Tectonic versus magmatic extension in the presence of core complexes at slow-spreading ridges from a visualization of faulted seafl oor topography Hans Schouten1, Deborah K. Smith1, Johnson R. Cann2, and Javier Escartín3 1Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA 2School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 3Groupe de Geosciences Marines, CNRS–IPGP, F-75252 Paris, France ABSTRACT assume symmetrical spreading. Taking a simple We develop a forward model of the generation of faulted seafl oor topography (visualiza- model that each dike is rooted in a magma body, tion) to estimate the relative roles of tectonic and magmatic extension in the presence of core we examine how gabbro would be partitioned to complexes at slow-spreading ridges. The visualization assumes fl exural rotation of 60° normal both sides of the axis as M varies through time. faults, a constant effective elastic thickness, Te, of young lithosphere, and a continuous infi ll of Our visualization of the 13°20′N topography pre- the depressed hanging wall by lava fl owing from the spreading axis. We obtain a new estimate dicts a high probability of fi nding gabbro beneath of Te = 0.5–1 km from the shapes of the toes of 6 well-documented oceanic core complexes. the surface of the 13°20′N core complex. We model an 80-km-long bathymetric profi le in the equatorial Atlantic across a core complex and the ridge axis at 13°20′N and estimate the variation in tectonic extension, which yields the OCEANIC CORE COMPLEX variation in the fraction of upper crust extension, M, by magmatic diking at the ridge axis. CURVATURE AND ESTIMATION OF Te Core complex formation appears to be stable for all values of M < 0.5. The visualization shows The locations of six core complexes are how gabbro emplaced at the base of the lithosphere during extension by magmatic diking is shown in Figure 1A; their profi les taken parallel partitioned to each side of the spreading axis, and predicts a high probability of fi nding gab- to the spreading direction are displayed in Fig- bros in the domes of core complexes. ure 1B. Four of the core complexes (13°30′N, 13°20′N, TAG [Trans-Atlantic Geotraverse], INTRODUCTION faulted seafl oor topography on both sides of and 22°30′N) intersect the seafl oor (termina- Over the past 10 years it has been recognized the spreading axis of the Mid-Atlantic Ridge tion) at the edge of the median valley fl oor, and that fault extension at slow-spreading ridges is at 13°20′N, which includes the 13°20′N core their association with high rates of seismicity much more important than previously thought, complex (Smith et al., 2006, 2008; MacLeod et suggests that they are active (deMartin et al., and that large offset faults involving signifi cant al., 2009), and estimate the variation in tectonic 2007; Escartin et al., 2008; Smith et al., 2008). rotation and the formation of core complexes extension at the axis over the past 3.2 m.y. We The extinct Kane megamullion core complex are common, accounting for >50% extension take the remainder of the extension to arise by at 23°33′N formed at the intersection of the along large sections of the ridge (e.g., Cannat et diking, and hence calculate the variation in the Mid-Atlantic Ridge and the Kane transform al., 2006; Escartin et al., 2008; Garcés and Gee, fraction of upper crust extension by diking at fault ca. 3.3 Ma (Dick et al., 2008; Tucholke 2007; Smith et al., 2006, 2008; Tucholke et al., the axis (M). Magnetic anomalies are poor in et al., 1998). An extinct core complex in the 1998). The outward rotation of normal faults with this area (Smith et al., 2008) so for simplicity we Eastern Atlantic that formed at the Mid-Atlan- increasing extension, from small offset faults to core complexes, has been described using mod- els of fault fl exure (e.g., Buck, 1988). Numerical b Te = 1 km V.E. = 1 models of core complex formation (e.g., Buck B r C 0.25 km 2240N a 0.5 km et al., 2005; Behn and Ito, 2008; Tucholke et t b al., 2008) require an input of M (0–1), the frac- A r 40˚N EATL 1 km tion of upper crust extension accommodated by t EATL r r b magmatic diking, and output a synthetic bathy- MAR 2 km r 2 km TAG TAG t KMM a metric profi le together with a tectonic cross sec- 22 40 20˚N 4 km r tion. Here we derive an alternative type of model 13 30 Africa b KMM t r based on fault fl exure that we term a visualiza- 13 20 8 km 0˚ r b 1330N tion, which uses a bathymetric profi le as input t 16 km 60˚W 30˚W a r and outputs a tectonic cross section as well as the b 1320N 32 km variation of M as a function of time. a t V.E. = 3 20 km The fault fl exure model requires an estimate -10 0 10 20 30 of the effective elastic thickness of the litho- Distance from axis (km) sphere, Te, specifying the fl exural wavelength. Smith et al. (2008) obtained an estimate of Te in Figure 1. A: Location map. B: Topographic profi les of six North Atlantic core complexes in A. See text for references. Active core complexes at 13°20′N, 13°30′N, TAG (Trans-Atlantic Geo- the range 0.5–1.0 km, from the outward slopes traverse), and 22°40′N are plotted against distance from spreading axis. Inactive core com- of what were assumed to be outward rotated plexes KMM (Kane megamullion) and EATL (Eastern Atlantic) are aligned on terminations, t, fault blocks. Here we obtain a more robust but of active core complexes. MAR—Mid-Atlantic Ridge. Bold lines—exposed detachment sur- similar estimate of Te from the curvature of six face. Gray lines—third-order polynomial fi t to exposed detachment surfaces between break- away b and termination t. r—rafted blocks. C: Primary fault model for fl exural rotation of well-documented core complexes. 60° normal fault developing into core complex at larger offsets. V.E.—vertical exaggeration. We use the fl exural rotation of 60° normal 11 2 ρ Model parameters: Young’s modulus = 10 N/m ; Poisson’s ratio = 0.5; tcrust = 6.0 km; water = 3 ρ 3 ρ 3 faults (e.g., Buck, 1988) and Te = 1 km to model 1030 kg/m ; crust = 2700 kg/m ; mantle = 3300 kg/m ; effective elastic thickness Te = 1.0 km. © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, July July 2010; 2010 v. 38; no. 7; p. 615–618; doi: 10.1130/G30803.1; 4 fi gures. 615 tic Ridge during the Cretaceous (ca. 130 Ma) A decoupling of stresses across the axis. Except (Ranero and Reston, 1999), is based on multi- Termination 2240N for infi lling of the depressed hanging wall, no channel seismic data. In Figure 1B, the profi les 2 km morphologic effects of volcanism are included EATL of the six core complexes were aligned at their (Thatcher and Hill, 1995). inferred terminations. A map of seafl oor centered on the Mid-Atlan- TAG Figure 1C shows a model for the fl exural tic Ridge at 13°20′N is shown in Figure 3A. Lin- rotation of a 60° normal fault developing into a KMM ear ridges, taken to be the crests of fault scarps, core complex (Buck, 1988). The exposed fault are marked with thin black lines. On the east (detachment) between the initiation (break- 1330N side of the axis the morphology suggests numer- away) and termination exhibits a characteristic ous steep normal faults. The 13°20′N core com- curvature. To emphasize the similarity between 1320N plex is immediately west of the axis. West of the profi les and the fault model in Figure 1C, V.E.= 2 that there are several distinctive linear ridges. we added a third-order polynomial fi t (gray B We interpret the breakaway of the 13°20′N line) to the exposed detachment surfaces (bold core complex to be marked by the linear ridge 10 line) between breakaway and termination in along 45°04′W, ~25 km from the spreading axis Figure 1B. The polynomials isolate what we 0.5 (Smith et al., 2008). If this is the case, the linear interpret as rafted blocks atop the primary 0 0.25 ridges along 44°59′W would be rafted blocks fault. Rafted blocks are sections of hanging resting on the long-lived primary fault. 1.0 wall (median valley fl oor) that are chopped Dots above the bathymetric profi le in Fig- off by successive normal faults that root in −10 ure 3B match the dots in Figure 3A at the inter- the same primary fault (Buck, 1988). They are section of the east-west profi le and the crests of Slope (deg) uplifted with the footwall, rotated, and carried −20 identifi ed fault scarps. Dotted lines in Figure 3B away from the axis (Smith et al., 2008; Ranero Te = 2.0 km that dip 60° toward the axis symbolically indi- and Reston, 1999). We use the similarity in cate the faults. We digitize fault locations and wavelength and surface curvature of the six −30 shift their across-axis distance from the axis to 0 10 20 core complexes to obtain a new estimate of Te the nearest multiple of 100 m.