed-age zone in the Ortega the north (Horsethief Creek) and south (Rito itic petrofacies is overlain by the arkosic petro- Quartzite belt. The lack of clasts derived from Oscuro and Windsor Creek). facies ~1 km south of Cave Creek. The arkosic other parts of the Ortega belt implies that major Two distinct clastic petrofacies are pres- petrofacies consists of feldspathic sandstone strike slip did not occur during deposition of the ent in Pennsylvanian strata near Cave Creek. and sparse pebbly sandstone that are interbed- Flechado and lower Alamitos Formations (Mor- The quartzarenitic petrofacies is expressed in ded with marine limestone and mudstone, a rowan(?) to early Desmoinesian; time interval outcrop and fl oat near the Picuris-Pecos fault lithology that supports correlation to the Alami- of ~13 m.y. or less; Davydov et al., 2010). along Cave Creek and in areas to the north, and tos Formation. A fossiliferous limestone in the consists of quartzarenitic sandstone, pebbly northern part of the arkosic petrofacies yielded Cave Creek Area sandstone, and rare conglomerate. Maximum Desmoinesian brachiopods (Table 1, sample E), clast size is ~8 cm. Clasts are mostly (~90%) and fossils in the arkosic petrofacies ~5 km to In this section we document paleocurrent Ortega Quartzite; the remainder are schist and the southeast are middle to late Desmoinesian trends and clast lithologies in Pennsylvanian vein quartz (Fig. 19). No granite or gneiss clasts (Windsor Creek locality of Sutherland and sandstones near Cave Creek, south of the Tru- were observed. Harlow, 1973, interpreted by them to be lower chas uplift (Fig. 3). Metasedimentary clasts The quartzarenitic petrofacies interfi ngers Alamitos Formation). The maximum clast size present in the Flechado and La Pasada Forma- eastward with mudstone and marine limestone is 3 cm; pebbles consist of varying proportions tions were derived from the Uncompahgre uplift that contain Desmoinesian fossils (Sutherland of granite, vein quartz, and feldspar megacrysts. west of the Picuris-Pecos fault. The mismatch and Montgomery, 1975, p. 89). Limestone is between these conglomeratic strata and their rare near the Picuris-Pecos fault, but becomes Cave Creek Area Interpretation nearest possible source in the Picuris Mountains abundant 2–3 km east of the fault. Using the A western source for the quartzarenitic petro- requires at least ~40 km of post-Desmoinesian nomenclature of Sutherland (1963), the quartz- facies is indicated by (1) decreased abundance dextral slip on the fault. arenitic Pennsylvanian beds near the fault are of limestone and mudstone intercalated within the quartzarenitic petrofacies near the Picuris- Pecos fault, and (2) paleocurrent data from clast imbrication that record paleofl ow toward the east. These paleocurrent data indicate that quartzite clasts in the quartzarenitic petrofacies were derived from metasedimentary rocks exposed on the Uncompahgre uplift, west of the

Figure 20. Paleocurrent directions for Figure 19. Quartzite clasts in Flechado Formation conglomerate, Horsethief Creek (UTM, quartzarenitic petrofacies near Cave Creek North American Datum, 1927, 436698E, 3969963N). from pebble imbrications.

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Picuris-Pecos fault. Because of poor exposure, east-up shear; Luther et al., 2008). These folds few dextral minor faults parallel the dextral we were unable to obtain paleocurrent infor- refolded metasedimentary rocks in both limbs Punchbowl fault of California, a well-studied, mation from the arkosic petrofacies. The pres- of the east-west–trending synclinorium exposed extinct strand of the San Andreas fault system ence of limestone and mudstone and the lack in the Picuris and Truchas Mountains. Mont- (Chester and Logan, 1986; Chester and Chester, of coarse conglomerate in this facies near the gomery (1963, p. 16) reasoned that this refold- 1998, Wilson et al., 2003). For these reasons, we Picuris-Pecos fault, however, indicate it was not ing required “extreme plasticity” and therefore question the rationale of Erslev’s (2001) method derived from the adjacent Uncompahgre uplift. occurred during Proterozoic regional meta- for determination of slip of a major fault using More likely, arkose was distally derived from morphism, and that dextral slip on the adjacent adjacent minor faults, particularly if the major granitic rocks in the Cimarron arch or Sierra Picuris-Pecos fault was therefore probably also fault is weak (e.g., Ghisetti, 2000). Grande uplift to the north and east, as demon- Proterozoic. Peak metamorphic conditions were Paleomagnetism has been used to test for strated for arkose of the Alamitos Formation attained ca. 1.4 Ga in northern New Mexico vertical-axis rotations associated with dextral at the Rio Santa Barbara (Fig. 13) and arkose with temperatures of 500–550 °C and pressures faulting along the eastern Colorado Plateau of the overlying Sangre de Cristo Formation of 3.5–4.0 kbar (Williams et al., 1999a; Karl- boundary. Although several areas of clockwise (Soegaard and Caldwell, 1990). strom et al., 2004). Fault slip under such condi- vertical-axis rotation in Permian–Triassic rocks Regional stratigraphy and fi eld relationships tions should produce mylonitic foliation, but to have been documented in the Southern Rocky indicate that the arkosic petrofacies (Alamitos date no mylonites have been reported anywhere Mountains of New Mexico and Colorado (e.g., Formation) overlies the quartzarenitic petro- along the Picuris-Pecos fault. Moreover, out- Wawrzyniec et al., 2002; Geissman, 2004), facies (Flechado and La Pasada Formations, crop-scale fracturing and quartz microstructural a locality on the Picuris-Pecos fault ~35 km undivided) south of Cave Creek. The ultimate fabrics in the refolds developed in brittle and south of the study area shows no evidence of southward extent of the quartzarenitic petrofa- brittle-ductile regimes (McDonald and Nielsen, signifi cant vertical-axis rotation since the late cies adjacent to the Picuris-Pecos fault is thus 2004; Luther et al., 2008), not during peak meta- Paleozoic and has been interpreted to indicate obscured by the overlying arkosic petrofacies morphism. The signifi cance of these refolds to that no major strike slip has occurred since then (Alamitos Formation) south of Cave Creek. the slip history of the Picuris-Pecos fault is not (Wawrzyniec et al., 2007). However, several Nonetheless, it is clear that the quartzarenitic yet clear. Although initiation of the fault near localities that lack paleomagnetically defi ned, petrofacies extends southward along the Picuris- peak metamorphic conditions can be ruled out, vertical-axis rotations exist along the dextral Pecos fault at least to the Cave Creek area. The subsequent Proterozoic (Grenville and younger) San Andreas and San Gabriel faults (e.g., Tavar- southern boundary of the metasedimentary ter- initiation remains possible (Cather et al., 2006). nelli, 1998; Levi et al., 2005), and show that rane west of the fault was therefore also near, or Several workers have interpreted Proterozoic strike-slip faulting is not everywhere associated south of, the latitude of Cave Creek area during dextral slip on the Picuris-Pecos fault using pro- with large rotations. the Desmoinesian. Today, this boundary (the cess-of-elimination arguments against Ancestral The mismatch between the mineralogic com- Plomo fault in the Picuris Mountains) is dex- Rocky Mountain and Laramide dextral slip. position of Pennsylvanian clastic rocks in the trally separated from the southernmost extent of Yin and Ingersoll (1997) interpreted Ancestral western Taos trough and source terranes west of the quartzarenitic petrofacies by at least 40 km, Rocky Mountain and Laramide faults in the the Picuris-Pecos fault was fi rst noted by Suther- and perhaps by as much as 50 km if our infer- Southern Rocky Mountains as mostly dip slip, land (1963). He accepted Montgomery’s (1963; ences based on monazite ages are correct. These and that the observed dextral basement separa- see above) hypothesis that dextral separation on estimates exceed the ~37 km dextral separa- tions therefore must be Proterozoic. They cited a the Picuris-Pecos fault probably occurred dur- tion that exists today (as measured between the lack of defi nitive evidence for strike slip during ing the Proterozoic. So, to explain the mismatch, originally continuous Hondo and Brazos Cabin the Phanerozoic orogenies, but did not produce Sutherland (1963, p. 41) argued that west-up synclines; Montgomery, 1963), and imply that compelling evidence for their preferred dip-slip Pennsylvanian slip caused the northward ero- a sinistral separation existed on the fault dur- model (see Cather, 2004, p. 236, for a critique of sional retreat of metasedimentary rocks west ing the middle Pennsylvanian. This possibil- their Laramide model). of the Picuris-Pecos fault. Sutherland (1963), ity is supported by evidence for pre-Permian, Erslev (2001) collected numerous kinematic following Montgomery (1963), envisioned the ~20°–30° anticlockwise rotation of gneissic data from minor faults in Permian and younger southern boundary of metasedimentary terrane foliation in the damage zone of the Picuris- rocks throughout north-central New Mexico; in the Picuris Mountains to be marked by an σ Pecos fault at Deer Creek ~35 south of the study he calculated an east-northeast Laramide 1 intrusive contact with younger granitic rocks to area (Cather et al., 2008), and is compatible with direction that is nearly perpendicular to the the south; if this intrusive contact dipped gently the predicted sense of shear on the fault during major dextral faults in the region (including the to the north, Sutherland reasoned, then Pennsyl- the Grenville and Neoproterozoic deformations Picuris-Pecos fault), and noted relatively few vanian uplift and erosion on the west side of the (Cather et al., 2006). dextral minor faults parallel to the major faults. Picuris-Pecos fault would cause the northward From these relationships, Erslev (2001, p. 63) retreat of the metasedimentary terrane boundary DISCUSSION interpreted only “limited” (but unspecifi ed) dex- and could explain the mismatch. tral slip on north-striking faults in the Southern There are several questionable aspects of Several workers have advocated a Protero- Rocky Mountains during the late Laramide. We Sutherland’s (1963) erosional retreat model. σ zoic origin for the dextral separation on the note, however, that elsewhere 1 is nearly per- First, Pennsylvanian uplift and erosion west Picuris-Pecos fault. The fi rst such interpreta- pendicular to known strike-slip faults, such as of the Picuris-Pecos fault would necessarily tion was by Montgomery (1963), who mapped the San Andreas fault (e.g., Mount and Suppe, have been deep and extensive, as no remnants folds near the Picuris-Pecos fault in the Picuris 1987; Zoback et al., 1987; Provost and Houston, of the metasedimentary rocks are preserved Mountains and interpreted them to have resulted 2001; Wilson et al., 2003) and the Great Suma- at even the highest elevations of the Santa Fe from dextral-sense drag on the fault (the geom- tran strike-slip fault (Mount and Suppe, 1992). Range (3847 m above sea level). However, as etry of these folds is also compatible with Moreover, detailed studies have shown that very described previously, a variety of geologic and

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thermochronologic data argue against major sediment shed from metasedimentary rocks gest that no early Desmoinesian uplift occurred dip slip (Pennsylvanian or otherwise) on the exposed in a Pennsylvanian precursor to the in the Truchas area. Paleocurrent data provide Picuris-Pecos fault. Second, recent mapping Laramide Truchas uplift east of the Picuris- an additional argument against a Pennsylvanian and geochronologic studies have revealed errors Pecos fault, rather than being derived from precursor to the Truchas uplift. Pebble imbrica- in the original interpretation by Montgomery west of the Picuris-Pecos fault. The Truchas tions in metasedimentary-sourced Pennsylva- (1963) of the Proterozoic geology in the Picuris uplift developed in response to west-up slip on nian deposits show no evidence of paleofl ow Mountains. Although local, younger gran- the Jicarilla fault, a steep (~75°) west-dipping away from a precursor uplift, but instead show ites (ca. 1.4 and 1.65 Ga) were intruded in the reverse fault that juxtaposes metasedimentary that paleofl ow was from the west (Figs. 3, 10, region south of the metasedimentary terrane in rocks (mostly Ortega Quartzite) with folded and 20). the Picuris Mountains, the basement underly- Pennsylvanian strata on the east (Fig. 3). To test Although the slip history of the Picuris-Pecos ing the metasedimentary rocks of northern New the possibility of Pennsylvanian orogenesis in fault is likely complex (Cather et al., 2006), we Mexico is mostly Yavapai age (ca. 1.76–1.72 the Truchas area, we examined well-exposed now have two snapshots into this history. The Ga; Karlstrom et al., 2004) and thus cannot be Pennsylvanian beds cut by the Jicarilla fault fi rst was during regional metamorphism in the in intrusive contact with the overlying, younger north of Pecos Baldy Lake and at Chimayosos Mesoproterozoic (ca. 1.4 Ga). At this time the (ca. 1.69 Ga) metasedimentary rocks. More- Peak. In both areas, these beds contain fos- Picuris-Pecos fault did not exist. The second over, detailed mapping and kinematic analysis sils of early Desmoinesian age (see measured snapshot is during the middle Pennsylvanian, in the Picuris Mountains (Bauer, 1993, 2004) sections 10 and 40 of Sutherland and Harlow, when there is clear evidence for both the exis- have shown that the southern boundary of the 1973), and thus are the same age as the upper tence of the fault as a basin-bounding struc- metasedimentary terrane is a steep, south- part of the conglomeratic section at Rio Chiq- ture and for the paleolocation of a distinctive dipping ductile thrust (the Plomo fault), not a uito. Despite their proximity to the Jicarilla metasedimentary terrane in the uplands west gently north-dipping intrusive contact. Thus, fault, the measured sections of Sutherland and of the fault (Fig. 22). There are three lines of uplift and erosion west of the Picuris-Pecos Harlow (1973) are dominated by mudstone and evidence that suggest ~40–50 km of post–early fault would cause a southward migration of the marine limestone (Fig. 21). Conglomerate and Desmoinesian dextral slip occurred on the metasedimentary terrane boundary, not north- pebbly sandstone occur only as rare, thin beds Picuris-Pecos fault: (1) Ortega Quartzite clasts ward as envisioned by Sutherland (1963). in which the maximum size of quartzite clasts in the lower part of the Rio Chiquito section Baltz and Myers (1999) speculated that the is typically 1–3 cm. Such lithologic attributes are separated by at least 20 km from their near- Pennsylvanian provenance mismatch fi rst noted are atypical for sediments deposited adjacent est possible source in the Picuris Mountains; by Sutherland (1963) could be explained by to an active mountain-front fault, and thus sug- (2) monazite ages within these clasts further suggest they are separated from their probable source in the middle part of the Ortega Quartzite belt by ~40–50 km; and (3) metasedimentary-derived detritus near Cave Creek is separated from its Jf nearest potential source area by at least ~40 km. This ~40–50 km estimate of post–early Desmoi- nesian dextral slip exceeds the present ~37 km dextral separation of Proterozoic features by the Picuris-Pecos fault, and suggests that no dextral separation, but rather a small sinistral separation (~3–13 km), existed on the Picuris-Pecos fault during the early Desmoinesian. Thus, either no major pre-Desmoinesian dextral slip occurred on the Picuris-Pecos fault, or such slip was effec- tively cancelled by other episodes of sinistral slip prior to the Desmoinesian. Our results imply that the present ~37 km dextral separation on the Picuris-Pecos fault accumulated late in the Ancestral Rocky Moun- tain orogeny and/or during the Laramide orogeny. With the exception of at least a few kilo- meters of dextral slip that must be Laramide (Cather and Lucas, 2004, based on lithofacies of the Late Cretaceous Dakota Sandstone that are mismatched across the fault), the apportionment of dextral slip between these two orogenies is unresolved.

Figure 21. View to west of ﬁ ne-grained early Desmoinesian strata on Chimayosos Peak. ACKNOWLEDGMENTS Jicarilla thrust fault (Jf) juxtaposes Pennsylvanian strata in foreground with Proterozoic Research was funded by the New Mexico Bureau Ortega Quartzite. Note chevron folds and lack of coarse, proximal deposits in Pennsylva- of Geology and Mineral Resources (Peter A. Scholle, nian beds. Director). We thank Bruce Allen and Daniel Vachard

1190 Geosphere, October 2011

A detritus

Figure 22. Simpliﬁ ed block diagrams showing interpreted lateral slip history of Picuris-Pecos fault. North is to the right. (A) Early Desmoines ian. Quartzite-clast conglomerates in the western Taos trough are derived from a metasedimentary terrane west of the Picuris- Pecos fault. An inherited, small (3–13 km) sinistral separation of Proterozoic lithotypes is present on the fault. (B) Today. Major dextral slip (~40–50 km) during the late Ancestral Rocky Mountains orogeny (post–early Desmoinesian) and/or the Laramide orogeny produced the provenance mismatch seen today.

for fusulinid identifi cation. We benefi tted from dis- Armstrong, A.K., Mamet, B.L., and Repetski, J.E., 2004, Mis- central United States: Rocky Mountain paleogeogra- cussions with Gary Axen, Elmer Baltz, Paul Bauer, sissippian system of New Mexico and adjacent areas, in phy symposium 1: Denver, Colorado, Rocky Mountain Chris Daniel, Matt Heizler, Mike Jercinovic, Karl Mack, G.H., and Giles, K.A., eds., The geology of New Section, Society of Economic Paleontologists and Karlstrom, Amy Luther, David McDonald, Mike Mexico: A geologic history: New Mexico Geological Mineralogists, p. 181–196. Society Special Publication 11, p. 77–93. Cather, S.M., 1999, Implications of Jurassic, Cretaceous, and Timmons, and Mike Williams. We thank Amy Luther Baars, D.L., and Stevenson, G.M., 1984, The San Luis Uplift, Proterozoic piercing lines for Laramide oblique-slip fault- for Ortega Quartzite samples from the Picuris Moun- Colorado and New Mexico: An enigma of the Ancestral ing in New Mexico and rotation of the Colorado Plateau: tains. The Cameca SX-100 electron microprobe at Rockies: Mountain Geologist, v. 21, p. 57–67. Geological Society of America Bulletin, v. 111, p. 849– New Mexico Institute of Mining and Technology Baltz, E.H., and Myers, D.A., 1999, Stratigraphic frame- 868, doi: 10.1130/0016-7606(1999)111<0849:IOJCAP> was partially funded by NSF Grant STI-9413900. work of upper Paleozoic rocks, southeastern Sangre 2.3.CO;2. The Pecos Wilderness part of this study would not de Cristo Mountains, New Mexico with a section on Cather, S.M., 2004, Laramide orogeny in central and north- have been possible without the assistance of Curtis speculations and implications for regional interpreta- ern New Mexico and southern Colorado, in Mack, Verploegh. The manuscript was improved by infor- tion of ancestral Rocky Mountains paleotectonics: G.H., and Giles, K.A., eds., The geology of New New Mexico Bureau of Mines and Mineral Resources Mexico; a geologic history: New Mexico Geological mal reviews from Gary Axen, Amy Luther, and Mike Memoir 48, 269 p. Society Special Publication 11, p. 203–248. Timmons. We appreciate careful editing by Bill Bauer, P.W., 1993, Proterozoic tectonic evolution of the Cather, S.M., 2009, Tectonics of the Chupadera Mesa region, Thomas, Associate Editor Mike Williams, and an Picuris Mountains, northern New Mexico: Journal of central New Mexico: New Mexico Geological Society, anonymous reviewer. Geology, v. 101, p. 483–500, doi: 10.1086/648241. 60th Annual Field Conference Guidebook, p. 127–137. Bauer, P.W., 2004, Proterozoic rocks of the Pilar cliffs, Cather, S.M., and Harrison, R.W., 2002, Lower Paleozoic REFERENCES CITED Picuris Mountains, New Mexico: New Mexico Geo- isopach maps of southern New Mexico and their impli- logical Society, 55th Annual Field Conference Guide- cations for Laramide and ancestral Rocky Mountain Aby, S., and Timmons, J.M., 2005, Preliminary geologic book, p. 193–205. tectonism: New Mexico Geological Society, 53rd map of the El Valle 7.5′ quadrangle, Rio Arriba, Santa Bauer, P.W., and Ralser, S., 1995, The Picuris-Pecos fault— Annual Field Conference Guidebook, p. 85–101. Fe, and Taos Counties, New Mexico: New Mexico Repeatedly reactivated, from Proterozoic to Neogene: Cather, S.M., and Lucas, S.G., 2004, Comparative stratigra- Bureau of Geology and Mineral Resources Open-File New Mexico Geological Society, 46th Annual Field phy of the Dakota Sandstone across the Picuris-Pecos Geologic Map 105, scale 1:24,000. Conference Guidebook, p. 111–115. fault system south of Lamy, New Mexico: Defi nitive Armstrong, A.K., 1967, Biostratigraphy and regional relations Casey, J.M., 1980, Depositional systems and paleogeo- evidence of Laramide strike slip: Geological Society of of the Mississippian Arroyo Peñasco Formation, north- graphic evolution of the late Paleozoic Taos trough, America Abstracts with Programs, v. 36, no. 5, p. 269. central New Mexico: New Mexico Bureau of Mines and northern New Mexico, in Fouch, T.D., and Magathan, Cather, S.M., Karlstrom, K.E., Timmons, J.M., and Heizler, Mineral Resources Memoir 20, 80 p. E.R., eds., Paleozoic paleogeography of the west- M.T., 2006, Palinspastic reconstruction of Proterozoic

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