Tectonic Versus Magmatic Extension in the Presence of Core Complexes at Slow-Spreading Ridges from a Visualization of Faulted Seaﬂ Oor Topography

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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 models 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 breakaway 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. We adjust the off- near the axis where most faults and core com- Distance from axis (km) sets on the faults in 100 m increments by trial plexes are postulated to originate. Figure 2. A: Details of core complexes in Fig- and error until a satisfactory match in terms of Details of the six core complexes are shown ure 1B. Interpreted rafted blocks are shaded seafl oor elevation and outward slopes is reached. in Figure 2A; shading identifi es the inferred in gray; symbols at respective terminations Each side of the axis is calculated separately identify source of slope data in B. V.E.—verti- rafted blocks. The slopes of the exposed parts cal exaggeration; MAR—Mid-Atlantic Ridge; in successive steps of 100 m fault extension or of the detachment surfaces are plotted against TAG—Trans-Atlantic Geotraverse); KMM— 100 m extension by magmatic diking. Assuming distance from the axis in Figure 2B. The curves Kane megamullion; EATL—Eastern Atlantic. symmetric spreading at 12.5 km/m.y. half rate in Figure 2B represent the slopes of model B: Comparison of slopes of exposed detach- (Smith et al., 2008), each step (8 k.y.) will give detachments with very large offsets (e.g., the ment surfaces in A to theoretical curves de- us one of three modes of extension at the axis, scribing slopes of primary faults, calculated exposed fault with 32 km offset in Fig. 1C) for effective elastic thickness Te = 0.25, 0.5, i.e., 100 m diking on both sides (M = 1; left panel that were calculated for Te = 0.25, 0.5, 1.0, and 1.0, and 2.0 km. Parameters as in Figure 1C. in Fig. 3C), 100 m diking on one side and 100 m 2.0 km. At offsets >20 km, the curves in Fig- Slopes suggest Te = 0.5–1 km. fault extension on the other (M = 0.5; middle ure 2B are constant. panel in Fig. 3C), or 100 m fault extension on Following Smith et al. (2008), we place the both sides (M = 0; right panel in Fig. 3C). fault origin at 3.5 km from the spreading axis. FLEXURAL MODEL OF FAULTED Figures 4A–4D show four stages in the calcu- This assumes a 60° fault that roots at 6 km BATHYMETRY AND ESTIMATE OF lation of the fi nal synthetic profi le in Figure 4D, beneath the axis as imaged at TAG (deMartin et TECTONIC EXTENSION progressing from 2.4 Ma (Fig. 4A) until present al., 2007). The true fault origin of the core com- To construct the model visualization, we iden- (Fig. 4D). The square wave in the top of each plexes is uncertain, so we have added horizontal tify the position of faults on the bathymetric pro- panel shows the location of the top of each fault error bars of ±1.5 km to the slope data points, fi le. For each fault we assume fl exural rotation and the length of its offset. Note, for example, in suggesting that the fault origin may be 2–5 km of a 60° normal fault near the axis (Fig. 1C) with Figure 4B that the offset of the fault at −7 km, as from the axis. Assuming an original 60° fault, a continuous infi ll of the depressed hanging wall indicated by the length of the square wave mini- its root would then be 3.5–8.7 km beneath the by lava (Buck, 1988; Thatcher and Hill, 1995) mum, is double the exposed offset of the fault axis. The error bar includes the uncertainty in and a constant Te = 1 km. Magmatic accretion face in the underlying profi le. This is the effect axis location. pushes the fault away from the rift axis, a new of volcanic infi ll that has buried the lower half Figure 2B shows that the slopes of the six fault is formed, and the process of fl exure and of the fault face. detachment surfaces are between the curves volcanic infi lling is repeated (Thatcher and Hill, Fault extension on the fi nal synthetic profi le Te = 0.5 and 1.0 km, which is the same as the 1995). The result is a complete synthetic profi le. is 43% (M = 0.57), which would be higher if estimate obtained by Smith et al. (2008). Our The quantitative calculations use a formula- a lower Te is used and lower with a higher Te, estimate of Te is more robust because the identi- tion of the defl ection caused by faulting in a thin because the magnitude of the fl exural response fi cation of the detachment surfaces (Fig. 1B) is elastic plate fl oating on an inviscid fl uid (e.g., scales with Te 3/4. The fi t of the observed topog- less ambiguous than the identifi cation of rotated Buck, 1988; Weissel and Karner, 1989). The raphy is good except for the core complex dome fault blocks by Smith et al. (2008). The similar- model is identical to that described in Thatcher at −11 km, where 750 m was added to the syn- ity between the two estimates of Te suggests that and Hill (1995), except we assume a fault dip thetic profi le to match the elevation of the dome, seafl oor faults and core complexes may form of 60° instead of 45°. Each side of the axis is and at −31.5 km, where 750 m was subtracted to under conditions of similar fl exural rigidity of calculated separately. We take the hanging-wall match the depth of the swale. The two regional young lithosphere near the axis. block to be a semi-infi nite plate and ignore any corrections do not infl uence the extension.

616 GEOLOGY, July 2010 A 13°25′N

b t r Figure 3. A: Bathymetric map of seafl oor centered on spreading axis (dashed line) at 13°20′N. Bold black line is location of profi le shown in B. Thin black lines are linear ridges with dots at 13°15′N their intersection with profi le. The 13°20′N core 45° 10′W45° 00′W44° 50′W 44° 40′W44° 30′W complex is between breakaway, b, and termination, t. B: East-west topographic profi le with B dots showing fault locations from A; dashed 2.0 b Axis lines symbolically indicate faults. Linear ridge ~5 km west of axis is likely rafted block, r. V.E.— t 3.0 r vertical exaggeration. C: Modes of spreading. M = 1: extension by magmatic diking on both 4.0 13˚20’N CORE COMPLEX sides of axis; M = 0.5: diking on one side, fault V.E. = 4 Normal faults extension on other side; M = 0: fault extension

Water depth (km) -40 -30 -20 -10 0 10 20 30 40 on both sides. Dikes root in melt lens (red) at Distance from the axis (km) same constant level as root of active faults. Schematic gabbro sills (black) show opposite C Modes of spreading age progression with depth in core complexes M = 1 M = 0.5 M = 0 than at base of crust. Dike Crust Melt lens Gabbro sills Mantle

A Axis of spreading 2.4 Ma Volcanics Dike Sheeted dikes Normal faults Crust V.E. = 1 Melt lens Gabbro sills Mantle B 1.6 Ma Figure 4. Visualization of faulted seafl oor calculated in steps of 0.1 km (8 k.y). A–D: Progressive stages in calculation of fi nal synthetic profi le in D. Gray is upper crust formed by magmatic diking and lava deposition (not shown). Bold verti- C cal and horizontal lines at axis are dike and melt 0.8 Ma lens. White is material drawn up from below. Thin black lines below gray crust are sills emplaced at axis. Square wave indicates tops of faults (from Figs. 3A and 3B) and offset on each fault. Thin black lines cutting subseafl oor are faults. Dikes and faults are rooted at constant level of 6 km below seafl oor. V.E.—vertical ex- D aggeration. Model parameters as in Figures 1C present and 1E. Thin line is fraction of magmatic diking at axis (M = 1, 0.5, 0) assuming symmetric spreading. Bold line is 50 point (0.4 m.y.) moving average of M. Note that 13°20′N core complex initiated west of axis; east of axis topography indicates signifi cant extension (1.8– −40 −30 −20 −10 0 10 20 30 40 1.6 Ma), and M ≈ 0. E Distance (km) 1.0

.5 M

0 −3.2 −2.4 −1.6 −0.8 0 0.8 1.6 2.4 3.2 Age (Ma)

GEOLOGY, July 2010 617 Figure 4E shows the variation of M at the axis increasing age of gabbro sills with depth (Grimes Geosystems, v. 9, Q05014, doi: 10.1029/ from 3.2 Ma until present in steps of 8 k.y., with et al., 2008), from which it was concluded that 2007GC001645. Escartín, J., Mevel, C., MacLeod, C.J., and McCaig, values of the three modes: 1, 0.5, and 0. The the sills were emplaced at random depths, rather A.M., 2003, Constraints on deformation conditions bold line shows a 50 point (0.4 m.y.) moving than a constant depth, beneath the axis. and the origin of oceanic detachments: The Mid- average. Our model results provide an estimate Gabbro sills that are emplaced simultane- Atlantic Ridge core complex at 15°450N: Geo- of M as a function of time (or distance from the ously with each step of diking at the axis are chemistry Geophysics Geosystems, v. 4, 1067, doi: 10.1029/ 2002GC000472. axis) derived from faulted seafl oor topography. shown schematically as thin black lines at the Escartín, J., Smith, D.K., Cann, J., Schouten, H., Lang- This is different from existing numerical models base of the crust in Figures 4A–4D. The den- muir, C.H., and Escrig, S., 2008, Central role of of core complexes that specify a value of M that sity of sills correlates with M and shows a high detachment faults in accretion of slow-spread oceanic lithosphere: Nature, v. 455, p. 790–794, doi: remains constant through time (e.g., Buck et al., probability of encountering gabbro bodies in the 10.1038/nature07333. 2005; Tucholke et al., 2008). Figure 4E displays upper parts of core complexes as long as M > 0. Garcés, M., and Gee, J.S., 2007, Paleomagnetic evidence large fl uctuations of M at the axis over the past Only in the rare event of prolonged faulting on of large footwall rotations associated with low-angle faults at the Mid-Atlantic Ridge: Geology, v. 35, 3.2 m.y. At ~22 km on the east side of the axis, both sides of the axis (e.g., M = 0 at 1.8–1.6 Ma p. 279–282, doi: 10.1130/G23165A.1. for example, the topography requires a signifi - in Figs. 4E and 4D) will the probability of Grimes, C.B., John, B.E., Cheadle, M.J., and Wooden, cant amount of offset on a series of faults. On encountering gabbros be low. This idea could be J.L., 2008, Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from the west side of the axis at the same distance, tested at spreading ridges such as the Southwest 206 238 ′ Pb/ U zircon ages from Atlantis Massif and the 13°20 N detachment fault initiated. This Indian Ridge, where prolonged faulting on both IODP Hole U1309D (30°N, MAR): Geochemis- resulted in a period of very low values of M, and sides of the axis has accommodated spreading try Geophysics Geosystems, v. 9, Q08012, doi: indicates that a detachment fault can be stable over extended periods of time (Dick et al., 2003; 10.1029/2008GC002063. ≈ Ildefonse, B., Blackman, D.K., John, B.E., Ohara, Y., when M 0 (1.6 Ma in Fig. 4B). Numerical Cannat et al., 2006). Miller, D.J., MacLeod, C.J., and Integrated Ocean experiments (Buck et al., 2005: Tucholke et al., This visualization of faulted seafl oor derived Drilling Program Expeditions 304/305 Science 2008), by contrast, have suggested that detach- from a bathymetric profi le thus allows estima- Party, 2007, Oceanic core complexes and crustal ment faults are most stable when the extension accretion at slow-spreading ridges: Geology, v. 35, tion of the relative contributions of faulting and p. 623–626, doi: 10.1130/G23531A.1. on the opposite fl ank is entirely by diking (M = diking during the construction of the crust as a MacLeod, C.J., Escartín, J., Banerji, D., Banks, G.J., 0.5), and not stable when M drops below ~0.3. function of time. The results depend strongly Gleeson, M., Irving, D.H.B., Lilly, R.M., McCaig, Even though the fraction of magmatic exten- on our identifi cation of normal faults and the A., Niu, Y.-L., Allerton, S., and Smith, D.K., 2002, ′ Direct geological evidence for oceanic detach- sion during core complex formation is low (M < 13°20 N core complex in the bathymetric pro- ment faulting: The Mid-Atlantic Ridge, 15°45′N: 0.5), there is a frequent sampling of gabbro bod- fi le. Nevertheless, the visualization provides a Geology, v. 30, p. 279–282, doi: 10.1130/0091 ies in the upper parts of core complexes (Black- useful means of linking theory and observation. -7613(2002)030<0879:DGEFOD>2.0.CO;2. MacLeod, C.J., Searle, R.C., Murton, B.J., Casey, J.F., man et al., 2002; Dick et al., 2008; Escartín et Mallows, C., Unsworth, S.C., Achenbach, K.L., and al., 2003; Ildefonse et al., 2007; MacLeod et al., ACKNOWLEDGMENTS Harris, M., 2009, Life cycle of oceanic core com- 2002; Tucholke et al., 1998). This indicates that This study was undertaken with National Science Foun- plexes: Earth and Planetary Science Letters, v. 287, dation support to Schouten and Smith, and Centre National there is still a signifi cant plutonic contribution to p. 333–344, doi: 10.1016/j.epsl.2009.08.016. de la Recherche Scientifi que support to Escartín. We had Ranero, C., and Reston, T.J., 1999, Detachment fault- core complexes during their formation, and that fruitful conversations with M. Tivey and C. Williams. This ing at ocean core complexes: Geology, v. 27, M is not a measure of actual magmatic supply to paper benefi tted from the reviews of B. Ildefonse, N. Hay- p. 983–986, doi: 10.1130/0091-7613(1999)027< the ridge axis. We visualize this phenomenon by man, and two anonymous reviewers. 0983:DFAOCC>2.3.CO;2. 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