Quaternary Rift Flank Uplift of the Peninsular Ranges in Baja and Southern California by Removal of Mantle Lithosphere

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TECTONICS, VOL. 28, TC5003, doi:10.1029/2007TC002227, 2009 Click Here for Full Article 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, and and Rockwell, 1992; Muhs et al., 2002]. The cause of uplift, northern Baja California, Mexico, is interpreted to however, has not been studied in detail, nor has it appeared result from flexure of the elastic lithosphere driven particularly significant until the recognition of active blind largely by heating and thinning of the upper mantle thrust faults in offshore regions of the southern California beneath the Gulf of California and eastern Peninsular borderland by Rivero et al. [2000]. They attribute the Ranges. The geometry and timing of faulting in the observed coastal uplift in southern California to slip on a blind thrust system that includes one segment (the Ocean- Salton Trough and Gulf of California, the history of side detachment) extending downdip beneath the coastline, recent rock uplift along the Pacific coastline, and implying significant seismic hazard for this region. In geophysical data constrain models of lithospheric contrast, Johnson et al. [1976], Muhs et al. [1992] and heating and thinning based on unloading of a Orme [1998] have argued that regional uplift in coastal continuous elastic plate. High topography that marks southern California and northern Baja California is due to the 400-km-long rift shoulder in northern Baja aseismic tectonic or epirogenic processes Table 1. California mimics the pattern of uplift observed [3] In this paper, we explore whether, and to what extent, along the Pacific coastline as defined by marine Miocene to Recent thinning and/or heating of the litho- terraces. We interpret this to indicate that recent rock sphere beneath the Gulf of California and Salton Trough is uplift has occurred across the entire width of northern likely to have produced rock uplift similar to that observed Baja Peninsula and increases from west to east. in the adjacent Peninsular Ranges. We first test whether rifting of a thin elastic plate as constrained by present Pliocene strata deposited at sea level along the geological and geophysical conditions in the Gulf of Pacific coastline in southern California have not California can accurately predict observed topography. been uplifted significantly above Quaternary marine Second, we determine whether modeled rift shoulder uplift terrace deposits. This suggests the onset of rock uplift produces a signal of total rock uplift along the Pacific along the Pacific coast here is post-Pliocene and coastline that is consistent with observed uplift of Quater- occurs after Miocene crustal extension in the Salton nary marine terraces. We then compare the timing of coastal Trough and Gulf of California. Strong heating of the uplift with the history and style of extension in the Salton mantle lid beneath the Peninsular Ranges in northern Trough and Gulf and California and its implications for Baja California thus coincides with crustal extension processes that drive Quaternary rock uplift in the Peninsular limited to localized oceanic spreading in the Gulf Ranges. of California. Citation: Mueller, K., G. Kier, T. Rockwell, and C. H. Jones (2009), Quaternary rift flank uplift of the 2. Geologic Setting and the Gulf of California Peninsular Ranges in Baja and southern California by removal of mantle lithosphere, Tectonics, 28, TC5003, doi:10.1029/ [4] The Peninsular Ranges of southern California and 2007TC002227. northern Baja California form the high-relief, western boundary escarpment of the northern Gulf of California and Salton Trough. This region, termed the Gulf Extensional 1. Introduction Province by Gastil et al. [1975] is characterized by low topography [Larsen and Reilinger, 1991] (Figure 1a) that [2] Quaternary uplift of coastal southern California and can be related to a history of extensional and transform northern Baja California has long been recognized by the faulting that began in the Miocene [Stock and Lee, 1994; presence of well-preserved flights of marine terraces along Axen and Fletcher, 1998; Axen et al., 2000; Oskin et al., the Pacific coast [Arnold, 1903; Kennedy et al., 1982; Kern 2001; Oskin and Stock, 2003]. [5] The Gulf of California rift system first developed as the Pacific-Farallon spreading ridge neared North America 1 Department of Geological Sciences, University of Colorado at Boulder, at the end of the Middle Miocene [Atwater, 1970; Lonsdale, Boulder, Colorado, USA. 2Department of Geological Sciences, San Diego State University, San 1989; Stock and Hodges, 1989]. Following the cessation of Diego, California, USA. spreading between ca. 12.5 and 14 Ma along the Pacific 3CIRES, University of Colorado at Boulder, Boulder, Colorado, USA. coast of Baja California, subsequent divergence of the Pacific Plate from North America was accommodated by Copyright 2009 by the American Geophysical Union. extension of continental crust farther east occurring as slip 0278-7407/09/2007TC002227$12.00

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

5a 5e Location Latitude Longitude Elevation Elevation Terrace Locality Number (deg) (deg) (m) (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. on segmented low-angle normal faults [e.g., Fletcher et al., deposited along the western margin of the Sierra Mayor 2000a]. [Axen et al., 1999]. [6] Early extensional strain in the Salton Trough at the [7] Quaternary strain in eastern Baja California, the northern head of the Gulf of California was accommodated Salton Trough and Gulf of California is dominated by by an east rooted system of detachment faults that have been dextral shear across the Pacific–North America plate subsequently dismembered by NW trending dextral faults boundary. This is manifest as dextral transform faults in along the western margin of the Salton Trough (Figure 1b the Gulf of California, and a broad zone of right lateral [Axen, 1995; Axen and Fletcher, 1998]). The age of faulting that extends across the Peninsular Ranges and syntectonic alluvial strata preserved in the hanging walls Salton Trough in southern California and northern Baja of these low-angle normal faults suggest they were active California. Faults that accommodate dextral Quaternary from late Miocene through Pliocene time (beginning ca. 5– plate motions in the Salton Trough include the active NW 8.3 Ma [Axen and Fletcher, 1998; Axen et al., 2000; Dorsey trending San Jacinto, Elsinore–Laguna Salada and San et al., 2007]). Early extension in the Salton Trough is also Andreas/Imperial/Cerro Prieto fault systems; the latter are defined in northernmost Baja California by a separate array interpreted to be linked across the Brawley and Mexicali/ of low-angle normal faults rooted to the west beneath the Cerro Prieto seismic zones [Fuis et al., 1984; Lonsdale, Sierra Juarez [Axen, 1995; Axen et al., 2000]. The cooling 1989] (Figure 1b). These dextral fault systems also bound history of rocks exhumed in the footwalls of these detach- actively extending basins such as the Imperial and ments suggest they were active between 4–15 Ma during Mexicali Valleys that are infilled with thick successions of early rift-related extension [Axen et al., 2000]. Evidence for Pleistocene and younger strata and marked by earthquake modern extension along this fault system has also been seismicity, Quaternary volcanism and locally elevated heat suggested on the basis of offset Quaternary sediments flow [Larsen and Reilinger, 1991; Mueller and Rockwell,

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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 m contours modified from Axen [1995]. Normal faults of San Pedro Martir Fault (SPMF) and Sierra Juarez Fault 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 Peninsular Ranges slopes gently toward the Pacific Ocean from rift shoulder segments and includes the Laguna Mountains (Laguna), the Sierra San Pedro Martir (Martir), and Sierra Juarez (Juarez).

1995; Axen, 1995; Axen and Fletcher, 1998; Axen et al., Tiburon, Upper and Lower Delfıń and other basins 1999, 2000; Dorsey and Martıń-Barajas, 1999]. (Figure 1b [Lonsdale, 1989; Stock, 2000; Aragon-Arreola [8] Dextral shear has been accommodated south of the and Martıń-Barajas, 2007]). Rapid extension in this part of Salton Trough in the northern Gulf of California by closely the Gulf began after ca 6.2 Ma [Oskin et al., 2001]. spaced transform faults that are linked and separated by Transform faults near the mouth of the Gulf of California highly extended sedimentary basins such as the Guaymas, are linked by short, actively spreading ridges composed

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Figure 2. Oblique view of Baja Peninsula from 90 m SRTM data. View to south. Note Gulf Escarpment is defined by 1–3 km high fault scarps shown in shadow. Illumination is from southwest. mostly of igneous rocks of mafic composition [Lonsdale, ment that extends the length of the Baja Peninsula. These 1989]. These spreading centers are marked by relatively structures include the east rooted Sierra San Pedro Martir little sedimentary infill and deep bathymetry, in contrast to fault system [Gastil et al., 1975; Dokka and Merriam, 1982; similar regions of highly extended crust formed further Stock, 2000], which bounds the highest rift shoulder seg- north in the Gulf of California and Salton Trough that are ment along the Baja California Peninsula (Figures 1a, 1b, covered with thick sequences of Pliocene (?)-Quaternary and 2). The San Pedro Martir fault has a steep, continuous sediment. Quaternary strata in the Salton Trough are meta- footwall scarp with large (5 km) displacement and scarps morphosed at greenschist facies conditions at shallow, upper that offset late Quaternary alluvium (Figure 2 [Dokka and crustal levels [Fuis et al., 1984; Muffler and White, 1969], Merriam, 1982; Brown, 1978]). The San Pedro Martir fault as a result of the high heat flow in this region [Lachenbruch segment of the Main Gulf Escarpment terminates abruptly to et al., 1985]. the south at the Puertecitos Volcanic Province that is associ- [9] Quaternary extension in the Salton Trough is likely ated with the Motamı´ accommodation zone (Figures 1b and 2 associated with mafic magmatism at deeper crustal levels [Stock, 2000]). [Fuis et al., 1984], which can be argued as beginning at ca. [12] The Peninsular Ranges rise steeply west of the Gulf 4Ma[Axen et al., 2006]. Thin crust (21–22 km) in the Escarpment between 30°–34°N latitude (Figures 1a and 2) Salton Trough [Parsons and McCarthy, 1996; Zhu and to a maximum height of 3095 m with elevations gradually Kanamori, 2000], and a temperature gradient of 33.3°Ckm1 decreasing westward along a concave upward slope [Lee et [Lachenbruch et al., 1985] all suggest that crustal extension, al., 1996]. Maximum elevations along the crest of the range magmatism and heating of the upper mantle are active vary between 1500 and 3000 m with the highest topography processes beneath the region [Schubert and Garfunkel, located in the Sierra San Pedro Martir (Figure 3a). The 1984]. Peninsular Ranges are composed primarily of Mesozoic [10] Spreading in the northern Gulf of California appar- igneous rocks that were exhumed and cooled through zircon ently underwent a westward shift along much of its length fission track closure temperatures during subduction in late in Pliocene time, as first identified by Nagy and Stock [2000] Cretaceous to Eocene time [Cerveny et al., 1991; Ortega- in the Tiburon-Delphin and Guaymas Basins at ca. 3 Ma, and Rivera, 2003; Lovera et al., 2006]. subsequently by Aragon-Arreola and Martı´n-Barajas [13] Igneous rocks of the Peninsular Ranges were subse- [2007] in this same region. A similar westward shift in quently covered by Eocene conglomerate derived from a active spreading has also been recognized near the mouth of source in northern Sonora and deposited in a channel the Gulf near the Alarcon Rise [Lizarralde et al., 2007]. system that extended west across southern California to [11] Quaternary extension in the Gulf of California is also the California borderlands [Minch et al., 1976; Kies and accommodated by normal faults that bound the eastern edge Abbott, 1983; Abbott and Smith, 1989]. Now largely of the Peninsular Ranges and dip east, forming an escarp- stripped of the overlying conglomerate across the top of

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Figure 3. (a) Plot of maximum topography of Baja California Peninsula (e.g., drainage divide; scale on left in kilometers) and uplift rate of marine stage 5a and 5e marine terraces (scale on right in millimeters per year) as measured between the Los Angeles Basin and the Viscaino Peninsula. (b) Shaded relief map showing location of terrace uplift surveyed at points along coastline listed in Table 1. Arrows showing uplift include solid examples that define regional uplift related to lithospheric flexure; dashed arrows define local uplift next to active fault zones. No uplift or subsidence occurs along the Pacific coast of Baja between Viscaino and the tip of the peninsula. the Peninsular Ranges, the nonconformity, or basement pret this as additional evidence for low relief prior to recent cover contact can be identified as a low-relief, eroded rock uplift otherwise paleochannels cut into batholithic surface that dips gently toward the Pacific coast and is rocks would be more deeply incised at higher elevations mapped as high as 1830 m, approximately 60 km east of the in rift shoulder segments. On the basis of this evidence, we Pacific coast at N32°500 [Gastil, 1961; Lovera et al., 2006]. assume that prior to uplift, the Peninsular Ranges lay at a [14] The Peninsular Ranges must have been relatively relatively low elevation (sloping very gently toward the low in elevation prior to extension and rift flank uplift, on Pacific coast), and therefore initial topography in our the basis of the distribution of Eocene conglomerate trans- models is approximated as sea level. The onset of surface ported from Sonora westward across southern California uplift in the eastern Peninsular Ranges in northern Baja [Kies and Abbott, 1983] and northern Baja California California is not well constrained, but desert soils, or [O’Connor and Chase, 1989]. The provenance of clasts in aridisols of mid Pliocene age exposed in the Salton Trough Eocene conglomerates also suggests little input from local is interpreted to form in a rain shadow consistent with an granitic sources in the Peninsular Ranges when they were existing rift flank uplift at this time [Peryam et al., 2008]. deposited [Abbott and Smith, 1989], consistent with a lack [15] Modeling presented later in this paper attempts to fit of significant relief. In addition, the modern river channel the total uplift of the initially gently west dipping Eocene network developed on the Laguna Mountain, Sierra Juarez surface, thus we are only concerned with the net change in and Sierra San Pedro Martir rift shoulder segments is not buoyancy since the Eocene. This uplift is presumed to be a deeply incised except near the Pacific coastline. We inter-

5of17 TC5003 MUELLER ET AL.: QUATERNARY RIFT FLANK UPLIFT OF BAJA CA TC5003 product of Miocene and younger normal faulting, erosion, [19] Although the absence of terrace uplift along the and lithospheric thinning or heating. Pacific coast is largely the result of a narrower rift flank uplift as defined by regional topography in the southern peninsula, the east rooted Santa Margarita and San Lazaro 3. Marine Terrace Uplift Along the Pacific faults normal faults defined just offshore south of the Coast Vizcaino Peninsula [Fletcher and Eakins, 2001; Fletcher et al., 2000b, 2000c, 2007] may also affect the record of [16] The Pacific coast west of the Peninsular Ranges, from 30 to 33°N, is characterized by flights of marine coastal uplift here. We therefore focus our attention on the terraces that imply continuous uplift during the late uplift signal north of about 28.6°N. Quaternary. Many of the most prominent terrace platforms [20] The highest observed marine terrace in southern are present in areas where faults locally influence and California located away from active structures is the Rifle control coastal structure and geomorphology, such as in the Range terrace in San Diego County that now stands at 155 m Palos Verdes Peninsula [Woodring et al., 1946; Woodring, above sea level [Kern and Rockwell, 1992]. Higher terraces 1948; Muhs et al., 1992], San Joaquin Hills [Grant et al., only exist within the Mount Soledad, Palos Verdes and San 1999], the Mt Soledad region of San Diego [Kern and Joaquin uplifts in southern California where active faulting Rockwell, 1992], and the Punta Banda region of Baja produces locally higher topography. Pliocene to early California [Rockwell et al., 1989] (Figures 3a and 3b). Pleistocene marine sediment of the San Diego Formation Shortening above blind thrusts, or in restraining bends of deposited at sea level are preserved in the San Diego region strike-slip faults, produces local uplift in these areas that is at nearly the same elevation of the highest Quaternary superposed on regional uplift that occurs at a lower rate. terraces. This can be interpreted to suggest that total Quaternary uplift does not exceed about 155 m, on the [17] It is the regional and remarkably uniform uplift signal along the Pacific coast, however, that is of primary basis of the eustatic record for Pliocene time [e.g., Dowsett interest in this study. Local structures could only produce and Cronin, 1990]. Given the present elevation of the San such a signal if a fault paralleled the coast and had near- Diego Formation and the Quaternary marine terraces in the constant displacements. A west facing escarpment mapped San Diego region, we thus argue that coastal southern offshore of the Pacific coast in northern Baja California California has undergone about 155 m of total surface uplift could reflect vertical motion just offshore, but existing since middle to late Pliocene time. earthquake focal mechanisms indicate strike-slip motion [21] Southward into Baja California, the highest marine [Rebollar et al., 1982]. Although we cannot completely terraces along the Pacific coast have been described as reject the possibility that this fault contributes in some way Pliocene in age on the basis of the presence of extinct fauna to coastal uplift, the great length and uniformity of the [Emerson and Hertlein, 1960; Hertlein and Allison, 1959]; regional uplift of terraces seems incompatible with local fault the maximum elevation of Pliocene marine fauna occur at control. While rock and surface uplift in Baja California is elevations greater than 100 m in northern Baja California, certainly modified by local faulting on the scale of tens of and decrease to about 20–30 m elevation at Santa Rosalalita kilometers or more, we seek to characterize and constrain near the Vizcaino Peninsula. Age controls for these deposits the larger-scale pattern of elevation change that extends are poor, and it is possible that some of the extinct fauna across and along the northern peninsula. described in these strata are early Quaternary in age. Nevertheless, these observations are consistent with the [18] The late Quaternary rate of regional, or background uplift is well known for much of the Pacific coast in much better described terraces and their inferred ages southern California and northern Baja California and aver- described in southern California. We use 155 m as the ages from 0.13 to 0.14 mm/a from southern California maximum post-Pliocene surface uplift where terraces are southward to the Ensenada region (Figures 3a and 3b). not affected by local structures. South of the Agua Blanca fault, the uplift rate is similarly [22] An extensive terrace including the Mesa Los Indios low at about 0.13 mm/a to 30° north latitude (interpreted and Mesa El Tigre surfaces located near the international from Ellis and Lee [1919], Emerson [1956], Valentine border in northernmost Baja California, is preserved at [1957], Emerson and Addicott [1958], Addicott and elevations of 350–380 m. Remnant benches above 400 m Emerson [1959], Emerson and Hertlein [1960], Valentine may represent older and poorly preserved terrace remnants, and Rowland [1969], Orme [1974], Emerson [1980], but the 350 m surface northwest and inland from La Lindberg et al. [1980], Emerson et al. [1981], Ortlieb et Mision is the highest, well-formed and well-preserved al. [1984], Kennedy et al. [1986], Ashby et al. [1987], marine terrace in the vicinity. The surface is capped by a Rockwell et al. [1989], Kern and Rockwell [1992], Muhs et cobble lag, as observed from an aerial overflight of the area, al. [1992, 2002], and G. L. Kennedy and T. K. Rockwell, although no fieldwork has yet been undertaken to locate unpublished survey data, 1995). Terraces along the Pacific marine fauna. This terrace may be very old, and possibly coastline south of 28.6° north latitude to the southern tip of Tertiary in age. the Baja California Peninsula have not been uplifted except [23] Uplifted marine deposits preserved along the Gulf of where they are locally deformed along strike slip faults California also provide constraints on regional strain in the (Figures3aand3b[Muhs et al., 1992; Kennedy and Peninsular Ranges. U-series dating of corals in southern Rockwell, unpublished data, 1995]). Baja California along the Gulf of California indicate low rates of uplift that range from 0.3 to 0.19 mm/a [DeDiego-

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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 qualitatively compared to other rift shoulders around the world (Figure 3a [Johnson et al., 1976]) or related to a suppressed root [O’Connor and Chase, 1989]. The maximum elevations along the range crest adjacent to the western edge of the Salton Trough vary from 3000 m in the south to 1500 m in the north. These regions of high-relief form distinct rift segments that are coincident with both east and west rooted extensional fault systems of Neogene age [Axen, 1995] and younger Quaternary pull-apart basins and spreading centers (Figures 1b and 3a; see separate rift shoulder segments labeled Laguna, Juarez and Martir).

5. Origin of Buoyancy Forces

[26] To explore the relationship between extension-driven uplift and the uplift of the terraces along the California and Baja California coasts, we first consider simple flexural models with single line loads and then develop solutions Figure 4. Diagram illustrating specific loads that con- that incorporate 2-D loads in cross section. The simple line tribute to a generalized line load that produces flexure of a load models help reveal the tradeoffs in determining the continuous plate comparable to topography in the Penin- contributions of loads produced by different sources needed sular Ranges in northern Baja California. to produce the observed flexure; they are a good initial approximation to reality because the topography we seek to match is outside much of the area where the load is created. Forbis et al., 2004]. Near Mulage, Ashby et al. [1987] We presume that any loads created by variations in the document a rate of about 0.05 mm/a for the past 120 ka. composition of the batholith [e.g., Gastil, 1983] were These values are more than an order of magnitude lower isostatically compensated before the initiation of rifting than geodetically determined rates derived from the crest of and before the development of the broad gentle surface the eastern Peninsular Ranges, in northern Baja California we use as a reference, and therefore we are only concerned (e.g., 4.8 ± 1.5 mm/a [Outerbridge et al., 2005]). We do not with the change in loads during the Neogene. consider published uranium series age determinations using [27] For an elastic plate subjected to an upward force in molluscs in Baja California [e.g., Ortlieb, 1991] owing to the vicinity of the rift, the upward force represents removal problems associated with the open system exchange of of rock by erosion and normal faulting at the top of the isotopes known to occur in samples of this type. crust, thermal expansion of the crust and thinning of the mantle lithosphere at depth. This upward force is reduced by the deposition of sediments, the shallower depth of the 4. Rift Shoulders Moho along the rift axis and any metamorphism of, or [24] Rift shoulders are common topographic features intrusion of mafic material into the crust. Contributions to along extensional basins and continental rift margins the total load from different sources are shown in Figure 4. throughout the world [Vening Meinesz, 1950]. Examples The resulting deformation is determined by the magnitude include the Transantarctic Mountains in Antarctica [Bott of the upward force, V, which controls the magnitude of and Stern, 1992; Stern and Ten Brink, 1989], and the flanks uplift, the flexural parameter a, which determines the width of the Rhine Graben [Weissel and Karner, 1989], the Rio of uplift or flexural wavelength, and the distance of the Graben in Greece [Poulimenos and Doutsos, 1997], and the force from the coast xl, which simply translates the defor- East African Rift [Vening Meinesz, 1950; Bott, 1992; Zeyen mation in the direction perpendicular to the rift. The upward et al., 1997]. As the hanging wall subsides to form a rift deflection, w(x), is then defined basin, the flanking footwall block is uplifted. Maximum x x l xl x xl x subsidence occurs along the rift axis, while uplift of rift wxðÞ¼w0e a cos þ sin shoulders occurs beyond the area offset by normal faults a a [Che´ry et al., 1992]. Rift shoulders range in height from 1000 to 5000 m along continental rift systems and vary in where the maximum deflection w0 is related to the force shape and amplitude as a function of both the uplift force through and the rigidity of the lithosphere [Che´ry et al., 1992]. Rift Va3 shoulders are asymmetric, with a steep escarpment facing w ¼ 0 8D

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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 varying magnitude 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 between buoyancy forces and their locations in the crust and upper mantle. (b) Simple model similar to Figure 5a but 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 underpredict elevation of higher parts of rift flank. where D, the flexural rigidity of the plate, and a,the and Te is the elastic plate thickness [e.g., Turcotte and flexural wavelength, are Schubert, 2002]. We assume a Young’s modulus E of vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 1011 Nm2 and Poisson’s ratio n of 0.25. For this u4D 3 a ¼ t4 calculation we use mantle density rm of 3250 kg m and rm rf g a fill density of 0. [28] If the aim of the models is only to fit the western ET 3 D ¼ e flank of the rift (i.e., the topography of the Peninsular 12ðÞ 1 n2

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Figure 5. (continued)

Ranges), a tradeoff exists between these three parameters: has undergone and uncertainty regarding the prerift topog- xl, V, and a can be varied to generally fit the overall slope of raphy that may have been present there. the profile. The main difference between these parameters [29] As an alternative end-member, a broken plate model within the Peninsular Ranges is a slight change in the approximates the case where strength in the lithosphere curvature of predicted rift flank topography (Figure 5a). In becomes very low within the rift. For a given load position, all cases the point where uplift begins to occur (moving the elastic plate thickness must be 80% greater and the from west to east) near the Pacific coastline is located load slightly smaller to produce a deflection comparable to offshore by some 40 km; moving it farther east misfits the that obtained from a continuous plate. Unless the plate edge topography near the coastline. In general, a similar shape is located well to the west of the main basin edge, the across the Sierra Juarez is obtained by shifting the load effective elastic plate thickness Te 40 km required to fit farther east while increasing both the flexural rigidity and the topography on the northern profile seems too high for the load. Because we do not know the preflexure topogra- lithosphere in southern California [e.g., Lowry et al., 2000]. phy in detail, we cannot fully resolve the tradeoff between The combination of the higher Te and steeper topography force, position, and flexural parameter without additional associated with the broken plate model make it a poorer information. We also do not consider the effect of loading choice for the northern profile than the continuous model of and flexure on the Sonoran side of the Gulf of California, Figure 5a. owing to the more complex extensional history this region [30] The origin of the buoyancy driving flexure can be determined by considering the forces in these simple mod-

9of17 TC5003 MUELLER ET AL.: QUATERNARY RIFT FLANK UPLIFT OF BAJA CA TC5003 els. For the profile across the Salton Trough, a line force of producing rift flank uplift with more realistic 2-D models. about 1013 N/m applied to a continuous plate within the rift To better determine the source, location, and magnitude of acceptably reproduces the topography (Figures 4 and 5a). the load uplifting the Sierra Juarez, we modeled the uplift as The upward buoyancy force from replacing granitic crust the result of extensional thinning of the crust from normal with the sedimentary fill in the Salton Trough only amounts faulting and variable thinning of the mantle lithosphere to between about 2.5 1011 N/m and 10 1011 N/m, (Figures 6, 7, and 8). The basin, Moho and lithosphere roughly an order of magnitude too small to produce loads are specified for each trial, and the deflection of the observed rift flank uplift. The surface load caused by plate is calculated. The material removed by erosion or removing rock in the rift through either erosion or extension crustal tectonism is then estimated to be equal to the mass is proportional to the area between the flexural curve and between the deflected position of the plate top and the the present topography; this varies with the flexural rigidity modern topography. The load from this estimate is then assumed. In all cases, the surface load is about 40–80% of added to the original loads and a new deflection is calcu- the necessary load, with the greater deficiencies for loads lated, which in turn leads to a new estimate of the load from placed farther west (Figure 5a). In addition, the crust thins erosion or crustal tectonism. We continue to iterate until no and crustal density increases from west to east across the new forces are generated. We force our flexural uplift near western edge of the Salton Trough [Fuis et al., 1984], the coastline to match the 155 m maximum elevation of producing a significant downward force in the crust along shoreline terraces described above and seek to match the rift perhaps twice the magnitude of the upward force deflections of an early Tertiary, low-relief surface. from the sedimentary basin. Thus much of the buoyancy [33] The load produced from variation in the thickness of force needed to drive flexural uplift (i.e., that matches the crust are specified from seismological inferences of observed rift flank topography) must be derived from the crustal thickness [Ichinose et al., 1996]. We use a 15 km mantle under the rift (Figure 4). This is also true of the thinner crust under the sediment-filled Salton Trough with broken plate models because the load required from exten- thinning tapering to zero 50 km from the edges of the basin. sion is (mis)located 65 km west of active normal faults in Our model of crustal thickness mimics that of Ichinose et al. the Salton Trough. [1996] who defined crustal thickness across the Peninsular [31] A similar analysis for the southern profile across the Ranges and Salton Trough of southern California near our Sierra San Pedro Martir yields comparable results though northern transect (see crustal depth points defined in Figure the required load is roughly twice that of the northern 8). Their work was based on using P-to-S converted phases profile for any particular load position. With a continuous of teleseismic body waves on a broadband array and has plate, loads between 1.2 and 3.5 1013 N/m applied to a been found to be representative of the Peninsular Ranges to plate with an elastic plate thickness from 17 to 30 km will the north and south [Lewis et al., 2000, 2001; Yan and match the topography of the Sierra San Pedro Martir, with Clayton, 2007]. We have assigned a density contrast of the thicker plate and larger load corresponding to a load 400 kg m3 to the Moho for our flexure models. This near the center of the Gulf of California. A slightly better fit presumes a relatively flat Moho prior to the Neogene. If to the topography is obtained by using a broken plate model more crustal thinning occurred, then a more buoyant mantle with somewhat smaller loads and 80% greater elastic plate is required, and if less thinning, then a less buoyant mantle. thicknesses than the continuous plate model (Figure 5b). We [34] Thinning of the upper mantle is considered by lack observations here comparable to those along the replacing lithosphere with less dense material similar to northern profile constraining the sedimentary thickness, the asthenosphere beneath the Salton Trough and eastern but it seems unlikely that load generated by replacing Peninsula Ranges. The replaced region is modeled by a crystalline rock with sediments will be more than double 80- to 100-km-thick, rectangular or trapezoidal body that in the Salton Trough. The main surface load is that 100 kg/m3 less dense than the mantle lithosphere (Figures 6, produced by removing crust through faulting and erosion; 7, and 8). Note that as the load is in proportion to the density the load could be somewhat higher than to the north, contrast and thickness, a thicker (or thinner) variation will perhaps as high as 2 1013 N/m for a load centered in produce the same load if the density contrast varies propor- the Gulf of California. Although this is almost 80% of the tionally. Both for simplicity and because there is an insig- load needed, the downward load from a shallower Moho nificant difference between a rectangular body and a and probable crustal density increases into the Gulf of trapezoid with an edge less than about 60 km wide, most California again indicate an important, if not dominant, models assume a rectangular body. contribution from buoyancy in the mantle. Furthermore, [35] The fit of models to observed topography is judged the comparison of the two profiles suggests a somewhat principally by the overall fit to the topography of the west weaker plate and a noticeably greater load in the south than slope of the Sierra Juarez. Because we force each model to the north. pass 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 below 6. Flexural Modeling of Sierra Juarez and modern topography, making the Eocene surface dip gently Salton Trough toward the ocean before flexure had occurred. We use the west edge of the deepest basin in the Salton Trough (140 km [32] Because the actual loads are not line loads but in fact distributed along the profile, we investigate the loads east of the coast) as our zero point (x = 0) in this modeling.

10 of 17 TC5003 MUELLER ET AL.: QUATERNARY RIFT FLANK UPLIFT OF BAJA CA TC5003

Figure 6. (a–c) Series of flexural models across northern transect assuming varied amounts of mantle lithosphere replaced by asthenosphere and different values for Te. Location of transect and all subsequent models 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 constant Te for a given model, with increasing Te from Figures 6a to 6c. All models maintain an elevation of 155 m at the Pacific coastline, as constrained by uplift of marine terraces. Figure 6a illustrates solutions for a relatively narrower and deeper region of mantle lid replaced by asthenosphere beneath the eastern Salton Trough for a relatively thick Te of 30 km. Figure 6b shows intermediate solutions for replaced mantle lid centralized beneath the Salton Trough and a Te of 20 km. Figure 6c defines region of replaced mantle lid that is thinner and wider but with a relatively thinner Te. Parameters in models illustrate the tradeoff between the location and magnitude of various loads and resulting rock uplift as illustrated by rift flank topography in the Sierra Juarez.

6.1. Elastic Plate Thickness [37] Smaller values of Te require loads more consistent with lithospheric variations expected in the region. A Te of [36] We first consider plausible plate thicknesses (Te)in southern California and northern Baja California. There is a 20 km requires a load extending to the west of the western edge of thinned crust and into the eastern Sierra Juarez, with tradeoff between the loads imposed and Te: a thickness of 0 requires a load exactly reflecting topography, while a about 95 km of lithosphere removed (Figure 6b). Unlike the stiffer plate permits the load to be located farther east. An Te = 30 km case, the eastern edge of the load can be located far enough to the west to allow for minimal removal of crust upper bound for Te in this region is 30 km [e.g., Lowry et in the Chocolate Mountains. A Te of 15 km represents the al., 2000]. Using a constant Te of 30 km and holding the eastern edge of the mantle load at x = 100 km, we find that lowest possible flexural rigidity capable of reproducing the the mantle load should not extend west of the west edge of topography above a sharp-edged mantle load (Figure 6c). In the deep basin (Figure 6a). These solutions require large this case, the load magnitude trades off dramatically with mantle loads equivalent to the removal of 170 km or more the amount of material that must be removed within the rift of mantle lithosphere 100 kg m3 denser than astheno- to match the topography, thus greatly limiting the range of 13 acceptable solutions. These results indicate that for any T sphere (equivalent to line loads of about 2 10 N/m), and e these require the tectonic removal of 3 or more km of crust less than or equal to about 20 km, mantle loading must in the eastern Chocolate Mountains. We find the magnitude extend west of the major basin bounding fault by at least of the load too large to be plausible given lithospheric 50 km and west of the westernmost range-bounding faults thicknesses in California [e.g., Yang and Forsyth, 2006]. by more than 15 km.

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

[38] A more plausible scenario is that the effective elastic 6.2. Timing of Uplift and Loading plate thickness thins into the rift. In general, elastic plate [40] We noted above that the Salton Trough has experi- thickness thins with increased heat flow. We focus on a enced multiple episodes of extensional deformation since model with Te = 24 km away from the rift and Te =10kmin the Miocene. However, the coastal uplift is post-Pliocene. the rift (from x =0tox = 33 km) with a gradient in elastic One possibility is that the entire mantle load causing uplift plate thickness about 30 km wide on each side of the rift. In of the rift shoulders postdates the Pliocene. This would this case (Figure 7), we find that topography is quite require that earlier crustal thinning be rooted into the mantle sensitive to the position of the left edge of the mantle load. west of the Sierra Juarez and the Salton Trough. The least This is in part because of the feedback of forcing material to amount of mantle loading can be found by noting that the be removed in the rift: as the mantle load grows, the terraces are near the nodal point for flexural uplift from a erosional load grows as well. The right edge of the load load in the interior. In Figure 8, we show that a 20–30 km has little effect on the topography in the Sierra Juarez, but westward shift in the west edge of the mantle load is this does change the amount of material that must be sufficient to generate the 155 m uplift at the coast. This removed from the Chocolate Mountains. If the Chocolate lateral growth of the load also produces about half of the Mountains are an equivalent flexural shoulder to the rift, topography of the Sierra Juarez, suggesting that the eleva- then the mantle load ends somewhere between 65 and 80 km tion of this range could also be mostly Pliocene and east of the axis of the rift basin. younger. However, the magnitude of uplift of the crest of [39] Just as with solutions with a constant Te below about the Sierra Juarez depends on the dip of the west edge of 20 km, the variable Te case requires the mantle load to buoyant mantle; if the dip allows buoyant material farther extend about 50 km to the west of the main basin bounding west than shown in Figure 8, then it is possible to raise the fault and about 20 km west of the eastern flank of the Sierra coastal areas while only raising the crest a few hundred Juarez. Significantly, this is also 20–25 km west of the meters, or, in the most extreme case, only 30 m. decrease in elastic plate thickness. These results indicate [41] Oxygen isotope analysis of paleosols and magneto- that the mantle load must extend well to the west of a stratigraphy in the Fish Creek–Vallecito basin in the west- thinner elastic plate, which is equivalent to the part of the ern Salton Trough suggest that a rain shadow existed there crust that has been heated and damaged by rifting. The prior to ca. 3.7 Ma [Peryam et al., 2008]. Aridisol soil magnitude of the load is equivalent to removal of about chemistry also suggests general aridity in this region be- 80 km of mantle lithosphere. Thus we find that the buoy- tween ca. 1.0 and 3.7 Ma [Peryam et al., 2008]. While this ancy in the mantle extends a minimum of 20 km (and does not tightly constrain the elevation of a supposed flank probably closer to 50 km) west compared with the western uplift, it is generally consistent with our arguments for limit of substantial crustal deformation.

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

13 of 17 TC5003 MUELLER ET AL.: QUATERNARY RIFT FLANK UPLIFT OF BAJA CA TC5003 the timing of rift flank uplift driven by increased mantle older terraces with amino acid racemization results and buoyancy. uplift rates). [45] Unpublished geodetic data [Outerbridge et al., 2005] support our work by suggesting that rapid rates (4.8 ± 7. Discussion 1.5 mm/a) of modern uplift may occur in the higher parts [42] Thinning, or strong heating of the upper mantle of the Peninsular Ranges province. The remarkably consis- under the eastern Peninsular Ranges is consistent with the tent rate of slow background uplift of marine terraces along previous results of Lewis et al. [2000] who require a the Pacific coastline and negative gravity along the range significant mantle component to support topography west crest is interpreted as evidence for large-scale flexure of the of the Main Gulf Escarpment. Ichinose et al. [1996] also lithosphere driven largely by a broad mantle upwelling observe the lack of correlation between topography and beneath eastern Peninsular Ranges, Salton Trough and crustal thickness (e.g., depths to the Moho) and suggest northern Gulf of California. At the scale of the width of compensation from lateral density variations in the lower the Baja Peninsula, this suggests that coastal uplift is crust and upper mantle. relatively unaffected by the segmentation of late Quater- [43] All models include removal of upper crustal material nary extensional faults in the Salton Trough and Gulf of by either erosion or normal faulting so that the modern California [Axen, 1995]. A lower rift flank uplift preserved topography within the rift is honored. We have not attemp- south of the Sierra San Pedro Martir in southern Baja also ted to explicitly include erosion to the west of the rift, nor suggests buoyant mantle adjacent to the Gulf of California, have we tried to estimate a priori the total amount of although to a lesser extent than farther north. material removed from within the rift. Erosion west of the [46] Our work thus suggests a fundamental increase in rift is volumetrically minor and does not significantly affect buoyancy in the upper mantle in the northern Gulf of our results. Estimating the total volume of material removed California that is presumably related to the thermal con- within the rift is problematic, largely because the original ditions that have affected this region. Rift flank uplift of the Moho geometry is unknown and any surficial geologic Peninsular Ranges is thus partly coeval with crustal exten- constraints are complicated by poor control on fault geom- sion driven by transform tectonics, as the timing of coastal etry at depth and possible lateral flow of lower crust. uplift does not match well-documented evidence for prior Despite these difficulties, the magnitude of buoyancy in Miocene extension along low-angle detachment faults in the mantle is only overestimated if the crust was originally this region. We envision the mantle load that drives rift thinner at the site of the Salton Trough prior to extension flank uplift as migrating westward from the Salton Trough (relative to the region to the west). and Gulf of California by some combination of lithospheric [44] Simple models of crustal extension coupled with thinning and thermal erosion (Figure 8). The onset of uplift near complete replacement of mantle lithosphere predict at the Pacific coastline must, however, be Quaternary in age. relief that broadly matches observed basin geometry and rift Our models also hint that the current topography of the flank swath topography across the Peninsular Ranges and Chocolate Mountains may be related to the relatively recent Salton Trough in southern California and Baja California development of the Gulf of California, although there are (Figures 5, 6, 7, and 8). Thinning of the lithosphere in this essentially no constraints on the true timing of uplift in this region has produced a western rift flank uplift composed of region. We also have not considered the effects of substan- the Laguna Mountains, Sierra Juarez and Sierra San Pedro tial Miocene thinning along the east rooted Colorado River Martir that rise to an average elevation of 2 km above sea extensional corridor farther to the east and its effect on the level. Uplift of the western rift flank apparently extends current topography of the Chocolate Mountains. An inter- westward to the Pacific coastline, where late Quaternary esting aspect of the models presented in this paper is the marine terraces are raised as much as 155 m above sea level. enormous magnitude of mantle lid thinning required to A narrow and lower rift flank uplift bounds the eastern side reproduce rift flank topography. For example, the best fit of the Salton Trough that is also predicted by our models solutions suggest 80 km of lid loss over a 120–150 km and marked by the Chocolate Mountains. While little wide region, perpendicular to the Gulf of California. While evidence exists to exactly constrain the timing of uplift of a number of previous workers have argued for significant the higher parts of the rift flanks, the age of uplifted marine heating of the mantle, or loss of the mantle lid [Fuis et al., terraces and shallow marine deposits along the Pacific coast 1984; Lewis et al., 2001] beneath parts of the rift, none have suggest regional uplift may be very young and largely suggested that crust in the northern Gulf of California and Pleistocene in age. This is supported by late Pliocene to Salton Trough overlies asthenosphere over the width of the Pleistocene soils developed in the western Salton Trough rift floor and adjacent eastern Peninsular Ranges as indicated that imply the presence of a rain shadow and hence by our models. However, Park et al. [1992] suggested the significant relief in the eastern Peninsular Ranges at this imposition of a major heat source in the past 5 My under the time. It is important to note that rates defined by the marine Peninsular Ranges and Lewis et al. [2001] argued for a stage 5e terrace only provide an average uplift rate for the significant mantle component to support topography east of last 120 Ka, and that total uplift defined in southern the Main Gulf Escarpment (located about 20 km east of the California of 155 m must have been initiated well before range crest). this time (but after shallow marine Pliocene strata were [47] Whereas the western rift shoulder is marked by deposited, and at about 1.3 Ma on the basis of dating of discrete segments along the length of Baja California, it

14 of 17 TC5003 MUELLER ET AL.: QUATERNARY RIFT FLANK UPLIFT OF BAJA CA TC5003 appears to be uplifting at significantly lower rates south of [49] Our hypothesis links uplift of the Pacific coast and the Sierra San Pedro Martir, on the basis of the morphology Sierra Juarez to either conductive or convective heating in of deeply embayed range fronts, lower overall rift flank the mantle lithosphere under the Peninsular Ranges. Further relief and rates of uplift defined by Late Pleistocene marine tests of this idea will either constrain the thermal evolution deposits near La Paz and Los Cabos [Muhs et al., 1992; of the mantle lithosphere under the Peninsular Ranges or DeDiego-Forbis et al., 2004]. Therefore, while the southern better constrain the temporal evolution of topography to- Gulf of California has undergone crustal extension across ward the crest of the range. Our interpretation requires spreading ridges comparable to similar transform and ridge substantial post-Pliocene removal or modification of mantle segments farther to the north, mantle buoyancy changes in lithosphere well beyond the edges of Neogene crustal strain the Quaternary appear to be lower to the south, on the basis recorded in the western Salton Trough; the mechanism(s) of rift shoulder relief. This is consistent with recent surface accommodating such deformation remain undetermined but wave studies in the region that show very low mantle wave could include low-angle normal faulting, lateral heat con- speeds in the northern Gulf of California but higher wave duction, or convective instabilities in the lithosphere. speeds in the south [Zhang et al., 2007]. [48] Our work suggests that the regional background [50] Acknowledgments. We thank Garry Karner for his comments on uplift of the Pacific coastline extends from southern our early modeling work and Tim Melbourne and Mike Oskin for their discussions of mantle structure and the Neogene tectonic history of the California to nearly the Vizcaino Peninsula and is not inner California borderland and the Gulf of California, respectively. Kelin related to local shortening above active blind thrusts [Rivero Whipple, Kip Hodges, Suzanne Jannecke, Rebecca Dorsey, Brian Wernicke, et al., 2000] but is instead related to a combination of crust and, in particular, John Fletcher and Gary Axen provided many helpful and mantle thinning centered on the Gulf of California and suggestions, resulting in a much improved final version of this paper. This research was supported by the Southern California Earthquake Center eastern Peninsular Ranges. This broad uplift appears to best (SCEC). SCEC is funded by NSF cooperative agreement EAR-8920136 correspond to a phase in development of the new spreading and USGS cooperative agreements 14-08-0001-A0899 and 1434-HQ- system in the Gulf of California when early regional 97AG01718. The SCEC contribution number for this paper is 580. Research was further supported by NEHRP grant 01 HQGR0031 to extension becomes more localized but before the system K. Mueller. The views and conclusions contained in this document are evolves into a better defined seafloor spreading boundary. those of the authors and should not be interpreted as necessarily represent- ing official policies of the U.S. government.

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