Rift topography linked to magmatism at the intermediate spreading Juan de Fuca Ridge Suzanne M. Carbotte Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA Robert S. Detrick Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Alistair Harding Scripps Institution of Oceanography, La Jolla, California 92093, USA Juan Pablo Canales Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Jeffrey Babcock Scripps Institution of Oceanography, La Jolla, California 92093, USA Graham Kent Emily Van Ark Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Mladen Nedimovic Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA John Diebold ABSTRACT ridge morphology along the Juan de Fuca New seismic observations of crustal structure along the Juan de Fuca Ridge indicate Ridge to non-steady-state accretion through that the axial rift topography reflects magma-induced deformation rather than alternating alternating magmatic-tectonic phases. Since phases of magmatism and tectonic extension, as previously proposed. Contrary to predic- then, the Juan de Fuca Ridge has been widely tions of the episodic models, crustal magma bodies are imaged beneath portions of all cited as a type example for episodic magma- ridge segments surveyed at average depths of 2.1–2.6 km. The shallow rift valley or axial tism in ongoing debate over the steady-state graben associated with each Juan de Fuca segment is ϳ50–200 m deep and 1–8 km wide versus non-steady-state nature of crustal for- and is well correlated with a magma body in the subsurface. Analysis of graben dimen- mation (e.g., Pezard et al., 1992; Wilcock and sions (height and width) shows that the axial graben narrows and graben height dimin- Delaney, 1996; Perfit and Chadwick, 1998). ishes where the magma body disappears, rather than deepening and broadening, as ex- The Juan de Fuca Ridge comprises seven pected for rift topography due to tectonic extension. We propose an evolutionary model segments, each with a distinct axial morphol- of axial topography that emphasizes the contribution of dike intrusion to subsidence and ogy (Fig. 1). Cleft, the southernmost segment, fault slip at the seafloor. In this model an evolving axial topography results from feedbacks has a shallow and broad axial high notched by between the rheology of the crust above a magma sill and dike intrusion, rather than a 2–3-km-wide axial rift flooded with recent episodic magma delivery from the mantle. lavas. Within Vance, the next segment to the north, an axial volcanic ridge (AVR) is located Keywords: Juan de Fuca Ridge, magma chambers, dikes, faulting, spreading centers. in the center of a much broader, ϳ8 km wide, valley. A narrow 1–2-km-wide depression bi- INTRODUCTION and possibly subcrustal melt sills build the sects the crest of a narrow and deeper axial The formation of oceanic crust at mid- layered gabbros of the lower crust (e.g., Ke- high at the Northern Symmetric (or Cobb) ocean ridges involves the production of melts lemen et al., 1997). Melt transport and extrac- segment. At Endeavour segment, abundant in the upper mantle through what is probably tion from the mantle may be episodic; some faulting is observed in the floor of a 2–3-km- a continuous process of adiabatic melting and studies of ridge axis structure and lava chem- wide axial trough; there is little evidence for porous flow. Once melt reaches crustal levels, istry indicate crustal accretion during inter- recent eruptions. In the Kappel and Ryan melt is extracted during discrete events; dike mittent magmatic phases (e.g., Lewis, 1979; (1986) model, these ridge segments are each injection from mid-crustal melt bodies builds Tucholke and Lin, 1994). Kappel and Ryan in a different phase in a cycle of alternating the sheeted dike section of the upper crust, (1986) attributed the marked variations in magmatism and tectonism. An AVR is formed Figure 1. A: Cross-axis bathymetry profiles showing contrast in ridge morphology at adjacent ridge segments. Latitude of profile crossing at axis is indicated. B: Regional bathymetry for Juan de Fuca Ridge; track of seismic survey is overlain. ᭧ 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; March 2006; v. 34; no. 3; p. 209–212; doi: 10.113/G21969.1; 3 figures; Data Repository item 2006039. 209 at the axis during a magmatic phase (Vance), and Vance segments, which represent ex- ends. A prominent example of this relation- and is split tectonically as magmatism wanes tremes in ridge axis morphology along the ship is found at the Split or Surveyor sea- to form a shallow axial depression (Cleft). Juan de Fuca Ridge, differ by Ͻ50 m. Ex- mount, a rifted conical seamount as high as With ongoing tectonism and intermittent mag- cluding the overlapped portions of segments, 475 m, ϳ7 km in diameter, and centered on matism, this depression widens (Northern the magma body is detected beneath 60% of the axis at the north end of NSymm segment Symmetric and Endeavour) until a new mag- the ridge axis, remarkably similar to findings (Fig. 2; Fig. DR1 [see footnote 1]). No magma matic phase initiates construction of a new at the EPR (Detrick et al., 1987). body is detected for ϳ25 km south of Split AVR. The abyssal hills on the flanks of the The base of seismic layer 2A, often inter- seamount where the axis is subdued and a lin- Juan de Fuca Ridge were interpreted by Kap- preted as the boundary between the extrusive ear axial graben Ͼ25 m in relief is not present. pel and Ryan (1986) as split AVRs formed at and dike sections (e.g., Karson and Christe- However, directly beneath Split Seamount, the axis during these major magmatic phases. son, 2002), is well imaged throughout the re- where steep fault scarps bound a 3–5-km- In this study we present new observations gion. There are segment-to-segment differenc- wide, 250-m-deep graben, there is a strong on the crustal structure of the Juan de Fuca es in the average thickness of layer 2A along isolated AMC reflection. Similarly, at Endeav- Ridge from the first detailed seismic reflection the axis (Fig. 2E); the thinnest and most uni- our segment, a magma body is only detected study carried out in the region (Fig. 1). Data form is at Cleft segment (219, Ϯ40 m), and a under the central shallow portion of the ridge were collected along the entire length of the thicker and more variable 2A is beneath the segment where the axial graben is well defined ridge axis south from Endeavour segment, Endeavour segment (322, Ϯ90 m). Uncertain- by two parallel axis-facing scarps. In some with detailed surveys of Endeavour, Vance, ties in depth estimates are Ϯ50 m for layer cases, the fine-scale segmentation of the axial Cleft, and Axial Volcano, and along three long 2A and Ϯ70 m for the AMC (Canales et al., graben defined by along-axis changes in gra- ridge flank transects. Our observations of seis- 2005). ben dimensions coincides with segmentation mic layer 2A and the presence and depth of The shallow rift valley or axial graben of the AMC (Fig. 2). For example, a break in magma bodies provide insights into the mag- found at each Juan de Fuca Ridge segment is the AMC reflection suggests that separate matic state of each Juan de Fuca Ridge seg- defined by remarkably linear parallel walls magma sills underlie north and south Cleft ment and allow us to test the alternating that extend for part or all of each segment. (Canales et al., 2005). This discontinuity in magmatic-tectonic cycle model for ridge Here we prefer the term axial graben to dis- the AMC coincides with segmentation of the structure. Here we focus on segment to seg- tinguish this feature from the much larger re- axial graben defined by the graben height pro- ment differences in axial structure. Details on lief (500 m to several kilometers) rift valleys file, suggesting differences in the history of ridge-axis structure within Cleft and Vance found at slow spreading ridges. Axial graben graben growth associated with each magma segments and description of the seismic ex- dimensions (height and width) are measured sill. periment are in Canales et al. (2005; see also for comparison with observations on internal 1 the GSA Data Repository ). crustal structure (Figs. 2B, 2C). Graben width DISCUSSION Contrary to predictions of the magmatic- differs between segments with abrupt steps of RESULTS FROM SEISMIC tectonic model of Kappel and Ryan (1986), several kilometers marking segment boundar- OBSERVATIONS seismic observations reveal that magma bod- ies. Smaller steps in graben width (Ͻ1 km) Seismic reflection profiles reveal a strong ies are present beneath portions of each Juan are found within segments, separating sections reflection, believed to correspond with a mag- de Fuca Ridge segment, including those pre- of approximately uniform or continuously ma sill associated with the axial magma cham- viously assumed to be in a tectonic phase. varying width that extend for tens of kilome- ber (AMC; Detrick et al., 1987), beneath all Each segment is associated with a distinct ters. At the two segments with the widest gra- Juan de Fuca Ridge segments including magmatic system with a magma body at dif- bens (Ͼ6 km), Vance and Coaxial, low-relief Northern Symmetric (Nsymm), where the ax- ferent depths in the crust. The zero-age thick- (as much as ϳ100 m) AVRs mark the zone of ial high is narrowest and in places difficult to ness of the seismically inferred extrusive layer active volcanism. identify (Figs. 1 and 2). With the exception of (layer 2A) also varies from segment to seg- Axial Volcano (0.6–1.2 s two-way traveltime Graben height varies within each segment ment; thicker 2A is present where the magma [twtt]; Kent et al., 2003), the AMC at this in- and ranges from Ͻ50 m to ϳ250 m.