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Synclinal-Horst Basins: Examples from the Southern Rio Grande Rift and Southern Transition Zone of Southwestern New Mexico, USA Greg H

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Synclinal-Horst Basins: Examples from the Southern Rio Grande Rift and Southern Transition Zone of Southwestern New Mexico, USA Greg H Basin Research (2003) 15 , 365–377 Synclinal-horst basins: examples from the southern Rio Grande rift and southern transition zone of southwestern New Mexico, USA Greg H. Mack,n William R. Seagern and Mike R. Leederw nDepartment of Geological Sciences, New Mexico State University, Las Cruces, New Mexico, USA wSchool of Environmental Sciences, University of East Anglia, Norwich, Norfolk, UK ABSTRACT In areas of broadly distributed extensional strain, the back-tilted edges of a wider than normal horst block may create a synclinal-horst basin.Three Neogene synclinal-horst basins are described from the southern Rio Grande rift and southernTransition Zone of southwestern New Mexico, USA.The late Miocene^Quaternary Uvas Valley basin developed between two fault blocks that dip 6^81 toward one another. Containing a maximum of 200 m of sediment, the UvasValley basin has a nearly symmetrical distribution of sediment thickness and appears to have been hydrologically closed throughout its history.The Miocene Gila Wilderness synclinal-horst basin is bordered on three sides by gently tilted (101,151,201) fault blocks. Despite evidence of an axial drainage that may have exited the northern edge of the basin, 200^300 m of sediment accumulated in the basin, probably as a result of high sediment yields from the large, high-relief catchments.The Jornada del Muerto synclinal- horst basin is positioned between the east-tilted Caballo and west-tilted San Andres fault blocks. Despite uplift and probable tilting of the adjacent fault blocks in the latest Oligocene and Miocene time,sedimentwas transported o¡ the horst and deposited in an adjacent basin to the south. Sediment only began to accumulate in the Jornada del Muerto basin in Pliocene and Quaternary time, when an east-dipping normal fault along the axis of the syncline created a small half graben. Overall, synclinal-horst basins are rare, because horsts wide enough to develop broad synclines are uncommon in extensional terrains. Synclinal-horst basins may be most common along the margins of extensional terrains, where thicker, colder crust results in wider fault spacing. INTRODUCTION activity and constitutes the largest-scale and generally most prominent structural feature of the footwall Many continental rifts are long, narrow tectonic terrains uplift. characterized by co-linear or en echelon uplift-basin pairs In areas of broadly distributed extensional strain, such (Fig. 1a). The most common type of basin is the half gra- as the Basin and Range and southern Rio Grande rift, ben, which is bordered on only one side by a major normal Tibetan Plateau and Aegean region, faults may be situated fault (Rosendahl, 1987). In narrow rifts, individual fault such that horst blocks develop, potentially resulting in a blocks and their complementary basins only interact near structurally and topographically low area within the horst the tips of the border faults in regions of strain transfer re- block created by back-tilting of the edges. A sedimentary ferred to as accommodation zones (Rosendahl, 1987; basin in this setting is herein referred to as a synclinal- Ebinger, 1989; Morley et al., 1990; Mack & Seager, 1995). horst basin (Fig. 1b). A large-scale version of this appears As it is uplifted, the footwall block commonly tilts away to be the case for theTanzanian craton, a gentle sag be- from the border fault. Regardless of whether the driving tween the back-tilted eastern and western branches of the force is rotational normal faulting (Thompson, 1960; East African rift (Gregory,1921; Holmes,1945). Occupying Morton & Black, 1975; Pro¡ett, 1977), reverse drag the northern part of the sag is Lake Victoria, which devel- (Barnett et al., 1987) and/or isostatic e¡ects (Jackson & oped in the Pleistocene by drainage reversal associated McKenzie, 1983; Buck, 1988; Wernicke & Axen, 1988), with back-tilting of the western rift margin (Bishop & back-tilting of the footwall is contemporaneous with fault Trendall,1967; Scholz et al., 1998). Presented here are case studies of three synclinal-horst Correspondence: Greg H. Mack, Department of Geological basins in the southern Rio Grande rift and southern Sciences, New Mexico State University, Las Cruces, NM 88003, Transition Zone of southwestern New Mexico, USA, that USA. E-mail: [email protected] illustrate similarities and di¡erences in the history of these r 2003 Blackwell Publishing Ltd 365 G. H. Mack et al. (a) en echelon rift (b) widely dispersed rift uplifts unusual rift basins, as well as some of the variables that uplifts and basins and basins control their development. half graben B half graben GEOLOGIC SETTING Southwestern New Mexico has experienced latest Paleo- accommodation half zone graben gene and Neogene volcanism, block faulting and sedimen- tation resulting from crustal extension in three tectonic half provinces, the Basin and Range, southern Rio Grande rift graben and Transition Zone (Fig. 2). Unlike the northern Rio Grande rift, which consists of a series of right-stepping, en echelon uplift-basin pairs (Chapin & Cather, 1994), accommodation C D zone synclinal- strain in the southern Rio Grande rift and Basin and horst Range in southwestern New Mexico is distributed among basin dozens of faults in an area roughly 66 000 km2 (Seager et al., 1982, 1987; Seager, 1995; Hawley et al., 2000). In the part of the southern Rio Grande rift under consideration in this study (right half of Fig. 2), there are more than a dozen major fault blocks that are adjacent to one another in a line B C D perpendicular to the strike of the north-trending faults. West of the southernRio Grande rift and north of the Ba- sin and Range in southwestern New Mexico is a Transition Zone that separates those more highly extended terrains from the largely undeformed Colorado Plateau (left half of Fig. 1. Schematic geologic maps and cross-sections of (a) en Fig. 2). Block faulting and concomitant sedimentation in echelon rift uplifts and basins and (b) part of broad rift with the southern Transition Zone began after the Oligocene widely dispersed fault blocks, two of which dip toward each other and is of smaller magnitude than in the southern Rio and create a synclinal-horst basin. Grande rift. Like the southern Rio Grande rift, however, Miocene basin fill Plio-Quaternary Gila Wilderness basin fill basin pre-Miocene Mogollon N Mts bedrock Black Range 16 km Copperas uplift TorC San Andres Palomas Caballo Mts 33°N Pinos Altos basin Mts Range Animas Hills Jornada del Muerto basin Cobre Colorado uplift Plateau New 108°W Good Sight Mexico Mts Dona Ana Mts Sierra de 32°30' N Great Uvas las Uvas Plains TZ Valley Robledo Mts basin Rio Grande rift Las Organ 100 km Cruces Mts USA BR 107°W Fig. 2. Generalized geologic map of part of the southern Rio Grande rift and southern Transition Zone, southwestern New Mexico, adapted from Seager et al.(1982,1987).TZ5 Transition Zone; BR 5 Basin and Range. 366 r 2003 Blackwell Publishing Ltd, Basin Research, 15,365^377 Synclinal-horst basins strain in the southern Transition Zone is accommodated veloped, many of which inverted parts of Miocene basins. on several faults that result in adjacent, back-tilted fault The sedimentary record of this pulse of deformation is the blocks (Fig. 2; Black Range and Mogollon Mountains). coeval Camp Rice and Palomas Formations, which have a The stratigraphic record of extension in the southern maximum thickness of about 150 m (Fig. 3; Seager et al., Rio Grande rift and southern Transition Zone began with 1984; Mack & Seager, 1990). Basin incision and partial latest Eocene or Oligocene volcanism involving eruptions back¢lling by the ancestral Rio Grande and its tributaries of andesitic and basaltic lava £ows, as well as voluminous during the past 0.78 Ma deposited a few tens of metres of rhyolitic ignimbrite eruptions from ¢ve calderas within inset ¢ll and locally exposed all or part of the Camp Rice the study area and several outside of it (Fig. 3; McIntosh and Palomas Formations, as well as some older basin- ¢ll et al.,1991; McMillan et al., 2000). Block faulting and sedi- strata (Mack et al.,1993,1998). Faults cutting the Gila Con- mentation were contemporaneous with latest Eocene^ glomerate indicate post-Miocene tectonism in the south- early Oligocene volcanism in the southern Rio Grande rift ern Transition Zone, but there is little evidence in the (Mack et al., 1994a).This was followed by a major period of study area for large-scale Plio-Pleistocene sedimentation faulting and basin subsidence that began near the Oligo- (Fig. 3). Instead, the dominant process appears to have cene^Miocene boundary and continued throughout most been drainage integration and erosion (Mack, in press). of the Miocene epoch, resulting in deposition of about 2 km of basin ¢ll of the Hayner Ranch and Rincon Valley Formations (Fig. 3; Mack et al., 1994b). In contrast, large- SYNCLINAL-HORST BASINS IN THE scale block faulting and basin subsidence in the southern RIO GRANDE RIFTAND SOUTHERN Transition Zone did not begin until the Miocene, result- TRANSITION ZONE ing in deposition of the Gila Conglomerate (Fig. 3). UvasValley synclinal-horst basin A major pulse of deformation in the southern Rio Grande rift began near the Miocene^Pliocene boundary The Uvas Valley synclinal-horst basin is located between and continues with diminishing intensity to the present the west-tilted Sierra de las Uvas and the east-tilted Good day (Seager et al., 1984). During this period new faults de- Sight Mountains (Figs 2 and 4).The basin is late Miocene southern southern epochs Transition Rio Grande rift Zone inset alluvium Quaternary upper pumice 3.1, 1.5 1.8 Santa Camp Rice/ c Fe Palomas o Pliocene basalts 4.5, 3.1 n 2.9 t 5.3 i basalt 9.8 basalt 6.5 n lower Rincon Valley e Miocene Gila n Santa Conglomerate t Fe Hayner Ranch a younger basalts, andesites l 23.8 25.7-21.1 tuffaceous sandstone r 27.4 older andesites i Oligocene basaltic andesite f 28.1 rhyolite calderas t rhyolite calderas 34, 31.4, 29, 28.1 33.7 36.2-34.9 Eocene andesites andesites pre- rift Laramide basin fill 54.8 Fig.
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