Late-Stage Volcano Geomorphic Evolution of the Pleistocene San Francisco Mountain, Arizona (USA), Based on High-Resolution DEM Analysis and 40Ar/39Ar Chronology
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Bull Volcanol (2010) 72:833–846 DOI 10.1007/s00445-010-0365-8 RESEARCH ARTICLE Late-stage volcano geomorphic evolution of the Pleistocene San Francisco Mountain, Arizona (USA), based on high-resolution DEM analysis and 40Ar/39Ar chronology Dávid Karátson & Tamás Telbisz & Brad S. Singer Received: 18 September 2008 /Accepted: 1 April 2010 /Published online: 1 May 2010 # The Author(s) 2010. This article is published with open access at Springerlink.com Abstract The cone-building volcanic activity and subse- nating ∼100 ka earlier than indicated by previous K-Ar quent erosion of San Francisco Mountain, AZ, USA, were ages. Moreover, 40Ar/39Ar dating constrains the formation studied by using high-resolution digital elevation model of the Inner Basin, an elliptical depression in the center of (DEM) analysis and new 40Ar/39Ar dating. By defining the volcano initially created by flank collapse. A 530 ka age remnants or planèzes of the volcano flanks in DEM-derived (with a ±58.4 ka 2σ error) for a post-depression dacite images, the original edifice can be reconstructed. We suggests that the collapse event is geochronologically propose a two-cone model with adjacent summit vents indistinguishable from the termination of the andesitic which were active in different times. The reconstructed cone-building activity. According to our DEM analysis, cones were 4,460 and 4,350 m high a.s.l., corresponding to the original cone of San Francisco Mountain had a volume ∼2,160 and 2,050 m relative height, respectively. New of about 80 km3. Of this volume, ∼7.5 km3 was removed 40Ar/39Ar data allow us to decipher the chronological by the flank collapse and subsequent glacial erosion, details of the cone-building activity. We dated the Older creating the present-day enlarged Inner Basin, and ∼2km3 and Younger Andesites of the volcano that, according to was removed from the outer valleys by erosion. Based on previous mapping, built the stage 2 and stage 3 stratocones, volumetric analysis and previous and new radiometric ages, respectively. The new 40Ar/39Ar plateau ages yielded 589– the average long-term eruption rate of San Francisco 556 ka for the Older and 514–505 ka for the Younger Mountain was ∼0.2 km3/ka, which is a medium rate for Andesites, supporting their distinct nature with a possible long-lived stratovolcanoes. However, according to the new dormant period between. The obtained ages imply an 40Ar/39Ar dates for the last ≤100 ka period, the final intense final (≤100 ka long) cone-building activity, termi- stratovolcanic activity was characterized by a greater ∼0.3 km3/ka rate. Editorial responsibility: M.A. Clynne D. Karátson (*) : T. Telbisz Keywords San Francisco Mountain . DEM analysis . Department of Physical Geography, Eötvös University, Volcanic geomorphology. 40Ar/39Ar geochronology . H-1117 Budapest Pázmány s. 1/C, Stratovolcanic activity. Erosion . Colorado Plateau Hungary e-mail: [email protected] B. S. Singer Introduction: scope and goals Department of Geology and Geophysics, University of Wisconsin-Madison, The stratovolcano of San Francisco Mountain or San 1215 W. Dayton St., Madison, WI 53706, USA Francisco Peaks (SFM hereafter; 3,853 m a.s.l.) is the largest eruptive center of the Late Miocene to Holocene San Present Address: Francisco Volcanic Field (SFVF, Figs. 1 and 2) in Northern D. Karátson Arizona, USA. Despite last erupting ∼0.4 Ma ago accord- Geoscience Center Göttingen, Georg-August University, Goldschmidstrasse 1, ing to previous dating on cone-building andesites, the 37077 Göttingen, Germany stratovolcano has a well-preserved conical shape except for 834 Bull Volcanol (2010) 72:833–846 Fig. 1 Location of San Francisco Mountain (SFM) within the San map shows location of SFVF in Arizona. (FL Flagstaff, SP Spring- Francisco Volcanic Field (SFVF). UTM Zone is 12North (WGS84). erville, OL O'Leary Peak, SC Sunset Crater) Quadrangle includes the areas presented in Figs. 3, 4, 5, and 7. Inset its summit region. There, the cone is truncated by the 5×3-km nants, the latest configuration of the cone can be elongated, U-shaped Inner Basin. Although the volcanic reconstructed. evolution of SFM including the peculiar Inner Basin has been (2) We tie the reconstructed geomorphological elements to studied in detail (Holm 1988, 2004; Duffield 1997), the final previously mapped geological units (Wolfe et al. 1987; volcanic stage as well as the final cone morphology are Holm 1988). The chronology of the SFM eruptions is poorly known. known mainly from geologic mapping, relative stra- The aim of this paper is to better understand the tigraphy, 20 K-Ar or fission-track determinations and evolution of the final cone-building stage and subsequent 1 40Ar/39Ar age that range from 2.78±0.26 Ma to 91± erosion of SFM. Our hypothesis is twofold: 2 ka at the ±2σ level of analytical uncertainty (Damon et al. 1974; Wolfe et al. 1987; McKee et al. 1998; (1) We suggest that the present morphology of the cone Morgan et al. 2004). Here, we use precise 40Ar/39Ar (e.g., outer flanks) includes some portions of the incremental heating methods to better constrain the original volcanic landform. By using these rem- sequence of late-stage volcanic events. Fig. 2 View of the Inner Basin from its northwest rim near Humphreys Peak Bull Volcanol (2010) 72:833–846 835 σ Methods Digital elevation model (DEM) applications in geomorphom- etry are widespread (Burrough and McDonnell 1998), and DEM analysis is occasionally used in studying volcanic areas as well (e.g., Favalli et al. 1999; Székely and Karátson 2004). In our study, we used DEM and DEM derivatives (e.g., slope, MSWD Age (ka) ± 2 σ aspect, and ridge maps) to quantify the volcanic geomor- ). Preferred ages are in bold. ±2 phology of SFM. The effect of resolution and DEM source i 1977 Ar on measurements impacts quality of DEM studies (Zhang and 36 Montgomery 1994; Ludwig and Schneider 2006;Wrightet Ar/ al. 2006); our reference DEM was the best available 10 m 40 spatial resolution US Geological Survey data base. Constructing shaded relief images, slope and aspect maps N are common products derived using GIS software. In order to 6 of 1010 of 11 290.6±0.99 294.0±1.3 of 108 of 0.40 88 1.20 293.4±3.3 of 11 596.1±19.5 563.2±17.1 293.1±6.1 5.69 292.4±7.4 519.7±23.8 1.01 0.56 508.5±17.0 698.2±356.9 emphasize the ridge and valley network, several methods σ 11.1 13.3 20.9 9.1 58.4 (e.g., plan curvature, median difference, modified Hammond ± ± ± ± ± method: Hammond 1964;Dikau1989) could be applied. (ka) ± 2 589.0 556.2 514.3 505.2 530.2 Here, we used the median difference, in which the elevation difference between each pixel and the median of its 150 m neighborhood is computed by applying a moving filter. If this difference is greater than a chosen positive threshold, ) using decay constants of Steiger and Jäger ( that pixel is classified as a ridge; if it is lower than another Ar% MSWD Age 1998 chosen negative threshold, it is classified as a valley. 39 Volumetric calculations of the volcanic edifice and the 1,2601,300 80.4 99.9 0.47 1,260 1.29 1,220 99.3 100.01,060 5.61 0.91 88.7 0.58 eroded material have been made by using standard GIS – – – – – Increments used (°C) functions that make it possible to determine volumes Plateau age (age spectrum) Isochron age between two distinct surfaces. For GIS-analysis, we used σ ESRI ArcView 3.2 and Golden Software Surfer 8 programs. 40Ar/39Ar ages were determined on groundmass sepa- fusion age rates from five andesitic and dacitic lava flows (represent- Age (ka) ± 2 ing the main stratigraphic units of SFM; Table 1) by using furnace incremental heating methods at the University of total Wisconsin-Madison Rare Gas Geochronology Laboratory. Sample preparation, mass spectrometry, and data reduction procedures are identical to those of Singer et al. (2008); throughout this paper, ages are reported at the ±2σ level of analytical uncertainty relative to 1.194 Ma Alder Creek rhyolite sanidine (Renne et al. 1998) using decay constants of Steiger and Jäger (1977). Isochron analyses revealed that none of the five samples contained significant levels of UW62D2T 104 0.510 550.3±24.1 780 excess argon, thus the inverse-variance weighted-mean ) ages, calculated from concordant plateau steps comprising 2008 >80% of the gas, give the best estimate of time elapsed since eruption (e.g., Singer et al. 2008). Ar furnace/laser incremental heating experiments of groundmass separates of San Francisco Mountain lava flows 39 Inner Basin Ar/ . Ages calculated relative to 1.194 Ma Alder Creek rhyolite sanidine (Renne et al. 3 40 Geological background Volcanic evolution Summary of Located in North Central Arizona, SFVF is one of 612 Older andesite642 Older andesite Agassiz Peak106 West Younger andesite rim of 645 Doyle UW62C5T Peak Younger andesite188 Humphreys Peak Younger dacite UW64D3TFor UW62C6T localities, 104 NE see of Fig. Sugarloaf 0.735 UW62C3T 595.4±12.7 97 104 960 0.644 0.981 104 504.0±10.4 501.9±43.8 780 810 0.575 585.3±64.9 710 Sample Total Table 1 Methods are those of Singer et al. ( several intracontinental, primarily basaltic volcanic fields ID Map unit Locality Experiment no. wt. (mg) K/Ca 836 Bull Volcanol (2010) 72:833–846 distributed along the southern margin of the Colorado 0.1 Ma, interrupted by long repose periods (Damon et al.