Kurras et al., Marine Geophy. Res., 21 (1):23-41, 2000 1 Volcanic … · Kurras et al., Marine Geophy. Res., 21 (1):23-41, 2000 2 Figure 1 3-D bathymetry model of the East Pacific

Kurras et al., Marine Geophy. Res., 21 (1):23-41, 2000 1

Volcanic Morphology of the East Pacific Rise Crest 9° 49’-52’ N: Implications

for Volcanic Emplacement Processes at Fast-Spreading Mid-Ocean Ridges

Gregory J. Kurras1, Daniel J. Fornari2, Margo H. Edwards3, Michael R. Perfit4, and Matthew C. Smith1

1 Dept. of Marine Geology and Geophysics, School of Ocean Earth Science and Technology, University of Hawai’i, Honolulu, HI 96822

2 Dept. of Geology and Geophysics, Woods Hole Oceanographic Inst., Woods Hole, MA 02543

3 Hawaii Institute of Geophysics and Planetology, School of Ocean Earth Science and Technology, University of Hawai’i, Honolulu, HI 96822

4 Dept. of Geology, University of Florida, Gainesville, FL, 32611

Abstract Deep sea photographs were collected for several camera-tow transects along and across the axis at the East Pacific

Rise crest between 9° 49’ and 9° 52’N, covering terrain out to 2.0 km from the ridge axis. The objective of the surveys was to utilize fine-scale morphology and imagery of seafloor volcanic terrain to aid in interpreting eruptive history and lava emplacement processes along this fast-spreading mid-ocean ridge. The area surveyed corresponds to the region over which seismic layer 2A, believed to correspond to the extrusive oceanic layer, attains full thickness [Christeson et al., 1994a, 1994b, 1996; Hooft et al., 1996; Carbotte et al., 1997]. The photographic data are used to identify the different eruptive styles occurring along the ridge crest, map the distribution of the different morphologies, constrain the relative proportions of the three main morphologies and discuss the implications of these results.

Morphologic distributions of lava for the area investigated are 66% lobate lava, 20% sheet lava, 10% pillow lava, and 4% transitional morphologies between the other three main types. There are variations in inferred relative lava ages among the different morphological types that do not conform to a simple increase in age versus distance relationship from the spreading axis, suggesting a model in which off-axis transport and volcanism contribute to the accumulation of the extrusive layer. Analysis of the data suggests this ridge crest has experienced three distinctly different types of volcanic emplacement processes: (1) axial summit eruptions within a ~1 km wide zone centered on the axial summit collapse trough (ASCT); (2) off-axis transport of lava erupted at or near the ASCT through channelized surface flows; and (3) off-axis eruptions and local constructional volcanism at distances of ~0.5-1.5 km from the axis. Major element analyses of basaltic glasses from lavas collected by Alvin, rock corer and dredging in this area indicate that the most recent magmatic event associated with the present ASCT erupted relatively homogeneous and mafic (>8.25 weight percent {wt.%} MgO) basalts compared to older, off-axis lavas which tend to be more chemically evolved [Perfit and Chadwick, 1998; Perfit and Fornari, unpublished data]. The more primitive lavas have a more extensive distribution within and east of the ASCT. More evolved basalts (MgO <8.0 wt.%) are concentrated in a broad area a few kilometers east of the axis, and in an oval-shaped area south of 9° 50’N, west of the ASCT. Transitional and enriched (T- and E-) mid-ocean ridge basalts exist in relatively small areas (<1 km2) on the crestal plateau and correlate with scarps or fissures where pillow lavas were erupted. Mafic lavas in this area are primarily related to the youngest magmatic events. Geochemical analysis of samples collected at distances ~>500m from the ASCT suggests that regions of off-axis volcanism may be sourced from older and cooler sections of the axial magma lens.

Analysis of these data suggests that this portion of the EPR has not experienced large scale volcanic overprinting in the past ~30ka. The predominance of lobate flows (66%) throughout much of the crestal region, and subtle variations in sediment cover and apparent age between flows, suggest that eruptive volumes and effusion rates of individual eruptions have been similar over much of the last 30ka and that most of the eruptions have been small, probably similar in volume to the 1991 EPR flow which had an estimated volume of ~1 x 106 m3 [Gregg et al., 1996].


Figure 1 3-D bathymetry model of the East Pacific Rise crest (9° 49’-52’ N) showing locations of camera tows conducted in November 1997 and May 1999, rock core samples collected between 1993 and 1998, and 1994 Alvin dive tracks. Seabeam bathymetry collected in 1994 is contoured at 10 m intervals. Track lines are color coded to show lava morphology. Inset shows the location of study area and major plate boundaries.

Introduction The global mid-ocean ridge (MOR) crest is the

largest active volcanic system on the Earth, yet only a very small portion of it has been studied in sufficient detail to understand its volcanic and magmatic history. During the past ~15 years marine geological, geophysical, and geochemical data have provided valuable insights into the processes that influence volcanism and tectonism at the fast-spreading East Pacific Rise (EPR) [e.g. Lonsdale, 1977; Macdonald et al., 1984; Thompson et al., 1985; Langmuir et al., 1986; Macdonald and Fox, 1988; Edwards et al., 1991; Sinton and Detrick, 1992; Harding et al., 1993; Perfit et al., 1994; Rubin et al., 1994; Fornari et al., 1998a; 1998b; Perfit and Chadwick, 1998; Chadwick et al., 1999]. Recent seismic data indicates that at the northern East Pacific Rise (EPR) between 9° 49’ and 52’N, the thickness of layer 2A, suggested to be the accumulation of extrusive volcanics, increases from ~300 m at the axis to ~600 m at ~2 km off-axis [Christeson et al., 1992; 1994a; 1994b; 1996; Hooft et al., 1996; Carbotte et al., 1997]. Geological and petrological evidence indicates that off-axis eruptions occur and that these eruptions may contribute significant volumes of material to the

formation of the extrusive oceanic layer [e.g. Perfit et al., 1994; Goldstein et al., 1994; Gregg et al., 1996; Macdonald et al., 1996; 1989; Sims et al., 1997; Perfit and Chadwick, 1998; Reynolds and Langmuir, in press].

Deep sea digital photographs were collected on several phototransects along and across the EPR crest between 9° 49’-52’N in the region over which crustal seismic layer 2A attains full thickness (Figure 1). The purpose of this work was to examine submarine emplacement processes and the contribution of off-axis volcanism to the accumulation of the extrusive layer (Figure 1). The line spacing of the survey provides along-axis traverses of volcanic terrain on the east flank of the crestal plateau at ~5 -10 ka intervals assuming a half-spreading rate during the last million years of 55 mm/yr [Carbotte and Macdonald, 1992]. Data from these phototransects were analyzed for information on lava morphology distribution, sediment cover, sediment infill, contact relationships, and other geologic features at the meter to sub-meter scale, to characterize eruptive activity along this portion of the EPR crest during the past ~30 ka.


Data Acquisition The photographic data were acquired using the

Woods Hole Oceanographic Institution (WHOI) camera system, a self-contained digital and 35 mm camera system towed from a surface ship [Fornari and Spencer, 1998; Fornari et al., 1998a]. Six survey lines were run parallel to the ridge axis at relatively fixed distances from the ASCT (Figure 1) plus one survey line (line a3-34-10) consisting of a set of parallel tracks over a small area on the east side of the ridge crest between 9° 49’-50’N to investigate the extent of a sheet flow (Figure 1). Digital images were acquired every 15 seconds at altitudes 5 - 10 m above the seafloor as determined using a 12kHz pinger on the sled. Typical tow speeds of ~0.25 knots (~8 m/min) produced a spacing of 1 - 2 m between pictures with 80 - 100% coverage in the along track direction (Figure 2). Navigation of the towed camera was accomplished using a layback calculation and P-Code GPS positions of the surface ship every 15 seconds, typical camera layback during the surveys was found to be 100 – 200 m behind the ship. The tow camera data were supplemented by visual observations and down-looking video collected during Alvin dives 2770, 2771, and 2772 which traversed the EPR crest and upper flanks in 1994 [Cochran et al., 1996; 1999] (Figure 1). Alvin dives were navigated using bottom-moored transponders, accurate to ~5 - 8 m [Haymon et al., 1991; Fornari et al., 1998b]. Imagery from each camera tow and Alvin dive was analyzed and compiled into a database containing volcanic and morphologic identifiers and relevant topographic and structural information (Figure 3). Slope values for the Alvin data are calculated as slope along the survey track based on Alvin near-bottom depth profiles.

Lavas in the images were classified into five main morphological categories and four transitional categories (Figure 4 and Table 1). The forms and nomenclature of the lava morphology classifications used in this paper are consistent with those used in Ballard and Van Andel [1977] and Ballard et al. [1982a, 1982b; 1984]. The three categories of sheet lava (lineated, ropy, and hackly lava) all indicate high effusion rates with varying topographic slopes [Bonatti et al., 1988; Gregg and Fink, 1995; Gregg et al., 1996; Head et al., 1996; Chadwick et al., 1999] and are therefore combined into a single sheet lava category for analysis and discussion of effusion rate at the time of emplacement. The occurrence of apparent flow contacts, variations in sediment cover and sediment infilling between the volcanic forms, fault and fissure locations, and regions of volcanic collapse were noted and cross-referenced based on image location.

Analysis & Discussion Lava flow morphology is the result of interactions

between the conditions of lava emplacement (i.e. pre-existing terrain, slope, roughness, etc.), rheology of the lava (viscosity and crystallinity), and eruption dynamics (i.e. effusion rate, and gas content) (Figure 5). The

results of this analysis are used in conjunction with theoretical modeling of lava flows and the geochemistry of the lavas in this field area to interpret the volcanic history of the study area [e.g. Bonatti et al., 1988; Perfit

Figure 2 Photomosaic of a lineated sheet flow showing local-scale variations in flow morphology. The flow is ~ 4.5 m wide by 12 m in length, and is located ~300 m west of the ASCT running from east (bottom) to west (top). Photographic coverage along survey lines (Figure 1) is 80-100%.



et al., 1994; Gregg and Fink, 1995; Gregg et al., 1996; Head et al., 1996; Perfit and Chadwick, 1998] (Figure 6). The correlations between ridge topography, lava chemistry, morphologic distributions, and relative morphologic percentages with recent work on the dependency of lava morphology upon slope angle, flow volume, and effusion rate are used to interpret seafloor eruptive mechanisms and make geologic interpretations [e.g. Rowland and Walker, 1990; Griffiths and Fink, 1992; Hon et al., 1994; Gregg and Fink, 1995; Gregg et al., 1996; Gregg and Keszthelyi, submitted].

Figure 3 Example (Line a3-10-6) of image analysis performed on data from each camera survey line and Alvin dive. Images were classified into one of five main or four transitional categories of lava morphology and assigned a slope based on Alvin data and depth based on location and correlation to 1994 Seabeam data. The occurrence of geologic features such as fissures, faults, and collapse pits were also noted.

Seafloor Topography Analysis of lava morphology as a function of

topographic slope derived from near-bottom Alvin data indicates that at the local scale 99% of the data fall within slope angles that range from 0° to 4°; only 1% of the data correspond to slopes >4° (Figure 7). Visual observations and bathymetric data from Alvin throughout the ridge crest and flank area [Haymon et al., 1991, 1993; Kurras et al., 1998; Fornari et al., 1998b] show that local scale topography and gradients can vary greatly. Locally, lava flowing over the seafloor is greatly affected by restrictions or disruptions in flow resulting in dramatic morphologic changes over short distances; however, these dramatic changes represent only 1% of the data analyzed in this study. All types of lava morphology can be found over the entire range of observed slope values suggesting that topography is not a dominant control of regional lava morphology within the area studied.

Distribution of Lava Morphologies and the Nature of Axial Volcanism

Lobate lava is ubiquitous over the area surveyed and comprises the largest percentage (66%) of the three morphologies classified along the EPR crest. Sheet lava comprises 20% of the morphologies observed, pillow lava is less abundant comprising 10% of the lava in the surveyed area, and 4% of the lava is transitional. At

distances >0.5 km from the ASCT, pillow lava occurrences increase (Figure 9 and Table 1).

Past investigations have shown that the axial-high near 9° 50.25' N (Figure 1) is the area of most recent volcanic and magmatic activity along this 4th order ridge segment [e.g., Macdonald et al., 1984; Scheirer and Macdonald, 1993; Haymon et al., 1991; 1993; Carbotte and Macdonald, 1994; Perfit and Chadwick, 1998; Fornari et al., 1998b] (Figure 1). Regional distributions and relative ages of lava morphologies shown in Figures 9 and 12 indicate a concentration of younger less viscous sheet lavas along the axial high reflecting a focusing of magmatic activity; however the data also shows evidence for emplacement of anomalously young lava along the flanks of the axial high at distances 0.5 – 2 km east of the axis. Analysis of the photographic data shows that while sheet lava occurs throughout the region, exposures of this type of lava along the camera tracks are usually small and isolated, except for the axial high and the area just east of the ASCT (Figures 1 and 9).

Distinct variations in sediment cover and sediment infill between lava forms and differences in the appearance of glassy outer crusts of the flows lead us to conclude that for the past ~30 ka this portion of the EPR axis has not experienced large scale volcanic overprinting. The distributions and percentages of lobate lava argue for a series of smaller eruptions, each with similar effusion rates producing largely lobate flows on the EPR axis in this area over the past ~30 ka (Figure 9). This contrasts with the EPR crest between 9° 14’ and 9° 18’N which is believed to have experienced a large eruption sometime in the 1987-1989 time frame; the eruption covered the axis for ~6-8 km along strike and ~1-2 km across axis with a very black and glassy lobate to pillowed flow that was imaged by the Argo camera and sonar system in late 1989 [Haymon et al., 1991; Fornari et al., 1998b]. The observational and photographic data from the 9° 49-52’N area indicate that the volcanic terrain on this portion of the EPR crest and flank represents a patchwork of volcanic flows created by eruptive fissures located in or proximal to the ASCT, as well as eruptions localized at some distance from the axis. An abrupt increase in sediment cover on the lava surfaces at distances >1500 m from the ASCT suggests that the extent of recent ASCT eruptions may not have traveled more than ~1500 m from the eruptive vent.

The occurrence of sheet lava within an area dominated by lobate lava, and the distribution of these sheet flows largely around the axial high may indicate discrete volcanic episodes separate from, and at higher effusion rates, than those that produced the lobate flows (Figure 9). The rare occurrence of off-axis sheets may be evidence of eruptive volumes large enough to locally overflow and spill out of the ASCT. If this is true, then the lack of off-axis sheet flow exposure suggests that many of the eruptions that have occurred in this area for the past ~10ka have had similar volumes and effusion


rates, with little or no voluminous spill out from the ASCT.

There are relatively few comprehensive studies of MOR crest morphology; the Ballard et al. [1982a] study

of the Galapagos Rift and the Embley et al. [1990] study of the caldera on Axial Seamount are some of the best examples of comprehensive camera and submersible mapping of seafloor lava flows. While the Galapagos

Figure 4 Digital photographs showing examples of lava morphologies and surface texture, nature of the glassy crusts, and variability in sediment cover. Classifications and approximate dimensions are listed for each photograph; (a) Pillow lava (~4.5m x 3.0m). (b) Hackly lava (~4.0m x 2.5m). (c) Lobate lava (~4.0m x 2.5m). (d) Lineated sheet lava (~4.5m x 3.0m). (e) Ropy sheet lava (~4.5m x 3.0m). (f) Collapse feature (~4.5m x 3.0m, ~1m of relief). (g) A flow contact (~5.0m x 3.5m). (h) Heavily sediment-covered lobate flow with a small fissure (~4.5m x 3.0m).

study suggests a largely bimodal distribution of lava morphology between pillows and sheet lavas in the rift valley, the data for Axial caldera suggest that about 40%

of the flows are some form of sheet flow, 50-55% are lobate lavas, and the remaining flows are pillows. This distribution of lobate lavas is similar to what is found at


the 9° 49-52’N segment of the EPR, but with a lower proportion of sheet lavas. The mapped morphologic distributions reported here indicate that sheets are more abundant near-axis while pillows are generally more abundant at distances >~0.5km from the ASCT (Figure 9). This is also consistent with observations from Alvin in the 9° 30’N region of the EPR where pillow lavas are rare in the ASCT but comprise a larger percentage of the surficial lavas on the crestal plateau out to ~4 km [Perfit et al., 1995; Perfit and Chadwick, 1998].

Figure 5 Schematic of surface and subsurface factors effecting submarine basaltic lava morphology, listed in approximate order of importance from top to bottom. The upper diagrams indicate regions in which these factors occur. Lower chart describes general relationships between lava morphology and key physical and chemical properties as determined by empirical and theoretical modeling [Bonatti et al., 1988; Rowland and Walker, 1990; Griffiths et al., 1992; Hon et al., 1994; Gregg and Fink, 1995; Gregg et al., 1996, Gregg and Keszthelyi, 1998; Head et al., 1996].

Geochemistry of Lavas Petrological and geochemical data from dredged

rocks, rock cores, and Alvin samples collected from the EPR ~9° 30’ N [Perfit et al., 1994; 1995; 1996; Smith et al., 1997] and EPR ~9° 50’ N (Figure 6) provide information that can be used to estimate temperatures of eruption, the nature of the source (e.g. “normal” mid-ocean ridge basalt [N-MORB] vs. transitional [T-MORB] or enriched types [E-MORB]), the physical characteristics of the sub-axial magma lens, and the viscosity of the erupted lavas. In general, major element analyses of basaltic glasses from lavas recovered in the 9° 50’N region indicate that the most recent magmatic event or events associated with the present ASCT erupted relatively homogeneous and mafic (>8.25 weight percent {wt.%} MgO) basalts. Consequently the value of 8.25 wt. % is used to discriminate the regions most likely covered by recent eruptions (gray shaded region in Fig. 6) from older flows. Older off-axis lavas tend to be more chemically evolved than the aforementioned recent ASCT lavas (Figure 6).

The spatial distribution of MgO values and K2O/TiO2 ratios for basalt glasses serves to illustrate the distinction between lavas in axial and off-axis regions and demonstrates the asymmetry of lava compositions across the crestal plateau. Figure 6 shows the distribution of MgO content in lavas relative to the ASCT and the camera tow tracks. There is excellent agreement between the composition of lavas recovered by Alvin and the glasses sampled by rock corer from the same localities. The most MgO-rich lavas are more abundant east of the ASCT. The distribution of these mafic lavas outside of the ASCT is consistent with lava breakouts (both sheets and lobates) observed along the eastern flank of the ASCT in the 9° 50'N area [Fornari and Embley, 1995, Fornari et al., 1998b]. More evolved basalts (MgO <8.0 wt.%) are concentrated in a relatively broad area a few kilometers east of the axis and in an oval-shaped area south of 9° 50’ to the west of the ASCT. T- and E-type MORB exist in relatively small areas (<1 km2) on the crestal plateau and are commonly related to prominent scarps or fissures from which pillow lavas were erupted [Macdonald et al., 1996; Perfit et al., 1994; 1998].

Further examination of the distribution of MgO concentrations in lavas sampled throughout the crestal region from 9° 48’-55’N verifies that off-axis basalts are, on average, more fractionated than basalts recovered from within 500 hundred meters of the ASCT. Only 20% of axial lavas contain less than 8.25 wt.% MgO, whereas 87% of all off-axis lavas (recovered more than 500 m outside of the ASCT) contain less than 8.25 wt.% MgO. The average on-axis MgO=8.49 wt.%, the average off-axis MgO=7.73 wt.%.

The geochemical data distinguish not only differences in extents of fractionation between axial and off-axis lavas, but also chemical differences between different flow morphologies within a given region of the crestal plateau. An examination of major element data for off-axis lavas with known morphology and contact relations (Alvin samples) reveals that the average MgO content of pillow flows is 7.27 wt.%. Sheet and lobate flows from this same off-axis region average 7.85 wt.% MgO.

The viscosity of basaltic melts is controlled by their composition, temperature of eruption, crystal content and exolved gas (vesicle) content. MgO contents and mineral-liquid phase equilibria calculations [Weaver and Langmuir, 1990] indicate that the most primitive EPR basalts in this sample suite had eruption temperatures in excess of 1200°. In comparison, the most evolved off-axis basalts (MgO <7.0) have liquidus temperatures between approximately 1170°C and 1150°C and can contain significantly more phenocrysts. Using the extremes of composition and calculated eruptive temperatures observed in the entire sample suite, differences in viscosity were calculated based on the method of McBirney and Murase [1984]. Liquidus viscosities range from ~18 Pa-s in the most mafic


samples (1210°C, 0.1% H2O, no crystals) to ~40 Pa-s in the most evolved lavas (1150°C, 0.2% H2O, no crystals). True eruptive viscosities of the pillow basalts are likely to be higher than the model results because of their greater phenocryst abundance compared to sheet and lobate lavas. The maximum crystallinities observed in

glassy selvages of pillow lavas are less than 10% by volume and as such should not increase the effective viscosity by more than a factor of 10 [Dragoni, 1993]. Accordingly, total increase in viscosity between the most mafic and aphyric lavas and the most evolved crystal-rich lavas is no greater than a factor of 20.

Figure 6 MgO content of basaltic glasses recovered by Alvin (triangles) and rock core (circles) plotted relative to camera and Alvin tracks on the crestal plateau of the East Pacific Rise. The gray shaded region represents the extent of recent (<~30 ka) volcanic emplacement. The outline of the boundaries of the ASCT are shown by small black crosses. Note the asymmetric distribution of lava types and the extension of high-MgO lavas to the east of the ASCT, particularly in the 9°50’ area where a breakout of young-looking sheets and lobates was observed during dive #2771. Microprobe analyses represent 5-10 spot analyses per glass. The dividing MgO values between basalt chemical groups were set at 0.25 wt. % because the precision of microprobe analyses is better than that at the 95% confidence level and allows us to show the distribution of a reasonable number of basalt groups. Analytical precision is better than 2% of concentration at the 95% (2-sigma) confidence level.


Evaluating these viscosity differences using the models of Gregg and Fink [1995], it is unlikely that the calculated increase in viscosity alone could account for the greater abundance of pillow lavas observed off-axis as compared to locales more proximal to the ASCT. This suggests that the greater abundance of pillow lavas observed off-axis results from geologic factors other than lava chemistry or temperature. In this region of the EPR crest it is likely that pillow flows are formed primarily by low-effusion rate eruptions.

The hypothesis that geologic factors other than lava temperature and crystallinity (both related to extent of cooling) control the morphology of pillow flows located off-axis is supported by the observed tendency of these lavas to form short, ridge-parallel mounds and to have enriched incompatible trace element characteristics (e.g. high K/Ti). Between 9°48’ N and 9°55’ N, enriched-type basalts show a clear affinity to erupt as pillows; 63% of all pillow lava samples are enriched in trace elements whereas only 20% of the sheet and lobate flow samples have enriched characteristics. These observations are consistent with the model proposed by Perfit et al. (1994) in the 9° 30’ N region where E-MORB preferentially erupt off-axis thereby avoiding mixing with other melts in a subaxial magma lens.

Off-Axis Volcanism Regional mantle upwelling, a concentration of sub-

crustal magma along the axis, and focused eruptions at the seafloor produce an axial high at the center of this 4th order ridge segment (Figures 1 and 6). North and south

of the axial-high slopes become steeper with increasing distance from the inferred concentration of crustal magma. West and east of the axis the ridge becomes steeper with increasing distance across-axis. Contrary to this general trend of subsidence, two off-axis plateaus are present northeast and southeast of the axial summit high. The isolation and concentration of pillow lava along these off-axis benches (Figure 9) implies volcanic emplacement processes different from those that created the lobate lava which dominant the surrounding region. Small Bouger anomalies (~2 - 4 mGal) over areas of observed pillow ridges on the EPR summit plateau near 9° 31’N suggest that the pillow ridges represent localized volcanic construction along a fissure system fed from a dike [Cochran et al., 1996, 1999]. Combined with the geochemical data, the gravity and topographic data suggest that these pillow ridges represent small eruptive events which are temporally different from those occurring in the ASCT and that the effusion rate involved in these eruptions is lower than that which forms the wide expanses of lobate flows erupted throughout the EPR crestal region.

Figure 8 Histograms of percentages of lava morphologies for the entire surveyed region (Figure 1) from camera tow and Alvin dive data.

Off-Axis Transport of Lava The phototransects in Figures 10, 11, 12, and 13

show examples of lobate and sheet lava across the ridge observed at incremental distances from the ridge axis (Figure 9). The lobate lava depicted in Figures 10 and 11 (LOB-1 and LOB-2) show an increase in sediment cover and sediment infill between the lava forms, as well as an increase in surface roughness and decrease in lava glassiness. These trends in lava characteristics are interpreted to indicate that the age of the lava generally increases with increasing distance off-axis. However, the surface morphology and characteristics of flow SH-1 in Figure 12 and SED-1 in Figure 13 are an exception to this general trend. Phototransect SH-1 depicts similar surface texture and apparent lack of sediment cover on hackly sheet flows located in photographs taken 250 m, 460 m, 630 m, 800 m, 1 km, and 1.25 km east of the ASCT. Photographs of hackly sheet lava located 230 m and 1.25 km east of the axis are nearly identical in character and sediment cover. Phototransect SED-1 (Figure 13) shows a heavily sedimented pillow lava and a lightly sedimented hackly lava that are both ~1770 m east of the ASCT and separated by ~80 m along strike (Figure 9). Though the hackly sheet flows shown in SH-1 (Figure 12) and SED-1 (Figure 13) cannot be traced unequivocally from the axis to the flank, the regular off-axis progression of sheet lavas within this region and the character and inferred young age suggest these lava may be ~20-30 ka younger than the crust upon which it is

Figure 7 Lava morphology plotted as a function of local topographic slope as calculated from Alvin transects. Data show that all of the lava morphologies are seen over the 0°-4° range (99% of the region) of calculated slope angle, indicating that topographic slope is not the dominant controlling factor, but only one of many factors determining lava morphology. The histogram shows the distribution of slope angles for all three Alvin dives shown in Figure 1 based on a single slope value for each digital image collected. This distribution of slope angles indicates that the local gradient of the EPR crest at the 4th-order segment scale is ~1.6°.


emplaced. Chemical data (Figure 6) from Alvin rock samples collected on dive 2768 show a line of >8.5 wt.%

MgO lavas coinciding with this general region (Figure 6, M. Perfit and D. Fornari, unpublished data). Near

Figure 9 Camera tow and Alvin survey lines are color coded to morphology as in Figure 1 and plotted on Seabeam bathymetry contoured at 10 m intervals (see key at bottom of figure). ASCT is shown by black crosses along the ridge axis. Percentages were calculated from binned data representing lava morphologies contained in consecutive, along-axis 100 m segments for each survey line. Percentages of sheet, lobate, and pillow lava were color coded for each morphology and plotted versus latitude corresponding to the bathymetry map shown at left. The black lines labeled LOB-1, LOB-2, SED-1,and SH-1 represent phototransects shown in Figures 10, 11, 12, and 13.


Figure 10 Phototransect (LOB-1) over areas of lobate lava across-axis at ~9°50.5’N (see Figure 9 for locations) showing variations in lobate forms and sediment cover, lava surface texture and glassiness all of which suggest that the relative age of the lava flows is increasing with distance from the ASCT. The image ID, lava morphology, and distance from ASCT are indicated in the upper left corner of each image. As distance off-axis increases, sediment cover and infill increases, surface texture become rougher, and the glassiness of the lava decreases. The upper left image (a3-10-10:J081646) taken 250 m east of the ASCT and the lower right image (a3-34-9:I033750) taken ~1.76 km east of the ASCT illustrate these changes. The dimensions of these images range from ~4 m x 2.5 m to ~4.5 m x 3.0 m.


Figure 11 Phototransect (LOB-2) over areas of lobate lava across-axis at ~9°49.5’N (see Figure 9 for locations) showing variations in lobate forms and sediment cover, lava surface texture and glassiness. Again, the relative age of the lobate lava appears to increase with distance from the ASCT. Labels and dimensions are the same as those described for Figure 10.

bottom observations from the same Alvin dive east of the axis also document a what appears to be a young-looking sheet and lobate flow originating at the axis and flowing to the east from the ASCT.

Discussion and Conclusions This study represents one of the few systematic

photographic surveys of the accretionary zone on a fast-spreading MOR crest over the region where seismic layer 2A doubles in thickness [e.g. Christeson et al., 1994a, 1994b, 1996; Carbotte et al., 1997]. Data and analysis presented in this paper provide a quantitative assessment of surficial volcanic emplacement processes and document the percentage, distribution, and type of lava morphologies emplaced and the geochemistry of surficial lavas erupted over the past ~30-40 ka. Lobate lavas comprise 66% of the surficial morphologies and are

distributed uniformly along and across the axis. Sheet lavas comprise 20% of the volcanic terrain. Although off-axis sheet flows do appear to have traveled more than 1 km from their source near the ASCT, these lavas are principally concentrated in a zone <500 m from the ASCT. Pillow lavas comprise 10% of the terrain and is concentrated at sites >500 m from the EPR axis. We suggest that this portion of the EPR has not experienced any significant changes in either eruption volume or effusion rate during the past ~30ka based on the interpretation of the morphologic data and the distribution of different types of lava flows along and across the axis, and relative lava ages observed in the photographs.

Between 9° 49’N and 9° 52’N on the EPR crest three types of volcanic emplacement have occurred: (1) axial summit eruptions resulting in principally lobate


Figure 12 Phototransects (SH-1) demonstrating the apparent age of hackly sheet lava across the axis (see Figure 9 for locations). Phototransect SH-1 shows similar fresh, glassy surface texture and apparent lack of sediment cover on hackly sheet flows located 250 m, 460 m, 630 m, 800 m, 1 km, and 1.25 km east of the ASCT. Labels and dimensions are the same as those described for Figure 10.

and sheet flows within 500 m of the ASCT; (2) off-axis transport of lava erupted at or near the ASCT through channelized surface flows; and (3) off-axis eruptions and local constructional volcanism resulting in pillow ridges at distances between ~0.5 km and 1.5 km from the axis. These three volcanic emplacement processes result in a patchwork accumulation of lava flows over a neovolcanic zone that is at least 1.5 km wide, and could be as wide as 4 km [e.g. Perfit et al., 1994]. In addition, the abrupt increase in sediment cover on the lava surfaces at distances >1.5 km from the ASCT suggests that many of the recent flows erupted at the EPR axis only have sufficient volume to extend out to ~1.5 km from the eruptive vent.

These results are consistent with the observation that seismic layer 2A thickens to its maximum thickness over a ~2 km wide zone on either side of the EPR axis. Schouten et al. [1999] suggest that based on the width

and near-bottom character of the central anomaly magnetic high recorded at 9° 50’N, the zone of recent volcanism over which highly magnetic lavas are deposited extends out to ~1.5 km east of the axis and ~2 km west of the axis. Eruptions at the axis that get channeled repeatedly down the upper rise flanks and pond off-axis could result in the geometry of upper crustal thickening observed in the seismic data [Hooft et al., 1996]. Photographic data indicate that channelization of lava flows occurs, as evidenced by the distinct sheet flows photographed ~1.75 km from the ASCT that are inferred to be derived from near the axis. Channelization of lava in this area is also supported by high-resolution side-looking sonar data that show evidence for narrow (<50 m wide) channels extending away from the ASCT [Fornari et al., 1998b]. Observational data collected to date do not allow us to place further constraints on the extent or inferred volume


of axial or off-axis eruptions that contribute to the thickening of the extrusive crustal layer away from the axis. More detailed photographic and sonar surveys of large areas of the MOR crest are required in order to

Figure 13 Images in SED-1 demonstrate the contrast in sediment cover and character between a pillow lava and hackly sheet lava located ~1.77 km east of the ASCT and ~80 m apart (see Figure 9 for locations). The difference between these flows supports to the hypothesis that narrow channelized lavas reach at least as far as ~1.25 km from the EPR axis in this area. Labels and dimensions are the same as those described for Figure 10.

unequivocally identify the distribution of individual flows, where they erupt and the location of lava depocenters. Similarly, more detailed sampling of well-identified flows on the MOR crest and U-series dating of those flows will permit the geochronology of the lavas to be resolved so that the history of volcanism and magmatism along this region of the EPR can be better understood.

Acknowledgments We thank the coPI’s of the Adventure VII cruise, R. Lutz, M. Lilley,

T. Shank, K. Von Damm, the PI’s of the Extreme I cruise, C. Cary, G. Luther, A.L. Reysenbach, R. Lutz, and the officers and crew of R/V Atlantis for their cooperation and assistance in collecting the towed camera data. The shore-based and shipboard members of the Deep Submergence Group who operate Alvin provided excellent support of our in-situ sampling. J. Sinton and C. Morgan provided thoughtful reviews of early versions of the manuscript. We thank T. Gregg, R. Haymon, D. Smith, H. Schouten and M. Tivey for stimulating discussions on submarine volcanic processes. This work was supported by National Science Foundation

Grants: OCE9396238, OCE9403773 and OCE9711720 (DJF), OCE9402360 (MRP and MCS) and OCE9300508 (MHE). SOEST Contribution No. 4916, HIGP Contribution No. 1085, WHOI Contribution No. 10073.

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Kurras et al., Marine Geophy. Res., 21 (1):23-41, 2000 1 Volcanic … · Kurras et al., Marine Geophy. Res., 21 (1):23-41, 2000 2 Figure 1 3-D bathymetry model of the East Pacific