Resurgent Volcano-Tectonic Depression of Oligocene Age, South-Central New Mexico

WILLIAM R. SEAGER Department of Earth Sciences, Box 3AB, New Mexico State University, Las Cruces, New Mexico 88003 Resurgent Volcano-Tectonic Depression of Oligocene Age, South-Central New Mexico

ABSTRACT as other well-known resurgent cauldron cycles. Locally, deposition in the depression continued The Goodsight-Cedar Hills volcano-tectonic without interruption into early late Tertiary depression in south-central New Mexico time, when extensional faulting occurred. formed concurrently with and following depo- The depression is elongated parallel to the sition of about 550 km3 of Oligocene volcanic late Tertiary Rio Grande Rift and may be a and volcaniclastic rocks. The depression is an precursor of Basin and Range structure in the asymmetric basin about 80 km long and 38 km area. The late Tertiary fault pattern in the wide. It is filled to a maximum depth of about area indicates that earlier volcano-tectonic 550 m by early rhyolitic ash-flow tuff, medial structures had important effects on the pattern epiclastic strata derived partly from marginal of late Tertiary Basin and Range structures. raised rims (Bell Top Formation), and late Locally, late Tertiary faults were inherited basaltic andesite (Uvas Basalt). Much of the from and duplicate the position of earlier sub- 295 km3 of effusive rock was erupted from sidence faults or the synclinal moat. Elsewhere, vents located near the center of the depression the north-trending regional fault pattern was and from a major subsidence fracture zone considerably modified by structural and litho- along the eastern margin. Subsidence of the logic inhomogeneities of volcano-tectonic ori- depression floor was noncatastrophic and ap- gin, particularly the Sierra de las Uvas dome proximately kept pace with basin filling, except and its buried intrusive masses. along the eastern margin. Following eruption and broad, regional sub- INTRODUCTION sidence of the Uvas Basalt, the central floor of Eruption of about 295 km3 of Oligocene lava the depression was arched upward to form the and tephra from vents in the Sierra de las Sierra de las Uvas dome and adjacent synclinal Uvas-Cedar Hills area of Dona Ana County, moat. The fault pattern of the dome and its New Mexico, created the Goodsight-Cedar association with known vents and the thickest Hills volcano-tectonic depression (Fig. 1). The part of the volcanic pile suggest that it formed depression extends over an area of about 2,900 by vertical movement of magma from an km2 and is filled to a maximum depth of about underlying chamber. Its development, follow- 550 m by rhyolitic ash-flow tuff, andesite, ing subsidence of the Uvas Basalt, suggests that basaltic andesite, and associated epiclastic strata it is essentially resurgent in origin. Although no derived largely from raised rims. A prominent postdoming volcanism is known, indirect evi- domal uplift, the Sierra de las Uvas dome, dence indicates that intrusion of silicic magma formed near the center of the depression fol- probably caused resurgence. lowing eruption and subsequent subsidence of It is clear that the Goodsight-Cedar Hills the youngest basaltic andesite. The dome seems depression is not a resurgent cauldron of the similar to the central dome of well-known re- classic Valles or Toba types. Rather, it appears surgent cauldrons, but, as discussed in this to be a structure transitional in character be- paper, it differs in some important fundamental tween a cauldron and a fault trough. Subsid- ways. ence apparently followed volcanism. Resur- Late Tertiary fault blocks are superimposed gence followed eruption of basaltic andesite on the volcano-tectonic structures. Their ero- rather than the usual ash-flow sequence, and sion by the Rio Grande and its tributaries pro- the 10- to 12-m.y. history of the Goodsight- vides excellent exposures of the volcanic fill of Cedar Hills depression is 5 to 6 times as long the depression and of various structural and

Geological Society of America Bulletin, v. 84, p. 3611-3626, 10 figs., November 1973 3611

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Figure 1. Index map of Goodsight-Cedar Hills volcano-tectonic depressi on.

vent features related to its eastern margin and genesis of the Goodsight-Cedar Hills depres- central dome. sion differs from them. Finally, some possibly The main objectives in this paper are to de- significant relations between the extensional scribe the volcano-tectonic depression and its late Tertiary fault pattern and earlier volcano- central dome. Comparison with other resurgent tectonic structures will be discussed. volcanic structures will emphasize how the Until recent years, little was known of the

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10w Figure 2. Generalired gcologic nup of Goodught Mountaim-Cedar llill. Sierra de b» Urai arta, .outh-cemral New Meako.

SEACER, FIGURE 2 Geological Society oí America Bulleun, v. M, no. 11 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/11/3611/3428609/i0016-7606-84-11-3611.pdf by guest on 01 October 2021 OLIGOCENE VOLCANO-TECTONIC DEPRESSION, NEW MEXICO 3613

geology of the area although Darton (1928) volcanic fill in the Goodsight-Cedar Hills de- and Dunham (1935) both reported observa- pression. tions in the Sierra de las Uvas and Caballo South of Interstate 10, Quaternary basalt Mountain area. Kelley and Silver (1952) flows and cinder cones of the West Potrillo named the middle Tertiary Palm Park and Mountains overlap older volcanic fill of the Thurman Formations from the Caballo Moun- Goodsight-Cedar Hills depression. The Quat- tains, and Kottlowski (1953) identified the Bell ernary volcanic rocks cover a rectangular area Top Formation and Uvas Basalt from the of about 800 km2 that is in line with, and Sierra de las Uvas. The Thurman, Bell Top, possibly above, a southward extension of the and Uvas rock units comprise the volcanic fill- older volcano-tectonic depression. ing of the volcano-tectonic depression. More recent work in the area involved detailed map- VOLCANO-TECTONIC DEPRESSION ping at a scale of 1:24,000 by Seager and FILL others (1971), Seager and Hawley (1973), and As in many other parts of southwestern demons and Seager (1973). Hawley (1970) New Mexico and adjacent Arizona, the Ter- summarized Cenozoic stratigraphy of the area tiary volcanic section in the Goodsight-Cedar in a guidebook to the Rio Grande Valley be- Hills depression consists of a tripartite sub- tween Hatch and Las Cruces, New Mexico, and division of andesite, rhyolite, and basaltic Seager and Clemons (1973) described the andesite (Seager and Clemons, 1973). Basal stratigraphy of the volcanic rocks. andesitic to latitic rocks are referred to the REGIONAL SETTING Palm Park and Rubio Peak Formations (Kelley and Silver, 1952; Elston, 1957), the medial The Goodsight-Cedar Hills volcano-tectonic rhyolitic section to the Bell Top-lower Thur- depression and Sierra de las Uvas dome are man Formations (Kottlowski, 1953; Kelley located near the southern end of the late and Silver, 1952), and upper basaltic andesite Tertiary Rio Grande Rift zone of Kelley and to the Uvas Basalt (Kottlowski, 1953). The Silver (1952), Kelley (1953), and Chapin volcanic sequence is more complex than the (1971; Fig. 1). Figure 2 shows a generalized threefold subdivision suggests because of the geologic map of the region. The Caballo, Sierra mutual intertonguing of rhyolite with basaltic de las Uvas, Cedar Hills, and other fault blocks, andesite and andesite. The volcanic rocks range together with adjoining grabens, are Miocene in age from Eocene to Oligocene and generally to Holocene intrarift structures that are super- overlie gently folded Paleozoic or Mesozoic imposed across the older, mainly Oligocene, strata, although in many places they are sepa- volcano-tectonic features. The position of some rated from them by fanglomerates or gypsifer- late Tertiary faults, however, appears to have ous red beds of the Love Ranch Formation been inherited from older subsidence fractures. (Kottlowski and others, 1956). Locally, the An interval of 2 to 4 m.y. lay between active volcanic rocks grade upward into fanglomerate volcanism in the depression and regional Basin of the Santa Fe Group, but more commonly and Range faulting in northern Doña Ana the volcanic rocks are overlain unconformably County. However, sedimentation and subsid- by the Santa Fe, especially along late Tertiary ence in the depression continued until rifting fault-block margins and along the eastern raised began, as indicated by local conformable rela- rim of the volcano-tectonic depression. The tions between Miocene Santa Fe Group Tertiary rock units are summarized in Table 1. fanglomerates, derived from rising fault blocks, and underlying volcaniclastic strata. The Eocene Palm Park and Rubio Peak Formations form the floor and eastern and Figure 1 also shows the position of the Good- western raised rim of the Goodsight-Cedar sight-Cedar Hills depression with respect to Hills depression (Figs. 3 and 4). The depression the better known and larger Mogollon-Datil is filled by the Bell Top-lower Thurman For- volcanic province farther west (Elston, 1957, mations and Uvas Basalt, which collectively 1968; Elston and others, 1968, 1970). Similar have a volume of about 550 cu km (Fig. 5). rock sequences, rock types, and ages character- ize the two provinces but the Goodsight-Cedar Hills field appears to be separate both geo- Bell Top-Lower Thurman Formations graphically and geologically from the Mogol- The Bell Top-lower Thurman rock units lon-Datil volcanic province. This is indicated comprise about two-thirds of the volcano- primarily by the provincial distribution of tectonic depression fill. Figure 4 illustrates

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Figure 3. Diagrammatic section through Cood- dome; effects of late Tertiary faulting removed. sight-Cedar Hills depression and Sierra de las Uvas

stratigraphie relations between the Bell Top cate the formations are about 33 to 38 m.y. and lower Thurman Formations in the old (Kottlowski and others, 1969, and 1972, Caballo Mountains, Sierra de las Uvas, and written commun.). Cedar Hills. The formations consist primarily The approximate original distribution and of six rhyolitic ash-flow tuff sheets (Table 1 ; thickness of the Bell Top-lower Thurman ash- Fig. 6) and tuffaceous sediments, although im- flow tuff sheets are illustrated in Figure 7. Each portant rhyolite flows, domes, and ai- -fall of the six sheets appears to be a simple cooling deposits occur in the Cedar Hills along the unit averaging 25 to 50 m thick. Collectively, eastern margin of the depression. Potassium- the ash flows are thickest in the eastern Sierra argon dates from ash-flow tuffs 2, 3, and 5 indi- de las Uvas along the axis of the depression.

SERRA KEMADO SAO SIERRA DE LAS UVAS DOME SIERRA KEMADO SAG

Figure 4. Diagrammatic section through Good- dome; eifects of late Tertiary faulting removed. sight-Cedar Hills depression and Sierra de la> Uvas

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* Outcrop control

/ —loo-- -isopach,conto-ur interval 200ft

Known Uvas Basalt vent © Known Bell Top vent ~ """""Limit of information

0 SCALE I-

Figure 5. Isopach map, total \ thickness of Uvas Basalt and Bell Top-lower Thurman Formations.

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Individual flows are thickest in the samt arta, Even ash-flow tuff 3, thought to have er. ptcd from dike-filled fissures along the er. stern margin of the depression, is thickest aloni; the axis. Judging from their distribution and thickness, the ash-flow sheets were deposited jn a slowly subsiding surface of very low relief although the possibility that they formed a broad, flat, regional shield cannot be ruled Dut. Planimeter measurements of the total volume of ash-flow tuff (and minor air-fall :uff) from Figure 7 indicate that about 135 km3 of rhyolitic magma was erupted. Because upper nonwelded parts of the ash-flow tuffs are generally missing, presumably due to erosion, this figure may be low by several, possibly many, cubic kilometers. At any rate, the Bell op ash-flow tuff field falls within group 6 of Figure 5. Ash-flow tuffs of Bell Top Formation, Smith's (1960) major ash-flow fields and is southeastern Sierra de las Uvas. About 215 m exposed. comparable in terms of volume of ash-flow ma- Numbers refer to ash-flow tuff members described in terial with the Bandelier (second cycle) tuff Table 1. Tpp = Palm Park Formation.

TABLE 1. TERTIARY ROCK UNITS OF IKE SIERRA DE LAS UVAS AREA, MEW MEXICO

Age Thickn.ns ( ) Character:sties

Alluvium (not named) Vari« ile Stream, pediment and alluvial fan gravels, sand dunes, and colluvium

Miocene- Santa Fe Group 0 - ree Fanglorerate, sandstone, siltstone, gypsum Pleistocene

Upper Thurrcan rm. 0 - 313 Light-gray to tan tuffaceous thin-bedded sandstone Miocene and mudstone; intertongues with Uvas Basalt ijvas Basalt 0 - Zi i Black tD mediuT-gray vesicular basaltic ar.desite Oligocene flows, lapilli tuff, and agglomerate Ash-flow tuff 7 0 - Gray, dense, welded, vitrlc ash-flow tuff Upper sedinentary unit 60 - 152 Light-brown to gray tuffacesus sandstone containing numerour conglomerate lense.» and local fanglomerate; E interiorgues with Uvas c Ash-flow tuff 6 0 - 30 Pale-red welded vitrlc ash-flow tuff g Lower seditrentary unit 0 - 60 White to tan tuffaceous sane stone and mudstone with congloperate lenses t Ash-flow tuff 5 0 - 75 Gray to ?ink crystal-vitrie ash-flow tuff containing Oligocene a>2 abundant pumice Itnps -J Ash-flow tuff 4 0 - 27 Grayish-purple wslded vitrie-crystal ash-flow tuff with dark devitrifled, flattened, pumice fragments o to 1 ft in diameter Rhyolite corplex of 0 - 150 Air-fall tuff and breccia, intrusive and extrusive % Cedar Kills flow-banded rhyolite, and miror andesite 00 Ash-flow tuff 3 0 - 90 Pink to orange, porous vitrlc ash-flow tuff containing 20 percent punice Basalt 0 - 60 Black, ol1vine-bearing basalt flows Ash-flow tuf* 2 0 - 27 Pale-purp'e to tan welded vit"ic ash-flow tuff with well-developed microscopic euiaxitic texture

Palm Park Fct. 45 - 760 Andesitic sandstone, mudstone, conglomerate, tuff-breccia and miror andesite flows; probably eastern epiclastic fades of Fubio Peak Fm.

Eocene(?) Rubio Peak Fm. 600+ Andesite to latite flows in Goodsight Mts.; probably laterally equivalent to Palm Park Fm. Local unconf-ji.iit-i Early Tertiary Love Ranch Fir.. 0 - 90 Extremely coarse far and talus debris derived from under underlying lirestones Unaonform' : Paleozoic and Paleozoic and 1,200+ Mainly Hirestone, dolomite and shale, with lesser tfesozoic Cretaceous rocks amounts of sandstone and siltstone

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(Smith and Bailey, 1966), southeastern Idaho 8. The greatest thickness and largest number of field (Mansfield and Ross, 1935), Island Park flows are associated with several vents near the field (Hamilton, 1965), Bishop field (Gilbert, crest of the Sierra de las Uvas dome. North- 1938), and the Aso and Aira fields (Matumoco, ward, the formation pinches out within the 1965), among others. All are associated with Thurman Formation. Eastward, the unit ap- important subsidence structures, mostly cal- pears to be deposited against the block-faulted deras, but except for the Bandelier field, are eastern raised rim of the depression; it also lacking resurgent domes. interfingers with rhyolitic fan debris adjacent The Bell Top-lower Thurman Formations to the Cedar Hills domes. About 92 m of Uvas contain about twice as much rhyolitic epiclastic Basalt occurs near Aden in southernmost expo- rock (250 km3) as ash-flow tuffs (Table 1). sures, indicating that the formation probably These are generally thickest along the axis of continues southward beneath Quaternary ba- the depression. Fanglomerates derived from salts of the West Potrillo field. Thin Uvas flow-banded rhyolite domes and flows in the Basalt flows overlap underlying Bell Top units Cedar Hills are locally interbedded (Figs. 3 at the western margin of the depression near and 4), as are numerous fluvial conglomerate the axis of the Goodsight Valley syncline. They overlie Rubio Peak flows on the western raised lenses. Conglomerate clasts in the eastern part rim in the Goodsight Mountains (Figs. 2 and of the depression are primarily andesite por- 3). Thin basaltic andesite flows, which may be phyry reworked from Palm Park mudflow de- Uvas, occur as far west as the Cooks Range. If posits exposed in the eastern raised rim. Rubio these are Uvas flows, the volume of basaltic Peak andesite and latite clasts derived from andesite erupted is substantially more than the the western rim constitute much of the con- 3 158 km calculated in Figure 8. The effusion of glomeratic facies in the western part of the this volume of lava led to important regional depression. These relations indicate that older subsidence, discussed further under the section rocks were exposed in low rims adjacent to the on structure. eastern and western margins of the depression during emplacement of Bell Top-lower Thur- Uvas Basalt vents are present on the crest man ash flows and sedimentary units in the and flanks of the Sierra de las Uvas dome and depression. in the Cedar Hills. A diatreme, remnants of An important part of the Bell Top Forma- cinder cones, a plug, a domelike andesite mass tion is the wedge-shaped accumulation of flow- and associated flows, and numerous dikes are banded rhyolite flows, air-fall deposits, and associated with the dome. The distribution and minor andesite that occurs between ash-flow thickness of Uvas flows and position of Uvas tuffs 3 and 4 in the Cedar Hills. Sources of these vents suggest that the Uvas accumulated pri- rocks are within the inferred Oligocene Cedar marily as a broad shield volcano whose summit Hills fault zone shown in Figures 3 and 4. was located near the center of the volcano- Rhyolite domes, a diatreme, collapsed areas, tectonic depression at the site of the future and dikes are crowded together in this north- Sierra de las Uvas dome. The Cedar Hills vent trending zone that forms the eastern margin of is a cinder cone, about 1.6 km in diameter, the Goodsight-Cedar Hills depression. Proba- located on the eastern flank of the shield. The ble dikes and irregular masses of ash-flow tuff 3 cone clearly formed along the Oligocene Cedar locally occur in the same zone as does an Uvas Hills fault and intrusion zone: (Fig. 3). Basalt cinder cone. The sources of the other ash-flow tuffs are unknown. Ash-flow tuffs 4 STRUCTURE and 5 pinch out against the western side of the Volcano-Tectonic Depression intrusive complex and, except locally, apparently failed to spread across the zone onto the Figure 9 illustrates a paleotectonic map inter- raised rim to the east. pretation of the Goodsight-Cedar Hills depression and its central dome. Figures 3 and 4 Uvas Basalt diagrammaticady show east-west and north- The Uvas Basalt comprises about one-third southeast cross sections, respectively, through of the depression fill. A K-Ar age determined the depression. From these it can be seen that from one flow is 26 m.y. (F. E. Kottlowski, only the eastern margin is structurally con- 1970, written commun.). Distribution and trolled. North and south of the depression thickness of the formation is shown in Figure center, Uvas and Bell Top rock units, especially

the flows, thin gradually; no evidence of con- f.rched in Oligocene time. In contrast, deposi- temporaneous faulting or intrusions exists at tional slopes west of the Cedar Hills fault zone their distal edge. The western margin is charac- were inclined westward toward the depression terized by an east-dipping homoclinal slope axis as inferred by the direction of transport of off the Goodsight uplift to the Goodsight ash-flow tuff 3, Palm Park and Rubio Peak Valley syncline. Prior to development of the detritus, and boulders derived from the Cedar Sierra de las Uvas dome and the adjoining Hills rhyolite domes. Goodsight Valley syncline, this slope, formed Collectively, the foregoing indicates that by post-Uvas subsidence, presumably con- the eastern margin of the depression comprised tinued to the axis of the basin several kilom- a zone of overlapping normal faults, stepped eters farther east. The abrupt pinch-out of Bell down to the west, that were the site of sub- Top units within the Goodsight Valley syn- sidence contemporaneous with surface volcanic cline (Fig. 3) suggests structural control, but activity, primarily rhyolitic dome intrusions outcrop data several kilometers from the over- and extrusions. The inferred Oligocene Cedar lap give no indication of this. In contrast, the Hills fault was the most important of these eastern margin of the depression is one of pro- nounced faulting and intrusion. fractures. The footwall block of each fault constituted a segment of the eastern raised rim The eastern margin of the Goodsight-Cedar and was formed mostly by Palm Park strata. Hills depression comprises an 8.0-km-wide zone These exposures of Palm Park rocks apparently of north-trending overlapping normal faults were the source of the reworked Palm Park that apparently were stepped down to the clastic debris contained in the Bell Top-lower west in Oligocene time. Faults include the Thurman sedimentary units in eastern parts of East and West Selden Hills faults, the inferred the depression. Oligocene Cedar Hills fault zone, possibly the Latest subsidence of the floor of the volcano- West Robledo fault, and several unnamed tectonic depression followed effusion of the faults (Fig. 9). Each of these faults clearly Uvas Basalt. The amount of subsidence in the underwent substantial movement in late Ter- central Uvas vent areas is unknown, however, tiary time inasmuch as each displaces the Santa because of later doming. It may have been Fe Group. Their importance as subsidence substantial. The moatlike Goodsight Valley fractures active during at: least part of Bell and Sierro Kemado synclines (Figs. 2, 4, and 9) Top-lower Thurman and Uvas time, is inferred are primarily a result of later central doming from several lines of evidence. First, Oligocene within the subsidence basin. Late Tertiary flow-banded rhyolite domes, dikes, and associ- westward tilting along the Ward Tank fault ated features occur along some of the faults. also contributed to development of the south- This is especially true of the Cedar Hills fault ern part of the Goodsight Vdley structure. The zone, within which at least nine domes, a moat syncline locally contains thick post-Uvas diatreme, dikes, collapsed areas, and a cinder lake and alluvial flat sediments of the upper cone occur. Together, they span 10 to 12 m.y. Thurman Formation. of volcanic activity. Second, greater thicknesses of Bell Top-Uvas strata occur on the down- Sierra de las Uvas Dome thrown (western) side of the faults. Again, this In the Sierra de las Uvas, Uvas Basalt and is especially true of the Cedar Hills fault zone older strata are deformed into a broad north- where several hundred feet of Uvas and Bell west-plunging arch about 16 km in diameter Top flow and clastic units are present west of (Fig. 2). The dome is younger than the Uvas the fault in contrast to thinned or missing sec- Basalt (26 m.y.) but appears to entirely pre- tions to the east. Similarly, Bell Top and Uvas date Basin and Range faulting which com- rock units are present beneath Santa Fe strata menced between 24 and 20 m.y. ago in this west of the West Selden Hills fault where they area (Seager and Hawley, 1973; C. E. Chapin, apparently were deposited against an Oligocene 1972, oral commun.). In fact, the original West Selden Hills fault-line scarp. They are configuration of the dome has been considera- missing between angularly unconformable Palm oly modified by movement on the Uvas and Park and Santa Fe units in the lootwall block Ward Tank faults (Fig. 2). Removal of 7° to east of the fault (Figs. 3 and 4). Third, removal . 0° of late Tertiary north to northwesterly of late Tertiary tilt from fault blocks show that tilt along the Ward Tank fault indicates that most rim fault blocks were tilted east or gently the arch was originally domal, probably elon-

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->V

d- ' EXPLANATION

• Rhyolite domi

. $ Basait vent

: Ooiatreme

^ Norrwl fault

Itopach.Uvas Boialt & 6*11 H>pFm.,contour IfltarvalGOOft. ^^ Inferred direction of trone- . * part of Palm Park 8 Rublo Peak ondeelte elatt* In Ball Tap conglomerates.

I I Ura Basalt a Bell Top Fm.

[ • -. ftifm ftrt ft Rublo Peak Fme.

SCALE 0 1 % 3 « 5ml

Figure 9. Paleotectonic map of Goodsight-Cedar preted at latest Oligocene or early Miocene time. Hills depression and Sierra de las Uvas dome, inter-

gated somewhat to the northwest (Fig. 9). following facts suggest that the normal faults Structural relief between the dome and the in the Sierra de las Uvas are related to doming Goodsight Valley syncline to the west is about rather than to late Tertiary Basin and Range 615 m. faulting. First, the density of faults is greater The most striking aspect of the dome is the on the dome compared to adjacent areas complex faulting along its crest and northeast- affected only by Basin and Range deformation. ern flank (Fig. 2). The fault pattern is charac- Second, faulting on the dome tends to be terized by a bifurcating longitudinal graben downthrown toward the axial area in such a system along the axis (Fig. 9) that is remark- way as to accommodate lateral stretching of ably similar to grabens produced experimen- the dome. Third, many of the faults lose dis- tally on domes by Cloos (1930, 1931, 1932). placement away from the dome crest. The The pattern also closely resembles fault pat- lack of radial faulting on the Sierra de las Uvas terns on the central domes of the Valles and dome is noteworthy because of its conspicuous Creede resurgent cauldrons (Smith and others, development in model salt-dome studies 1970; Steven and Ratte, 1959; Ratte and (Parker and McDowell, 1955) and on the Steven, 1967; Smith and Bailey, 1968). The Timber Mountain resurgent dome in Nevada

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(Carr and Quinlivan, 196S). Apparently, elon- south. It is not a steep-sided collapse structure gation of the Sierra de las Uvas dome retarded but rather a broad tectonic basin formed by radial faulting but favored the development of slow subsidence in the central area and by longitudinal grabens. faulting along the eastern margin. In this re- The Sierra de las Uvas dome is interpreted spect the structure differs fundamentally from to be resurgent in origin. The dimensions, classic cauldrons of the Valles type. structural relief, and fault pattern of the dome Following eruption and subsidence of the are typical of well-known resurgent domes. Uvas Basalt, the central floor of the depression The dome is located within an area where was arched upward to form the Sierra de las Uvas and Bell Top flows are thickest and most Uvas dome and bordering synclines. The up- numerous and where several Uvas vents are lifted area is coincident with several vents and known. Collectively, these suggest that doming with the thickest and most numerous ash-flow and a subsurface magma body are related. tuffs and basaltic andesite flows in the depres- Apparently, the dome was created by the sion. The dome has mar y of the characteristics vertical resurgence of magma following erup- of the central domes of well-known resurgent tion and subsidence of the Uvas Basalt. No cauldrons. Most important are a central posi- large post-Uvas intrusive masses are exposed, tion in a large subsidence structure, typical however. The small basaltic andesite plugs and resurgent dome dimensions and structural dikes that are present on the flank and summit relief, the prominent development of a bifur- of the dome within the Uvas apparently pre- cating axial graben, and development following date doming. a period ot important subsidence. Also, the volume of extrusive and pyroclastic rocks SUMMARY AND DISCUSSION erupted from the depression prior to doming compares with the volume of effusive rocks The Goodsight-Cedar Hills depression is a associated with the resurgent Jemez caldera comparatively shallow asymmetric volcano- (Smith and Bailey, 1966). It seems reasonable tectonic sag about 80 km long in a north-south that the Sierra de las Uvas dome is resurgent direction and 38 km wide. It formed contem- also, and it is likely that the rise of silicic poraneously with and iollowing eruption of magma is responsible. about 295 km3 of older rhyolitic ash-flow tuffs (Bell Top Formation) and younger basaltic There is scant evidence of post-Uvas volcan- andesite (Uvas Basalt). These were erupted ism or intrusion in the Goodsight-Cedar Hills from vents located near the center of the de- depression that might have been connected pression and along the eastern, faulted margin. with rising magma. The close association of the They appear to have accumulated entirely on dome with Uvas vents, particularly one at the the floor of the depression. A volume of sedi- summit of the dome, suggests that doming is ment nearly equal to the volume of lavas and related to resurgent basaltic andesite magma. tuffs also was deposited within the depression. However, Uvas flows and vents clearly antedate Much of this was derived from the eastern and the dome. On the other hand, post-Uvas, western raised rims. upper Thurman beds in the synclinal moat The only tectonically active part of the de- north of the dome contain much rhyolitic pression was the eastern margin. Subsidence tuffaceous sediment and same pumiceous air- early in the history of ash-flow tuff eruption fall tuff. The source of the tuff is unknown and was accompanied in this area by intrusion of it may well have originated in the Mogollon- at least 10 flow-banded rhyolite domes, forma- Datil province. Indeed, intrusive rhyolite tion of local subsidence structures and a rhyo- younger than Uvas Basalt is common in the litic diatreme, and minor intrusions of ande- Mogollon-Datil province and several rhyolite site. No subsequent flow-banded rhyolites are bodies are exposed by erosion of the domed known but later eruptions of basaltic andesite basaltic rocks (W. E. Elston, 1973, written occurred in the same zone. In contrast, the commun.). It seems probable that the rise of a western raised rim is characterized by a simple large silicic mass formed the Sierra de las Uvas homoclinal dip eastward toward the depression dome, although erosion has not cut deep axis. Thus, the depression is asymmetrical, con- enough to confirm this. It is significant that no taining thick and numerous flows at the de- large positive magnetic anomaly is present pression axis and along the eastern side, but across the Sierra de las Uvas dome (P. M. thinner accumulations to the north, west, and Wright, 1973, written comr.nun.).

Figure 10. Rio Grande Rift fault pattern between Truth or Consequences and Las Cruces, New Mexico.

Some major differences exist between well- (Byers and others, 1968), Silverton and Lake known resurgent cauldrons and the Goodsight- City (Luedke and Burbank, 1966; 1968) Cedar Hills resurgent volcano-tectonic depres- calderas, and Toba depression (van Bem- sion. The Creede (Steven and Ratte, 1959; melen, 1939) all are characterized by a well- Ratte and Steven, 1967), Timber Mountain defined, steep-sided, circular or elliptical de-

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pression, and each is associated with two to the Sierra de las Uvas dome parallels the twenty times as much rhyolitic ash-flow tuff eastern edge of the Oligocene volcano-tectonic compared to that in the Goodsight-Cedar Hills depression. Several of these faults apparently depression. Collapse and resurgence followed constituted the step-faulted eastern raised rim eruption of the ash-flow tuff sheets. Each of the depression. Subsequent movement is cauldron also is typified by postcaldera silicic indicated by offset of Sants. Fe strata. Thus, volcanism occurring either contemporaneously their present position appears to be inherited with or following resurgence, and none is from an earlier volcano-tectonic origin. This is associated with volumes of basaltic andesite especially true of the late Te rtiary Cedar Hills comparable to that in the Goodsight-Cedar fault. The fault appears to follow closely the Hills depression. Finally, the life cycle of the strike of the Oligocene Cedar Hills fault zone classic rhyolite cauldrons appears to be about and line of vents. Although evidence suggests 2 m.y. or less, compared to the 10- to 12-m.y. that the Oligocene structure was down-to-the- span of the Goodsight-Cedar Hills depression. west, it is clear that late Tertiary movements It is clear that the Goodsight-Cedar Hills were in an opposite sense. depression is not a cauldron of the classic Valles From the foregoing discussion it can be seen or Toba types. Rather, it appears to be a long- that in some areas, at least, middle Tertiary lived structure transitional in character be- volcano-tectonic structures, including depres- tween a fault basin and a partly developed sions, subsidence fractures, and intrusive cauldron. It is apparent, however, that subsid- masses and associated domes, are precursors of, ence of the basin resulted from volcanism and or at least affect, late Tertiary Basin and Range that the basin is therefore volcano-tectonic in structures. Indeed, locally, the late Tertiary origin. The depression is elongated parallel to extensional fault patterns may be inherited the Rio Grande Rift (Figs. 1 and 10) and may from and duplicate to some degree older be a precursor to Basin and Range structure in volcano-tectonic structures. Elsewhere, the this area. At any rate there is evidence to sug- local fault pattern may be considerably modi- gest that several types of volcano-tectonic fied by and reflect structural or lithologic in- structures affected the pattern of Miocene and homogeneities of volcano-tectonic origin in younger faulting in this part of south-central the crust. New Mexico. Figure 10 shows the distribution of late ACKNOWLEDGMENTS Tertiary faults in the Rio Grande Rift be- W. E. Elston, R. E. Clemons, and F. E. tween Las Cruces and Truth or Consequences, Kottlowski critically reviewed the manuscript. New Mexico. The major half-graben between 3.. E. Clemons also contributed basic data to the Caballo Mountains and the Animas Moun- this report, including basic mapping and rock tains strikes southward toward the Sierra de descriptions. The New Mexico Bureau of las Uvas. Near Hatch, the graben bifurcates Mines and Mineral Resources provided finan- into southwest- and southeast-trending spurs cial support, and I thank Don H. Baker, Jr., which pass around the central Sierra de las past Director, for permission to publish this Uvas dome. This behavior is interpreted to be article. a response by the extensional stress field to a large resistant knot of intrusive rocks beneath REFERENCES CITED the dome. Rather than continue southward Bemmelen, R. YV. van, 1939, The volcano-tectonic through a resistant intrusive complex, late origin of Lake Toba (north Sumatra): De Tertiary faulting apparently by-passed the Ingenier Nederlandsch-Indie, v. 6, p. 126-140. dome entirely, the adjusted course being deter- 3yers, F. M., Jr., Orkild, P. P.. Carr, W. J., and mined partly by the comparatively weak- Quinlivan, W. D., 1968, Timber Mountain bedded rocks adjacent to the dome and partly tuff, southern Nevada and its relation to by the configuration of the already structurally cauldron subsidence, in Eckel, Edwin B., ed., low synclinal moat. The southeastern spur Nevada test site: Geol. Soo. America Mem. becomes north trending again on the eastern 110, p. 87-97. side of the dome and continues southward to Carr, W. J. and Quinlivan, W. E>., 1968, Structure Mexico where it gives way to northwest- of Timber Mountain resurgent dome, in Eckel, Edwin B., ed., Nevada test site: Geol. trending fault blocks. Soc. America Mem. 110, p. 99-108. The group of north-trending faults east of Chapin, C. E., 1971, The Rio Grande rift, Pt. I:

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ser. 2, v. 29, p. 83-104. Steven, T. A., and Ratte, J. C., 1959, Caldera sub- • 1968, Resurgent cauldrons, in Coats, R. R., sidence in the Creede area, San Juan Moun- Hay, R. L., and Anderson, C. A., eds., St.. dies tains, Colorado [abs.]: Geol. Soc. America in volcanology: Geol. Soc. America Mem. 116, Bull., v. 70, p. 1788-1789. p. 613-662. Smith, R. L„ Bailey, R. A., and Ross, C. S„ D70; MANUSCRIPT RECEIVED I,Y THE SOCIETY OCTOBER Geologic map of the Jemez Mountains, .\Tew 5, 1S72 Mexico: U.S. Geol. Survey Misc. Geol. Inv. REVISED MANUSCRIPT RECEIVED FEBRUARY 15, Map 1-571, scale: 1:62,500. 1973

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