The Fifteen-Twenty or 15°20' Fracture Zone (FTFZ), also known as the Cabo Verde Fracture Zone, is a fracture zone located on the Mid-Atlantic Ridge (MAR) in the central Atlantic Ocean between 14–16°N. It is the current location of the migrating triple junction marking the boundaries between the North American, South American, and Nubian plates.[1][2] The FTFZ is roughly parallel to the North and South America—Africa spreading direction and has a broad axial valley produced over the last ten million years by the northward-migrating triple junction.[1][2] Offsetting the MAR by some 175 km, the FTFZ is located on one of the slowest portions of the MAR where the full spreading rate is 25 km/Myr.[3]
Location of the Fifteen-Twenty Fracture Zone and the triple junction between the North American, South American, and African (Nubian) plates. The Azores–Gibraltar Transform Fault is in the top right corner.
North and south of the FTFZ the axis of the MAR is near-perpendicular to the spreading direction and the spreading rate is 2.6 mm/yr. The axial valley south of the FTFZ is composed of short axial volcanic ridges separated by 8–18 km-long en echelon deeps, while north of the FTFZ the axial ridges are much longer and more linear.[4]
North and south of the FTFZ the ocean floor is relatively smooth with long abyssal hills, probably detachment faults, aligned near-parallel to the ridge axis. In contrast, close to the FTFZ the terrain is more rugged and adorned with short, oblique fault scarps. Associated with the transition between these two types of terrains (at about 15°50'N and 14°30'N respectively) are V-shaped, south-propagating structures. These transitional structures disappear away from the ridge. Within the rugged terrain serpentinizedperidotite and gabbro are capped with a thin layer of extrusive basalt. In the smooth areas the lithosphere is more magmatic in composition.[4]
The FTFZ is flanked by two negative gravity anomalies associated with the accretion of igneous crust. The anomaly south of the FTFZ is twice as large as the northern one. There are also geochemical variations across the fracture zone. On the southern side basalts are enriched MORB (mid-ocean ridge basalt) but on the northern side basalts change from enriched to depleted away from the FTFZ. Peridotites collected from south of the FTFZ have an uncommon composition ascribed to a H2O-rich or hot mantle source.[3]
Megamullions
Corrugated surfaces known as megamullions or oceanic core complexes measure 25 km along-axis and 10–15 km across. When found along other mid-ocean ridges such structures occur at the inside corners of ridge discontinuities, but at the FTFZ they occur on both sides of the ridge away from any non-transform discontinuities.[4] These structures and ultramafic rocks outcropping on either side of the MAR (in contrast to other parts of the ridge) indicate considerably reduced magma supply near the FTFZ. Paradoxically, geochemical analyses of basalts near the FTFZ instead suggest an enriched mantle source and the presence of a mantle hotspot.[2]
Two models can explain these contradictions. A westward ridge jump could relocate an older megamullion on the original western flank to the opposite flank after which a new megamullion start to form on the new western flank. Near the FTFZ this would place the older megamullion in an outside corner while the younger develop in an inside corner. Alternatively, an eastward ridge jump or migration could turn a west-dipping detachment fault into an east-dipping fault, which would also result in an older abandoned and a younger active megamullion. Which is the case is currently not known.[5]
Superimposed on the larger corrugated surfaces are two systems of smaller scale corrugations: one on a 1–3 km-scale, roughly 200 m high, and another finer about 100–500 m wide. The latter occur up to 1 km from the ridge and is covered by elevated ridges running parallel to the spreading direction, about 10 m wide, hundreds of metres long, and 10 m tall. These in turn are covered with cm-scale striations running in the same direction.[6]
Triple junction
The North American–South American–African triple junction is associated with the initial opening of the Atlantic Ocean and has had a complex tectonic history. It probably migrated from near 10°N to its current location near the FTFZ between 72.5 and 35.5 Ma. Both its location and that of the North America–South-American–Caribbean triple junction are debated, however.[7] The initiation and evolution of triple junctions is often associated with mantle plumes, but, if this is the case near the FTFZ, the limited supply of magma suggest an embryonic plume or a local, anomalous mantle composition. The relative movement between the North American and South American plates is very small, but the resulting deformation could possibly explain both off-axis seismicity and the odd mantle composition.[8]
Escartín, J.; Cannat, M. (1999). "Ultramafic exposures and the gravity signature of the lithosphere near the Fifteen-Twenty Fracture Zone (Mid-Atlantic Ridge, 14–16.5 N)". Earth and Planetary Science Letters. 171 (3): 411–424. doi:10.1016/S0012-821X(99)00169-7. Retrieved 22 October 2016.
Fujiwara, T.; Lin, J.; Matsumoto, T.; Kelemen, P. B.; Tucholke, B. E.; Casey, J. F. (2003). "Crustal Evolution of the Mid-Atlantic Ridge near the Fifteen-Twenty Fracture Zone in the last 5 Ma". Geochemistry, Geophysics, Geosystems. 4 (3): 1024. Bibcode:2003GGG.....4.1024F. doi:10.1029/2002GC000364. hdl:1912/5774. S2CID 17597399. Retrieved 22 October 2016.
Godard, M.; Lagabrielle, Y.; Alard, O.; Harvey, J. (2008). "Geochemistry of the highly depleted peridotites drilled at ODP Sites 1272 and 1274 (Fifteen-Twenty Fracture Zone, Mid-Atlantic Ridge): Implications for mantle dynamics beneath a slow spreading ridge". Earth and Planetary Science Letters. 267 (3): 410–425. Bibcode:2008E&PSL.267..410G. doi:10.1016/j.epsl.2007.11.058. Retrieved 22 October 2016.
MacLeod, C. J.; Escartín, J.; Banerji, D.; Banks, G. J.; Gleeson, M.; Irving, D. H. B.; Lilly, R. M.; McCaig, A. M.; Niu, Y.; Allerton, S.; Smith, D. K. (2002). "Direct geological evidence for oceanic detachment faulting: The Mid-Atlantic Ridge, 15 45′ N" (PDF). Geology. 30 (10): 879–882. Bibcode:2002Geo....30..879M. doi:10.1130/0091-7613(2002)030<0879:DGEFOD>2.0.CO;2. Retrieved 22 October 2016.
Smith, D. K.; Escartín, J.; Schouten, H.; Cann, J. R. (2008). "Fault rotation and core complex formation: Significant processes in seafloor formation at slow-spreading mid-ocean ridges (Mid-Atlantic Ridge, 13–15 N)". Geochemistry, Geophysics, Geosystems. 9 (3): n/a. doi:10.1029/2007GC001699. hdl:1912/3266. S2CID 56238897. Retrieved 22 October 2016.
fifteen, twenty, fracture, zone, fifteen, twenty, fracture, zone, ftfz, also, known, cabo, verde, fracture, zone, fracture, zone, located, atlantic, ridge, central, atlantic, ocean, between, current, location, migrating, triple, junction, marking, boundaries, . The Fifteen Twenty or 15 20 Fracture Zone FTFZ also known as the Cabo Verde Fracture Zone is a fracture zone located on the Mid Atlantic Ridge MAR in the central Atlantic Ocean between 14 16 N It is the current location of the migrating triple junction marking the boundaries between the North American South American and Nubian plates 1 2 The FTFZ is roughly parallel to the North and South America Africa spreading direction and has a broad axial valley produced over the last ten million years by the northward migrating triple junction 1 2 Offsetting the MAR by some 175 km the FTFZ is located on one of the slowest portions of the MAR where the full spreading rate is 25 km Myr 3 Location of the Fifteen Twenty Fracture Zone and the triple junction between the North American South American and African Nubian plates The Azores Gibraltar Transform Fault is in the top right corner Contents 1 Geological setting 1 1 Megamullions 2 Triple junction 3 References 3 1 Notes 3 2 SourcesGeological setting EditNorth and south of the FTFZ the axis of the MAR is near perpendicular to the spreading direction and the spreading rate is 2 6 mm yr The axial valley south of the FTFZ is composed of short axial volcanic ridges separated by 8 18 km long en echelon deeps while north of the FTFZ the axial ridges are much longer and more linear 4 North and south of the FTFZ the ocean floor is relatively smooth with long abyssal hills probably detachment faults aligned near parallel to the ridge axis In contrast close to the FTFZ the terrain is more rugged and adorned with short oblique fault scarps Associated with the transition between these two types of terrains at about 15 50 N and 14 30 N respectively are V shaped south propagating structures These transitional structures disappear away from the ridge Within the rugged terrain serpentinized peridotite and gabbro are capped with a thin layer of extrusive basalt In the smooth areas the lithosphere is more magmatic in composition 4 The FTFZ is flanked by two negative gravity anomalies associated with the accretion of igneous crust The anomaly south of the FTFZ is twice as large as the northern one There are also geochemical variations across the fracture zone On the southern side basalts are enriched MORB mid ocean ridge basalt but on the northern side basalts change from enriched to depleted away from the FTFZ Peridotites collected from south of the FTFZ have an uncommon composition ascribed to a H2O rich or hot mantle source 3 Megamullions Edit Corrugated surfaces known as megamullions or oceanic core complexes measure 25 km along axis and 10 15 km across When found along other mid ocean ridges such structures occur at the inside corners of ridge discontinuities but at the FTFZ they occur on both sides of the ridge away from any non transform discontinuities 4 These structures and ultramafic rocks outcropping on either side of the MAR in contrast to other parts of the ridge indicate considerably reduced magma supply near the FTFZ Paradoxically geochemical analyses of basalts near the FTFZ instead suggest an enriched mantle source and the presence of a mantle hotspot 2 Two models can explain these contradictions A westward ridge jump could relocate an older megamullion on the original western flank to the opposite flank after which a new megamullion start to form on the new western flank Near the FTFZ this would place the older megamullion in an outside corner while the younger develop in an inside corner Alternatively an eastward ridge jump or migration could turn a west dipping detachment fault into an east dipping fault which would also result in an older abandoned and a younger active megamullion Which is the case is currently not known 5 Superimposed on the larger corrugated surfaces are two systems of smaller scale corrugations one on a 1 3 km scale roughly 200 m high and another finer about 100 500 m wide The latter occur up to 1 km from the ridge and is covered by elevated ridges running parallel to the spreading direction about 10 m wide hundreds of metres long and 10 m tall These in turn are covered with cm scale striations running in the same direction 6 Triple junction EditThe North American South American African triple junction is associated with the initial opening of the Atlantic Ocean and has had a complex tectonic history It probably migrated from near 10 N to its current location near the FTFZ between 72 5 and 35 5 Ma Both its location and that of the North America South American Caribbean triple junction are debated however 7 The initiation and evolution of triple junctions is often associated with mantle plumes but if this is the case near the FTFZ the limited supply of magma suggest an embryonic plume or a local anomalous mantle composition The relative movement between the North American and South American plates is very small but the resulting deformation could possibly explain both off axis seismicity and the odd mantle composition 8 References EditNotes Edit a b Fujiwara et al 2003 Bathymetry and Geological Features p 4 a b c Fujiwara et al 2003 Introduction pp 2 3 a b Godard et al 2008 Geological setting p 414 a b c Escartin amp Cannat 1999 Geological setting seafloor morphology and ultramafic outcrop distribution pp 415 417 Fujiwara et al 2003 Development of Megamullions on Ridge Flanks pp 20 26 MacLeod et al 2002 Morphology of the Striated Surface at 15 45 N pp 879 880 Smith et al 2008 Study area pp 2 3 Smith et al 2008 Equatorial Atlantic p 20 Sources Edit Escartin J Cannat M 1999 Ultramafic exposures and the gravity signature of the lithosphere near the Fifteen Twenty Fracture Zone Mid Atlantic Ridge 14 16 5 N Earth and Planetary Science Letters 171 3 411 424 doi 10 1016 S0012 821X 99 00169 7 Retrieved 22 October 2016 Fujiwara T Lin J Matsumoto T Kelemen P B Tucholke B E Casey J F 2003 Crustal Evolution of the Mid Atlantic Ridge near the Fifteen Twenty Fracture Zone in the last 5 Ma Geochemistry Geophysics Geosystems 4 3 1024 Bibcode 2003GGG 4 1024F doi 10 1029 2002GC000364 hdl 1912 5774 S2CID 17597399 Retrieved 22 October 2016 Godard M Lagabrielle Y Alard O Harvey J 2008 Geochemistry of the highly depleted peridotites drilled at ODP Sites 1272 and 1274 Fifteen Twenty Fracture Zone Mid Atlantic Ridge Implications for mantle dynamics beneath a slow spreading ridge Earth and Planetary Science Letters 267 3 410 425 Bibcode 2008E amp PSL 267 410G doi 10 1016 j epsl 2007 11 058 Retrieved 22 October 2016 MacLeod C J Escartin J Banerji D Banks G J Gleeson M Irving D H B Lilly R M McCaig A M Niu Y Allerton S Smith D K 2002 Direct geological evidence for oceanic detachment faulting The Mid Atlantic Ridge 15 45 N PDF Geology 30 10 879 882 Bibcode 2002Geo 30 879M doi 10 1130 0091 7613 2002 030 lt 0879 DGEFOD gt 2 0 CO 2 Retrieved 22 October 2016 Smith D K Escartin J Schouten H Cann J R 2008 Fault rotation and core complex formation Significant processes in seafloor formation at slow spreading mid ocean ridges Mid Atlantic Ridge 13 15 N Geochemistry Geophysics Geosystems 9 3 n a doi 10 1029 2007GC001699 hdl 1912 3266 S2CID 56238897 Retrieved 22 October 2016 Coordinates 15 19 12 N 45 52 16 W 15 320 N 45 871 W 15 320 45 871 Retrieved from https en wikipedia org w index php title Fifteen Twenty Fracture Zone amp oldid 1097336730, wikipedia, wiki, book, books, library,
