Mid-ocean ridge spreading at the East Pacific Rise (EPR) from 9°N to 10°N typically has been viewed as a geologically continuous process involving eruptions and dike emplacement within a narrow zone of magmatism defined by the axial summit trough (Figure 1) [Fornari et al., 1998; Gregg et al., 1996; Haymon et al., 1991, 1993; Perfit and Chadwick, 1998; Schouten et al., 2001, 2002; Sims et al., 2002, 2003; Soule et al., 2009]. However, seismic studies suggest that seismic layer 2A, interpreted as the extrusive crust, doubles in thickness within ∼2–4 km from the axial summit trough [Christeson et al., 1994, 1996; Harding et al., 1993; Schouten et al., 1999; Sohn et al., 2004; Vera and Diebold, 1994], requiring that a significant component of volcanic crustal accretion occurs outside of this region [Goldstein et al., 1994; Hooft et al., 1996; Kurras et al., 2000; Perfit et al., 1994; Sims et al., 2003; Soule et al., 2005; White et al., 2002]. Magnetic surveys, Autonomous Benthic Explorer (ABE) 675 kHz microbathymetry, and DSL-120A side-scan sonar imagery in the 9°50′N region reveal that a highly magnetized and high acoustic backscatter region defined as the “neo-volcanic zone” extends to ∼2–4 km on either side of the axial summit trough and is dominated by a shingle-patterned lava terrain (Figure 2) [Fornari et al., 2004, Plate 2; Schouten et al., 1999; Sims et al., 2003, Figure 1b; Williams et al., 2008]. Consequently, the neo-volcanic zone appears to be produced primarily by transport of lavas away from the axial summit trough, either from lava overflow of the axial summit trough or flow through subterranean tubes [Fornari et al., 2004; Haymon et al., 1993; Kurras et al., 2000; Soule et al., 2005]. This inference is consistent with more recent studies of an eruption in 2005–2006 that show that, in places, lava flowed up to ∼3 km from the axial summit trough (Figure 1) [Fundis et al., 2010; Soule et al., 2007]. Off-axis pillow mounds, which have also been proposed to contribute to extrusive layer thickening [Perfit et al., 1994], appear to make up a less significant component of neo-volcanic zone accretion [cf., Sims et al., 2003, Figure 1b].
 Although high-resolution spatial observations can elucidate the mechanisms of volcanic accretion, they are limited in their ability to quantify temporal aspects of ridge evolution. For example, sediment thickness variations can be useful for making first-order observations of relative lava ages for lava flows of significantly different ages and sediment cover. However, absolute ages of lava flows in years or relative ages of similarly sedimented lava flows estimated from sediment thicknesses are less reliable due to differences in sediment appearances on different lava morphologies (flat surfaces generally appear more sedimented than textured surfaces) as well as local variations in deposition from hydrothermal venting and sediment winnowing by deep ocean currents [e.g., Ballard et al., 1979; Cann and Smith, 2005; Perfit and Chadwick, 1998]. As a result, little is known about the age and compositional and stratigraphic relationships among adjacent “shingles” (interpreted to be lava flow lobes) within the neo-volcanic zone; yet, this information is necessary for determining whether the shingled terrain is produced by the overlapping of flows from multiple eruptions sourced within the axial summit trough or elsewhere, or reflects ridge crest repaving by single, large eruptions. This type of information is crucial for understanding the processes that produce the neo-volcanic zone at fast-spreading ridges, and it has important implications for spreading-rate dependent models of mid-ocean ridge behavior (e.g., it is presumed that fast-spreading ridges produce smaller volume eruptions than intermediate- and slow-spreading ridges, Sinton et al. ).
 Determining stratigraphic relationships among mid-ocean ridge lava flows and eruptive units using solely submersible/geological observations can be problematic due to logistical difficulties (e.g., limitations in dive time, areal coverage, field of view, and area of illuminated seafloor), variability in sedimentation, and the overall complexity of volcanic terrain (e.g., tube flow and localized breakouts through the flow front; intraflow variations in lava morphology; superposition and interfingering of flow units or flow lobes; braiding of lava tubes and channels within flows; ponding or channeling of flows due to faulting and topography) [e.g., Ballard et al., 1979; Cann and Smith, 2005; Escartin et al., 2007; Fornari et al., 2004; Fundis et al., 2010; Gregg and Fink, 1995, 2000; Hon et al., 1994; Thordarson and Self, 1993]. When combined with geological and remote sensing observations, lava geochemistry can be critical for distinguishing among different lava flows. However, because geochemical variability is known to exist within flows of both individual submarine [Bergmanis et al., 2007; Goss et al., 2010; Perfit and Chadwick, 1998; Rubin et al., 2001; Sims et al., 2002] and subaerial basaltic eruptions [e.g., Maclennan et al., 2003; Rhodes, 1983], and because lava flow compositions are nonunique and may recur over time, particularly in an area with lava as homogeneous as 9°50′N EPR, lava age is a key parameter for identifying distinct eruptive episodes and thus determining lava stratigraphy. An interesting counterpoint to observations of geochemical heterogeneity within mid-ocean ridge type flows is the extremely voluminous (∼15 km3) 1783–1784 Laki eruption in Iceland that displayed remarkably homogeneous abundances of Th (1.12 ± 0.02 ppm (2σ), n =11) and U (0.344 ± 0.007 ppm (2σ), n =11), Th/U (3.27 ± 0.01), 87Sr/86Sr (0.70324, n=3), and δ18O (3.13 ‰, n=4) [Sigmarsson et al., 1991].
 Uranium decay series (U-series) dating techniques provide a more accurate means for dating mid-ocean ridge basalts (MORB) on time scales of ∼0.1–375 ka [Cooper et al., 2003; Goldstein et al., 1992-1994; Lundstrom, 2003; Rubin and MacDougall, 1990; Rubin et al., 1994; Sims et al., 2003; Standish and Sims, 2010; Sturm et al., 2000; Waters et al., 2011; Elkins et al., 2011]. The presence or absence of disequilibria between daughter and parent nuclides (e.g., 238U-230Th- 226Ra-210Pb, and 235U−231Pa) can place absolute age limits on lava samples, and under certain conditions disequilibria can provide finer temporal resolution by model age dating. Sims et al.  used U-series model ages at 9°50′N EPR to show that volcanic construction is not limited to the axial summit trough and occurs over the full width of the neo-volcanic zone, which is consistent with both geological and geophysical observations [Christeson et al., 1994, 1996; Fornari et al., 2004; Harding et al., 1993; Hooft et al., 1996; Perfit and Chadwick, 1998; Perfit et al., 1994; Schouten et al., 1999; Sohn et al., 2004]. However, this study did not provide sufficient geographical coverage to identify age relationships between adjacent lava flows. We addressed this issue by collecting new sample suites from 9°N to 10°N EPR using the DSV Alvin during the cruise AT11-7 in 2004 that were explicitly identified to have come from different flow lobes (i.e., shingles). Samples were collected along two dive transects (dives 3963 and 3974) that traversed the east and west sides of the ridge crest from ∼0.7 to 2.0 km from the axial summit trough, spanning a major portion of the neo-volcanic zone (Figures 1-3) [Schouten et al., 2004]. These samples were collected directly from pillow lavas and adjacent lobate and sheet flows that correspond to, respectively, the fronts and bodies of flow shingles observed in side-scan sonar and ABE microbathymetry, and they provide more appropriate sample spacing and continuity for determining the geochemical and age relationships among a stratigraphically related series of flow units.
 In this study, we combine geological and remote sensing observations, U-series age constraints, and geochemical and isotopic data in an effort to unravel volcanic stratigraphy along the ridge crest at 9°50′N EPR. We evaluate the resolution of the U-series model age techniques, namely 230Th-226Ra model age dating, for dating dive 3974 and 3963 samples, as exact age determinations have the potential to provide the strongest complementary constraints to geological observations of complex volcanic stratigraphy. In addition, we examine the geochemical variability in dive 3974 and 3963 samples and compare this to data for the 1991–1992 and 2005–2006 eruptions, which currently provide the best constraints on the degree of intra- and inter-eruption natural geochemical variability at 9°50′N EPR. Finally, we evaluate the relative utility of each method and how coupling of these independent constraints might lead to a more comprehensive understanding of ridge crest volcanic accretionary processes.