Structure and evolution of an oceanic megamullion on the Mid-Atlantic ridge at 27°N
type: CreativeWork
description: Megamullions in slow-spreading oceanic crust are characterized by smooth "turtle-back" morphology and are interpreted to be rotated footwalls of long-lived detachment faults. Megamullions have been analyzed in preliminary studies, but many questions remain about structural and tectonic details of their formation, in particular how the hanging wall develops in conjugate crust on the opposing side of the rift axis. This study compares the structure of an off-axis megamullion complex and its conjugate hanging wall crust on the Mid-Atlantic Ridge near 27°N. Two megamullion complexes, an older (Ml) and younger (M2), formed successively on the west side of the rift axis in approximately the same location within one spreading segment. Megamullion M1 formed while the spreading segment had only one inside comer on the west flank, and megamullion M2 formed after the segment developed double inside comers west of the axis and double outside comers east of the axis. The older megamullion formed between ~22.3 and ~20.4 Ma, and the younger megamullion formed between ~20.6 and ~18.3 Ma; they are presently ~200-300 km off-axis. Reconstruction poles of plate rotation were derived and plate reconstructions were made for periods prior to initiation of the megamullion complex (anomaly 6Ar, ~22.6 Ma), after the termination of mega mullion M1 and during the development of megamullion M2 (anomaly 5E, ~19 9 Ma), and shortly following the termination of megamullion M2 (anomaly 5C, ~17.6 Ma). These reconstructions were used to compare morphological and geophysical features of both flanks at each stage of the megamullions' development. Megamullion M1's breakaway occurred at ~22.3 Ma and slip along this detachment fault continued and propagated northward at ~20.6 Ma to form the northern portion ofM2. The exhumed footwall of mega mullion M1 has weak spreading-parallel lineations interpreted as mullion structures on its surface, and it forms an elevated plateau between the enclosing segment boun…
abstract: Megamullions in slow-spreading oceanic crust are characterized by smooth "turtle-back" morphology and are interpreted to be rotated footwalls of long-lived detachment faults. Megamullions have been analyzed in preliminary studies, but many questions remain about structural and tectonic details of their formation, in particular how the hanging wall develops in conjugate crust on the opposing side of the rift axis. This study compares the structure of an off-axis megamullion complex and its conjugate hanging wall crust on the Mid-Atlantic Ridge near 27°N. Two megamullion complexes, an older (Ml) and younger (M2), formed successively on the west side of the rift axis in approximately the same location within one spreading segment. Megamullion M1 formed while the spreading segment had only one inside comer on the west flank, and megamullion M2 formed after the segment developed double inside comers west of the axis and double outside comers east of the axis. The older megamullion formed between ~22.3 and ~20.4 Ma, and the younger megamullion formed between ~20.6 and ~18.3 Ma; they are presently ~200-300 km off-axis. Reconstruction poles of plate rotation were derived and plate reconstructions were made for periods prior to initiation of the megamullion complex (anomaly 6Ar, ~22.6 Ma), after the termination of mega mullion M1 and during the development of megamullion M2 (anomaly 5E, ~19 9 Ma), and shortly following the termination of megamullion M2 (anomaly 5C, ~17.6 Ma). These reconstructions were used to compare morphological and geophysical features of both flanks at each stage of the megamullions' development. Megamullion M1's breakaway occurred at ~22.3 Ma and slip along this detachment fault continued and propagated northward at ~20.6 Ma to form the northern portion ofM2. The exhumed footwall of mega mullion M1 has weak spreading-parallel lineations interpreted as mullion structures on its surface, and it forms an elevated plateau between the enclosing segment boun…
author: McKnight, Amy R.
language: en_US
keywords: Seafloor spreading
date published: 2001-02
date modified: 2026-07-27T22:05:16Z
provider: Woods Hole Oceanographic Institute
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: Massachusetts Institute of Technology and Woods Hole Oceanographic Institution

PropertyValue
property ID: Handle
value: https://hdl.handle.net/1912/3039

PropertyValue
property ID: DOI
value: 10.1575/1912/3039