Geophysical study of the San Juan Mountains batholith complex, southwestern Colorado

Benjamin J. Drenth1,*, G. Randy Keller1, and Ren A. Thompson2 1ConocoPhillips School of Geology and Geophysics, 100 E. Boyd Street, University of Oklahoma, Norman, Oklahoma 73019, USA 2U.S. Geological Survey, MS 980, Federal Center, Denver, Colorado 80225, USA

ABSTRACT contrast of the complex. Models show that coincident with the San Juan Mountains (Fig. 3). the thickness of the batholith complex var- This anomaly has been interpreted as the mani- One of the largest and most pronounced ies laterally to a signifi cant degree, with the festation of a low-density, upper crustal granitic gravity lows over North America is over the greatest thickness (~20 km) under the west- batholith complex that represents the plutonic rugged San Juan Mountains of southwest- ern SJVF, and lesser thicknesses (<10 km) roots of the SJVF (Plouff and Pakiser , 1972). ern Colorado (USA). The mountain range is under the eastern SJVF. The largest group of Whereas this interpretation remains essentially coincident with the San Juan volcanic fi eld nested calderas on the surface of the SJVF, unchallenged, new gravity data processing tech- (SJVF), the largest erosional remnant of a the central caldera cluster, is not correlated niques, digital elevation data, and constraints widespread mid-Cenozoic volcanic fi eld that with the thickest part of the batholith com- from seismic refraction studies (Prodehl and spanned much of the southern Rocky Moun- plex. This result is consistent with petrologic Pakiser , 1980) enable reassessment and improve- tains. A buried, low-density silicic batholith interpretations from recent studies that the ment of the previous model. complex related to the volcanic fi eld has been batholith complex continued to be modifi ed We employed a three-dimensional forward the accepted interpretation of the source of after cessation of volcanism and therefore is modeling approach to analyze the gravity sig- the gravity low since the 1970s. However, not necessarily representative of synvolcanic nal of the region using density constraints from this interpretation was based on gravity data magma chambers. The total volume of the previous studies (Table 1) and topographic relief processed with standard techniques that are batholith complex is estimated to be 82,000– represented by digital elevation data. These problematic in the SJVF region. The combi- 130,000 km3. The formation of such a large data are expressed in a modifi ed Bouguer-like nation of high-relief topography, topography felsic batholith complex would inevitably regional gravity map. The upper crustal den- with low densities, and the use of a common involve production of a considerably greater sity structure is signifi cantly better constrained reduction density of 2670 kg/m3 produces volume of residuum, which could be present using this modifi ed gravity map, resulting in spurious large-amplitude gravity lows that in the lower crust or uppermost mantle. The modifi cation of both lateral and depth extents of may distort the geophysical signature of interpreted vertically averaged density con- the modeled batholith complex and estimation deeper features such as a batholith complex. trast (–60 to –110 kg/m3), density (2590–2640 of its total volume. Our analyses are undertaken We applied an unconventional processing kg/m3), and seismic expression of the batho- within the context of recent interpretations of procedure that uses geologically appropriate lith complex are consistent with results of underlying mantle structure (Roy et al., 2004) densities for the uppermost crust and digi- geophysical studies of other large batholiths and petrologic evolution of the SJVF (Lipman, tal topography to mostly remove the effect in the western United States. 2007; Farmer et al., 2008). of the low-density units that underlie the topography associated with the SJVF. This INTRODUCTION REGIONAL GEOLOGY approach resulted in a gravity map that pro- vides an improved representation of deeper The topographically elevated and deeply dis- The modern San Juan Mountains are under- sources, including reducing the amplitude of sected San Juan Mountains of southwestern lain by geologic units ranging from Precambrian the anomaly attributed to a batholith com- Colo rado, USA (Fig. 1), are principally the ero- crystalline rocks to middle to late Cenozoic plex. We also reinterpreted vintage seismic sional remnants of the mainly Oligocene San volcanic rocks of the SJVF that dominate the refraction data that indicate the presence of Juan volcanic fi eld (SJVF) (Fig. 2), the laterally surface geology (Fig. 2). Structurally high Pre- low-velocity zones under the SJVF. Assum- and volumetrically largest subarea of the South- cambrian rocks crop out to the north and south- ing that the source of the gravity low on the ern Rocky Mountain volcanic fi eld (Steven, west of the SJVF (Gunnison uplift and Needle improved gravity anomaly map is the same 1975; Lipman, 2007). The San Juan Mountains Mountains, respectively; Fig. 2). The oldest as the source of the low seismic velocities, are bordered by the topographic and structural exposed Precambrian rocks in the study area are integrated modeling corroborates the inter- San Juan and San Luis Basins, to the south and ca. 1.7 Ga metasedimentary and metavolcanic pretation of a batholith complex and then east, respectively, the Colorado Plateau to the rocks (included in undifferentiated Precambrian defi nes the dimensions and overall density west, and the high Rocky Mountains of central rocks; Fig. 2). Subsequent magmatism ca. 1.7 Ga Colorado to the north (Fig. 1). and ca. 1.4 Ga left behind large plutons and pos- *Present address: U.S. Geological Survey, MS 964, One of the largest and most pronounced Bou- sibly batholiths in the region (units Xg and Yg; Federal Center, Denver, Colorado 80225, USA. guer gravity anomaly lows over North America is Fig. 2) (Tweto, 1979; Bickford and Anderson,

Geosphere; June 2012; v. 8; no. 3; p. 669–684; doi: 10.1130/GES00723.1; 11 fi gures; 2 tables.

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Juan Mountains, and the region was affected by the Ancestral Rocky Mountain orogeny (Kluth W and Coney, 1981; Kluth, 1986), during which the DENVER Uncompahgre uplift formed. This uplift extends to the northwest of the SJVF (Casillas , 2004). The Laramide orogeny was responsible for uplift GRAND ASPEN of the Needle dome and Gunnison uplift (Kelley, JUNCTION COLORADO 1955; Tweto, 1975; Cather, 2004) and formation of the San Juan sag, a northward extension of the SPRINGS C′ GUNNISON San Juan Basin into the eastern SJVF region that A′ is fi lled with as much as ~2 km of Laramide-age (ca. 70 to ca. 40 Ma) sedimentary rocks (Gries, 1985; Brister and Chapin, 1994). San Juan San Most of the study area is underlain by the B 2 B′ SJVF, a 25,000 km erosional remnant of an Mountains Oligocene volcanic province that was active Luis across the southern Rocky Mountains of UTAH COLORADO southern Colorado and northern New Mexico C A (Steven, 1975; Lipman, 2007). Magmatism that produced the SJVF is interpreted as having NEW MEXICO Valley resulted from a long-term, complex interaction of mantle source melts and continental crust related to subduction beneath the western U.S. ARIZONA (Lipman et al., 1978; Farmer et al., 2008). The SJVF is predominantly andesitic, but ranges

Colorado Plateau Colorado SANTA FE compositionally from basalt to rhyolite (Lip- man et al., 1970; Lipman, 2007, and references therein). Early andesitic volcanism was fol- ALBUQUERQUE lowed by voluminous ignimbrite eruptions of dacite and rhyolite, which resulted in the for- mation of multiple nested caldera complexes (see following). Mixing of basaltic magmas with crustal rocks is inferred to be the source of Mogollon-Datil the silicic products emplaced into and erupted Volcanic Field from the upper crust (Lipman et al., 1978). The minimum volume of basaltic magma required to be emplaced into the crust to produce the SJVF rocks is estimated to be 384,000 km3 (Farmer et al., 2008). There may be a signifi - W cant amount of mafi c residuum left behind in the upper mantle and lower crust, based on analogy with other regions of the southwestern U.S. (Keller et al., 2005) and bounds on pos- sible magma volumes from petrologic model- ing (Farmer et al., 2008). SJVF volcanism began ca. 35 Ma, with erup- Figure 1. Physiography and geography of the San Juan Mountains region. Yellow box defi nes tion of andesitic and dacitic lavas of the Conejos area of this study and subsequent fi gures. Locations of gravity profi les that were modeled are Formation from stratovolcanoes until ca. 30 Ma, shown as A–A′, B–B′, and C–C′. Red dots show locations of shot points for seismic refraction when eruption of silicic ash-fl ow tuffs from profi les (Fig. 10). numerous calderas began (Lipman et al., 1970; Lipman, 2007, and references therein). The Conejos Formation accounts for about two- 1993; Anderson and Cullers, 1999; Karlstrom the southern Oklahoma aulacogen, may have thirds of the pre-erosional volume of the vol- et al., 2004; Gonzales and Van Schmus, 2007). extended into and through southern Colorado, canic fi eld (unit Tpl; Fig. 2) (Lipman et al., These plutons may be volumetrically signifi cant including the SJVF region (Larson et al., 1985; 1970). The younger caldera complexes formed in the subsurface, although their lateral and ver- Casillas, 2004). Rocks associated with rift- within and around clusters of these strato- tical extents are unknown. ing include mafi c and ultramafi c intrusions that volcanoes (Steven and Lipman, 1976). Silicic A number of tectonic events since the Pre- crop out immediately north of the SJVF (Fig. 2). ignimbrites (unit Taf; Fig. 2) erupted from 18 cambrian have affected the SJVF region. Phanerozoic sedimentary rocks of the Colorado calderas and range in estimated volume from 25 Cambrian rifting in Oklahoma, refl ected as Plateau are along the western margin of the San to 5000 km3 (Lipman, 2000, 2007; Lipman and

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108 W 107 106

% Montrose Xg Gunnison uplift M NC BZ Tpl Taf CP NP Tbb UN SL 38 SL 38 LC SL LG Tpl SV SJ B San Tmi C Luis UC LL Xg Basin Taf Yg SR Needle Mtns. LG Yg Taf Alamosa TKi % S P Figure 2. Simplified geology of the study area modified % Durango Tpl from Tweto (1979), Lipman Tbb (2000, 2006), and Lipman and McIntosh (2008). Approximate San Juan Basin outline of San Juan sag is after 37 37 108 W 107 106 Brister and Chapin (1994). 050 Map Units km Undifferentiated sediments, Calderas (age, Ma) sedimentary rocks Lake City (23.0) San Juan (28.4) Tbb Rio Grande rift-age volcanic rocks LC* SJ* Cochetopa Park (26.9) Lost Lakes (28.5) Ash-flows of Lake City caldera CP LL C** Creede (26.9) UC Ute Creek (28.6) Taf Main ash-flow tuffs SL** San Luis calderas (26.9) S Summitville (28.8) Tpl Conejos Formation SR** South River (27.1) P Platoro (29.4-28.3) Tmi mid-Tertiary intrusive rocks B** Bachelor (27.5) NC Needle Creek complex (29.8) TKi Laramide intrusive rocks SV* Silverton (27.6) NP North Pass (32.2)

Cambrian mafic/ultramafic rocks LG** La Garita (27.8) BZ Bonanza (33.0)

Uncompahgre (28.4) Marshall (33.7) Yg ca. 1.4 Ga granitic rocks UN* M * calderas of western cluster Xg ca. 1.7 Ga granitic rocks ** calderas of central cluster

Undifferentiated Precambrian rocks Approximate outline of San Juan sag

McIntosh, 2008), and individual eruptions are 26.9 Ma (Lipman, 2000, 2006). Eruptions of cluster. The ignimbrites (including those of thought to have been among the most violently andesites similar to the Conejos Formation con- the Lake City caldera) account for about one- explosive events in Earth’s history (Mason et al., tinued throughout the stage of ignimbrite vol- third of the original total volume of the SJVF 2004). The fi rst eruptions, from 33.7 to 29.8 Ma, canism, such that these rocks are interbedded (Lipman et al., 1970), but have been eroded to occurred in the northeastern portion of the SJVF, with the ignimbrites. the point that they now only represent about followed by eruptions from 29.4 to 28.8 Ma Caldera-centered eruptions of ignimbrites one-sixth of the total volume (P. Lipman, 2007, at the Platoro and Summitville calderas in the continued until ca. 26 Ma, when volcanism written commun.). The total volume of volcanic southeast SJVF. Volcanic activity then shifted changed to a volumetrically minor assemblage rocks erupted in the SJVF, including both the to the western caldera cluster with formation of basalts and rhyolites, related to the forma- Conejos Formation and ignimbrites, is ~40,000 of fi ve calderas between 28.6 and 27.6 Ma. The tion of the Rio Grande rift (Steven and Lipman, km3 (Lipman, 2007, and references therein). massive La Garita caldera erupted at 27.8 Ma, 1976; Lipman, 2000, 2007). The Lake City A large-amplitude Bouguer gravity anomaly the fi rst of 7 nested calderas to form in the cen- caldera formed at 23 Ma as part of this latter low occurs over the SJVF (Figs. 3 and 4), and tral cluster, ending with the Creede caldera at episode within the preexisting western caldera has been interpreted to refl ect a low-density,

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W A′ C′

B B′

C A

W

Figure 3. Complete Bouguer anomaly map (reduction density 2670 km/m3) of the study area. Caldera outlines from Figure 2 are included for reference. Black dots are gravity stations. Locations of gravity profi les that were modeled are shown as A–A′, B–B′, and C–C′. Red polygon is interpreted extent of batholith from Plouff and Pakiser (1972).

composite granitic batholith in the upper crust are signifi cantly younger than the ignimbrites occurring over areas of relatively low density (Plouff and Pakiser, 1972). This interpretation (Bachmann et al., 2007; de Silva and Gosnold, rocks. Regional quality gravity data, with sta- is discussed in greater detail in the following, 2007; Lipman, 2007). A subvolcanic batholith tion spacings of 1–10 km, were extracted from but has been accepted in a broad sense by sub- complex remaining behind today is not neces- the PACES (Pan American Center for Earth and sequent studies in the region. Such a batholith sarily compositionally or volumetrically repre- Environmental Studies) database (maintained complex could be considered the accumulated sentative of large magma chambers that existed by the University of Texas at El Paso; http://gis remnants of magma chambers that sourced during or immediately following volcanism. .utep.edu, accessed December 2008) and supple- caldera-forming eruptions (Lipman, 1984). A It follows that a batholith complex would not mented with 323 stations recently acquired in more recent interpretation holds that batholith be expected to be compositionally uniform, the San Juan sag region (R. Gries, 2007, written complex growth continued incrementally after because it would have been assembled over a commun.). The PACES database consists of data caldera subsidence ended, as recorded by ages long period of time of magmas with differing collected over decades by many previous workers and compositions of localized, shallow intru- compositions, with the possibility of relatively and was compiled as the result of a major coop- sions (Bachmann et al., 2007; Lipman, 2007). mafi c lower portions (Bachmann et al., 2007; erative effort between federal agencies and uni- A number of granitic intrusions crop out in the de Silva and Gosnold, 2007; Lipman, 2007). versities (Keller et al., 2006). For comparison to SJVF region (unit Tmi; Fig. 2), and are asso- the nonstandard approach taken in this report, a ciated with the ignimbrite stage of volcanism. GRAVITY AND SEISMIC DATA standard Bouguer anomaly map was computed They are as much as millions of years younger using conventional techniques (Telford et al., than the volcanic units at the corresponding cal- Gravity anomalies refl ect lateral variations of 1990; Blakely, 1995). Corrections included those deras, and tend to be more mafi c in composition density, with gravity highs occurring over areas for predicted gravitational attraction at the eleva- than the volcanic rocks, especially those that of relatively high rock density, and gravity lows tion and latitude of the gravity stations (free air

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W

Montrose Gunnison

Ouray Lake City

Telluride Silverton Creede

Del Norte

Alamosa

Cortez Pagosa Springs Durango

W

Figure 4. Color shaded relief elevation map of the study area (90 m; Shuttle Radar Topography Mission). Black contours are complete Bouguer anomaly values (in mGal). Cities are shown as black circles.

and theoretical corrections), effects of tabular the effect of the gravity fi eld was calculated at although in the southern Rocky Mountain region, and homogeneous rocks masses between the a level of 10 km above the ground, in order to anomalously low velocities have been observed stations and sea level (Bouguer slab correction), enhance the signature of relatively deep/broad in the crust and mantle, geographically cor- and effects of topographic masses (terrain correc- geologic features (Fig. 5). related with granitic batholiths (Schneider and tions). The standard reduction density of 2670 Seismic refraction data indicate the broad Keller, 1994). kg/m3, assumed to represent an average upper velocity structure of the crust and upper mantle, crustal density (Hinze, 2003), was used for the highlighting variations with depth. Whereas PREVIOUS GEOPHYSICAL Bouguer and terrain corrections. Application these data sets do not provide detailed images of INTERPRETATIONS of these corrections yields complete Bouguer lateral variations of velocities or of stratigraphic anomalies (Fig. 3). layering, they do give direct estimates of seismic Densities of igneous rocks vary as functions The wavelengths of gravity anomalies can be P-wave velocity for geologic situations where of both composition and texture (Telford et al., related to the depths of the causative sources, velocity increases with depth. A seismic refrac- 1990). Silicic rocks generally have lower den- because deep and/or broad sources produce tion profi le was recorded over the eastern SJVF sities than mafi c rocks, and for a given com- longer wavelength anomalies than those caused by the U.S. Geological Survey during 1965, position volcanic rocks typically have lower by shallow and/or narrow sources. One method with shotpoints on the south and north fl anks of densities than their plutonic equivalents (Tel- used to enhance the signature of deep and/or the SJVF (Fig. 1) (Prodehl and Pakiser, 1980). ford et al., 1990). Granitic plutons commonly broad sources at the expense of shallow and/or By modern standards these data are low quality, have lower densities than the country rocks narrow sources is upward continuation, in which with low signal to noise ratio and a small num- they intrude, producing gravity lows (Bott and the gravity anomalies measured on the ground ber of imprecisely located receivers, yet they are Smithson, 1967). A number of Bouguer gravity surface are mathematically transformed to a still useful for studies of gross velocity structure. anomaly lows in the southern Rocky Mountain higher measurement level (Blakely, 1995). Here Seismic velocities typically increase with depth, region have been interpreted to refl ect low-

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less on average (Plouff and Pakiser, 1972). This means that Airy-type isostatic compensation W (e.g., thickened crust under mountains ranges) cannot be responsible for a signifi cant portion of the gravity low, unlike anomalies observed over many other mountain ranges (e.g., Blakely, C′ 1995). (4) Precambrian granites exposed in the A′ Needle Mountains and in the Gunnison region may be expected to contribute to the gravity low, as is the case in the southern Sangre de Cristo B Mountains (Quezada et al., 2004). However, the B′ Bouguer gravity anomaly low does not extend over the entire Needle Mountains, only over its northern portion (Figs. 2 and 3), and at local scale, Precambrian granites there do not appear to correlate with gravity anomalies. This led Plouff and Pakiser (1972) to interpret that the batholith complex was intruded underneath COLORADO the northern portion of the Needle Mountains. C A NEW MEXICO The gravity low also does not extend over gra- nitic rocks in the Gunnison region. Whereas Pre- cambrian granites may account for some of the gravity low and their presence under the SJVF cannot be ruled out, it seems unlikely from these observations that they are a signifi cant source for the gravity low. Individual calderas are not associated with W gravity anomalies within the larger gravity low, indicating that the densities of rocks that fi ll the calderas are not signifi cantly different from the surrounding volcanic rocks. The Cochetopa cal- dera is an exception, with a geographically coin- cident gravity low (Figs. 2 and 3). The source of the low is unknown, but may be caused by a density contrast with relatively dense Precam- brian rocks surrounding much of the caldera Figure 5. Upward-continued complete Bouguer anomaly values for the San Juan volcanic margin (Plouff and Pakiser, 1972). fi eld region. Continuation distance is 10 km. Locations of gravity profi les that were modeled Plouff and Pakiser (1972) placed the margin ′ ′ ′ are shown as A–A , B–B , and C–C . Dashed polygon shows interpreted extent of source of of the batholith complex at approximately the long-wavelength gravity low. –300 mGal contour. They used two-dimensional forward modeling (close to profi le A–A′ of this study) and an assumed density contrast of –100 density granitic batholiths and plutons, includ- Pakiser, 1972; Bachmann et al., 2007). (1) The kg/m3 to estimate a batholith complex thickness ing an Oligocene batholith in central Colorado boundaries of the gravity low correlate well with of ~24 km (Plouff and Pakiser, 1972), but did (Isaacson and Smithson, 1976), Oligocene and the mapped extent of calderas (Fig. 2), which not estimate its volume. Precambrian plutons in the Sangre de Cristo would be expected for a batholith complex that A regional gravity low, 400–500 km in wave- Mountains (Cordell and Keller, 1984; Cordell is related to surface manifestations of magma- length and therefore encompassing the gravity et al., 1985; Grauch and Keller, 2004; Quezada tism (Lipman, 1984). (2) Low-density volcanic low related to the batholith complex, is over et al., 2004), and an Oligocene batholith under rocks of the SJVF extend well beyond the area the region surrounding the SJVF (Fig. 5). Roy the Mogollon-Datil volcanic fi eld (Schneider of the gravity low and tend to have small density et al. (2004) interpreted the source of this low as and Keller, 1994). contrasts with surrounding sediments and sedi- a low-density zone in the upper mantle, devel- The Bouguer gravity anomaly low associ- mentary rocks (Table 1) of the Colorado Plateau, oped as a product of Tertiary magmatism by ated with the SJVF has dimensions of ~100 by San Juan Basin, and San Luis Basin, meaning extraction of basaltic magmas from the mantle 150 km and laterally is contained nearly com- that the surfi cial rocks cannot be the main source into the lower crust. A solution for the source pletely within the boundaries of the SJVF (Figs. of the anomaly. (3) Whereas there is a profound of the long-wavelength gravity low is an 2 and 3). An upper crustal composite Oligocene inverse correlation between surface elevation and ~300-km-wide, ~200-km-thick zone assuming granitic batholith complex related to the SJVF the complete Bouguer gravity anomaly values a density contrast of –33 kg/m3 with surround- has been interpreted as the source of the anom- (Fig. 3), the gradients at the margins of the grav- ing mantle rocks, on the basis of a model that aly (Plouff and Pakiser, 1972). A number of lines ity low are too sharp to be caused by a source corresponds in location to profi le B–B′ herein of evidence support this conclusion (Plouff and >~15–20 km depth at most and probably 5 km or (Roy et al., 2004).

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Seismic low-velocity zones are present that are incorrect, if the entire gravity low is could be removed and the resulting gravity in the upper crust of the SJVF and Aspen- attributed to the complex. anomalies would not include density variations Gunnison regions. Seismic refraction data col- To compute a gravity anomaly map that better related to surfi cial rocks. In practice, however, lected by the U.S. Geological Survey during refl ects the batholith complex, we used a method the depth to the top of the batholith complex the 1960s revealed signifi cant thicknesses of that combines data reduction (corrections simi- is not well known, nor is the overlying three- relatively low velocity rocks within the upper lar to standard Bouguer slab and terrain cor- dimensional distribution of densities. Also, 20 km of the crust (Prodehl and Pakiser, 1980). rections) and modeling of surface geology into adjacent to the SJVF are a number of complex Rapid terminations of upper crustal refractions one step, and allowed variable surface densities structures, including the San Juan and San Luis were interpreted to refl ect two stacked and lat- to be used (Wingrav program, M. Baker and Basins and San Juan sag, that we did not attempt erally extensive zones (with velocities <6 km/s K. Crain, 2003–2005, personal communs.). to model in three dimensions. Thus, we pro- between rocks with velocities >6 km/s), This method calculates the gravitational effect ceeded by assigning variable densities (Fig. 6) including a small zone of high-velocity rocks of mass between two layers at each gravity and calculating the gravitational effect of rocks between the low-velocity zones (Prodehl and station, where the layers are defi ned by digital only above an elevation of 2286 m (7500 ft), Pakiser, 1980). Whereas the rocks associated elevation models. Different calculation meth- the approximate lowest surface elevation of the with the low-velocity zones were interpreted ods, following those of Hammer (1974), were adjacent San Luis Basin. The structures of to occupy the same upper ~20 km of the crust used depending on the distance from the grav- basins and uplifts of Precambrian basement occupied by the batholith complex beneath the ity station to different masses, where bound- rocks around the margin of the SJVF were not SJVF (Prodehl and Pakiser, 1980), we are not aries between different masses are defi ned in built into the model. For simplicity we further aware of any literature that has commented on terms of a grid of density values (Fig. 6). The assumed that all density bound aries modeled the relationship between the source of the seis- edges of individual masses were treated as ver- above 2286 m extended vertically down to mic low-velocity zones and the source of the tical bound aries, extending from the top of the 2286 m, such that no dipping contacts are used. gravity low. upper layer to the top of the bottom layer (Crain, All rocks beneath 2286 m were assigned a den- 2006). For masses >500 km away from a grav- sity of 2670 kg/m3. The gravitational effect was GRAVITY REDUCTION ity station, a row mass approximation (Hammer, calculated for this model, the upper surface of AND MODELING 1974) was used, and for distances <500 km to a which is defi ned by 90 m Shuttle Radar Topog- gravity station, vertical line element approxima- raphy Mission digital elevation data. This effect Gravity data processing (or reduction) involves tions (M. Baker, 2003, written commun.) were was subtracted from the original free air gravity the removal of several predictable effects that employed. Groups of gravity stations were com- fi eld (corrected for distance above the center of infl uence observed gravity values, with the goal bined, or binned, into groups of masses for ease the Earth but not masses of rock), in order to of enhancing the geologic signal to be investi- of calculation, with the spatial extent of station yield a residual map (Fig. 7) that is conceptually gated. As discussed herein, conventional gravity bins depending on the distance to masses. similar to the standard complete Bouguer anom- maps (such as that shown in Fig. 3) are derived Density information for the model comes aly map (Fig. 3). To summarize, data reduction and interpreted assuming that an upper crustal from published data and typical values based and three-dimensional modeling have effec- density of 2670 kg/m3 (Hinze, 2003) is appro- on lithologies (Popenoe and Steven, 1969; tively been combined to remove the effect of priate for the Bouguer slab and terrain correc- Popenoe and Luedke, 1970; Grauch and Hud- rocks above 2286 m. Thus, this map gives an tions over large areas. son, 1987; Jenkins, 1989; Telford et al., 1990), improved representation of buried anomaly Interpretation problems arise from conven- as well as information from borehole density sources. tional reduction procedures when the densi- logs (R. Gries, 2007, written commun.). The The difference (Fig. 8) between the standard ties of rocks above sea level differ from 2670 most important density assignment was that complete Bouguer anomaly map from the new kg/m3. The Bouguer slab and terrain corrections used for the SJVF, because these volcanic rocks map is as much as 10–15 mGal, the greatest dif- are of special interest in this study, because the form most of the high topography in the study ference occurring over the highest elevations of andesites and ignimbrites that compose the high area. Andesites of the Conejos Formation, as the SJVF region (Figs. 4 and 8). The cumula- elevations (Figs. 2 and 4) of the SJVF are much well as andesites that continued to erupt during tive error level within the new gravity map is less dense than the standard reduction density of the ignimbrite phase of volcanism, have typical estimated to reach ~3 mGal based on observa- 2670 kg/m3, and because the San Juan Moun- densities of 2500 kg/m3 (R. Gries, 2007, writ- tions of solutions at single stations thought to tains contain extremely rugged topography and ten commun.) and make up ~5/6 of the total be spurious, likely the combined result of using therefore large-magnitude terrain corrections SJVF volume. Ignimbrites make up 1/6 of the relatively coarse (90 m) digital elevation data are required. The Bouguer slab correction is SJVF volume and have typical densities of 2200 and the inherent resolution of the Wingrav pro- proportional in magnitude to the assumed den- kg/m3. A weighted average of 2450 kg/m3 was gram with the chosen calculation parameters. sity and algebraically negative in effect, mean- used to represent the entire SJVF, because the The map is therefore only interpreted in terms ing that calculated Bouguer gravity anomalies subsurface distribution of the different volcanic of broad sources that produce anomalies with will be lower for larger assumed densities. Inter- rocks is not known well enough to be built into amplitudes much greater than 3 mGal, and sur- mediate and silicic composition volcanic rocks, the model. Geologic units of small thickness, fi cial sources are not studied. like those that compose the SJVF, normally have such as Rio Grande rift–age volcanic fl ows, The margins of the batholith complex were densities signifi cantly <2670 kg/m3. This means were not included in the model because they interpreted from the horizontal gradient mag- that the gravity low over the SJVF (Fig. 3) has a are volumetrically insignifi cant. Density assign- nitude of the new gravity map. This method signifi cant contribution from the vol canic rocks, ments are summarized in Table 1. is based on the principle that for the case of and is not entirely due to the batholith complex. For this study, we ideally wanted to calculate vertical boundaries, the gravity fi eld’s largest Thus, quantitative interpretation of the anomaly the gravitational effect of all masses above the magnitude gradients in the horizontal direction will result in models of the batholith complex level of the batholith complex, so that this effect are located over the edges of density contrasts

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W

density = 2670 Gunnison uplift (Precambrian rocks)

density = Density = 2670 2450

silicic intrusions

San Luis Basin Needle uplift Density = (Precambrian rocks) 2450

7500 ft/2286 m

density = 2670

W 3

Figure 6. Density model used for all rocks above elevation of 2286 m. Heavy black line indicates 2286 m (7500 ft) elevation contour; all rocks below this level were assigned a standard density of 2670 kg/m3 (units for all densities are kg/m3).

(Cordell, 1979; Blakely and Simpson, 1986). gin, assumed to be vertical, was interpreted from the new gravity map. First break times of The method is therefore useful for detecting from the result, and the location is similar to the refracted and refl ected phases were picked from geologic contacts. Here, the horizontal gradi- previously interpreted extent for the western, analog paper records provided by C. Prodehl ent magnitude was calculated from the new southern, and northern portions of the complex (2007, written commun.), and modeled using gravity map after upward continuation of 5 km, (Fig. 3) (Plouff and Pakiser, 1972). The inter- MacRay (Luetgert, 1992), a ray-tracing for- in order to attenuate effects of near-surface preted eastern margin of the batholith complex ward-modeling program (Fig. 10). The picking sources (Fig. 9). The batholith complex mar- is extended signifi cantly to the east, such that it error of phase arrival times was estimated to nearly reaches the western margin of the San be 0.2–0.3 s, based on resolution of the origi- Luis Basin (cf. Figs. 3 and 7). It is possible that nal records. Picks were not signifi cantly differ- TABLE 1. SURFICIAL DENSITIES USED interference from other anomaly sources, such ent from those originally made by Prodehl and TO CALCULATE NEW GRAVITY MAP as the San Juan sag, may be present in the new Pakiser (1980). Density G(kg/meologic unit 3)* gravity anomaly and horizontal gradient maps. Gravity and seismic refraction data were Oligocene volcanic rocks of San Juan 2450 Nevertheless, the interpreted areal extent of the modeled in an integrated fashion along profi les volcanic field (SJVF) complex is 8200 km2. A–A′, B–B′, and C–C′ (Fig 5). The fundamental Plutons of SJVF 2550 assumption underlying the following discus- Mafic plutons of Needle Mountains (small 2900 volumes) INTEGRATED SEISMIC AND sion is that the source of the gravity low is the Default density for unassigned units and 2670 GRAVITY MODELING same as the source of the seismic low-velocity granitic plutons of Needle Mountains zones. Whereas not specifi cally addressed by *Values from R. Gries (2007, written commun.), Grauch and Hudson (1987), Jenkins (1989), Popenoe The vintage analog seismic refraction pro- previous studies, this assumption makes sense, and Luedke (1970), Popenoe and Steven (1969), and fi les (A–A′; Fig. 1) were reinterpreted to provide as the source body interpreted by gravity data Telford et al. (1990). subsurface constraints on gravity models made analysis (Plouff and Pakiser, 1972) occupies the

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W A′ C′

B B′

C A

W

Figure 7. New residual (Bouguer-like) gravity anomaly map of the study area. Brown polygon is new interpretation of batholith margin based on horizontal gradient magnitude trends (see Fig. 9).

same portion of the upper crust occupied by the a short distance to the east of the central caldera using a variety of different velocities, and veloc- sources of seismic low-velocity zones (Prodehl cluster (Fig. 7). This model was constructed ities ranging from 5.8 to 6.0 km/s were found and Pakiser, 1980), and low seismic velocities from the shallow subsurface downward, adjust- to provide reasonable fi ts to the observed data. are normally correlated with low densities. This ing layer thicknesses and velocities of refractors Attempts to model the low-velocity zones with common source is interpreted to be a silicic until the observed data were fi t in a broad sense values outside of this range resulted in refrac- Oligocene batholith complex. Because seismic (Fig. 10). It was not possible to fi t the arrival tor crossover distances and traveltimes that were refraction is a useful method for detecting broad time of every fi rst break pick, although this was inconsistent with the observed data. Because the variations of velocity with depth, the seismic not necessarily desirable due to the low spatial velocity of rocks composing the low-velocity modeling was used to defi ne the top and bot- resolution and large possible time errors inher- zones cannot be uniquely determined, there was tom of the complex, and the gravity horizontal ent in the data. Velocities averaging 4.1 km/s a tradeoff between velocity and low-velocity gradient magnitude map (Fig. 9) was used to were used, extending from the surface to a depth zone thickness such that higher velocities result defi ne its lateral boundaries in the models. Other corresponding roughly to sea level, where a very in a larger required thickness and lower veloci- assumptions used to make the models presented high velocity (6.2 km/s) refractor is interpreted. ties result in smaller required thickness. This here are that the complex is entirely below an This may indicate the presence of Precambrian tradeoff is expressed in terms of different batho- elevation of 2286 m (7500 ft), which the seis- rocks under the eastern margin of the central lith complex thicknesses for different velocities mic model supports (see following), and that caldera cluster. (Table 2). The best fi t and therefore preferred its density contrast with country rocks remains Two low-velocity zones were modeled to be seismic model used a velocity of 5.9 km/s for constant with depth. within the upper crust (0–20 km depth below the low-velocity zones that are interpreted to The fi rst step was to develop a crustal P-wave sea level), based on traveltime modeling and the defi ne the batholith complex (Fig. 10). velocity model that satisfi ed the fi rst break picks rapid terminations of two branches of refracted The low-velocity zones are separated by a made from the seismic data along profi le A–A′, arrivals (Fig. 10). These zones were modeled re frac tor with a velocity of 6.3 km/s. The shal-

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W

WNW

ESE

W

Figure 8. Difference between residual and complete Bouguer gravity anomaly maps. (Top) In map form. (Bottom) In profi le. -240 V.E.—vertical exaggeration. conventional complete Bouguer gravity (all 2670 kg/m 3 ) -270

-300 Gravity (mGal) -330

30

0 new complete Bouguer-like based on density model

-30 Gravity (mGal) -60

20 difference = new - conventional = excess signal removed

10

Gravity (mGal) 0

Silverton Lost La Garita caldera caldera Lake Creede caldera

4 WNW caldera ESE

12,500

t)

3 10,000 Colorado San Luis Plateau densitydensity = vvariableariable Valley 7500 Elevation (f Elevation (km) (km) Elevation 2 densitydensity = 22670670 kkg/mg/m3

0 90 180 270 V.E.=20 Distance (km)

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W

W

Figure 9. Horizontal gradient magnitude of residual gravity anomaly map (Fig. 7) after upward continuation of 5 km. Thick black line is the interpreted margin of the batholith complex.

lowest low-velocity zone is apparent in reversed fi rst defi ned. The same source of low densities in the mantle was initially left loosely defi ned arrivals (e.g., shown by seismic arrivals from in the mantle was used here: an ~300-km-wide, along A–A′. both north and south directions) and is therefore ~200-km-thick zone with a density contrast of The next step was to use the top and bottom a robust feature of the model. The same is true –33 kg/m3 with surrounding mantle rocks. How- contacts of the batholith complex interpreted by of the 6.3 km/s refractor. However, the deeper ever, Roy et al. (2004) only defi ned the regional seismic modeling in the gravity model along low-velocity zone is only detected in seismic fi eld in an east-west direction. The upward con- A–A′, using the lateral position of the batholith data from the northern shot, due to a lack of tinued gravity data were used as a rough guide complex edges from the horizontal gradient receivers south of the southern shot (Fig. 10), to extend the interpreted regional source to the magnitude analysis (Fig. 9). These contacts were and so is a less robust feature, despite provid- south and northwest (Fig. 5) to profi le C–C′. The extended to B–B′ and C–C′ (Fig. 11). Modeling ing a reasonable fi t to the observed data. Deeper northern extent of the regional source (end of of the three profi les together allowed an overall portions of the crust and upper mantle are not A–A′) was more diffi cult to defi ne because the best fi t density contrast for the batholith com- well imaged by the seismic data. The gravity regional gravity low over the SJVF merges with plex to be determined, as well as variations in model along profi le A–A′ was constructed to a broader regional gravity low over much of the thickness away from A–A′. Once the gravity sig- match the upper crustal confi guration indicated Rocky Mountains of central Colorado. The con- nature of the complex was determined across all by the seismic model (see following discus- tinuation of the gravity low to the north of A–A′ three profi les, the location of the northern edge sion; Fig. 11). may be related to a number of different sources, of the mantle low-density zone was adjusted For profi le B–B′, the fi rst step of the gravity including crustal thickening under the Rocky until the model along A–A′ fi t the observed data modeling was to defi ne the regional gravity fi eld, Mountains of central Colorado, and therefore as shown in Figure 11. The entire process of because this profi le is coincident in location does not necessarily indicate a continuation using the seismic model to constrain the gravity with the model of Roy et al. (2004), wherein the of low mantle densities north of A′. Given this interpretation was repeated using the range of regional gravity fi eld and interpreted source were uncertainty, the north edge of the low densities reasonable velocities for the batholith complex

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Figure 10. Seismic refraction fi rst break 5 picks and predicted response (top panel), ray A A′ paths through the model (middle panel), and interpretation of the best fi t seismic model (bottom panel). Seismic data plotted with 4 reduction velocity of 6 km/s. COC—north- ern shot point (Cochetopa). LUM—southern shot point (Lumberton). Black colors indi- Moho reflection (PmP) 3 PmP cate northward-traveling seismic energy, green colors indicates southward-traveling seismic energy. Pick sizes were chosen to

represent amount of uncertainty. Zero eleva- T - X/6 (s) 2 tion corresponds to sea level. Note that seis- mic model length is greater than A–A′ length. Refraction 6.3 km/s Refra Refraction ction 6.3 km/s 1 Direct 6.3 km/s (Table 2), although only the preferred models Refra ction km/s are shown here (Fig. 11). The seismic refractor 6.2 km/s Refraction 6.2 Di rect that is between the two low-velocity zones was 0 included in all the gravity profi les because it is 0 240 a robust feature of the seismic model, although LUM Distance (km) COC this is not required by the gravity data. South North Neither the gravity nor seismic refraction data are capable of resolving changes of density 0 and/or velocity with depth within the batholith complex. For the preferred model, the computed density contrast is a vertically averaged value 10 of –80 kg/m3. This corresponds to a batholith complex density of 2620 kg/m3 if a background 20 upper crustal density of 2700 kg/m3 is assumed (only the density contrast can be uniquely deter- mined). This contrast can reasonably be varied Depth (km) 30 from –60 to –110 kg/m3 for seismic velocities of 6.0 and 5.8 km/s, respectively (Table 2). All modeling runs showed the batholith complex 40 to be signifi cantly thicker under the western portion of the SJVF than under the central and MOHO eastern portions (Fig. 11). The preferred model shows a maximum thickness of ~20 km under 0 the western SJVF that decreases eastward to <10 km under the eastern SJVF. The model that A A′ 4.1 km/s produces a maximum thickness of 23 km, cor- 0 6.2 km/s 3 responding to a density contrast of –60 kg/m , is Batholith 5.9 km/s consistent with the thickness estimate of Plouff 6.3 km/s and Pakiser (1972). However, the thickness esti- 10 Batholith 5.9 km/s mate range presented here is generally thinner 6.3 km/s than the previous estimate (Plouff and Pakiser, 1972), due to the modeling approach taken here 20 that resulted in a smaller amplitude gravity low. Assuming an area of 8200 km2 (from the hori- zontal gradient magnitude analysis; Fig. 9) and Depth (km) 30 Middle and Lower Crust using thickness estimates from the gravity mod- eling, the volume of the complex is estimated to be 82,000–130,000 km3 (Table 2). 40 VE = 3 7.0 km/s MOHO DISCUSSION 7.9 km/s Mantle

The Southern Rocky Mountain volcanic fi eld 04080 120 160 200 240 is generally considered to be a manifestation of Distance (km) the Cenozoic ignimbrite fl are-up that affected

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TABLE 2. RANGE OF BATHOLITH COMPLEX PROPERTIES FROM JOINT SEISMIC AND GRAVITY MODELING lions of years), as opposed to refl ecting plutonic Average Minimum Maximum Average residuum associated with any given caldera. This A–A′ overall overall Total density Density of thickness thickness thickness volume contrast batholith is consistent with interpretations that the batho- Model (km) (km)* (km)* (km3)† (kg/m3) (kg/m3)§ lith complex was characterized by accumula- Seismic model A–A′, 5.8 km/s 8.3 4.5 15.0 82,000 –110 2590 tion of magma after volcanism ended, and that assigned to low-velocity zone Seismic model A–A′, 5.9 km/s 9.8 7.6 19.9 110,000 –80 2620 the batholith complex remaining today is not assigned to low-velocity zone necessarily representative of magma chambers (preferred model) that existed during any specifi c caldera-forming Seismic model A–A′, 6.0 km/s 11.8 7.7 23.0 130,000 –60 2640 assigned to low-velocity zone eruption (Lipman, 2007). A more direct relation *Across all three gravity profile models. between batholith complex thickness and cal- †Based on interpreted total batholith area of 8200 km2 and average of modeled thicknesses along gravity profi les. dera clustering would be expected if the complex § 3 Assumes surrounding rocks have density of 2700 kg/m . represented fossil magma chambers that were tapped by surfi cial volcanism. Despite the fact that the batholith complex is much of southwestern North America. The fl are- of ~100,000 km3 (Lipman, 2007). Taking into shown here to have constant physical properties up is characterized by a high volume of erupted account an erupted volume of ~40,000 km3, and simple boundaries, it almost certainly was intermediate and silicic volcanic rocks, the lat- ratios of intrusive volumes (presumed to consti- assembled over several million years (e.g., Lip- ter typically associated with nested caldera col- tute magma chamber residuum following a major man, 2007), is composed of rocks emplaced in lapse and regional emplacement of far-traveled eruption) to volcanic volumes range from 2:1 to multiple episodes with different densities, and ash-fl ow tuffs and associated deposits (Lipman, ~3:1. These values are signifi cantly lower than has complex boundaries. The simple models 2007, and references therein). The subarea of bounding estimates of 10:1 proposed for conti- presented here are only approximations of its the SJVF, those erosional remnants constituting nental magmatic systems (Smith, 1979; Crisp, properties and dimensions, which are chosen the San Juan Mountains, has been interpreted to 1984). Lower values appear to be more reason- to be consistent with the seismic model. For preserve the predominantly volcanic record of able for large systems where ignimbrite erup- example, batholiths are thought to become mainly Oligocene high-power magmatic input tions may drain the upper several kilometers of more mafi c in composition and thus denser with (Lipman, 2007). This magmatism overlaps in any given magma chamber (White et al., 2006; increasing depth, although with no geophysi- time with the transition to extensional tectonism Lipman, 2007), as appears to be the norm in the cal constraints on increasing density or velocity of the Rio Grande rift. Regardless of differing Southern Rocky Mountain volcanic fi eld. How- with depth, any models with this characteristic opinions of tectonic controls on mantle source ever, our ratios based on calculated batholith com- would be quantitatively arbitrary and were not materials, mechanism of melting, magma plex volumes may refl ect minimums, because the attempted here. Further, the top and bottom of ascent, and differentiation models, the general lateral extent of the batholith complex is based the geophysically imaged batholith complex consensus remains that Oligocene supervol- on horizontal gradient trends of gravity data that may not correspond well to bound aries that canoes of the SJVF refl ect an almost unprece- are fi ltered to refl ect the effect of the main mass would be picked based on petrologic analyses. dented input of mafi c magma into the crust, a of the batholith complex, excluding intrusions Despite these limitations, the overall (i.e., ver- necessary thermal requirement to generate the that almost certainly underlie calderas outside of tically averaged) densities (2590–2640 kg/m3) estimated 40,000 km3 of erupted materials (Lip- the main gravity low in the northeast and south- determined for the batholith complex, assuming man, 2007). Only the Altiplano-Puna volcanic east San Juan Mountains (Fig. 9). Perhaps more an upper crustal background density of 2700 complex in the central Andes (de Silva, 1989; important, relatively deep and dense rocks of or kg/m3, are consistent with published values for Lindsay et al., 2001; Schmitt et al., 2003; de related to the batholith complex may not be geo- granite densities (Daly et al., 1966; Telford et al., Silva et al., 2006; de Silva and Gosnold, 2007) physically well imaged, meaning that they are not 1990), as well as density measure ments on gran- provides a reasonable late Cenozoic to Quater- included in the volume estimates presented here. ite outcrops of the SJVF (Popenoe and Steven, nary corollary with respect to eruptive style, Our underlying assumptions are that the 1969; Popenoe and Luedke, 1970; Grauch and petrologic diversity, and scale (although total source of the gravity low is the same as the source Hudson, 1987). erupted volumes are less, ~15,000 km3). of the seismic low-velocity zones and that the The interpreted density range of 2590–2640 Given the extraordinary volumes of erupted density of the interpreted batholith complex does kg/m3 assumes an overall upper crustal den- magma, much of it cataclysmically erupted from not change signifi cantly with lateral position. sity of 2700 kg/m3, a value that is commonly multiple caldera sources, the nature of associ- Integrated modeling can thus be used to infer the assumed for gravity studies but may or may ated subvolcanic magma chambers solicits con- batholith complex’s thickness variations in addi- not be valid in the SJVF region. The vertically siderable discussion and debate. Constraints tion to volume. The thickest part (15–23 km, averaged density contrast between the batholith on volume and geometry are critical to these Table 2) of the batholith complex underlies the complex and surrounding rocks, however, can be discussions. We focus this discussion on the western SJVF and the western caldera cluster, uniquely determined. The range of permissible geophysically observable characteristics of the where there are four nested calderas on the sur- values, –60 to –110 kg/m3, particularly the pre- subsurface environment of the SJVF, particularly face. The complex is thinner under the central ferred value of –80 kg/m3, compares favorably crystallized magma chambers that coalesced as caldera cluster, with seven calderas, showing that to the few other studies that have examined the a large batholith complex. there is not a direct relationship between batho- geophysical expressions of large batholiths and Total volume estimates for the sub volcanic lith complex thickness and mapped calderas. We have included joint seismic and gravity model- batholith complex of the SJVF subarea of interpret this as refl ecting the integrated tem- ing. The inferred batholith under the Mogollon- the southern Rocky Mountain volcanic fi eld poral and spatial development of subvolcanic Datil volcanic fi eld of New Mexico and Arizona range from 82,000 km3 to 130,000 km3, con- magma chambers and their crystallized plutonic (Fig. 1) is interpreted to have a density contrast sistent with estimates of total intrusive volume equivalents over protracted periods of time (mil- of –100 kg/m3 (Schneider and Keller, 1994).

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Figure 11. Gravity profi le models along A–A′ AA′ ′ (top panel), B–B′ (middle panel), and C–C′ 0 B-B (bottom panel). The units for all densities –20 regional field (D) are kg/m3. Yellow regions indicate rocks –40 South North

above 2286 m elevation. WC—western cal- Gravity (mGal) =Observed, =Calculated dera cluster; CC—central caldera cluster; –60 CC VE—vertical exaggeration. Green lines San Juan Basin labeled regional fi eld indicate the gravita- sedimentary rocks/volcanics D = 2500 tional effect of a low-density zone within the 0 upper mantle (see text). batholith D = 2620 crust

10 D = 2700 batholith D = 2620

The Boulder batholith of Montana, similar in Depth (km) dimensions and interpreted thickness to the 20 SJVF batholith complex (Biehler and Bonini, 1969), was found by joint seismic refl ection and 0 100 180 gravity modeling to have a density contrast of Distance (km) –80 kg/m3 (Vejmelek and Smithson, 1995). VE = 3

Although the main source of the gravity low ′ is interpreted to be an Oligocene batholith com- 0 plex, elsewhere in the southern Rocky Moun- –20 B A-A tain region Precambrian granitic intrusions are B′ –40 regional field interpreted to cause gravity lows (Quezada et al., West East 2004). As Precambrian granites crop out on the Gravity (mGal) –60 northern and southwestern margins of the SJVF =Observed, =Calculated (Fig. 2) and may fl oor the magmatic complex, WC CC significant contributions to the gravity low Colorado Plateau San Luis Basin from low-density Precambrian granites buried 0 San Juan sag D = 2350 under the SJVF cannot be ruled out, especially batholith D = 2620 because their distribution at depth is not known. crust However, this possibility seems unlikely to us, 10 given that Precambrian granites are normally D = 2700 D = 2620 not directly associated with gravity lows where batholith

they crop out in the SJVF region. Depth (km) Another assumption in the gravity modeling 20 and the interpreted thickness distribution is that the density of the batholith complex does not 0 100 200 vary vertically or laterally. Although batholith VE = 3 Distance (km) density may reasonably be expected to vary ver- tically (Miller and Miller, 2002), it is diffi cult ′ to model these variations in a meaningful way 0 ′

because little is known of the batholith complex C B-B C –20 regional field composition at depth. Lateral density varia- North tions are also possible, as observed dramatically –40 South Gravity (mGal) within the Sierra Nevada batholith in California –60 =Observed, =Calculated (Oliver, 1977; Oliver et al., 1993). Such varia- Needle Mtns WC tions within the SJVF batholith complex would San Juan Basin change the interpretation of lateral thickness 0 patterns. However, this is diffi cult to address batholith quantitatively because the batholith complex crust D = 2620 cannot be sampled for densities, and assignment of variable densities would be arbitrary and 10 D = 2700 batholith unsubstantiated. Existing density measurements D = 2620

on Oligocene pluton outcrops are confi ned to Depth (km) the western portion of the study area and further 20 may not be representative of the deeper batho- lith complex (Lipman, 2007). 0 100 200 The batholith complex underlying the SJVF is expressed seismically by relatively VE = 3 Distance (km)

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low velocities compared to surrounding upper elevations, was used to compute a new complete improved the quality of the models presented here. crustal rocks. Numerous studies have inter- Bouguer anomaly–like gravity map of the San Reviews from V.J.S. Grauch, Rick Saltus, Chris Frid- preted seismic low-velocity zones to indicate Juan Mountains region. Traditional methods rich, Michael Cheadle, Scott Paterson, Peter Styles, and two anonymous reviewers greatly improved the magma chambers under active caldera systems of gravity data processing cause interpretation quality of the manuscript. Any use of trade, product, of similar scale to the SJVF (de Silva, 1989; problems there due to high elevations being or fi rm names is for descriptive purposes only and Chmielowski et al., 1999; Zandt et al., 2003; composed of rocks of the SJVF that have densi- does not imply endorsement by the U.S. Government. de Silva et al., 2006; Bachmann et al., 2007), ties signifi cantly lower than the standard Bouguer REFERENCES CITED although this study is one of few to interpret low correction density (2670 kg/m3). A large-ampli- velocities for a batholith complex (i.e., a crystal- tude gravity low over the region has long been Anderson, J.L., and Cullers, R.L., 1999, Paleo- and Meso- lized magma chamber or chambers). A similar interpreted to refl ect a silicic Oligocene batholith proterozoic granite plutonism of Colorado and Wyo- ming: Rocky Mountain Geology, v. 34, p. 149–164, interpretation was made for the batholith under complex that is related to the SJVF. Although doi: 10.2113/34.2.149. the Mogollon-Datil volcanic fi eld (Schneider the processing and interpretation scheme used Bachmann, O., Miller, C.F., and de Silva, S.L., 2007, The and Keller, 1994) that is comparable in age to here resulted in a residual gravity anomaly that volcanic-plutonic connection as a stage for understand- ing crustal magmatism: Journal of Volcanology and the SJVF, and also along the western margin is 10–15 mGal smaller than that observed on a Geothermal Research, v. 167, p. 1–23, doi: 10.1016 of the Rio Grande rift (Fig. 1). A 1–2-km-thick standard Bouguer anomaly map, a large gravity /j.jvolgeores.2007.08.002. Bickford, M.E., and Anderson, J.L., 1993, Middle Protero- refractor (e.g., high-velocity zone) between two low remained. zoic magmatism, in Reed, J.C., et al., eds., Precambrian: low-velocity zones was found to be a robust fea- The enhanced gravity expression of the Conterminous U.S.: Boulder, Colorado, Geo logi cal ture of the seismic model along A–A′. The geo- interpreted batholith complex was modeled Society of America, Geology of North America, v. C2, p. 281–292. logic signifi cance of the refractor is unknown, quantitatively in conjunction with reinter- Biehler, S., and Bonini, W.E., 1969, A regional gravity study although it has a higher velocity than low- preted seismic refraction data that image thick of the Boulder batholith, Montana, in Larson, L.H., velocity zones that are above and below. Its low-velocity zones in the upper crust beneath et al., eds., Igneous and metamorphic geology: Geo- logical Society of America Memoir 115, p. 401–422. presence and geometry are not constrained by the SJVF. Assuming that the source of both the Blakely, R.J., 1995, Potential theory in gravity and magnetic the gravity data and modeling, although gravity gravity low and the source of the low seismic applications: Cambridge, Cambridge University Press, ′ ′ 441 p. models along B–B and C–C show it in order velocities is an Oligocene batholith, a series of Blakely, R.J., and Simpson, R.W., 1986, Approximating to maintain consistency with the models along two-dimensional modeling runs indicated that edges of source bodies from magnetic or gravity anom- A–A′ (Fig. 11). Although it is compelling to try the batholith has an overall vertically averaged alies: Geophysics, v. 51, p. 1494–1498, doi: 10.1190 3 /1.1442197. to interpret this high-velocity zone within the density contrast of –60 to –110 kg/m with sur- Bott, M.H.P., and Smithson, S.B., 1967, Gravity investiga- broader low-velocity mass as a manifestation rounding upper crustal rocks, an overall density tions of subsurface shape and mass distributions of of magmatic processes (e.g., resulting from a of 2590–2640 kg/m3, and an estimated volume granite batholiths: Geological Society of America Bulletin, v. 78, p. 859–878, doi: 10.1130/0016-7606 3 two-stage process of magma emplacement), any of 82,000–130,000 km . Comparison to erupted (1967)78[859:GIOSSA]2.0.CO;2. such interpretation would be poorly constrained volumes yields ratios of intruded to extrusive Brister, B.S., and Chapin, C.E., 1994, Sedimentation and tectonics of the Laramide San Juan sag, southwestern and largely conjecture. magma volumes of ~2:1 or 3:1, although these Colorado: The Mountain Geologist, v. 31, p. 2–18. The timing of batholith complex emplace- likely represent a minimum because the deep- Casillas, H., 2004, An integrated geophysical study of the ment is not directly addressed by the geophysical est and most dense parts of the batholith com- Uncompahgre uplift, Colorado and Utah [M.S. thesis]: El Paso, University of Texas, 151 p. analyses presented here. However, the relatively plex may not be geophysically well imaged. Cather, S.M., 2004, Laramide orogeny in central and north- old calderas of the northeastern and south- The interpreted density contrasts, densities, and ern New Mexico and southern Colorado, in Mack, eastern SJVF (Fig. 2) are outside of the area inferred seismic expression of the batholith are G.H., and Giles, K.A., eds., The geology of New Mexico: A geologic history: New Mexico Geological of interpreted batholith extent (Fig. 9). While consistent with other similar intrusions in the Society Special Publication 11, p. 203–248. speculative, we suggest that batholith complex western United States. Chmielowski, J., Zandt, G., and Haberland, C.H., 1999, The central Andean Altiplano-Puna magma body: emplacement was more closely associated with The batholith complex is thickest under the Geophysical Research Letters, v. 26, p. 783–786, doi: the formation of the younger central and western western SJVF, as much as ~20 km in the best 10.1029/1999GL900078. caldera clusters. fi t model, and thins to <10 km under the eastern Cordell, L., 1979, Gravimetric expression of graben fault- ing in Santa Fe country and the Espanola basin, New A relationship may exist between batholith SJVF. The thickness pattern is not correlated Mexico, in Ingersoll, R.V., ed., Guidebook to Santa Fe complex thickness and the elevation of the over- with clustering of calderas on the surface, in Country: New Mexico Geological Society 30th Field lying ground surface. The thickest part of the that the largest caldera cluster does not overlie Conference Guidebook, p. 59–64. Cordell, L., and Keller, G.R., 1984, Regional structural trends complex is interpreted to underlie the western the thickest part of the complex. This is consis- inferred from gravity and aeromagnetic data in the New SJVF, spatially coincident with the highest ele- tent with the latest petrologic interpretations that Mexico–Colorado border region, in Baldridge, W.S., et al., eds., Rio Grande Rift—northern New Mexico: vations of the San Juan Mountains (Figs. 4 and the complex continued to be modifi ed following New Mexico Geological Society 35th Field Conference 11). This observation is consistent with a previ- cessation of volcanism, and therefore is not nec- Guidebook, p. 21–23. ous hypothesis that thick, low-density batholiths essarily representative of synvolcanic magma Cordell, L., Long, C.L., and Jones, D.W., 1985, Geophysi- cal expression of the batholith beneath Questa caldera, may cause uplift of overlying rocks on the order chambers. New Mexico: Journal of Geophysical Research, v. 90, of hundreds to several hundreds of meters (Lip- p. 11263–11269, doi: 10.1029/JB090iB13p11263. man, 1988, 2007). However, we consider this to ACKNOWLEDGMENTS Crain, K.D., 2006, Three-dimensional gravity inversion with a priori and statistical constraints [Ph.D. thesis]: El Paso, be speculative, and do not address it further. This work was supported by the University of University of Texas, 62 p. Oklahoma and the University of Texas at El Paso. Crisp, J.A., 1984, Rates of magma emplacement and vol- CONCLUSIONS Numerous discussions with Peter Lipman and Tom canic output: Journal of Volcanology and Geothermal Steven helped to sharpen our arguments. Kevin Crain Research, v. 20, p. 177–211, doi: 10.1016/0377-0273 (84)90039-8. and Steve Holloway provided critical support to the A new approach to gravity data processing, Daly, R.A., Manger, E., and Clark, S.P., 1966, Density of three-dimensional gravity modeling effort, and Jef- rocks, in Clark, S.P., ed., Handbook of physical con- one that used geologically appropriate densities ferson Chang assisted with the seismic modeling. stants (revised edition): Geological Society of America and digital topography to represent areas of high Robbie Gries contributed gravity and density data that Memoir 97, p. 19–26.

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