Quaternary rift flank uplift of the Peninsular Ranges in Baja

and southern California by removal of mantle lithosphere

Karl Mueller,1 Grant Kier,1 Thomas Rockwell,2 and Craig H. Jones1,3

Received 2 November 2007; revised 13 January 2009; accepted 12 May 2009; published 9 September 2009.

[1] Regional uplift in southern California, USA, andnorthern Baja California, Mexico, is interpreted toresult from flexure of the elastic lithosphere drivenlargely by heating and thinning of the upper mantlebeneath the Gulf of California and eastern PeninsularRanges. The geometry and timing of faulting in theSalton Trough and Gulf of California, the history ofrecent rock uplift along the Pacific coastline, andgeophysical data constrain models of lithosphericheating and thinning based on unloading of acontinuous elastic plate. High topography that marksthe �400-km-long rift shoulder in northern BajaCalifornia mimics the pattern of uplift observedalong the Pacific coastline as defined by marineterraces. We interpret this to indicate that recent rockuplift has occurred across the entire width of northernBaja Peninsula and increases from west to east.Pliocene strata deposited at sea level along thePacific coastline in southern California have notbeen uplifted significantly above Quaternary marineterrace deposits. This suggests the onset of rock upliftalong the Pacific coast here is post-Pliocene andoccurs after Miocene crustal extension in the SaltonTrough and Gulf of California. Strong heating of themantle lid beneath the Peninsular Ranges in northernBaja California thus coincides with crustal extensionlimited to localized oceanic spreading in the Gulfof California. Citation: Mueller, K., G. Kier, T. Rockwell,

and C. H. Jones (2009), Quaternary rift flank uplift of the

Peninsular Ranges in Baja and southern California by removal

of mantle lithosphere, Tectonics, 28, TC5003, doi:10.1029/

2007TC002227.

1. Introduction

[2] Quaternary uplift of coastal southern California andnorthern Baja California has long been recognized by thepresence of well-preserved flights of marine terraces alongthe Pacific coast [Arnold, 1903; Kennedy et al., 1982; Kern

and Rockwell, 1992; Muhs et al., 2002]. The cause of uplift,however, has not been studied in detail, nor has it appearedparticularly significant until the recognition of active blindthrust faults in offshore regions of the southern Californiaborderland by Rivero et al. [2000]. They attribute theobserved coastal uplift in southern California to slip on ablind thrust system that includes one segment (the Ocean-side detachment) extending downdip beneath the coastline,implying significant seismic hazard for this region. Incontrast, Johnson et al. [1976], Muhs et al. [1992] andOrme [1998] have argued that regional uplift in coastalsouthern California and northern Baja California is due toaseismic tectonic or epirogenic processes Table 1.[3] In this paper, we explore whether, and to what extent,

Miocene to Recent thinning and/or heating of the litho-sphere beneath the Gulf of California and Salton Trough islikely to have produced rock uplift similar to that observedin the adjacent Peninsular Ranges. We first test whetherrifting of a thin elastic plate as constrained by presentgeological and geophysical conditions in the Gulf ofCalifornia can accurately predict observed topography.Second, we determine whether modeled rift shoulder upliftproduces a signal of total rock uplift along the Pacificcoastline that is consistent with observed uplift of Quater-nary marine terraces. We then compare the timing of coastaluplift with the history and style of extension in the SaltonTrough and Gulf and California and its implications forprocesses that drive Quaternary rock uplift in the PeninsularRanges.

2. Geologic Setting and the Gulf of California

[4] The Peninsular Ranges of southern California andnorthern Baja California form the high-relief, westernboundary escarpment of the northern Gulf of California andSalton Trough. This region, termed the Gulf ExtensionalProvince by Gastil et al. [1975] is characterized by lowtopography [Larsen and Reilinger, 1991] (Figure 1a) thatcan be related to a history of extensional and transformfaulting that began in the Miocene [Stock and Lee, 1994;Axen and Fletcher, 1998; Axen et al., 2000; Oskin et al.,2001; Oskin and Stock, 2003].[5] The Gulf of California rift system first developed as

the Pacific-Farallon spreading ridge neared North Americaat the end of the Middle Miocene [Atwater, 1970; Lonsdale,1989; Stock and Hodges, 1989]. Following the cessation ofspreading between ca. 12.5 and 14 Ma along the Pacificcoast of Baja California, subsequent divergence of thePacific Plate from North America was accommodated byextension of continental crust farther east occurring as slip

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1Department of Geological Sciences, University of Colorado at Boulder,Boulder, Colorado, USA.

2Department of Geological Sciences, San Diego State University, SanDiego, California, USA.

3CIRES, University of Colorado at Boulder, Boulder, Colorado, USA.

Copyright 2009 by the American Geophysical Union.0278-7407/09/2007TC002227$12.00

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on segmented low-angle normal faults [e.g., Fletcher et al.,2000a].[6] Early extensional strain in the Salton Trough at the

northern head of the Gulf of California was accommodatedby an east rooted system of detachment faults that have beensubsequently dismembered by NW trending dextral faultsalong the western margin of the Salton Trough (Figure 1b[Axen, 1995; Axen and Fletcher, 1998]). The age ofsyntectonic alluvial strata preserved in the hanging wallsof these low-angle normal faults suggest they were activefrom late Miocene through Pliocene time (beginning ca. 5–8.3 Ma [Axen and Fletcher, 1998; Axen et al., 2000; Dorseyet al., 2007]). Early extension in the Salton Trough is alsodefined in northernmost Baja California by a separate arrayof low-angle normal faults rooted to the west beneath theSierra Juarez [Axen, 1995; Axen et al., 2000]. The coolinghistory of rocks exhumed in the footwalls of these detach-ments suggest they were active between �4–15 Ma duringearly rift-related extension [Axen et al., 2000]. Evidence formodern extension along this fault system has also beensuggested on the basis of offset Quaternary sediments

deposited along the western margin of the Sierra Mayor[Axen et al., 1999].[7] Quaternary strain in eastern Baja California, the

Salton Trough and Gulf of California is dominated bydextral shear across the Pacific–North America plateboundary. This is manifest as dextral transform faults inthe Gulf of California, and a broad zone of right lateralfaulting that extends across the Peninsular Ranges andSalton Trough in southern California and northern BajaCalifornia. Faults that accommodate dextral Quaternaryplate motions in the Salton Trough include the active NWtrending San Jacinto, Elsinore–Laguna Salada and SanAndreas/Imperial/Cerro Prieto fault systems; the latter areinterpreted to be linked across the Brawley and Mexicali/Cerro Prieto seismic zones [Fuis et al., 1984; Lonsdale,1989] (Figure 1b). These dextral fault systems also boundactively extending basins such as the Imperial andMexicali Valleys that are infilled with thick successions ofPleistocene and younger strata and marked by earthquakeseismicity, Quaternary volcanism and locally elevated heatflow [Larsen and Reilinger, 1991; Mueller and Rockwell,

Table 1. Locations and Elevations of Marine Terraces Measured Along the Pacific Coastlinea

Terrace LocalityLocationNumber

Latitude(deg)

Longitude(deg)

5aElevation

(m)

5eElevation

(m) Uplift Rate (mm/a) References

San Joaquin Hills 1 33.61 117.92 19 32 0.21–0.24 Grant et al. [1999]Oceanside 2 33.35 117.52 9 22 0.13 Kern and Rockwell [1992]North San Diego County 3 33.17 117.35 9 22 0.13 Kern and Rockwell [1992]San Diego (outside fault zone) 4 32.91 117.24 9 22 0.13 Kern and Rockwell [1992]Mt. Soledad 5 32.85 117.27 14 0.18 Kern and Rockwell [1992]Point Loma 6 32.69 117.25 9–10 22 0.13 Kern and Rockwell [1992],

Ku and Kern [1974],and Muhs et al. [1992]

Tijuana Playa 7 32.5 117.15 20–23 �0.13 Valentine and Rowland [1969]Rosarito Beach 8 32.3 117.08 23 0.14 Valentine [1957] and Valentine and

Rowland [1969]Punta Descanso 9 32.24 117.03 23 0.14 Valentine [1957]Alisitos/La Fonda 10 32.1 116.91 27 0.17 Kennedy et al. [1986]

(SLA in fault zone)Ensenada 11 31.85 116.65 �20 �0.12 Kennedy and Rockwell

(unpublished data, 1995)Punta Banda 12 31.73 116.77 27–43 0.16–0.29 Rockwell et al. [1989]South Maximinos 13 31.65 116.7 29–30 0.18 Rockwell et al. [1989]

and Kennedy et al. [1986]Punta Santo Tomas 14 31.54 116.72 8–9 0.13 Emerson [1956]Punta Cabras 15 31.3 116.48 6 17 0.08–0.10 Addicott and Emerson [1959]

and G. L. Kennedy(personal communication, 2004)

Punta Baja 16 29.94 115.81 8–10 >10 0.13 Emerson and Addicott [1958],Ortlieb et al. [1984],

and Kennedy (unpublished data, 1995)Isla de Guadalupe 17 28.95 118.14 6 0 Lindberg et al. [1980]Punta Rosalilita 18 28.66 114.27 n.o. 6 0 Emerson and Hertlein [1960]

and Kennedy and Rockwell(unpublished data, 1995)

Turtle Bay, Viscaino Peninsula 19 27.67 114.88 12 24–27 0.15–0.16 Emerson [1980],Emerson et al. [1981],and Ortlieb et al. [1984]

Mulege (On Gulf of California) 20 26.9 112 n.o. 12 0.04–0.05 Ashby et al. [1987]Cabo San Lucas 21 22.85 109.9 n.o. 6 0 Muhs et al. [1992]

aLocations of measurements along coastline shown in Figure 3 by location number. Stage 5a is 83 ka, and stage 5e is 122 ka. Here n.o., not observed.

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1995; Axen, 1995; Axen and Fletcher, 1998; Axen et al.,1999, 2000; Dorsey and Martın-Barajas, 1999].[8] Dextral shear has been accommodated south of the

Salton Trough in the northern Gulf of California by closelyspaced transform faults that are linked and separated byhighly extended sedimentary basins such as the Guaymas,

Tiburon, Upper and Lower Delfın and other basins(Figure 1b [Lonsdale, 1989; Stock, 2000; Aragon-Arreolaand Martın-Barajas, 2007]). Rapid extension in this part ofthe Gulf began after ca 6.2 Ma [Oskin et al., 2001].Transform faults near the mouth of the Gulf of Californiaare linked by short, actively spreading ridges composed

Figure 1. (a) Digital topography from 90 m SRTM data. Dotted boxes represent locations of swath-averaged topography for north and south models (see Figure 5). (b) Simplified geologic map with 500 mcontours modified from Axen [1995]. Normal faults of San Pedro Martir Fault (SPMF) and Sierra JuarezFault Zone (SJFZ) are marked by dots on downthrown sides. Active dextral faults are marked by arrows.Hanging walls of low-angle normal faults are marked by double tick marks. Topography in the PeninsularRanges slopes gently toward the Pacific Ocean from rift shoulder segments and includes the LagunaMountains (Laguna), the Sierra San Pedro Martir (Martir), and Sierra Juarez (Juarez).

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mostly of igneous rocks of mafic composition [Lonsdale,1989]. These spreading centers are marked by relativelylittle sedimentary infill and deep bathymetry, in contrast tosimilar regions of highly extended crust formed furthernorth in the Gulf of California and Salton Trough that arecovered with thick sequences of Pliocene (?)-Quaternarysediment. Quaternary strata in the Salton Trough are meta-morphosed at greenschist facies conditions at shallow, uppercrustal levels [Fuis et al., 1984; Muffler and White, 1969],as a result of the high heat flow in this region [Lachenbruchet al., 1985].[9] Quaternary extension in the Salton Trough is likely

associated with mafic magmatism at deeper crustal levels[Fuis et al., 1984], which can be argued as beginning at ca.4 Ma [Axen et al., 2006]. Thin crust (21–22 km) in theSalton Trough [Parsons and McCarthy, 1996; Zhu andKanamori, 2000], and a temperature gradient of 33.3�C km�1

[Lachenbruch et al., 1985] all suggest that crustal extension,magmatism and heating of the upper mantle are activeprocesses beneath the region [Schubert and Garfunkel,1984].[10] Spreading in the northern Gulf of California appar-

ently underwent a westward shift along much of its lengthin Pliocene time, as first identified by Nagy and Stock [2000]in the Tiburon-Delphin and Guaymas Basins at ca. 3 Ma, andsubsequently by Aragon-Arreola and Martın-Barajas[2007] in this same region. A similar westward shift inactive spreading has also been recognized near the mouth ofthe Gulf near the Alarcon Rise [Lizarralde et al., 2007].[11] Quaternary extension in the Gulf of California is also

accommodated by normal faults that bound the eastern edgeof the Peninsular Ranges and dip east, forming an escarp-

ment that extends the length of the Baja Peninsula. Thesestructures include the east rooted Sierra San Pedro Martirfault system [Gastil et al., 1975; Dokka and Merriam, 1982;Stock, 2000], which bounds the highest rift shoulder seg-ment along the Baja California Peninsula (Figures 1a, 1b,and 2). The San Pedro Martir fault has a steep, continuousfootwall scarp with large (�5 km) displacement and scarpsthat offset late Quaternary alluvium (Figure 2 [Dokka andMerriam, 1982; Brown, 1978]). The San Pedro Martir faultsegment of the Main Gulf Escarpment terminates abruptly tothe south at the Puertecitos Volcanic Province that is associ-ated with theMotamı accommodation zone (Figures 1b and 2[Stock, 2000]).[12] The Peninsular Ranges rise steeply west of the Gulf

Escarpment between 30�–34�N latitude (Figures 1a and 2)to a maximum height of 3095 m with elevations graduallydecreasing westward along a concave upward slope [Lee etal., 1996]. Maximum elevations along the crest of the rangevary between 1500 and 3000 m with the highest topographylocated in the Sierra San Pedro Martir (Figure 3a). ThePeninsular Ranges are composed primarily of Mesozoicigneous rocks that were exhumed and cooled through zirconfission track closure temperatures during subduction in lateCretaceous to Eocene time [Cerveny et al., 1991; Ortega-Rivera, 2003; Lovera et al., 2006].[13] Igneous rocks of the Peninsular Ranges were subse-

quently covered by Eocene conglomerate derived from asource in northern Sonora and deposited in a channelsystem that extended west across southern California tothe California borderlands [Minch et al., 1976; Kies andAbbott, 1983; Abbott and Smith, 1989]. Now largelystripped of the overlying conglomerate across the top of

Figure 2. Oblique view of Baja Peninsula from 90 m SRTM data. View to south. Note Gulf Escarpmentis defined by 1–3 km high fault scarps shown in shadow. Illumination is from southwest.

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the Peninsular Ranges, the nonconformity, or basementcover contact can be identified as a low-relief, erodedsurface that dips gently toward the Pacific coast and ismapped as high as 1830 m, approximately 60 km east of thePacific coast at N32�500 [Gastil, 1961; Lovera et al., 2006].[14] The Peninsular Ranges must have been relatively

low in elevation prior to extension and rift flank uplift, onthe basis of the distribution of Eocene conglomerate trans-ported from Sonora westward across southern California[Kies and Abbott, 1983] and northern Baja California[O’Connor and Chase, 1989]. The provenance of clasts inEocene conglomerates also suggests little input from localgranitic sources in the Peninsular Ranges when they weredeposited [Abbott and Smith, 1989], consistent with a lackof significant relief. In addition, the modern river channelnetwork developed on the Laguna Mountain, Sierra Juarezand Sierra San Pedro Martir rift shoulder segments is notdeeply incised except near the Pacific coastline. We inter-

pret this as additional evidence for low relief prior to recentrock uplift otherwise paleochannels cut into batholithicrocks would be more deeply incised at higher elevationsin rift shoulder segments. On the basis of this evidence, weassume that prior to uplift, the Peninsular Ranges lay at arelatively low elevation (sloping very gently toward thePacific coast), and therefore initial topography in ourmodels is approximated as sea level. The onset of surfaceuplift in the eastern Peninsular Ranges in northern BajaCalifornia is not well constrained, but desert soils, oraridisols of mid Pliocene age exposed in the Salton Troughis interpreted to form in a rain shadow consistent with anexisting rift flank uplift at this time [Peryam et al., 2008].[15] Modeling presented later in this paper attempts to fit

the total uplift of the initially gently west dipping Eocenesurface, thus we are only concerned with the net change inbuoyancy since the Eocene. This uplift is presumed to be a

Figure 3. (a) Plot of maximum topography of Baja California Peninsula (e.g., drainage divide; scale onleft in kilometers) and uplift rate of marine stage 5a and 5e marine terraces (scale on right in millimetersper year) as measured between the Los Angeles Basin and the Viscaino Peninsula. (b) Shaded relief mapshowing location of terrace uplift surveyed at points along coastline listed in Table 1. Arrows showinguplift include solid examples that define regional uplift related to lithospheric flexure; dashed arrowsdefine local uplift next to active fault zones. No uplift or subsidence occurs along the Pacific coast of Bajabetween Viscaino and the tip of the peninsula.

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product of Miocene and younger normal faulting, erosion,and lithospheric thinning or heating.

3. Marine Terrace Uplift Along the Pacific

Coast

[16] The Pacific coast west of the Peninsular Ranges,from 30 to 33�N, is characterized by flights of marineterraces that imply continuous uplift during the lateQuaternary. Many of the most prominent terrace platformsare present in areas where faults locally influence andcontrol coastal structure and geomorphology, such as in thePalos Verdes Peninsula [Woodring et al., 1946; Woodring,1948; Muhs et al., 1992], San Joaquin Hills [Grant et al.,1999], the Mt Soledad region of San Diego [Kern andRockwell, 1992], and the Punta Banda region of BajaCalifornia [Rockwell et al., 1989] (Figures 3a and 3b).Shortening above blind thrusts, or in restraining bends ofstrike-slip faults, produces local uplift in these areas that issuperposed on regional uplift that occurs at a lower rate.[17] It is the regional and remarkably uniform uplift

signal along the Pacific coast, however, that is of primaryinterest in this study. Local structures could only producesuch a signal if a fault paralleled the coast and had near-constant displacements. A west facing escarpment mappedoffshore of the Pacific coast in northern Baja Californiacould reflect vertical motion just offshore, but existingearthquake focal mechanisms indicate strike-slip motion[Rebollar et al., 1982]. Although we cannot completelyreject the possibility that this fault contributes in some wayto coastal uplift, the great length and uniformity of theregional uplift of terraces seems incompatible with local faultcontrol. While rock and surface uplift in Baja California iscertainly modified by local faulting on the scale of tens ofkilometers or more, we seek to characterize and constrainthe larger-scale pattern of elevation change that extendsacross and along the northern peninsula.[18] The late Quaternary rate of regional, or background

uplift is well known for much of the Pacific coast insouthern California and northern Baja California and aver-ages from 0.13 to 0.14 mm/a from southern Californiasouthward to the Ensenada region (Figures 3a and 3b).South of the Agua Blanca fault, the uplift rate is similarlylow at about 0.13 mm/a to 30� north latitude (interpretedfrom Ellis and Lee [1919], Emerson [1956], Valentine[1957], Emerson and Addicott [1958], Addicott andEmerson [1959], Emerson and Hertlein [1960], Valentineand Rowland [1969], Orme [1974], Emerson [1980],Lindberg et al. [1980], Emerson et al. [1981], Ortlieb etal. [1984], Kennedy et al. [1986], Ashby et al. [1987],Rockwell et al. [1989], Kern and Rockwell [1992], Muhs etal. [1992, 2002], and G. L. Kennedy and T. K. Rockwell,unpublished survey data, 1995). Terraces along the Pacificcoastline south of 28.6� north latitude to the southern tip ofthe Baja California Peninsula have not been uplifted exceptwhere they are locally deformed along strike slip faults(Figures 3a and 3b [Muhs et al., 1992; Kennedy andRockwell, unpublished data, 1995]).

[19] Although the absence of terrace uplift along thePacific coast is largely the result of a narrower rift flankuplift as defined by regional topography in the southernpeninsula, the east rooted Santa Margarita and San Lazarofaults normal faults defined just offshore south of theVizcaino Peninsula [Fletcher and Eakins, 2001; Fletcheret al., 2000b, 2000c, 2007] may also affect the record ofcoastal uplift here. We therefore focus our attention on theuplift signal north of about 28.6�N.[20] The highest observed marine terrace in southern

California located away from active structures is the RifleRange terrace in San Diego County that now stands at 155 mabove sea level [Kern and Rockwell, 1992]. Higher terracesonly exist within the Mount Soledad, Palos Verdes and SanJoaquin uplifts in southern California where active faultingproduces locally higher topography. Pliocene to earlyPleistocene marine sediment of the San Diego Formationdeposited at sea level are preserved in the San Diego regionat nearly the same elevation of the highest Quaternaryterraces. This can be interpreted to suggest that totalQuaternary uplift does not exceed about 155 m, on thebasis of the eustatic record for Pliocene time [e.g., Dowsettand Cronin, 1990]. Given the present elevation of the SanDiego Formation and the Quaternary marine terraces in theSan Diego region, we thus argue that coastal southernCalifornia has undergone about 155 m of total surface upliftsince middle to late Pliocene time.[21] Southward into Baja California, the highest marine

terraces along the Pacific coast have been described asPliocene in age on the basis of the presence of extinct fauna[Emerson and Hertlein, 1960; Hertlein and Allison, 1959];the maximum elevation of Pliocene marine fauna occur atelevations greater than 100 m in northern Baja California,and decrease to about 20–30 m elevation at Santa Rosalalitanear the Vizcaino Peninsula. Age controls for these depositsare poor, and it is possible that some of the extinct faunadescribed in these strata are early Quaternary in age.Nevertheless, these observations are consistent with themuch better described terraces and their inferred agesdescribed in southern California. We use 155 m as themaximum post-Pliocene surface uplift where terraces arenot affected by local structures.[22] An extensive terrace including the Mesa Los Indios

and Mesa El Tigre surfaces located near the internationalborder in northernmost Baja California, is preserved atelevations of 350–380 m. Remnant benches above 400 mmay represent older and poorly preserved terrace remnants,but the �350 m surface northwest and inland from LaMision is the highest, well-formed and well-preservedmarine terrace in the vicinity. The surface is capped by acobble lag, as observed from an aerial overflight of the area,although no fieldwork has yet been undertaken to locatemarine fauna. This terrace may be very old, and possiblyTertiary in age.[23] Uplifted marine deposits preserved along the Gulf of

California also provide constraints on regional strain in thePeninsular Ranges. U-series dating of corals in southernBaja California along the Gulf of California indicate lowrates of uplift that range from 0.3 to 0.19 mm/a [DeDiego-

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Forbis et al., 2004]. Near Mulage, Ashby et al. [1987]document a rate of about 0.05 mm/a for the past �120 ka.These values are more than an order of magnitude lowerthan geodetically determined rates derived from the crest ofthe eastern Peninsular Ranges, in northern Baja California(e.g., 4.8 ± 1.5 mm/a [Outerbridge et al., 2005]). We do notconsider published uranium series age determinations usingmolluscs in Baja California [e.g., Ortlieb, 1991] owing toproblems associated with the open system exchange ofisotopes known to occur in samples of this type.

4. Rift Shoulders

[24] Rift shoulders are common topographic featuresalong extensional basins and continental rift marginsthroughout the world [Vening Meinesz, 1950]. Examplesinclude the Transantarctic Mountains in Antarctica [Bottand Stern, 1992; Stern and Ten Brink, 1989], and the flanksof the Rhine Graben [Weissel and Karner, 1989], the RioGraben in Greece [Poulimenos and Doutsos, 1997], and theEast African Rift [Vening Meinesz, 1950; Bott, 1992; Zeyenet al., 1997]. As the hanging wall subsides to form a riftbasin, the flanking footwall block is uplifted. Maximumsubsidence occurs along the rift axis, while uplift of riftshoulders occurs beyond the area offset by normal faults[Chery et al., 1992]. Rift shoulders range in height from1000 to 5000 m along continental rift systems and vary inshape and amplitude as a function of both the uplift forceand the rigidity of the lithosphere [Chery et al., 1992]. Riftshoulders are asymmetric, with a steep escarpment facing

the rift and gradually decreasing elevation along a concave-up slope away from the rift [Stern and Ten Brink, 1989].[25] As early as 1976, topography observed across the

Peninsular Ranges and Salton Trough was qualitativelycompared to other rift shoulders around the world (Figure3a [Johnson et al., 1976]) or related to a suppressed root[O’Connor and Chase, 1989]. The maximum elevationsalong the range crest adjacent to the western edge of theSalton Trough vary from �3000 m in the south to �1500 min the north. These regions of high-relief form distinct riftsegments that are coincident with both east and west rootedextensional fault systems of Neogene age [Axen, 1995] andyounger Quaternary pull-apart basins and spreading centers(Figures 1b and 3a; see separate rift shoulder segmentslabeled Laguna, Juarez and Martir).

5. Origin of Buoyancy Forces

[26] To explore the relationship between extension-drivenuplift and the uplift of the terraces along the California andBaja California coasts, we first consider simple flexuralmodels with single line loads and then develop solutionsthat incorporate 2-D loads in cross section. The simple lineload models help reveal the tradeoffs in determining thecontributions of loads produced by different sources neededto produce the observed flexure; they are a good initialapproximation to reality because the topography we seek tomatch is outside much of the area where the load is created.We presume that any loads created by variations in thecomposition of the batholith [e.g., Gastil, 1983] wereisostatically compensated before the initiation of riftingand before the development of the broad gentle surfacewe use as a reference, and therefore we are only concernedwith the change in loads during the Neogene.[27] For an elastic plate subjected to an upward force in

the vicinity of the rift, the upward force represents removalof rock by erosion and normal faulting at the top of thecrust, thermal expansion of the crust and thinning of themantle lithosphere at depth. This upward force is reducedby the deposition of sediments, the shallower depth of theMoho along the rift axis and any metamorphism of, orintrusion of mafic material into the crust. Contributions tothe total load from different sources are shown in Figure 4.The resulting deformation is determined by the magnitudeof the upward force, V, which controls the magnitude ofuplift, the flexural parameter a, which determines the widthof uplift or flexural wavelength, and the distance of theforce from the coast xl, which simply translates the defor-mation in the direction perpendicular to the rift. The upwarddeflection, w(x), is then defined

w xð Þ ¼ w0e�xl�x

a cosxl � x

a

� �þ sin

xl � x

a

� �� �

where the maximum deflection w0 is related to the forcethrough

w0 ¼Va3

8D

Figure 4. Diagram illustrating specific loads that con-tribute to a generalized line load that produces flexure of acontinuous plate comparable to topography in the Penin-sular Ranges in northern Baja California.

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where D, the flexural rigidity of the plate, and a, theflexural wavelength, are

a ¼ 4

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4D

rm � rf� �

g

vuut

D ¼ ET 3e

12 1� n2ð Þ

and Te is the elastic plate thickness [e.g., Turcotte andSchubert, 2002]. We assume a Young’s modulus E of1011 N m�2 and Poisson’s ratio n of 0.25. For thiscalculation we use mantle density rm of 3250 kg m�3 anda fill density of 0.[28] If the aim of the models is only to fit the western

flank of the rift (i.e., the topography of the Peninsular

Figure 5. (a) Simple model that illustrates swath topography in the Sierra Juarez and Salton Trough(i.e., northern transect; see location in Figure 1a) in comparison to general point loads of varyingmagnitude for a continuous plate. Values for parameters used in models given below each cross section;the position of the line load is marked by an upward-pointing arrow. See text for tradeoff betweenbuoyancy forces and their locations in the crust and upper mantle. (b) Simple model similar to Figure 5abut for a broken plate solution that illustrates swath topography across the Sierra San Pedro Martir (i.e.,southern profile) in comparison to point loads of different magnitude. Solutions generally underpredictelevation of higher parts of rift flank.

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Ranges), a tradeoff exists between these three parameters:xl, V, and a can be varied to generally fit the overall slope ofthe profile. The main difference between these parameterswithin the Peninsular Ranges is a slight change in thecurvature of predicted rift flank topography (Figure 5a). Inall cases the point where uplift begins to occur (movingfrom west to east) near the Pacific coastline is locatedoffshore by some 40 km; moving it farther east misfits thetopography near the coastline. In general, a similar shapeacross the Sierra Juarez is obtained by shifting the loadfarther east while increasing both the flexural rigidity andthe load. Because we do not know the preflexure topogra-phy in detail, we cannot fully resolve the tradeoff betweenforce, position, and flexural parameter without additionalinformation. We also do not consider the effect of loadingand flexure on the Sonoran side of the Gulf of California,owing to the more complex extensional history this region

has undergone and uncertainty regarding the prerift topog-raphy that may have been present there.[29] As an alternative end-member, a broken plate model

approximates the case where strength in the lithospherebecomes very low within the rift. For a given load position,the elastic plate thickness must be �80% greater and theload slightly smaller to produce a deflection comparable tothat obtained from a continuous plate. Unless the plate edgeis located well to the west of the main basin edge, theeffective elastic plate thickness Te � 40 km required to fitthe topography on the northern profile seems too high forlithosphere in southern California [e.g., Lowry et al., 2000].The combination of the higher Te and steeper topographyassociated with the broken plate model make it a poorerchoice for the northern profile than the continuous model ofFigure 5a.[30] The origin of the buoyancy driving flexure can be

determined by considering the forces in these simple mod-

Figure 5. (continued)

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els. For the profile across the Salton Trough, a line force ofabout 1013 N/m applied to a continuous plate within the riftacceptably reproduces the topography (Figures 4 and 5a).The upward buoyancy force from replacing granitic crustwith the sedimentary fill in the Salton Trough only amountsto between about 2.5 1011 N/m and 10 1011 N/m,roughly an order of magnitude too small to produceobserved rift flank uplift. The surface load caused byremoving rock in the rift through either erosion or extensionis proportional to the area between the flexural curve andthe present topography; this varies with the flexural rigidityassumed. In all cases, the surface load is about 40–80% ofthe necessary load, with the greater deficiencies for loadsplaced farther west (Figure 5a). In addition, the crust thinsand crustal density increases from west to east across thewestern edge of the Salton Trough [Fuis et al., 1984],producing a significant downward force in the crust alongthe rift perhaps twice the magnitude of the upward forcefrom the sedimentary basin. Thus much of the buoyancyforce needed to drive flexural uplift (i.e., that matchesobserved rift flank topography) must be derived from themantle under the rift (Figure 4). This is also true of thebroken plate models because the load required from exten-sion is (mis)located 65 km west of active normal faults inthe Salton Trough.[31] A similar analysis for the southern profile across the

Sierra San Pedro Martir yields comparable results thoughthe required load is roughly twice that of the northernprofile for any particular load position. With a continuousplate, loads between 1.2 and 3.5 1013 N/m applied to aplate with an elastic plate thickness from 17 to 30 km willmatch the topography of the Sierra San Pedro Martir, withthe thicker plate and larger load corresponding to a loadnear the center of the Gulf of California. A slightly better fitto the topography is obtained by using a broken plate modelwith somewhat smaller loads and �80% greater elastic platethicknesses than the continuous plate model (Figure 5b). Welack observations here comparable to those along thenorthern profile constraining the sedimentary thickness,but it seems unlikely that load generated by replacingcrystalline rock with sediments will be more than doublethat in the Salton Trough. The main surface load is thatproduced by removing crust through faulting and erosion;the load could be somewhat higher than to the north,perhaps as high as 2 1013 N/m for a load centered inthe Gulf of California. Although this is almost 80% of theload needed, the downward load from a shallower Mohoand probable crustal density increases into the Gulf ofCalifornia again indicate an important, if not dominant,contribution from buoyancy in the mantle. Furthermore,the comparison of the two profiles suggests a somewhatweaker plate and a noticeably greater load in the south thanthe north.

6. Flexural Modeling of Sierra Juarez and

Salton Trough

[32] Because the actual loads are not line loads but in factdistributed along the profile, we investigate the loads

producing rift flank uplift with more realistic 2-D models.To better determine the source, location, and magnitude ofthe load uplifting the Sierra Juarez, we modeled the uplift asthe result of extensional thinning of the crust from normalfaulting and variable thinning of the mantle lithosphere(Figures 6, 7, and 8). The basin, Moho and lithosphereloads are specified for each trial, and the deflection of theplate is calculated. The material removed by erosion orcrustal tectonism is then estimated to be equal to the massbetween the deflected position of the plate top and themodern topography. The load from this estimate is thenadded to the original loads and a new deflection is calcu-lated, which in turn leads to a new estimate of the load fromerosion or crustal tectonism. We continue to iterate until nonew forces are generated. We force our flexural uplift nearthe coastline to match the 155 m maximum elevation ofshoreline terraces described above and seek to matchdeflections of an early Tertiary, low-relief surface.[33] The load produced from variation in the thickness of

the crust are specified from seismological inferences ofcrustal thickness [Ichinose et al., 1996]. We use a 15 kmthinner crust under the sediment-filled Salton Trough withthinning tapering to zero 50 km from the edges of the basin.Our model of crustal thickness mimics that of Ichinose et al.[1996] who defined crustal thickness across the PeninsularRanges and Salton Trough of southern California near ournorthern transect (see crustal depth points defined in Figure8). Their work was based on using P-to-S converted phasesof teleseismic body waves on a broadband array and hasbeen found to be representative of the Peninsular Ranges tothe north and south [Lewis et al., 2000, 2001; Yan andClayton, 2007]. We have assigned a density contrast of400 kg m�3 to the Moho for our flexure models. Thispresumes a relatively flat Moho prior to the Neogene. Ifmore crustal thinning occurred, then a more buoyant mantleis required, and if less thinning, then a less buoyant mantle.[34] Thinning of the upper mantle is considered by

replacing lithosphere with less dense material similar tothe asthenosphere beneath the Salton Trough and easternPeninsula Ranges. The replaced region is modeled by a80- to 100-km-thick, rectangular or trapezoidal body100 kg/m3 less dense than the mantle lithosphere (Figures 6,7, and 8). Note that as the load is in proportion to the densitycontrast and thickness, a thicker (or thinner) variation willproduce the same load if the density contrast varies propor-tionally. Both for simplicity and because there is an insig-nificant difference between a rectangular body and atrapezoid with an edge less than about 60 km wide, mostmodels assume a rectangular body.[35] The fit of models to observed topography is judged

principally by the overall fit to the topography of the westslope of the Sierra Juarez. Because we force each model topass through the 155 m elevation of the swath topography,much of the difference between models is near the crest.Ideally, acceptable models would pass somewhat belowmodern topography, making the Eocene surface dip gentlytoward the ocean before flexure had occurred. We use thewest edge of the deepest basin in the Salton Trough (140 kmeast of the coast) as our zero point (x = 0) in this modeling.

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6.1. Elastic Plate Thickness

[36] We first consider plausible plate thicknesses (Te) insouthern California and northern Baja California. There is atradeoff between the loads imposed and Te: a thickness of0 requires a load exactly reflecting topography, while astiffer plate permits the load to be located farther east. Anupper bound for Te in this region is 30 km [e.g., Lowry etal., 2000]. Using a constant Te of 30 km and holding theeastern edge of the mantle load at x = 100 km, we find thatthe mantle load should not extend west of the west edge ofthe deep basin (Figure 6a). These solutions require largemantle loads equivalent to the removal of 170 km or moreof mantle lithosphere 100 kg m�3 denser than astheno-sphere (equivalent to line loads of about 2 1013 N/m), andthese require the tectonic removal of 3 or more km of crustin the eastern Chocolate Mountains. We find the magnitudeof the load too large to be plausible given lithosphericthicknesses in California [e.g., Yang and Forsyth, 2006].

[37] Smaller values of Te require loads more consistentwith lithospheric variations expected in the region. A Te of20 km requires a load extending to the west of the westernedge of thinned crust and into the eastern Sierra Juarez, withabout 95 km of lithosphere removed (Figure 6b). Unlike theTe = 30 km case, the eastern edge of the load can be locatedfar enough to the west to allow for minimal removal of crustin the Chocolate Mountains. A Te of 15 km represents thelowest possible flexural rigidity capable of reproducing thetopography above a sharp-edged mantle load (Figure 6c). Inthis case, the load magnitude trades off dramatically withthe amount of material that must be removed within the riftto match the topography, thus greatly limiting the range ofacceptable solutions. These results indicate that for any Teless than or equal to about 20 km, mantle loading mustextend west of the major basin bounding fault by at least50 km and west of the westernmost range-bounding faultsby more than 15 km.

Figure 6. (a–c) Series of flexural models across northern transect assuming varied amounts of mantlelithosphere replaced by asthenosphere and different values for Te. Location of transect and all subsequentmodels shown in Figure 1a. Basin fill geometry in Salton Trough is derived from Fuis et al. [1984].Uppermost 6 km of crust is highly (>30 times) vertically exaggerated. Solutions are based on a constantTe for a given model, with increasing Te from Figures 6a to 6c. All models maintain an elevation of 155 mat the Pacific coastline, as constrained by uplift of marine terraces. Figure 6a illustrates solutions for arelatively narrower and deeper region of mantle lid replaced by asthenosphere beneath the eastern SaltonTrough for a relatively thick Te of 30 km. Figure 6b shows intermediate solutions for replaced mantle lidcentralized beneath the Salton Trough and a Te of 20 km. Figure 6c defines region of replaced mantle lidthat is thinner and wider but with a relatively thinner Te. Parameters in models illustrate the tradeoffbetween the location and magnitude of various loads and resulting rock uplift as illustrated by rift flanktopography in the Sierra Juarez.

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[38] A more plausible scenario is that the effective elasticplate thickness thins into the rift. In general, elastic platethickness thins with increased heat flow. We focus on amodel with Te = 24 km away from the rift and Te = 10 km inthe rift (from x = 0 to x = 33 km) with a gradient in elasticplate thickness about 30 km wide on each side of the rift. Inthis case (Figure 7), we find that topography is quitesensitive to the position of the left edge of the mantle load.This is in part because of the feedback of forcing material tobe removed in the rift: as the mantle load grows, theerosional load grows as well. The right edge of the loadhas little effect on the topography in the Sierra Juarez, butthis does change the amount of material that must beremoved from the Chocolate Mountains. If the ChocolateMountains are an equivalent flexural shoulder to the rift,then the mantle load ends somewhere between 65 and 80 kmeast of the axis of the rift basin.[39] Just as with solutions with a constant Te below about

20 km, the variable Te case requires the mantle load toextend about 50 km to the west of the main basin boundingfault and about 20 km west of the eastern flank of the SierraJuarez. Significantly, this is also 20–25 km west of thedecrease in elastic plate thickness. These results indicatethat the mantle load must extend well to the west of athinner elastic plate, which is equivalent to the part of thecrust that has been heated and damaged by rifting. Themagnitude of the load is equivalent to removal of about80 km of mantle lithosphere. Thus we find that the buoy-ancy in the mantle extends a minimum of 20 km (andprobably closer to 50 km) west compared with the westernlimit of substantial crustal deformation.

6.2. Timing of Uplift and Loading

[40] We noted above that the Salton Trough has experi-enced multiple episodes of extensional deformation sincethe Miocene. However, the coastal uplift is post-Pliocene.One possibility is that the entire mantle load causing upliftof the rift shoulders postdates the Pliocene. This wouldrequire that earlier crustal thinning be rooted into the mantlewest of the Sierra Juarez and the Salton Trough. The leastamount of mantle loading can be found by noting that theterraces are near the nodal point for flexural uplift from aload in the interior. In Figure 8, we show that a 20–30 kmwestward shift in the west edge of the mantle load issufficient to generate the 155 m uplift at the coast. Thislateral growth of the load also produces about half of thetopography of the Sierra Juarez, suggesting that the eleva-tion of this range could also be mostly Pliocene andyounger. However, the magnitude of uplift of the crest ofthe Sierra Juarez depends on the dip of the west edge ofbuoyant mantle; if the dip allows buoyant material fartherwest than shown in Figure 8, then it is possible to raise thecoastal areas while only raising the crest a few hundredmeters, or, in the most extreme case, only 30 m.[41] Oxygen isotope analysis of paleosols and magneto-

stratigraphy in the Fish Creek–Vallecito basin in the west-ern Salton Trough suggest that a rain shadow existed thereprior to ca. 3.7 Ma [Peryam et al., 2008]. Aridisol soilchemistry also suggests general aridity in this region be-tween ca. 1.0 and 3.7 Ma [Peryam et al., 2008]. While thisdoes not tightly constrain the elevation of a supposed flankuplift, it is generally consistent with our arguments for

Figure 7. (a–c) Series of flexural models across the Sierra Juarez, similar to Figure 6 except that variedTe is used across the rift flanks (24 km) and Salton Trough (10 km). Figure 7a replaces a wider region ofmantle lid, while Figures 7b and 7c use progressively narrower and shallower areas of replacedlithosphere. Basin geometry, location of swath topography, and vertical exaggeration of upper crust aresame as shown in Figure 6.

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Figure 8. Flexural model across Sierra Juarez that shows an east to west progression of rift flank upliftin the Peninsular Ranges. Model is intended to illustrate an evolving rift that migrates toward the Pacificfrom the Salton Trough with 155 m of coastal uplift. Basin geometry similar to previous models, with avariable Te across the Salton Trough. White circles denote base of crust as defined by receiver functionsfrom Ichinose et al. [1996].

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the timing of rift flank uplift driven by increased mantlebuoyancy.

7. Discussion

[42] Thinning, or strong heating of the upper mantleunder the eastern Peninsular Ranges is consistent with theprevious results of Lewis et al. [2000] who require asignificant mantle component to support topography westof the Main Gulf Escarpment. Ichinose et al. [1996] alsoobserve the lack of correlation between topography andcrustal thickness (e.g., depths to the Moho) and suggestcompensation from lateral density variations in the lowercrust and upper mantle.[43] All models include removal of upper crustal material

by either erosion or normal faulting so that the moderntopography within the rift is honored. We have not attemp-ted to explicitly include erosion to the west of the rift, norhave we tried to estimate a priori the total amount ofmaterial removed from within the rift. Erosion west of therift is volumetrically minor and does not significantly affectour results. Estimating the total volume of material removedwithin the rift is problematic, largely because the originalMoho geometry is unknown and any surficial geologicconstraints are complicated by poor control on fault geom-etry at depth and possible lateral flow of lower crust.Despite these difficulties, the magnitude of buoyancy inthe mantle is only overestimated if the crust was originallythinner at the site of the Salton Trough prior to extension(relative to the region to the west).[44] Simple models of crustal extension coupled with

near complete replacement of mantle lithosphere predictrelief that broadly matches observed basin geometry and riftflank swath topography across the Peninsular Ranges andSalton Trough in southern California and Baja California(Figures 5, 6, 7, and 8). Thinning of the lithosphere in thisregion has produced a western rift flank uplift composed ofthe Laguna Mountains, Sierra Juarez and Sierra San PedroMartir that rise to an average elevation of �2 km above sealevel. Uplift of the western rift flank apparently extendswestward to the Pacific coastline, where late Quaternarymarine terraces are raised as much as 155 m above sea level.A narrow and lower rift flank uplift bounds the eastern sideof the Salton Trough that is also predicted by our modelsand marked by the Chocolate Mountains. While littleevidence exists to exactly constrain the timing of uplift ofthe higher parts of the rift flanks, the age of uplifted marineterraces and shallow marine deposits along the Pacific coastsuggest regional uplift may be very young and largelyPleistocene in age. This is supported by late Pliocene toPleistocene soils developed in the western Salton Troughthat imply the presence of a rain shadow and hencesignificant relief in the eastern Peninsular Ranges at thistime. It is important to note that rates defined by the marinestage 5e terrace only provide an average uplift rate for thelast �120 Ka, and that total uplift defined in southernCalifornia of �155 m must have been initiated well beforethis time (but after shallow marine Pliocene strata weredeposited, and at about 1.3 Ma on the basis of dating of

older terraces with amino acid racemization results anduplift rates).[45] Unpublished geodetic data [Outerbridge et al., 2005]

support our work by suggesting that rapid rates (4.8 ±1.5 mm/a) of modern uplift may occur in the higher partsof the Peninsular Ranges province. The remarkably consis-tent rate of slow background uplift of marine terraces alongthe Pacific coastline and negative gravity along the rangecrest is interpreted as evidence for large-scale flexure of thelithosphere driven largely by a broad mantle upwellingbeneath eastern Peninsular Ranges, Salton Trough andnorthern Gulf of California. At the scale of the width ofthe Baja Peninsula, this suggests that coastal uplift isrelatively unaffected by the segmentation of late Quater-nary extensional faults in the Salton Trough and Gulf ofCalifornia [Axen, 1995]. A lower rift flank uplift preservedsouth of the Sierra San Pedro Martir in southern Baja alsosuggests buoyant mantle adjacent to the Gulf of California,although to a lesser extent than farther north.[46] Our work thus suggests a fundamental increase in

buoyancy in the upper mantle in the northern Gulf ofCalifornia that is presumably related to the thermal con-ditions that have affected this region. Rift flank uplift of thePeninsular Ranges is thus partly coeval with crustal exten-sion driven by transform tectonics, as the timing of coastaluplift does not match well-documented evidence for priorMiocene extension along low-angle detachment faults inthis region. We envision the mantle load that drives riftflank uplift as migrating westward from the Salton Troughand Gulf of California by some combination of lithosphericthinning and thermal erosion (Figure 8). The onset of upliftat the Pacific coastline must, however, be Quaternary in age.Our models also hint that the current topography of theChocolate Mountains may be related to the relatively recentdevelopment of the Gulf of California, although there areessentially no constraints on the true timing of uplift in thisregion. We also have not considered the effects of substan-tial Miocene thinning along the east rooted Colorado Riverextensional corridor farther to the east and its effect on thecurrent topography of the Chocolate Mountains. An inter-esting aspect of the models presented in this paper is theenormous magnitude of mantle lid thinning required toreproduce rift flank topography. For example, the best fitsolutions suggest 80 km of lid loss over a 120–150 kmwide region, perpendicular to the Gulf of California. Whilea number of previous workers have argued for significantheating of the mantle, or loss of the mantle lid [Fuis et al.,1984; Lewis et al., 2001] beneath parts of the rift, none havesuggested that crust in the northern Gulf of California andSalton Trough overlies asthenosphere over the width of therift floor and adjacent eastern Peninsular Ranges as indicatedby our models. However, Park et al. [1992] suggested theimposition of a major heat source in the past 5 My under thePeninsular Ranges and Lewis et al. [2001] argued for asignificant mantle component to support topography east ofthe Main Gulf Escarpment (located about 20 km east of therange crest).[47] Whereas the western rift shoulder is marked by

discrete segments along the length of Baja California, it

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appears to be uplifting at significantly lower rates south ofthe Sierra San Pedro Martir, on the basis of the morphologyof deeply embayed range fronts, lower overall rift flankrelief and rates of uplift defined by Late Pleistocene marinedeposits near La Paz and Los Cabos [Muhs et al., 1992;DeDiego-Forbis et al., 2004]. Therefore, while the southernGulf of California has undergone crustal extension acrossspreading ridges comparable to similar transform and ridgesegments farther to the north, mantle buoyancy changes inthe Quaternary appear to be lower to the south, on the basisof rift shoulder relief. This is consistent with recent surfacewave studies in the region that show very low mantle wavespeeds in the northern Gulf of California but higher wavespeeds in the south [Zhang et al., 2007].[48] Our work suggests that the regional background

uplift of the Pacific coastline extends from southernCalifornia to nearly the Vizcaino Peninsula and is notrelated to local shortening above active blind thrusts [Riveroet al., 2000] but is instead related to a combination of crustand mantle thinning centered on the Gulf of California andeastern Peninsular Ranges. This broad uplift appears to bestcorrespond to a phase in development of the new spreadingsystem in the Gulf of California when early regionalextension becomes more localized but before the systemevolves into a better defined seafloor spreading boundary.

[49] Our hypothesis links uplift of the Pacific coast andSierra Juarez to either conductive or convective heating inthe mantle lithosphere under the Peninsular Ranges. Furthertests of this idea will either constrain the thermal evolutionof the mantle lithosphere under the Peninsular Ranges orbetter constrain the temporal evolution of topography to-ward the crest of the range. Our interpretation requiressubstantial post-Pliocene removal or modification of mantlelithosphere well beyond the edges of Neogene crustal strainrecorded in the western Salton Trough; the mechanism(s)accommodating such deformation remain undetermined butcould include low-angle normal faulting, lateral heat con-duction, or convective instabilities in the lithosphere.

[50] Acknowledgments. We thank Garry Karner for his comments onour early modeling work and Tim Melbourne and Mike Oskin for theirdiscussions of mantle structure and the Neogene tectonic history of theinner California borderland and the Gulf of California, respectively. KelinWhipple, Kip Hodges, Suzanne Jannecke, Rebecca Dorsey, Brian Wernicke,and, in particular, John Fletcher and Gary Axen provided many helpfulsuggestions, resulting in a much improved final version of this paper. Thisresearch was supported by the Southern California Earthquake Center(SCEC). SCEC is funded by NSF cooperative agreement EAR-8920136and USGS cooperative agreements 14-08-0001-A0899 and 1434-HQ-97AG01718. The SCEC contribution number for this paper is 580.Research was further supported by NEHRP grant 01 HQGR0031 toK. Mueller. The views and conclusions contained in this document arethose of the authors and should not be interpreted as necessarily represent-ing official policies of the U.S. government.

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���������C. H. Jones, G. Kier, and K. Mueller, Department of

Geological Sciences, University of Colorado atBoulder, Boulder, CO 80309-0399, USA. (karl.mueller@colorado.edu)

T. Rockwell, Department of Geological Sciences,San Diego State University, San Diego, CA 92182,USA.

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