Structure and evolution of an oceanic megamullion on the Mid-Atlantic ridge at 27°N

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 boundaries (non-transform discontinuities). There was an expansion southward of the detachment fault forming megamullion M2 at ~ 20.1 Ma. It either cut a new detachment fault through megamullion M1, stranding a piece of megamullion M1 on the conjugate side (east flank), or it linked into the active detachment fault that was forming megamullion M1 or propagated into its hanging wall. The expanded detachment of mega mullion M2 and the termination of mega mullion M1 occurred during a time when the enclosing spreading segment roughly doubled in length and formed two inside comers. Megamullion M2 developed prominent, high-amplitude (~600 m) mullion structures that parallel the spreading direction for more than 20 km at each inside comer. Its detachment fault was abandoned ~ 18.6 Ma in the south and ~ 18.3 Ma in the north. The gravity of this area demonstrates a consistent pattern of higher gravity corresponding to inside comers with thinner crust, apparently caused by fault exhumation of deep lithosphere, and lower gravity values corresponding to outside comers, indicating thicker crust, most likely a result of volcanic accretion. The gravity pattern of the area also helps with interpreting evolution of the megamullion complex. The southern section of megamullion M1 exhibits a series of inside-comer highs and elevated gravity values while the northern section has lower gravity values until megamullion M2 began to form. This change coincides with the change of the northern segment edge from an outside comer to an inside comer. During the formation of megamullion M2, a gravity high developed over the center of the megamullion. After the termination of megamullion M2, the gravity values of both the northern and southern sections of the spreading segment decrease. This pattern suggests exhumation of higher-density lithosphere during formation of M1 and M2, and a return to more normal ridge-axis conditions following termination of the megamullion complex. The gravity of conjugate crust is consistently more negative, slightly decreasing in value during the formation of megamullion M2. This suggests that crust on the east flank is significantly thicker than that on the west flank, and that rift-axis magmatism may have slightly increased at the time that megamullion M2 formed. We modeled gravity of an idealized structural cross-section of megamullion M2 to investigate possible structure and composition of the megamullion. Models with different detachment-fault angles and degrees of serpentinization of exhumed mantle that may be present in the megamullion were compared to Residual Mantle Bouguer Anomaly (RMBA) profiles. All models show gravity peaks slightly skewed towards the termination because higher-density rock is exposed closer to the termination than to the breakaway. Four models that varied the detachment fault angle show small variations that are unresolvable in the actual gravity data. Thus, the gravity profile of a megamullion is not diagnostic of its detachment fault angle from 30° to 60°/90°. Models that varied the degree of serpentinization of a lithospheric wedge beneath the megamullion show that slight variations in density give rise to large changes in the modeled gravity profiles. Comparison of model results against gravity profiles taken across megamullion M2 indicate that the magnitude of the gravity high associated with the megamullion is best explained by densities between 2800 kg/m3 and 3000 kg/m3 in the main body of the megamullion. This corresponds to peridotite serpentinized approximately 50%, or to gabbro (~2800 kg/m3).