JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B9, 2181, doi:10.1029/2000JB000051, 2002

Internal structure of uppermost oceanic crust along the Western Blanco Transform Scarp: Implications for subaxial accretion and deformation at the Jeffrey A. Karson Division of Earth and Ocean Sciences, Duke University, Durham, North Carolina, USA

Maurice A. Tivey Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

John R. Delaney School of Oceanography, University of Washington, Seattle, Washington, USA Received 10 November 2000; revised 19 October 2001; accepted 7 November 2001; published 10 September 2002.

[1] The uppermost 2 km of oceanic crust created near the southern end of the Juan de Fuca Ridge (60 mm/yr, full spreading rate) is exposed in a ‘‘tectonic window’’ created along the north wall of the Blanco Transform . A total of 27 submersible dives transect this escarpment over a distance of >30 km, corresponding to 1 m.y. of spreading history. In this exposure, weakly deformed basaltic lavas grade downward into much more intensely fractured lavas cut by basaltic dikes. Lava flow surfaces dip persistently to the northwest (toward the spreading center) and the dikes dip southeast (away from the spreading center). The underlying sheeted dike complex is composed of subparallel dikes that dip moderately to the southeast. Discrete faults and more distributed cataclastic deformation zones separate the panels of sheeted dikes from one another. Disruption of the lava and dike units are interpreted as a result of dramatic, subaxial, vertical mass transport that created the ‘‘accommodation space’’ for the accumulation of a thickness of 1000–1500 m of basaltic lava. The observed patterns of tilting of lavas and dikes imply subsidence beneath the region of active volcanism (3 km wide) that is most rapid near the axis and diminishes quickly off-axis. Deformation that accommodates this subsidence has generated extensive fracture porosity in the subaxial region that could strongly influence upper crustal hydrothermal pathways, alteration history, seismic properties, and the distribution of subsurface biological communities. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 8010 Structural Geology: Fractures and faults; 8429 Volcanology: Lava rheology and morphology; 3640 Mineralogy and Petrology: Igneous petrology; 3045 Marine Geology and Geophysics: Seafloor morphology and bottom photography; KEYWORDS: oceanic crust, mid-ocean ridges, seafloor spreading, faulting, lavas, sheeted dikes Citation: Karson, J. A., M. A. Tivey, and J. R. Delaney, Internal structure of uppermost oceanic crust along the Western Blanco Transform Scarp: Implications for subaxial accretion and deformation at the Juan de Fuca Ridge, J. Geophys. Res., 107(B9), 2181, doi:10.1029/2000JB000051, 2002.

1. Introduction [3] Major tectonic escarpments on the ocean floor occur along spreading centers, transform faults, propagating rifts, [2] The geology of the oceanic crust is the end product of etc. and provide ‘‘tectonic windows’’ into the internal the interplay among magmatic accretion, tectonic deforma- structure and composition of the oceanic crust. Although tion, and hydrothermal alteration beneath mid-ocean ridge these exposures are complicated to some extent by faulting spreading centers, a region that cannot be directly observed. related to the origin of the escarpments, they provide Surface geology and geophysical studies provide important important opportunities for documenting structural and constraints on subaxial processes, but many questions compositional relationships in the oceanic crust [Karson, remain because of the nonuniqueness of results or the scale 1998]. Exploiting tectonic windows complements deep and limited depth range of resolution of these investigations. crustal drilling and also provides a two- or three-dimensional view of geological relationships. In many tectonic windows, Copyright 2002 by the American Geophysical Union. extensive outcrops along escarpments several tens of kilo- 0148-0227/02/2000JB000051$09.00 meters in length reveal crustal structures generated at mid-

EPM 1 - 1 EPM 1 - 2 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST ocean ridge spreading centers over tens of thousands to a few mm/yr [Delaney et al., 1981; Riddihough, 1984]. The million years. lithosphere along the north wall has a complicated history [4] The upper 2 km of oceanic crust is generally consid- involving transform displacement punctuated by ridge prop- ered to be a carapace of fine-grained, basaltic material that agation events revealed by complex patterns of magnetic formed above subaxial magma chambers. It consists of two anomalies and bathymetry across the transform and inter- distinctly different units that reflect different styles of secting pseudofaults. The western 50 km of the transform accretion at different crustal depths. The uppermost unit is (Western Blanco Trough), between the JdFR and a prom- between 500 and 1000 m thick and consists of basaltic lavas. inent pseudofault, is essentially free of these complications The lavas overlie a sheeted dike complex 1000 m thick [Delaney et al., 1981; Juteau et al.,1995].Prominent [Becker et al., 1988; Altetal., 1993] consisting of a abyssal hill lineaments north of the are continuous array of ridge-parallel, vertical, intrusive basaltic sharply truncated by the transform valley wall, suggesting sheets. Each dike is 1 m wide and represents a discrete that transform-related tectonic effects may be slight along increment of seafloor spreading accommodated by mag- the valley wall. The transform may have recently cut into matic accretion [Moores and Vine, 1971; Cann, 1974; the north wall [Embley and Wilson, 1992] thereby exposing Delaney et al., 1998]. These two upper units overlie an oceanic crust originally formed somewhat to the north of the extensive volume of coarse-grained gabbroic rock that Cleft/Blanco ridge-transform intersection (RTI). crystallized from subaxial magma bodies and makes up the [7] The northern wall of the transform valley from 20 to remaining 3–4 km of the oceanic crust [Sinton and Detrick, 50 km southeast of the JdFR is a linear escarpment, 1992]. This geological structure implies a continuous and typically with >2500 m of relief [Embley and Wilson, rather passive mode of crustal accretion upon which crustal 1992]. Slopes are extremely steep, commonly >50°, com- faulting is superimposed as the crust spreads laterally away pared with many other seafloor escarpments that lie in the from the axis. It is widely believed that seafloor spreading, at range of 30° to 40°. In detail, the northern wall is stair- least at intermediate to fast rates (and relatively high magma stepped with major cliff faces providing continuous out- budgets), involves little or no substantial tectonic deforma- crops up to hundreds of meters high. These are separated by tion. Surface investigations and geophysical studies limit the terraces usually no more than a few tens of meters wide that region of active volcanism to a very narrow region only a are covered with loose rock debris, talus, and pelagic ooze. few kilometers wide centered on the spreading axis [Perfit et [8] The Blanco Transform Fault is essentially orthogonal al., 1994; Macdonald, 1998; Perfit and Chadwick, 1998]. to the JdFR; therefore the crustal section exposed there is The most active volcanism is strongly concentrated near the oriented at right angles to the strike of dikes, lava flows, and axial summit graben or median valley, with minor eruptions faults expected to strike more or less parallel to the ridge also occurring within a few kilometers of the axis [Perfit et axis. The steep slopes and extensive exposures make this an al., 1994]. Thus the bulk of magmatic construction appears excellent natural cross section through the uppermost part of to take place in a very limited area not much more than 1 km the oceanic crust. However, because the crust exposed there to either side of the ridge axis. was generated near the southern end of the JdFR, near a RTI [5] In this paper, we review the geology of uppermost (or ‘‘inside corner’’), it may have been formed under oceanic crust generated at an intermediate spreading rate of conditions of relatively low magma supply [Detrick and 60 mm/yr at the Juan de Fuca Ridge that is exposed along Purdy, 1980; Bender et al., 1984; Fox and Gallo, 1984; the north wall of the Western Blanco Transform Fault. We Karson and Elthon, 1987]. About 60 km to the east of the augment initial reports with new structural data derived from intersection with the JdFR, intersecting pseudofaults are an extensive review of video images collected during two associated with compositional variations associated with major submersible programs. Our results provide additional ridge propagation events [Juteau et al., 1995]. Other detail and texture to the geology of the uppermost crust in structural complications may arise from transform-related this area and reveal a crustal structure that differs substan- strike-slip or normal faulting [Francheteau et al., 1976]. tially from expectations based on indirect information and Notwithstanding these caveats, the north wall of the Blanco ophiolite analogs. We find evidence of widespread brittle Transform Fault is one of the only tectonic windows into deformation and block rotations that accompanied magmatic intermediate-rate crust, and its geological structure is likely construction as well as evidence for temporal variations in to shed light on the processes beneath the JdFR. accretion. The processes attending seafloor spreading implied by our results would be difficult to determine from surface studies alone and provide a new framework within 3. Previous Work which to consider active processes on the Juan de Fuca [9] The north wall of the West Blanco Trough or Blanco Ridge and other intermediate-rate spreading centers. Transform scarp (Figure 2) has been the site of a number of studies aimed at documenting the geology and geophysics of the crust exposed in this tectonic window [Delaney et al., 2. Setting 1987; Juteau et al., 1995; Tivey, 1996; Naidoo, 1998; Tivey [6] The Blanco Transform Fault is a 360-km-long, left- et al., 1998b]. stepping (dextrally slipping) transform fault that links the [10] A high-resolution, deep-tow side-scan sonar image Gorda Ridge to the south and the Cleft Segment of the Juan provides a detailed backscatter map of a portion of this area de Fuca Ridge (JdFR) to the north (Figure 1) [McManus, [Delaney et al., 1987] (Figure 2). This image was used to 1967; Embley and Wilson, 1992]. Lithosphere along the guide a series of Nautile dives in 1991 (Blanconaute north wall of the Blanco Transform Fault was created near Program) [Juteau et al., 1995]. This program provided the southern end of the JdFR at a spreading half-rate of 30 ground data for the backscatter map, generated a composite KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 3

Figure 1. Bathymetric map (100-m contour interval) of Western Blanco Trough. Inset shows location and plate boundary setting [Tivey, 1996] with small box outlining main map area. Box outlines the submersible study area (Figure 2) on the north wall of the Trough between the Juan de Fuca Ridge and pseudofault to the east. view of the exposed upper crust, and obtained an extensive provided new observations that challenge the traditional suite of samples. In addition, near-bottom magnetic data view of oceanic crust on the basis of ophiolite analogs documented the form of magnetic anomalies with depth on [Nicolas, 1989 and references therein], inferences from the scarp face [Tivey, 1996; Tivey et al., 1998b]. An seismic studies [Bratt and Purdy, 1984; Vera et al., 1990; extensive compilation of available geological and petrolog- Christeson et al., 1994], and limited deep crustal drilling ical data is given by Naidoo [1998]. [Becker et al., 1988; Alt et al., 1993]. Lessons from large- [11] The 1995 Alvin dives (Blancovin Program) filled a scale digital mosaics acquired on steep cliffs at the Hess gap between the Nautile dives with a series of upslope Deep Rift [Karson et al., 2002] in particular motivated us to transects that further documented the geology and magnetic reexamine the existing dive data from the Blanco Transform anomaly structure of the upper crust (M. A. Tivey, Woods scarp for key structural features such as the orientations and Hole Oceanographic Institution, unpublished data, 1995). spatial relations of lava flows, dikes, and faults. The Hess The combined dive programs cover just over 30 km along Deep data show that these features commonly occur in the northern wall of the transform valley (Figure 3), orientations and on scales that would make them difficult to corresponding to 1.0 m.y. of crustal accretion at the JdFR anticipate or discern from the limited views available from from 20 km (crustal age 0.66 m.y.) to 50 km (1.66 m.y.) submersibles. west of the JdFR. [13] In this study, we reviewed video images from 1991 Blanconaute and 1995 Blancovin dives from the north 4. A New Look at Blanco Geology—Methods wall of the Blanco Transform scarp (Figure 4). These included a total of 27 dives and 120 hours of video and Approach coverage. In addition to building on the initial descriptions [12] Recent results from the Hess Deep Rift and other of these transects by previous workers, we constructed tectonic windows into the uppermost oceanic crust have numerous digital mosaics from video frame grabs using EPM 1 - 4 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST

Figure 2. Bathymetric map of the north wall of the Blanco Transform scarp (contour interval 50 m). Submersible tracks (bold lines) are from the Blanconaute (BN-Nautile) (T. Juteau, unpublished data, 1991) and Blancovin (BV- Alvin) (M. A. Tivey, Woods Hole Oceanographic Institute, unpublished data, 1995) dive programs. Box shows area of detailed side-scan sonar data [Delaney et al., 1987; Naidoo, 1998].

Adobe Photoshop1. These provide much larger views of structure along the scarp that imply temporal variations seafloor outcrops than are available instantaneously from in accretion. submersibles and provide a significantly broader context in which to view geological details. They commonly reveal important relationships that can be difficult or 5. Major Rock Units and Structures impossible to determine when moving rapidly over com- 5.1. Rock Units and Thicknesses plex seafloor outcrops. The dive transects were not [16] In this section, we describe the internal structure of designed to acquire video coverage most appropriate for the major rock units exposed in the study area. We defined the construction of digital mosaics, but many areas were rock units primarily on the basis of their outcrop appear- well covered during sampling or observing features of ance. Joint patterns, contacts of lava flows, and discordant special interest. dikes proved to be the most useful features. We outline [14] Cliff faces theoretically provide only apparent dips these characteristics for each unit in section 5.2. The of inclined planar features. These inclinations are a func- basement units we recognized include (1) upper basaltic tion of the true orientation of those features (always lavas, (2) lower basaltic lavas, (3) sheeted dike complex, steeper than the apparent dip) and the surfaces along and (4) massive (gabbroic?) rocks. From map and cross- which they are exposed. However, views of cliff faces sectional data for each dive we have constructed simplified along the Blanco Transform scarp commonly have suffi- columnar sections that represent the crustal structure along cient three-dimensional relief to permit good estimates of each dive transect (Figure 4). Our definitions of units true dips. Multiple apparent dips were also used locally to differ somewhat from those used by Juteau et al. [1995] calculate true dips. The strike of planar features is more and Naidoo [1998], as noted in section 5.1. In addition, we difficult to estimate. This is done using the azimuthal describe substantial exposures of sedimentary rocks and orientation displayed on the video images and is probably indurated colluvium that locally overlie the basement rock no better than ±10°. units. We describe deformation structures separately [15] Major results discussed below include documenta- because they generally are not confined to specific rock tion of (1) internal structures reflecting tilting and defor- units. Figures 5 and 6 summarize the general internal mation of major rock units, (2) local variations in the structure of the upper crustal units and structural features internal structure and thickness of identifiable rock units, within it. (3) widespread accumulations of sedimentary breccias and [17] The thicknesses of the units should be considered in colluvium, and (4) significant variations in the crustal light of several possible sources of error that are difficult to KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 5

assess quantitatively. The most important of these is prob- ably transform-parallel normal faulting along the scarp that would tend to increase the vertical outcrop extent of a given horizontal unit [Francheteau et al., 1976]. In addition, normal faulting related to spreading could tend to decrease the thickness of rock units by tectonic extension and rotation [Wernicke and Burchfiel,1982].Finally,trans- form-related strike-slip faults obliquely intersecting the dive transects could modify the internal structures and apparent thicknesses of units with nonhorizontal contacts. On the basis of inspection of side-scan sonar images [Delaney et al., 1987] and the dive data we believe that the effects of postaccretion faulting are minimal, at least for the upper part of the north wall examined in this study. [18] The gradational nature of rock unit contacts and the limited view from submersibles also hamper thickness estimates. A local, high-level dike swarm, for example, might determine the position of the lava/sheeted dike contact along a single dive track but will not necessarily reflect the crustal structure on a broader scale, perhaps only meters away. Locally intense fracturing also makes it very difficult to confidently assign some outcrop areas to appro- priate units. [19] In any case, the unit thicknesses should be consid- ered as only rough estimates with uncertainties of at least ±10%. Nevertheless, even these approximate thicknesses provide important constraints on the internal structure of oceanic crust of the JdFR and by inference, subaxial processes at this type of spreading center. 5.2. Upper Crustal Rock Units of the North Wall 5.2.1. Upper (Undeformed) Basaltic Lavas [20] The uppermost unit found on all dives that reached the top of the scarp consists of undeformed or weakly fractured basaltic lavas (Figures 7a and 7b). These vary in thickness from as little as 100 m to as much as 700 m but, for most transects, fall within the range of 200–500 m. In some dives the steep upper part of the scarp has apparently been degraded by downslope movement of the densely jointed basaltic material. On several dives the upper lavas merge smoothly with the hummocky, variably sedimented, constructional volcanic surface to the north. In cases where dive transects do not reach the top of the unit the thickness can be estimated by assuming that it continues to the top of the scarp. The top of the scarp is marked by a thin (2m thick), discontinuous layer of crudely bedded sedimentary rock. Bedding follows local basement topography but is overall horizontal and essentially undeformed except by local slumping. [21] This unit corresponds in part with the ‘‘volcanic unit’’ of Juteau et al. [1995]. Naidoo [1998] subdivided this unit into an upper pillow lava unit and underlying

Figure 3. (opposite) Generalized cross section (no vertical exaggeration) along the Blanco Transform scarp from Nautile and Alvin dive data. Magnetic anomaly traces and extrapolation to lava/dike contact beneath rubble from Tivey et al. [1998a]. This crustal section was generated at the southern end of the Juan de Fuca Ridge at a half-spreading rate of 30 mm/yr and represents 1.0 m.y. of spreading history. Average crustal age is 1.2 m.y. EPM 1 - 6 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST

Figure 4. (a) Geological strip map and (b) cross section for a typical dive on the Blanco Transform scarp (BN01); see Figure 2 for location. (c) Columnar section is constructed from these data with the assumption that structures are continuous normal the trend of the north wall. pillow lava plus dike unit. Our designation is essentially the Lobate and sheet flows have distinctive molded to cuspate same as the upper lava layer identified in the Blancovin bases and flat to gently undulating tops. The orientations of program (M. A. Tivey, Woods Hole Oceanographic Insti- flow surfaces in the upper part of the unit are highly tution, unpublished data, 1995). inconsistent but are typically within 20° of horizontal. [22] Throughout this unit lava flow, structures are well Through most of the unit, however, the lava flows, includ- preserved and can in most cases be distinguished unambig- ing interlayered sheet flows, dip persistently to the north- uously. Pillow morphologies with distinctive radial fracture west, locally as steeply as 40° (Figure 6a), and thus appear patterns in cross section (Figures 7a and 7d) are by far the to have been tilted (Figure 7). East dipping flows are very most common forms observed. Longitudinal sections uncommon except near the top of the scarp. through tongue-like lobate flows are widely preserved, [24] Dikes are rare (<5%) in this unit (Figure 6b). Where and sheet flows (Figure 7b) are sparse. Overall, the lava present, they tend to be sinuous, steeply dipping, and pile appears to be well compacted, but substantial porosity striking northeast, that is parallel to the trend of JdFR and remains. Cross sections of lava flows are exposed on cliff magnetic anomalies in adjacent crust. No sills were found in faces, but many slopes have been degraded by the removal this unit. of material along flow boundaries, leaving three-dimen- 5.2.2. Lower (Deformed) Basaltic Lavas sional constructional surfaces. [25] Most of the basaltic lavas are assigned to this unit. It [23] Lava flow surfaces are defined by flattened or consists of mostly intensely fractured basaltic lavas with collapsed pillows as well as elongate, lobate forms and dispersed outcrops in which flow morphologies are clearly sheet flows and can be laterally traced for several meters. preserved (Figures 7c and 7d). The thickness of the unit

Figure 5. (opposite) Generalized columnar section for upper crustal structure exposed on Blanco Transform scarp. This diagram summarizes observed relationships and does not necessarily represent any particular part of the outcrop area. Essentially undeformed lavas cover much more intensely fractured lavas, dikes, and probably some gabbro. Flow surfaces in the lavas and dikes have apparently been rotated from their original orientations to NW (inward) and SE (outward) dipping orientations, respectively. Rotations and fracturing may be the result of vertical mass transport that accommodated the thickening of the lava units. KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 7 EPM 1 - 8 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST

Figure 6. Histograms of apparent dips of lavas and dikes in upper crustal units of the Blanco Transform scarp. Bold line represents the mean; dashed lines represent ±1 standard deviation; N represents the number of observations. (a and b) Lava flow surfaces and dikes in the upper basaltic lavas. (c and d) Lava flow surfaces and dikes in the lower basaltic lavas. (e) Dikes in the sheeted dike complex. (f ) Lava flows from both units. (g) Dikes in all units. varies from 300 to 900 m, but rubble and talus in many places to the northwest. Dip angles are as high as 80° but hide the base of the unit. The typical thickness is >500 m. commonly lie in the range of 40°–50° (Figure 6c). In some [26] As in the upper unit, pillow morphologies appear to areas, sheet flows are interlayered with pillow and lobate dominate over subordinate lobate flows and rare sheet flows, indicating that these surfaces probably initially flows. In the lower lavas, flow surfaces dip ubiquitously formed in a more or less horizontal orientation. KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 9

Figure 7. Images of the basaltic lava units exposed on cliff faces of the north wall of the Blanco Transform scarp. All views are to the north of steep cliff faces. Horizontal dimension of individual images is 3 m. Mosaics of multiple images have been constructed to show larger-scale relationships. Insets show key features. See Figure 12 for locations. (a) Upper basaltic lava unit with relatively undeformed, flattened, pillow and lobate lavas with flow surfaces dipping 50° W. Scarp to E is along an east dipping dike margin. Slope to west parallels flow surfaces and has a dusting of pelagic ooze. Digital mosaic from video is from dive BN06 (2780 m). (b) Upper basaltic lava unit with sheet and lobate flows. Flow surfaces dip 35°W. Digital mosaic from video is from dive BN18 (2500 m). (c) Lower basaltic lava unit with densely fractured and compacted pillow and lobate lavas with flow surfaces dipping 45°W. Digital mosaic from video is from dive 2953 (3400 m). (d) Upper basaltic lava unit with moderately fractured pillow and lobate lavas with flattening surfaces dipping 45°W. Digital mosaic from video is from dive 2959 (2600 m). EPM 1 - 10 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST

Figure 7. (continued)

[27] Overall, this unit is characterized by closely spaced is so great that no unambiguous flow morphologies can be nonsystematic fractures, with spacing in the range of a few discerned. Minor fractures and small fault zones with offsets centimeters to 20 cm. All of the morphologies are variably of tens of centimeters to a few meters are also common. fractured and in some places intensely shattered. In many [28] Crosscutting dikes (Figure 8a) are very common cases, contractional cooling joints have been opened by (estimated at 10–20% for the unit as a whole) and occur later deformation. In some outcrops the density of fracturing locally in dense swarms. The dikes dip to the southeast, KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 11

Figure 8. Images of the sheeted dike unit exposed on cliff faces of the Blanco Transform scarp. All views to north of steep cliff faces unless otherwise indicated. Horizontal dimension of individual images is 3 m. (a) Massive, jointed (late?) dikes in shattered basaltic lavas. Dikes dip 60°SE. Scarp to E is along a dike margin. Digital mosaic from video is from dive BN04 (3119 m). (b) Moderately fractured sheeted dikes dipping 40°SE. This relatively old exposure has pelagic ooze on gently sloping surfaces, numerous sponges, and a cap of indurated rubble. Digital mosaic from video is from dive BN02 (3990 m). (c) Massive, undeformed, sheeted dikes dipping only 30°SE (apparent dip is less than true dip determined from other views). Digital mosaic from video is from dive BN02 (3902 m). (d) Fractured sheeted dike complex dipping 45°SE. Less fractured dikes are separated by and grade into fault zones that are subparallel to the dike margins. Smooth pelagic ooze covers talus at base of outcrop. Digital mosaic from video is from dive BN23 (3695 m). typically 40°–50° (Figure 6d). In many outcrops, south- smoothly weathering sheets of indurated colluvium that east dipping dikes are oriented at a high angle to lava drape the basement outcrops. flows that they cut (Figure 8a). Swarms of steeply dipping 5.2.3. Sheeted dike complex dikes cutting shattered and northwest dipping lavas were [31] A sheeted dike complex, similar to that described found in a few places. These appear to be less fractured from ophiolite complexes [Moores and Vine, 1971; Casey than nearby tilted dikes and may be relatively late intru- et al., 1981; Pallister, 1981] and other seafloor exposures sions. No tabular features could be unambiguously iden- [Auzende et al., 1989; Karson, 1998], was not found in tified as sills. initial investigations of this area. Instead, a ‘‘massive [29] Like the lavas, the dikes are also mostly densely diabase’’ unit was recognized, with sheeted dikes and sills fractured. Cooling joints and margins appear to have been reported in a few restricted areas [Juteau et al., 1995; exploited by opening and/or minor shear displacements. In Naidoo, 1998]. Our reexamination of video coverage and many places, the lavas are intensely deformed along cata- samples and experience from later investigations, however, clastic fault zones and arrays of shear fractures that are allowed us to recognize and map several areas of sheeted roughly parallel to the dikes (dipping southeast 40°–50°). dikes (>90% dikes) that we define as a discrete rock unit. [30] This unit corresponds in part to the ‘‘transition The awareness that dikes need not be vertical in some zone’’ between the volcanic and diabase units of Juteau regions of the oceanic crust, for example at the Hess Deep [1995]. In the Blanconaute dive area, Naidoo [1998] Rift [Karson et al.,1992;Karson et al.,2002],was recognized two subunits in this depth interval consisting essential in recognizing this unit (Figures 8b–8d). of thick sheet flows, sills, and dikes above and thin sheet [32] The thickness of the sheeted dike unit is rather flows and dikes below. These massive units and walls poorly constrained because its basal contact with massive interpreted as southwest dipping dikes appear to be (gabbroic?) rocks is indistinct (see section 5.1) and in EPM 1 - 12 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST

Figure 8. (continued) many places is obscured by rubble and talus. It crops out with crustal depth. There is a tendency for the dikes to at depths between 3000 and 4000 m and appears to be become steeper and somewhat less fractured and hence 300–700 m thick with the most complete-looking sections much more easily identified toward the upper part of the 500 m thick. unit and into the overlying lavas. Steeply dipping or cross- [33] Dikes in this setting are recognized by their contin- cutting dikes are present in a few outcrops and suggest uous, planar, systematic margins spaced at 0.5 to 2 m injections that postdated tilting of somewhat older dike (Figures 8b–8d). Chilled dike margin textures are not swarms. visible at this scale and are also obscured by ferromanga- [36] The panels of subparallel dikes grade into intensely nese hydroxide, which coats most rock surfaces. The dike fractured material or discrete fault zones (Figure 8d). The margins bound massive tabular rock with planar, systematic intensely fractured material commonly consists of dense joint patterns interpreted as the traces of columnar cooling arrays of anastomosing fractures that diminish laterally into joints. Joint spacings correlate roughly with dike width as in more massive, sparsely jointed dike rock. Bands of fault many subaerial outcrops. Samples of holocrystalline basal- gouge, cataclasite, and fault breccia occur locally. Networks tic material with subophitic to diabasic textures support of white hydrothermal veins commonly cut these areas. In these interpretations [Juteau et al., 1995; Naidoo, 1998]. some places the veins follow dike margins and columnar [34] The sheeted dike complex exposed on the Blanco joints in adjacent dikes. Transform wall is composed of dikes that dip persistently to 5.2.4. Gabbroic Rocks the southeast (Figure 6e), that is, away from the JdFR where [37] In the initial investigations of the area, no contin- they were intruded. If they were initially intruded vertically, uous gabbroic unit was recognized, though areas of their current orientation implies postintrusion tectonic rota- massive rock outcrops were reported and several coarse- tion. grained gabbroic rock samples were collected nearby (T. [35] Throughout the unit, dikes occur in subparallel Juteau, unpublished data, 1991). Upon further examina- clusters, or panels. They strike at a high angle to the wall, tion, we reinterpret some areas previously described as that is, to the north-northeast [see also Naidoo, 1998]. In massive rock as indurated colluvium or sedimentary brec- many places dips of dikes are as low as 30° (Figure 8c), but cias (see section 5.2.5). However, there are several areas throughout most of the unit dikes dip in the range of 30°– that consist of very massive, sparsely jointed basement 50° southeast (Figure 6e). The dips of dikes in nearby rock that are likely to be outcrops of gabbroic material panels are commonly highly variable (commonly by 10°– (Figure 9). The thickness of the massive gabbroic rock 20°), and there is no consistent systematic change in dip unit is unknown because its base is not exposed in the KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 13

Figure 9. Images of massive unit interpreted as gabbroic rocks exposed on cliff faces of the Blanco Transform scarp. All views to the north of steep cliff faces unless otherwise indicated. Horizontal dimension of individual images is 3 m. (a) Massive rock with widely spaced, planar, orthogonal joint pattern. Fault zone dips 20°E and has dense white veins. Digital mosaic from video is from dive BN05 (3863 m). (b) Weakly fractured dike with columnar joints dips 50°E in massive host rock (probably gabbro) with widely spaced joints. A steeply NW dipping fault zone cuts massive material and is offset where it crosses the dike. It is unclear if the dike postdates the fault or causes a slight deviation in its trace. Digital mosaic from video is from dive BN23 (3863 m). EPM 1 - 14 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST area. The largest exposures have vertical dimensions of formed near the JdFR axis where high-temperature hydro- 300–400 m. thermal mineralization could occur. 5.2.5. Sedimentary Breccias and Related Rocks 5.3.3. Strike-Slip and Oblique-Slip Faults [38] Previous reports noted extensive areas characterized [44] Several steeply dipping fault zones with northwest to by massive, uniform-looking material in steep cliff faces, north-northwest trending orientations have low-pitch slick- especially along the lower slopes of the north wall (T. enlines, striae, and grooves, with kinematic indicators show- Juteau, unpublished data, 1991). Careful reexamination of ing strike-slip to normal, oblique-slip (Figures 11a and 11d). these outcrops and adjacent areas shows that many of these Both dextral and sinistral faults are present. These faults surfaces belong to discontinuous layers of sedimentary affect both basement rocks as well as the sedimentary rocks. breccias and related clastic rocks (Figure 10). We interpret Slip on them is likely related to transform fault movements. these as the consolidated products of mass wasting (collu- vium) that have accumulated on the lower slopes and that lie nonconformably over the basement rock units. 6. Discussion [39] The thickness of these sedimentary rocks is not [45] Synthesizing the results of the Blanconaute and known, but steep scarps reveal local thicknesses commonly Blancovin dive programs in the context of available bathy- in the range of 1–4 m. Near-vertical scarps up to several metric and side-scan sonar studies along the north wall of tens of meters high as well as planar joints and discrete fault the Blanco Transform Fault provides a view of the upper- zones in this material demonstrate that it is very well most oceanic crust generated at the JdFR axis 1 m.y. ago. indurated. Fresh exposures of this material show that it The internal geologic structure of this crustal section implies consists of both framework- and matrix-supported breccias subaxial processes that are likely to continue beneath the as well as finer-grained clastic material. Samples apparently Cleft Segment and similar spreading centers today. pulled from these deposits include mostly cobbles of [46] Overall, the upper crustal units at the Blanco Trans- basaltic material with a range of grain sizes [Naidoo, 1998]. form scarp include essentially undeformed basaltic lavas grading downward into progressively more fractured, 5.3. Deformation Structures inward dipping lavas and outward dipping dikes. Under- [40] A number of distinct types of deformation structures lying sheeted dikes occur in panels of fractured, subparallel occur in the study area. We describe these separately because dikes that also dip outward. Underlying, weakly fractured they occur in more than one of the units described in section gabbroic rocks crop out only locally. The continuous down- 5.2 and probably cut across the contacts that separate. ward progression of structures and crosscutting relations 5.3.1. Distributed Deformation Zones strongly suggest that this entire assemblage was created by a [41] Regions of distributed brittle deformation with dis- more or less continuous accretionary process. placements of tens of centimeters to a few meters are very [47] Assuming that the uppermost oceanic crust exposed common, especially in both lava units and the sheeted dike along the Blanco Transform scarp was generated at a unit. These consist of swaths of closely spaced nonsyste- spreading center similar to the present-day Cleft Segment matic fractures that tend to follow the edges of massive rock of the JdFR, then the Upper Basaltic Lavas probably units such as sheet flows and dikes but can also be found to represent the last magmatic additions to the oceanic crust. cut across entire outcrops without regard for preexisting Judging from the distribution of young-looking lavas along structures. Adjacent material shows that contractional cool- the JdFR, they may include material erupted both inside and ing joints and minor fractures have been reactivated by outside the limits of the axial valley [Delaney et al., 1981; opening or minor shear displacements especially in areas of Kappel and Normark, 1987; Applegate, 1990; Embley et al., pronounced tilting (Figure 8d). No dikes were found to cut 1991; Embley and Chadwick, 1994]. If these undeformed these deformation zones, but in the upper lava unit, shat- lavas were erupted at or very near the JdFR axis, then the tered material is commonly interspersed with essentially underlying lava and sheeted dike units, as well as structures undeformed lavas, suggesting that this fracturing occurred inherent to them, must have been formed essentially beneath during accretion. the median valley floor. The decrease of tilting upward in 5.3.2. Discrete Fault Zones the lava units requires that subsidence and lava accumu- [42] Discrete fault zones on the order of 1–2 m wide lation diminished over a lateral distance that is less than that occur in all of the rock units (Figures 11a–11d). Steep of the width of the region of active volcanic construction. At southeast and northwest dipping normal fault zones are the JdFR axis today this essentially corresponds to the especially well exposed in the lava and dike units. They median valley or axial rise, an area only 3 km wide. create walls exposing smooth surfaces of fault gouge or The relatively steep dips (40° even in the upper basaltic fault breccia with steeply plunging striations or fault mul- lava unit) suggest that most of the tilting and thickening lions (Figures 11a and 11b). The fault rocks occur in layers occurred essentially beneath the median valley where all but up to a few tens of centimeters thick and grade into relatively minor off-axis lavas are erupted. The intense essentially coherent pillow lavas and/or dike rocks on either fracturing in the lavas and dikes, especially in the dike- side (Figures 11a and 11b). We interpret these as inward and parallel fault zones, could help accommodate the required outward dipping faults related to seafloor spreading. tectonic rotation. [43] In the massive gabbro and sheeted dike units, dis- [48] Overlying undeformed lavas and gradational relation- crete faults tend to dip gently to the southeast, subparallel to ships between the lava units resemble growth strata in fault- the dike margins. They are marked by anastomosing arrays controlled sedimentary rift basins. In rift basins, flat-lying of fractures and commonly include networks of white bedding in surface units commonly overlie deeper units in hydrothermal veins (Figure 11c). Thus they appear to have which bedding is progressively tilted and faulted downward KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 15

Figure 10. Images of indurated sedimentary material and colluvium exposed on cliff faces of the Blanco Transform scarp. All views are to the north of steep cliff faces unless otherwise indicated. Horizontal dimension of individual images is 3 m. (a) Low outcrop of rubbly, indurated colluvium over bedrock. Note lumpy, rounded form of outcrop contrasting with the sharp, angular character of most basement rock outcrops. Single video frame grab is from dive BN06 (3204 m). (b) Medium-bedded sedimentary material dipping moderately to the south over basement rock. Sheet failure is controlled by bedding surfaces and joints. Single video frame grab is from dive BN06 (3277 m). (c) Steeply south dipping fault zone cuts crudely bedded sedimentary breccias. Note that bedding is much more gently dipping in the hanging wall (hw-right) than in the footwall (fw-left). Digital mosaic from video is from dive BN10 (3331 m). (d) Nonconformity (dashed line) separates angular, jointed basement rock from overlying sedimentary breccia, partially removed by mass wasting. The sedimentary breccia is composed of basaltic clasts in a friable matrix of unidentified material (probably carbonate). It is massive, heterogeneous, and has a lumpy surface. The smooth area to the right is a continuous dip-slope (cliff face parallel to bedding). Digital mosaic from video is from dive BN24 (3976 m). EPM 1 - 16 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST

Figure 11. Images of deformation structures exposed on cliff faces of the Blanco Transform scarp. All views are to the north of steep cliff faces unless otherwise indicated. Horizontal dimension of individual images is 3 m. (a) Steep, SE dipping fault zone with downdip striae cuts basaltic lavas. Moderately fractured pillow lavas crop out in the footwall (left) and west dipping lobate flows in the hanging wall (far right). Fault breccia and gouge form a smooth, steep wall where the hanging wall has been removed. Digital mosaic from video is from dive BN24 (2473 m). (b) Steep, NE-SW fault surface in west dipping pillow lavas with low-plunge striae indicating oblique (left-lateral, normal) slip. Digital mosaic from video is from dive BN24 (3386 m). (c) A fault zone with moderately SE dipping anastomosing fractures filled with white vein material cuts several dikes dipping 50°SE (bottom). Overlying hanging wall rocks are shattered basaltic lavas. Loose rubble and pelagic ooze cover parts of the lava outcrop including the shoulder at the top of the cliff. Digital mosaic from video is from dive BN06 (3546 m). (d) Steeply dipping ENE trending fault zone in sedimentary breccia with pronounced striations plunging moderately to the east indicating oblique (right-lateral, normal) slip probably related to transform displacements. Digital mosaic from video is from dive BN10 (3471 m). KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 17

Figure 11. (continued)

[Rosendahl et al., 1986]. Similar relationships in the upper means of accommodating extensive tectonic rotations of crustal units described in section 5.2 strongly support the crustal blocks on the order of hundreds of meters across. interpretation that the tectonic rotation of dikes and lavas [49] We note that the rotations implied by the orientations occurred essentially beneath the ridge axis where they could of structures we observe, inward dipping lavas (Figure 6f ) be rapidly buried by later lavas. The extensive faulting and and outward dipping dikes (Figure 6g), are just the opposite distributed fracturing in this unit would provide a reasonable of what would be expected from slip on inward dipping, EPM 1 - 18 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST

rotational, normal faults that typify the JdFR [Davis and Lister, 1977] and other spreading centers and that are featured in many models of seafloor spreading based on ophiolites [Moores and Vine, 1971; Harper, 1982; Varga and Moores, 1985; Nicolas, 1989]. The observed sense of tilting demands that the rate of subsidence was much greater near the axis and decreased rapidly off-axis. Subaxial subsidence predicted for deeper level gabbroic rock units [Sleep, 1975; Dewey and Kidd, 1977; Phipps Morgan and Chen, 1993; Boudier et al., 1996] could be directly linked to the upper crustal subsidence inferred in our study. [50] Upper crustal structures with inward dipping lavas and outward dipping dikes have been anticipated in several ophiolite models of seafloor spreading [Cann, 1974; Dewey and Kidd, 1977; Rosencrantz, 1982]. More recent studies of spreading centers [Hooft et al., 1996; Schouten et al., 1999] and Ocean Drilling Program (ODP) drill cores [Pariso and Johnson, 1989; Schouten and Denham, 2000] from fast- spreading ridges predict similar relationships. Analog models of subaxial magma withdrawal also result in this general structural pattern [Lagabrielle et al., 2001]. Recent inves- tigations of a tectonic window into East Pacific Rise crust at Hess Deep shows a strikingly similar geological structure [Karson et al., 2002]. Collectively, these observations suggest that the general upper crustal structural geometry we describe for the Blanco Transform scarp may be common if not typical of crust generated at intermediate- to fast-spreading ridges. [51] The major faults that define the median valley of the present-day JdFR spreading center and that bound abyssal hill lineaments in older crust are the most obvious signs of tectonic deformation near the spreading center. However, our results suggest that a much more pervasive type of brittle deformation occurs in the subaxial region. Essentially undeformed surficial lavas bury crustal rock units that have experienced substantial brittle deformation and modification beneath the ridge axis. 6.1. Thickness of Lavas and Implications [52] One of the most important constraints on subaxial processes at mid-ocean spreading centers is the thickness of the lavas. Because even the lavas at the bottom of the basaltic lava unit must have been erupted on seafloor, the thickness of the lava units must equal the net vertical subsidence (relative to the seafloor) experienced by those lowest lavas. The total thickness of the lava units along the Blanco Transform scarp is 1000 m (and locally as much as 1500 m; Figures 12 and 13). This is substantially more than the thickness of lavas inferred from seismic studies in this area [Rohr et al., 1988; White and Clowes, 1990; Cudrack and Cowles, 1993] and from reconstructions of most

Figure 12. (opposite) Columnar sections of submersible dives across the Blanco Transform scarp. Each dive has been reduced to a single columnar section (Figure 4), representing the geologic structure where they crossed the wall. The dives only represent transects a few meters wide and are not shown with their true separations. See Figures 2 and 3 for locations and relative spacing. Ornament as in previous figures, except the horizontal lines represent foliated areas in massive rock and dark shaded areas represent massive-looking sedimentary rocks. KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 19

Figure 13. Schematic summary diagram of upper crustal structure of the southern edge of the , showing undulating contacts between upper crustal rock units and relation to magnetic anomaly polarity boundaries (dashed lines). Ornament as in previous figures. ophiolites [Nicolas, 1989 and references therein]. Such large subsidence histories. We could not substantiate these domain thickness may also be at odds with the widely held notion boundaries and also note that slip on vertical faults cannot that magmatism is attenuated at ridge-transform intersec- account for the observed tectonic rotations. tions [Detrick and Purdy, 1980; Bender et al., 1984; Fox [56] In retrospect it seems obvious that the gentle north- and Gallo, 1984; Karson and Elthon, 1987] where this crust west (inward) dip of the trace of magnetic anomaly polarity formed. Although, as discussed in section 5.1, there are boundaries on the Blanco Transform scarp [Tivey, 1996; some possible uncertainties in determining the unit thick- Tivey et al., 1998b] would be related to the inward dipping ness, they probably cannot change the apparent thickness of lava flow orientations we describe (Figure 13). The dip lavas enough to eliminate the need for hundreds of meters of patterns in the lavas are much more complicated than the subsidence of the lower lavas. smooth traces of magnetic reversal boundaries, but this is probably an artifact of the relatively wide spacing of the 6.2. Dip Patterns and Block Rotations magnetometer transects. Mapping magnetic anomaly boun- [53] Tectonic rotations experienced by lavas and dikes in daries in uppermost oceanic crust exposed on oceanic the upper crustal rock units of the Blanco Transform scarp are escarpments may be an effective means of mapping the difficult to quantify in detail because of uncertainties in geometry of lava flows and dikes in other tectonic windows unambiguously determining the original orientations of into the upper oceanic crust. These data could be extremely many lava flows and dikes. Originally horizontal sheet flows useful in placing constraints on subaxial deformation pro- and dikes that cut them tend to be oriented at high angles to cesses and the internal fabric of oceanic plates (Figure 13). one another, strongly supporting the interpretation that blocks of lavas and dikes have been tectonically rotated after they 6.3. Subaxial Subsidence Mechanisms were emplaced. Future paleomagnetic studies could further [57] About 1 km thickness of lava that is added to the constrain the kinematic patters of rotation in these units. crust in an area of comparable width demands dramatic, [54] The upslope or across-slope variations found in dip rapid, subaxial subsidence. Although the structural geome- angles of dikes and lavas suggest that adjacent blocks of try of lava flows and dikes described in section 5 reflects the upper crustal materials have been rotated to varying degrees style of subsidence in the upper crustal units, it does not (Figure 14). The size of blocks is difficult to determine uniquely constrain the processes controlling this subsidence. because of the spacing and paths of the dives and limited For example, the subsidence might be caused by loading of exposures. We note no sharp variations in dip between lavas on top of the axial crust or by the creation of nearby dives. In addition, side-scan sonar images [Delaney ‘‘accommodation space’’ for the thickening lava pile caused et al., 1987] show no obvious discontinuities that can be by processes deeper in the crust. The latter is analogous to correlated with regions of consistent dip. It is therefore syn-rift subsidence in sedimentary basins [e.g., Gibbs, difficult to distinguish the details of subsidence and rotation 1983] or the ‘‘collapse’’ of the internal structure of basaltic patterns. For example, it is not clear if rotation and creation calderas on land [e.g., Walker, 1993]. Models of magmatic of accommodation space is a continuous process (timescale intrusion [Kelemen et al., 1997], magma withdrawal of 10 kyr) that creates smoothly varying structures [Pa´lma- [Lagabrielle et al., 2001], and viscous flow [Sleep, 1975; son, 1973; Dewey and Kidd, 1977] (Figure 14) or a laterally Dewey and Kidd, 1977; Phipps Morgan and Chen, 1993; discontinuous process with more chaotic collapse structures Boudier et al., 1996] for gabbroic units in oceanic crust and [Karson et al., 2002] (Figure 15). ophiolites provide possible mechanisms of subsidence at [55] Naidoo [1998] interpreted side-scan sonar data in deeper crustal levels. terms of roughly equant ‘‘domains’’ of crustal materials 1 [58] Several different shallow crustal mechanisms, alone km across representing magmatic units with variable, vertical or in combination, could contribute to the creation of the EPM 1 - 20 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST

Figure 14. Diagrammatic representation of thickening of lavas and dike intrusion [after Pa´lmason, 1973; Dewey and Kidd, 1977; Karson et al., 2002] with geological constraints from the Blanco Transform scarp. (middle) The generalized trajectory of a pillow lava erupted at the axis is shown by a solid dot. This point must subside to 1000 m depth within the region of volcanic construction. (bottom) Both dike intrusion and rate of volcanic construction are likely to decrease rapidly within 1kmofthe axis because of narrow lava/dike transition [Kidd, 1977]. Subsidence that is more rapid near the axis than off-axis results in tilting of lava flows (inward) and dikes (outward). (top) Stages of progressive subsidence, rotation, and intrusion of upper crustal material near the lava/sheeted dike contact are shown from the axis (t0) across the region of volcanic construction (t1, t2). Solid, dashed, and shaded bold lines represent dikes intruded at progressively greater distances from the axis. accommodation space necessary for the thickening of the accomplish subsidence as well as burial of axial lavas. (4) lava units. In general, any of the processes that help to Subsurface dike intrusion beneath the axis [Chadwick and create space in the crust beneath the median valley floor Embley, 1998] would also result in local subsidence of axial could result in vertical mass transfer of shallow crustal lavas that could be subsequently buried. materials. There are at least four obvious processes. (1) Tectonic extension resulting from plate separation could 6.4. Temporal Variations in Spreading Processes result in slip on widely spaced normal faults or a more [59] Temporal variations in accretionary processes are chaotic style of subsidence by slip and block rotation in implied by the variations in upper crustal geology along highly fractured crust. (2) Thermal contraction [Wilcock and the Blanco Transform scarp (Figure 12). The total area Delaney, 1996] could also create space and hence subsi- covered by the two dive programs is 30 km along the dence. (3) The transfer of magma from subaxial magma scarp, corresponding to 1 m.y. of spreading history. The chambers to the seafloor via dike intrusion would both dives are spaced at 2 km, corresponding to 66 kyr of remove molten material from below the axis and load the spreading. This is comparable to the width of the median axial floor. The resulting vertical readjustments would valley at JdFR. This spacing limits the resolution of any KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST EPM 1 - 21

Figure 15. Schematic model of subaxial processes and resulting structures at the Juan de Fuca Ridge, interpreted from the internal structure of upper crustal rock units exposed along the Blanco Transform scarp. Dramatic subaxial subsidence (open arrows) is required to create ‘‘accommodation space’’ for the accumulation of >1000 m of basaltic lavas mostly beneath the median valley. Subsidence is accomplished by intense fracturing and block rotation in lower lavas and sheeted dikes. Underlying partially molten material may deform by viscous flow. The resulting crustal structure features inward dipping lava flows and outward dipping dikes [after Karson et al., 2002]. geological variations that may occur along the scarp. Sig- of the same types of structures and inferences seem to apply. nificant along-scarp variations in thickness of lava units Compared with the crustal section exposed at the Hess Deep (hundreds of meters) and subsidence patterns inferred from Rift, the Blanco Transform section differs in the following dip variations imply that crustal construction and subsi- ways: (1) The thickness of lavas at the Blanco Transform dence varied with time. The spacing of variations, for (1000 m) is about twice as thick as that at Hess Deep example, between dives 2963 and BN-24 (a few kilometers) (500 m). (2) The Blanco lavas include a much higher suggests changes in volcanism and subsidence occurring proportion of sheet flows (10%) than at Hess Deep (<5%). over a few tens of thousands of years. Although we note (3) The Blanco lavas appear to be less pervasively damaged, that this corresponds to the timescale implied by the wave- and tilting patterns are much more clearly defined. (4) The length of well-developed abyssal hill lineaments in the area, sheeted dike unit at the Blanco Transform scarp is intensely we cannot confidently correlate crustal structure with these fractured and much more poorly exposed than the spectac- morphologic features. Spreading flow line variations found ular exposures at Hess Deep. (5) Only small outcrops of at the Hess Deep Rift imply similar temporal variations in gabbroic rocks appear to be present along the Blanco spreading albeit on a somewhat different (<10,000 yr) Transform scarp in contrast to a much larger, 1000-m- timescale [Karson et al., 2002]. high exposure at Hess Deep. [61] The Hess Deep section was generated at the fast- 6.5. Comparison With Other Areas spreading equatorial East Pacific Rise (135 mm/yr, full rate) [60] The geology of upper crustal units exposed along the and the Blanco Transform section at the intermediate-rate Blanco Transform scarp is similar in many respects to that JdFR (60 mm/yr). Although the spreading rates and surface recently documented at the Hess Deep Rift [Francheteau et features of these two spreading centers are markedly differ- al., 1992; Karson et al., 1992; Karson et al., 2002]. Many ent, they appear to have generated upper crust with very EPM 1 - 22 KARSON ET AL.: STRUCTURE OF UPPERMOST OCEAN CRUST similar internal structures. Although ODP Hole 504B rep- ture of the upper oceanic crust strongly complement surface resents a spatially limited view of the oceanic crust, the studies along the JdFR axis. thickness of units and inferred structural geometry [Becker [67] Our review of the geology of the upper crustal rock et al., 1988; Alt et al., 1993] are very similar to those of the units provides a view of the internal structure of basaltic Blanco and Hess Deep tectonic windows. lava and sheeted dike units and, by inference, the processes [62] We note that these two similar upper crustal struc- that created them at the Juan de Fuca Ridge. The internal tures occur in tectonic windows in very different settings: structural geometry of these units is substantially more one along a major transform fault and the other along a complex than anticipated from ophiolite analogs and pre- propagating rift. Very different styles of faulting are likely to vailing models of intermediate-rate spreading centers. Pat- be responsible for these exposures. Similar spreading struc- terns of inward dipping lava flows and outward dipping tures found in the two areas gives us confidence that the dikes suggest dramatic subaxial subsidence that created deformation features that we describe are not strongly accommodation space for the growth of the lava unit on influenced by the tectonic processes responsible for their the order of 1000 m thick. Complex patterns of fracturing exposures. and block rotation are buried by a relatively thin layer (500 m thick) of much less deformed surficial lavas. 6.6. Implications for Other Subaxial Processes and These relationships indicate that mapping of surface lavas Evolution of Oceanic Crust and faults along spreading centers like the Juan de Fuca [63] The type of deformation described in uppermost Ridge may not reveal important processes that occur in the crust of the north wall of the Blanco Transform (Figure subaxial domain. 15) would create a substantial amount of fracture porosity [68] Brittle deformation resulting from subaxial deforma- and permeability. Although most models for the upper tion may have substantially modified the porosity structure oceanic crust anticipate a highly porous upper unit of of the subaxial crust and affected physical, chemical, and basaltic pillow lavas, intense fracturing associated with biological processes that are related to porosity. The subaxial subsidence could modify the porosity structure of observed structure is similar to that found in the upper crust the lavas and expand the region of high porosity to deeper at the Hess Deep Rift that was created at a much higher rock units, in particular, the sheeted dike unit. If the crustal spreading rate along the East Pacific Rise. structure that we describe for the Blanco Transform scarp is representative of crust created at intermediate rates, highly [69] Acknowledgments. This study was possible because of the fractured and porous basaltic material may extend to crustal sustained support of near-bottom geological investigations by the MG&G depths of 1.0–1.5 km in oceanic plate interiors. Program of NSF, in particular Accomplishment Based Renewal [64] Fracture porosity of this type would probably OCE907308 (JAK), OCE-8614644 (JRD), and OCE9400623 and OCE9115298 (MAT). We thank Thierry Juteau (chief scientist) for his strongly influence the hydrogeology and hence the hydro- comments and access to unpublished data from the Blanconaute dive series, thermal alteration and evolution of the upper crust. Porosity without which this study would not have been possible. Initial evaluation of generated by fractures and healed by hydrothermal miner- the Blanconaute dives by D. Naidoo and Blancovin dives by Cheryl Waters alization is very likely to dictate the seismic properties of were important starting points for this study. We thank G. Christeson, S. Hurst, P. Johnson, D. Kelley, and R. Varga for their insightful comments the oceanic crust. These effects must be considered as and discussions. W. Chadwick and Y. Lagabrielle provided thoughtful and important effects that overprint the velocity/porosity struc- constructive reviews. Jeffrey A. Karson thanks the School of Oceanography ture inherent to lavas and dikes. 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