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Cracking of Lithosphere North of the Galapagos Triple Junction

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Cracking of Lithosphere North of the Galapagos Triple Junction Cracking of lithosphere north of the Galapagos triple junction Hans Schouten Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Deborah K. Smith Massachusetts 02543, USA Laurent G.J. Montési Department of Geology, University of Maryland, College Park, Maryland 20742, USA Wenlu Zhu Emily M. Klein Division of Earth and Ocean Sciences, Duke University, Durham, North Carolina 27708, USA ABSTRACT evolution that probably affect terrestrial as The Galapagos triple junction is a ridge-ridge-ridge triple junction where the Cocos, Nazca, well as oceanic rifts. and Pacifi c plates meet around the Galapagos microplate. Directly north of the large scarps of Lonsdale (1988) proposed that the GTJ is the Cocos-Nazca Rift, a 250-km-long and 50-km-wide band of northwest-southeast–trending made up of a set of four rifts: the East Pacifi c cracks with volcanics at their western ends crosscuts and blankets the north-south–trending Rise, Incipient Rift, Cocos-Nazca Rift, and abyssal hills of the East Pacifi c Rise. It appears that the roughly northeast-southwest exten- Galapagos-Nazca Rift (referred to herein as Dietz sion of East Pacifi c Rise–generated seafl oor has been accommodated by a succession of minor Deep Rift), which enclose a rotating Galapagos rifts that, during at least the past 4 m.y., had their triple junctions with the East Pacifi c Rise microplate (Fig. 1A). The current tip of the at distances of 50–100 km north of the tip of the propagating Cocos-Nazca Rift. We propose Cocos-Nazca Rift (Hess Deep) does not intersect that the rift locations are controlled by stresses associated with the dominant Cocos-Nazca the East Pacifi c Rise; instead there are two ridge- Rift, and scaled by the distance of its tip to the East Pacifi c Rise. We speculate that similar ridge-ridge (RRR) intersections at 1°10′N and ephemeral rifts occurred south of the Cocos-Nazca Rift and were instrumental in the origin of 2°40′N, where Dietz Deep Rift and Incipient the rotating Galapagos microplate ca. 1.5 Ma. Rift, respectively, meet the East Pacifi c Rise. Lonsdale’s (1988) model for the GTJ has a Keywords: Galapagos triple junction, plate boundaries, lithospheric stress. simple major plate RRR triple junction until ca. 1.5 Ma, when Dietz Deep Rift developed at INTRODUCTION lithosphere responds to stress. In this paper we the East Pacifi c Rise, forming a short, east-west– Distributed deformation at oceanic triple investigate the kinematic history and nature trending spreading center. Over time, Dietz junctions shows that the lithospheric plates of distributed deformation at the Galapagos Deep Rift propagated northeast, approaching undergo signifi cant internal deformation triple junction (GTJ) (e.g., Bird et al., 1999; the southern scarps of the Cocos-Nazca Rift. as their boundaries rapidly evolve (e.g., Klein et al., 2005; Lonsdale, 1988; Searle and The Galapagos microplate took on its own Lonsdale, 1988; Mitchell, 1991; Mitchell and Francheteau, 1986; Zonenshain et al., 1980), motion, a clockwise rotation about a vertical Livermore, 1998). The nature of the deforma- a region that reveals the fundamental inter- axis, ca. 1 Ma; Incipient Rift also began opening tion provides important constraints on how the actions between stress and plate boundary around that time. 100W 80W A 10N Cocos B 102°00′W 101°40′W 101°20′W 3°00′N 1 My 0 Pacific 10S Nazca 102°W 101°W 100°W 2°50′N ER EPR 3°N EPR C TJ IR B Figure 1. A: Location maps showing major NGMP C-N 2°40′N 2°N TJ tectonic features of Galapagos triple junction GMP (after Karson et al., 2002). GMP—Galapagos Rift scarps microplate, NGMP—North Galpagos micro- ′ 1°N TJ DDR 2°30 N plate, ER—Extinct Rift, IR—Incipient Rift, DDR—Dietz Deep Rift, C-N—Cocos-Nazca spreading center, EPR—East Pacifi c Rise, C 101°20′W 101°00′W 100°40′W 100°20′W TJ—triple junction. B: Multibeam bathymetry ′ of Incipient Rift. C: Multibeam bathymetry of 3°20 N Extinct Rift. Shading—regions of vol canic highs and fl ows. White dashed lines— graben extending southeast from the vol- 3°10′N ? canic highs. White solid line—1 m.y. iso- ? chron (from Lonsdale, 1988). Red boxes show locations of maps in B and C. 3°00′N Extinct Rift 2°50′N 10 km m –3500 –3000 © 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, May May 2008; 2008 v. 36; no. 5; p. 339–342; doi: 10.1130/G24431A.1; 4 fi gures. 339 A 102°W 101°W 100°W the Cocos-Nazca Rift and was instrumental in the development of Dietz Deep Rift and the origin of the Galapagos microplate ca. 1.5 Ma. EPR NORTHERN TRIPLE JUNCTIONS At the western end of Incipient Rift (Fig. 1B), 3°N 1 My Extinct Rift a broad volcanic high extends east from its triple junction with the East Pacifi c Rise for ~15 km, where it changes to a narrow graben (~200 m deep, ~4 km wide) extending southeast (Figs. TJ Incipient Rift 1B and 2). Excess volcanism blankets East Pacifi c Rise–generated crust to the north and south of the volcanic high (Klein et al., 2005). The overall shape of Extinct Rift is similar to that of Incipient Rift: a volcanic high and fl ows 2°N blanketing East Pacifi c Rise–generated abyssal Water depth (m) hills changing to a graben trending southeast 50 km B –4000 –2000 (Figs. 1C and 2). This suggests that the west- ern end of the volcanism marks the location of the former junction of Extinct Rift with the EPR East Pacifi c Rise. Because Extinct Rift is deeper (to 600 m) and broader (~14 km) than Incipi- 3°N ent Rift, we infer that it opened for a longer Extinct Rift period of time (>0.5 m.y.) before it was aban- Rift scarps doned ca. 1.5 Ma (inferred from the location of its tip). If Extinct Rift bounded an edge-driven TJ microplate, the lithosphere to its south should have been noticeably rotated. As at the Incipi- ent Rift, however, no evidence of seafl oor fabric HD Cocos-Nazca SC rotation is observed, which challenges the model proposed by Klein et al. (2005) of a succession of rotating edge-driven microplates north of the 2°N 102°W 101°W 100°W Cocos-Nazca Rift. Between Incipient and Extinct Rifts and far- Figure 2. A: Multibeam bathymetry and global seafl oor topography (Smith and Sandwell, ther to the east, we recognize multiple volcanic 1997) north of Cocos-Nazca Rift. Red dashed line—inferred trace of northern triple junctions. ridges (Figs. 1B, 1C) that change to graben along B: Interpretation of features from A. Dark shading—outline of excess volcanism. Light gray shading—Cocos-Nazca Rift. Bold black lines—rift systems. Light black lines—lineations. their strike. The western edge of volcanism that Red dashed line—same as in A. SC—spreading center, HD—Hess Deep, TJ—triple junction, blankets and abuts the north-south–trending abys- EPR—East Pacifi c Rise. sal hills (Fig. 2) is identifi ed as the trace of many short-lived rifts where they intersected the East Pacifi c Rise. The associated graben have different It has been diffi cult to understand what role Klein et al. (2005) also described Extinct widths, and thus presumably opened for varying Incipient Rift plays in the triple junction kine- Rift, a trough that trends parallel to the Incipi- lengths of time. In places where volcanic highs matics. Lonsdale (1988) and Lonsdale et al. ent Rift, ~100 km to the northeast. They inter- are observed and graben are not well defi ned, (1992) concluded that Incipient Rift was a preted Extinct Rift to have been a larger version we infer that volcanism occurred along an initial small westward-propagating rift. In contrast, of Incipient Rift and suggested that the GTJ has crack, which was then quickly abandoned. Klein et al. (2005) concluded that the Incipient undergone a long and complex history of plate Thus it appears that Incipient Rift is just the Rift opens about a pivot at its eastern end. reorganization with the development and aban- latest and Extinct Rift perhaps the largest of a And, what was previously thought to be a donment of rotating microplates north and south sequence of more than 20 southeast-trending rifts single Galapagos microplate could, in fact, be of the Cocos-Nazca Rift. or cracks that progressively stepped southward composed of two counterrotating microplates: We present a new interpretation of the evolu- and westward and successively accommodated the northern portion of Lonsdale’s Galapagos tion of the GTJ. We observe that during the past approximately north-south extension of East microplate (North Galapagos microplate), 4–5 m.y., north-south–trending abyssal hills Pacifi c Rise–generated crust. At ~101°20′W, and the remaining portion of the microplate. of the Cocos plate have been cut and volcani- the strike of the inferred triple junction trace Applying the edge-driven microplate model of cally overprinted by a succession of short-lived changes abruptly from westward to southward. Schouten et al. (1993) to their proposed dual northwest-southeast–trending minor rifts and At ~101°35′W it changes back to westward. We microplate system, Klein et al. (2005) estimated associated triple junctions, but without signifi cant observe that a more westward trend implies a that the North Galapagos microplate would rotation.
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