Open AccessCrustal Evolution of the Mid‐Atlantic Ridge near the Fifteen‐Twenty Fracture Zone in the last 5 Ma
Author(s) -
Fujiwara Toshiya,
Lin Jian,
Matsumoto Takeshi,
Kelemen Peter B.,
Tucholke Brian E.,
Casey John F.
Publication year - 2003
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2002gc000364
Subject(s) - geology , fracture zone , crust , mid atlantic ridge , seismology , magnetic anomaly , seafloor spreading , mantle (geology) , ridge , detachment fault , tectonics , oceanic crust , transform fault , mid ocean ridge , bouguer anomaly , lithosphere , gravity anomaly , geophysics , paleontology , subduction , extensional definition , oil field
The Mid‐Atlantic Ridge around the Fifteen‐Twenty Fracture Zone is unique in that outcrops of lower crust and mantle rocks are extensive on both flanks of the axial valley walls over an unusually long distance along‐axis, indicating a high ratio of tectonic to magmatic extension. On the basis of newly collected multibeam bathymetry, magnetic, and gravity data, we investigate crustal evolution of this unique section of the Mid‐Atlantic Ridge over the last 5 Ma. The northern and southern edges of the study area, away from the fracture zone, contain long abyssal hills with small spacing and fault throw, well lineated and high‐amplitude magnetic signals, and residual mantle Bouguer anomaly (RMBA) lows, all of which suggest relatively robust magmatic extension. In contrast, crust in two ridge segments immediately north of the fracture zone and two immediately to the south is characterized by rugged and blocky topography, by low‐amplitude and discontinuous magnetization stripes, and by RMBA highs that imply thin crust throughout the last 5 Ma. Over these segments, morphology is typically asymmetric across the spreading axis, indicating significant tectonic thinning of crust caused by faults that have persistently dipped in only one direction. North of the fracture zone, however, megamullions are that thought to have formed by slip on long‐lived normal faults are found on both ridge flanks at different ages and within the same spreading segment. This unusual partitioning of megamullions can be explained either by a ridge jump or by polarity reversal of the detachment fault following formation of the first megamullion.