UPDATED ISOPACH MAP OF THE TUFF OF SAN FELIPEUPDATED ISOPACH MAP OF THE TUFF OF SAN FELIPEUPDATED ISOPACH MAP OF THE TUFF OF SAN FELIPEUPDATED ISOPACH MAP OF THE TUFF OF SAN FELIPEIN BAJA CALIFORNIA MEXICOIN BAJA CALIFORNIA MEXICOIN BAJA CALIFORNIA, MEXICOIN BAJA CALIFORNIA, MEXICO

L S bb th1 J St k2L Sabbeth1 J Stock2L Sabbeth , J Stock,1 i i f h h 2 lif i i f h l d1_ University of Rochester, Rochester NY USA 2_ California Institute of Technology, Pasadena CA USAUniversity of Rochester, Rochester NY USA California Institute of Technology, Pasadena CA USA

lsabbeth@u rochester edu jstock@gps caltech edulsabbeth@u.rochester.edu, jstock@gps.caltech.edu

Ab t tAbstractNORTH AMERICA

AbstractNORTH AMERICA

The 12 5 Ma Tuff of San Felipe is an ignimbrite layer often more than 10 m thick exposed in a number ofThe 12.5 Ma Tuff of San Felipe is an ignimbrite layer, often more than 10 m thick, exposed in a number of p g y poutcrops throughout Sonora and Baja California Mexico If we correctly calculate the volume distributionoutcrops throughout Sonora and Baja California, Mexico. If we correctly calculate the volume distribution of the Tuff of San Felipe on either side of the Gulf of California it will help us to place Baja California toof the Tuff of San Felipe on either side of the Gulf of California, it will help us to place Baja California to where it once was aligned with Sonora before the opening of the Upper Delfín Basin at 6 Ma. The volumewhere it once was aligned with Sonora before the opening of the Upper Delfín Basin at 6 Ma. The volume f th T ff ti t d t b t l t 54 bi k b St k t l (JVGR 1999) lth h t th t i tof the Tuff was estimated to be at least 54 cubic km by Stock et al. (JVGR, 1999), although at that point, y ( , ), g p ,

not all outcrops of the tuff in Baja California had been discovered and none had yet been identified innot all outcrops of the tuff in Baja California had been discovered and none had yet been identified in j ySonora We use the presently known extent and thickness of the Tuff in Baja California including outcropsSonora. We use the presently known extent and thickness of the Tuff in Baja California, including outcrops identified farther west and south in the stable part of the Baja California peninsula (Bennett 2009; Olguin-identified farther west and south in the stable part of the Baja California peninsula (Bennett, 2009; OlguinVill 2010 Ski l hi i ) d h l i f h T ff f S F li i B jVilla, 2010; Skinner et al., this meeting) to update the volume estimate for the Tuff of San Felipe in BajaVilla, 2010; Skinner et al., this meeting) to update the volume estimate for the Tuff of San Felipe in Baja C lif i O t i S till b i i ti t d b k d ill t b i l d d iCalifornia. Outcrops in Sonora are still being investigated by numerous workers, and will not be included in p g g y ,this presentation Using existing published maps and GoogleEarth we can zoom in to identify the Tuff ofthis presentation. Using existing published maps, and GoogleEarth, we can zoom in to identify the Tuff of San Felipe in Baja California and calculate the thicknesses of the outcrops in more locations than wereSan Felipe in Baja California and calculate the thicknesses of the outcrops in more locations than were

PACIFICpreviously reported With ArcGIS we create a layer of thicknesses and will present a final distribution and PACIFICpreviously reported. With ArcGIS, we create a layer of thicknesses and will present a final distribution and i h f th T ff f S F li i B j C lif i Thi lt i d t l l t d t d lisopach map of the Tuff of San Felipe in Baja California. This result is used to calculate an updated volumeisopach map of the Tuff of San Felipe in Baja California. This result is used to calculate an updated volume estimate after correcting for geological e tension that took place after 12 5 Ma in parts of the regionestimate, after correcting for geological extension that took place after 12.5 Ma in parts of the region. , g g g p p g

B k dBackgroundBackground gh ff f l / b f ll d f d l f d h hThe 12.5Ma Tuff of San Felipe / Ignimbrite of Hermosillo is identified in Baja California and throughout Sonora. On e 5 a u o Sa e pe / g b te o e os o s de t ed aja Ca o a a d t oug out So o a O

b th id f th G lf f C lif i th t ff i i l k th t b d t t k ti b t thboth sides of the Gulf of California, the tuff is a regional marker that can be used to track motion between the gNorth American and Pacific Plates (Oskin and Stock 2003) In his thesis (2002) Oskin estimated a minimum volumeNorth American and Pacific Plates (Oskin and Stock, 2003). In his thesis (2002), Oskin estimated a minimum volume of the tuff to be 118 1 cubic km Since then Bennett (2009) and Olguin-Villa (2010) found more outcrops to theof the tuff to be 118.1 cubic km. Since then, Bennett (2009) and Olguin-Villa (2010) found more outcrops to the South and minor changes have been identified in the map pattern of outcrops elsewhere. For example, Seiler et al. Figure 1: Map showing the boundary between the North American andSouth and minor changes have been identified in the map pattern of outcrops elsewhere. For example, Seiler et al. (2010) il d i h i l d d h Wi h b i f h di

Figure 1: Map showing the boundary between the North American and (2010) compiled mapping that included more northern outcrops. With new observations from these studies, we Pacific plates along the Ballenas transform fault (Stock et al 2008)( ) p pp g p ,

l l t d t i i l f th t ffPacific plates along the Ballenas transform fault (Stock et al., 2008) .

can calculate a new and more accurate minimum volume of the tuff. Outcrops of the Tuff of San Felipe (black and red dots) are found inOutcrops of the Tuff of San Felipe (black and red dots) are found in ll l f h d ° °Sonora as well as in Baja California. The red star near 30°N, 245°E j ,

h h fi 2 i l t d Th t ff i i t t i t tishows where figure 2 is located. The tuff is important in reconstructing g p gthe Gulf Extensional Province on the Pacific platethe Gulf Extensional Province on the Pacific plate.

Correlation of the Tuff of San FelipeCorrelation of the Tuff of San FelipeCorrelation of the Tuff of San FelipepThe Tuff of San Felipe/Ignimbrite of Hermosillo is a 12 5 Ma density currentThe Tuff of San Felipe/Ignimbrite of Hermosillo is a 12.5 Ma density current deposit presumably deposited as a continuous sheet Differences in thickness ofdeposit presumably deposited as a continuous sheet. Differences in thickness of the deposit result from travel away from the source and through river valleys Thethe deposit result from travel away from the source and through river valleys. The

t l i h d l ld d lk li t ff h 5 15% lk li f ld dcrystal-rich, densely welded peralkaline tuff has 5-15% alkali feldspar and a c ysta c , de se y e ded pe a a e tu as 5 5% a a e dspa a d acharacteristic black basal itroph re It also has a lo inclination and re ersecharacteristic black basal vitrophyre. It also has a low-inclination and reverse p ymagnetization (Stock et al 1999)magnetization (Stock et al., 1999).

MethodsMethodsMethods To calculate the volume of the Tuff of San Felipe we used previously published data toTo calculate the volume of the Tuff of San Felipe, we used previously published data to identify the latitude, longitude and thickness of the tuff. Often, the tuff was a top layer,identify the latitude, longitude and thickness of the tuff. Often, the tuff was a top layer,

ki hi k i i i i i U imaking our thickness measurement at various points a minimum estimate. Using g p gClaudia Lewis’ (1994) map and cross section and the Stock (1993) map we measuredClaudia Lewis’ (1994) map and cross-section, and the Stock (1993) map, we measured the thickness or minimum thickness of the tuff from the cross sections and determinedthe thickness or minimum thickness of the tuff from the cross-sections and determined the locations from the map Angel Olguín provided stratigraphic columns in his thesisthe locations from the map. Angel Olguín provided stratigraphic columns in his thesis (2010), giving us a more direct way to measure the thickness of the tuff. In other(2010), giving us a more direct way to measure the thickness of the tuff. In other i t h d l ll t d t ifi l ti d d G l E th tinstances, we had samples collected at specific locations and used GoogleEarth to , p p gmeasure the thicknesses GoogleEarth provided the elevation at the top and bottom ofmeasure the thicknesses. GoogleEarth provided the elevation at the top and bottom of the tuff which gave us a minimum thickness With larger dips this method was not asthe tuff, which gave us a minimum thickness. With larger dips, this method was not as effective

Figure 2: Using GoogleEarth, we find the elevation of the top and bottom of the Tuffeffective.

Figure 2: Using GoogleEarth, we find the elevation of the top and bottom of the Tuff f S F li hl i i hi k i l i d d l i d Thof San Felipe to roughly estimate its thickness at a given latitude and longitude. The

I E l d h t il d th l tit d l it d ti thip g y g g

location of this spot can be found by the red star in figure 1In an Excel spreadsheet, we compiled the latitude, longitude, easting, northing,

location of this spot can be found by the red star in figure 1. p p g g g

thickness and data from which we got our measurements In ArcGIS we imported thethickness and data from which we got our measurements. In ArcGIS, we imported the

C l i Excel spreadsheet as XY data which we saved as a point shapefile which was thenConclusions Excel spreadsheet as XY data, which we saved as a point shapefile, which was then Conclusions turned into a raster shapefile. We clipped the raster with a zero contour line, which weTh A GIS f l t l l l t d t ff l f b t 126 k 3 W

turned into a raster shapefile. We clipped the raster with a zero contour line, which we d li i f hi k di S h h hi k dThe ArcGIS surface volume tool calculated a tuff volume of about 126 km3. We made as a limit of zero thickness surrounding our outcrops. So that the thicknesses and

have not corrected for the extension of the Gulf Extension Province and areg p

XY d t ld b i th it t d l tit d d l it d t UTMhave not corrected for the extension of the Gulf Extension Province and are XY data would be in the same units, we converted our latitudes and longitudes to UTM. probably integrating over an area too large However in many cases the tuff is To calculate the final minimum estimate of the volume of the Tuff of San Felipe in Bajaprobably integrating over an area too large. However, in many cases, the tuff is To calculate the final minimum estimate of the volume of the Tuff of San Felipe in Baja the top unit and much of it has been eroded making many of our thicknesses California we used the tool surface volume in ArcGISthe top unit and much of it has been eroded, making many of our thicknesses t ll Th t ff t ill t t h th

California, we used the tool, surface volume in ArcGIS. too small. These two effects will counteract each other.too small. These two effects will counteract each other.

Our calculation of the volume is small compared to the actual volume of theOur calculation of the volume is small compared to the actual volume of the tuff All indications are that the volume may be larger in Sonora than in Bajatuff. All indications are that the volume may be larger in Sonora than in Baja California If less than half of the tuff is found in Baja California with more inCalifornia. If less than half of the tuff is found in Baja California with more in S d I l Tib th t d l i li ti ll th 200Sonora and on Isla Tiburon, the erupted volume is realistically more than 200 , p ykm3 Curiously no volcanic ash has been found and all of our outcrops havekm3. Curiously, no volcanic ash has been found, and all of our outcrops have y pbeen densely welded tuffs The volume of densely welded tuff is comparable tobeen densely welded tuffs. The volume of densely welded tuff is comparable to the volume of magma used in the Volcanic Explosity Index (Newhall 1982)the volume of magma used in the Volcanic Explosity Index (Newhall, 1982). Based on the VEI index, the eruption could have had a VEI >7, which isBased on the VEI index, the eruption could have had a VEI 7, which is

i l t t l f j t f 100 k 3equivalent to a volume of ejecta of 100 km3.q j

Additionally our zero contour line which we used to clip the raster and then toAdditionally, our zero contour line, which we used to clip the raster and then to calculate the volume of the tuff is probably inaccurate To improve the volumecalculate the volume of the tuff, is probably inaccurate. To improve the volume

i f h fi ld k h ld b d d fi d fi i d hestimate further, more field work should be done to define a definite end to theestimate further, more field work should be done to define a definite end to the T ff f S F li Si S F i d th S t R B i d t bTuff of San Felipe. Sierra San Fermin and the Santa Rosa Basin need to be pinvestigated further Olguin Villa (2010) proposed that the Tuff of San Felipeinvestigated further. Olguin-Villa (2010) proposed that the Tuff of San Felipe may underlie Pliocene tuffs in the Puertecitos region but this hypotheticalmay underlie Pliocene tuffs in the Puertecitos region, but this hypothetical additional volume has been excluded from this calculation Tuff of San Felipeadditional volume has been excluded from this calculation. Tuff of San Felipe h b f d I l A l d l G d b t i l di th ll i ihas been found on Isla Angel de la Guarda, but including the small remaining as bee ou d o s a ge de a Gua da, but c ud g t e s a e a gpreser ed o tcrop there onl increases o r ol me estimate b ro ghl 0 03preserved outcrop there only increases our volume estimate by roughly 0.03 p p y y g ykm3 (Skinner et al 2012)km3 (Skinner et al., 2012).

AcknowledgmentsAcknowledgments gThi h t d b th U S N ti l S i F d ti d G t NThis research was supported by the U.S. National Science Foundation under Grant No. EAR-0911761 Lisa Christiansen assisted with use of ArcGISEAR-0911761. Lisa Christiansen assisted with use of ArcGIS.

ReferencesReferencesReferencesB S E K 2009 T i l if i i h l G lf f C lif i B hí Ki S Mé i•Bennett, S.E.K., 2009. Transtensional rifting in the late proto-Gulf of California near Bahía Kino, Sonora, México.

MS Thesis, University of North Carolina, Chapel Hill, 132pp.MS Thesis, University of North Carolina, Chapel Hill, 132pp.•Bryant B A 1986 Geology of the Sierra Santa Rosa basin Baja California Mexico MS Thesis San Diego State•Bryant, B.A., 1986. Geology of the Sierra Santa Rosa basin, Baja California, Mexico. MS Thesis, San Diego State U i i S Di CA USAUniversity, San Diego, CA, USA. •Lewis, C., 1994. Miocene extensional tectonics and volcanism in the Sierra San Fermín, Baja California, México.Lewis, C., 1994. Miocene extensional tectonics and volcanism in the Sierra San Fermín, Baja California, México. Ph D Harvard University Cambridge MA USAPh.D., Harvard University, Cambridge, MA, USA.

h ll lf h l l d ( ) f l d f h l•Newhall, C.G., Self, S, 1982. The Volcanic Explosivity Index (VEI): An estimate of explosive magnitude for historical volcanism. Journal of Geophysical Research 87 (C2), p. 1231-1238.volcanism. Journal of Geophysical Research 87 (C2), p. 1231 1238. •Olguín Villa A E 2010 Estudio físico y químico del volcanismo hiperalcalino en la región de Cataviña Baja•Olguín-Villa A.E., 2010. Estudio físico y químico del volcanismo hiperalcalino en la región de Cataviña, Baja

l f d d d d áCalifornia. Tesis de Licenciatura, Universidad de Sonora, 84 páginas.•Oskin, M. E., 2002. Tectonic evolution of the northern Gulf of California, México, deduced from conjugate riftedOskin, M. E., 2002. Tectonic evolution of the northern Gulf of California, México, deduced from conjugate rifted margins of the upper Delfin Basin Ph D thesis California Institute of Technology Pasadena 487 ppmargins of the upper Delfin Basin. Ph.D thesis, California Institute of Technology, Pasadena, 487 pp.•Oskin, M., Stock, J., 2003. Marine incursion synchronous with plate-boundary localization in the Gulf of California. Geology 13, p. 23-26.California. Geology 13, p. 23 26. •Seiler C Fletcher J M Quigley M C Gleadow A J M Kohn B P 2010 Neogene structural evolution of the•Seiler, C., Fletcher, J.M., Quigley, M.C., Gleadow, A.J.M., Kohn, B.P., 2010. Neogene structural evolution of the Sierra San Felipe, Baja California: Evidence for proto-gulf transtension in the Gulf Extensional Province? Tectonophysics 488: p 87-109Tectonophysics 488: p. 87 109.•Skinner S Stock J Martin Barajas A 2012 Characterization of the Tuff of San Felipe on Isla Angel de la•Skinner, S., Stock, J., Martin-Barajas, A., 2012. Characterization of the Tuff of San Felipe on Isla Angel de la

hGuarda, Baja California, Mexico. 108th GSA Cordilleran Section Meeting. Paper 10-2. j g p•Stock J M 1993 Geologic map of Southern Valle Chico Baja California Mexico GSA map and chart series MCH-Stock, J.M., 1993. Geologic map of Southern Valle Chico, Baja California, Mexico. GSA map and chart series MCH076 i 3 f hi i d i f hi h h il d076. Figure 3: Map from ArcGIS: White points are data points for which we have compiled •Stock, J. M., C. J. Lewis and E. A. Nagy, 1999, The Tuff of San Felipe: An extensive middle Miocene pyroclastic flow

g p p p pl tit d l it d d thi k C t f thi k thi t t d

gy p pydeposit in Baja California Mexico Journal of Volcanology and Geothermal Research 93 p 53–74 latitude, longitude and thickness. Contours for thickness are every thirty meters and are deposit in Baja California, Mexico, Journal of Volcanology and Geothermal Research 93. p. 53 74.•Stock J M Martin A Chapman A Lopez Martinez M Dec 2008 Net slip across the Ballenas transform fault

g y ysurrounded by our zero contour line The thickest areas in red are around 30°45’N•Stock, J.M., Martin, A., Chapman, A., Lopez-Martinez, M., Dec 2008. Net slip across the Ballenas transform fault surrounded by our zero contour line. The thickest areas, in red, are around 30 45 N,

measured from offset. EOS Transactions AGU 89 (53). Abstract T11A-1853. 114°15’W presumably closest to the vent Here we measured the tuff to be 180 m thick( ) 114 15 W, presumably closest to the vent. Here, we measured the tuff to be 180 m thick.