Variations in structure and tectonics along the Mid-Atlantic Ridge, 23⁰N and 26⁰N

Abstract

The variation in the depth and width of the median valley along the Mid-Atlantic Ridge (MAR) suggests that the formation of ocean crust at slow spreading centers is not a simple two-dimensional process in which crustal accretion occurs uniformly both along the ridge axis and with time. Rather, it has been proposed that the ridge axis can be divided into a number of distinct segments or spreading cells. This thesis investigates the segmentation model by studying the variability in the structure and tectonics within spreading cells at 23°N and 26°N along the MAR. The results support the segmentation model in which accretion varies along the ridge, evolving as independent spreading cells or segments, with different portions of the ridge system being in different stages of volcanic and tectonic evolution. Chapter 2 presents an overview of morphologic and tectonic variations along a 100- km-length of the MAR south of the Kane Fracture Zone (MARK area). Sea MARC.I side scan sonar data and multi-beam Sea Beam bathymetry are used to document the distribution of crustal magmatism and extensional tectonism near 23°N. The data indicate a complex median valley composed by two distinct en echelon spreading cells which overlap in a discordant zone that lacks a well-developed rift valley or neovolcanic zone. The northern cell, immediately south of the fracture zone, is dominated by a large constructional volcanic ridge and is associated with active high-temperature hydrothermal activity. In contrast, the southern cell is characterized by a NNE-trending band of small fissured and faulted volcanos that are built upon relatively old, fissured and sediment-covered lavas; this cell is inferred to be in a predominantly extensional phase with only small, isolated volcanic eruptions. Despite the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10-17 km wide inner rift valley. Small-offset normal faulting along near-vertical planes begins within a few kilometers of the ridge axis and appears to be largely completed by the time the crust moves out of the median valley. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the major geological processes occurring outside the rift valley. In Chapters 3 and 4, the microearthquake characteristics and P wave velocity structure beneath the median valley of the Mid-Atlantic Ridge near 26°N are studied; this ridge segment is characterized by a large high-temperature hydrothermal field situated within the inner floor at the along-axis high. Chapter 3 explores the tectonic variations within the crust as evidenced from the distribution and source mechanisms of microearthquakes observed by a network of seven ocean bottom hydrophones and two ocean bottom seismometers over a three week period in 1985. Hypocenters were determined for 189 earthquakes, with good resolution of focal depth obtained for 105 events. Almost all events occurred at depths between 3 and 7 km beneath the seafloor, with earthquakes occurring at shallower depths beneath the along-axis high (<4 km). The distribution of hypocenters and the diversity of faulting associated with earthquakes beneath the inner floor and walls suggests a spatially variable tectonic state for the ridge segment at 26°N. These variations are presumably a signature of lateral heterogeneity in the depth region over which brittle failure occurs, and are a consequence of along-axis changes in the thermal structure and state of stress. We suggest that at present the hydrothermal activity and deposition of massive sulfides is being sustained by heat generated by a recent magmatic intrusion. A consequence of this scenario is that thermal stresses play a dominant role in controlling the distribution of earthquakes and nature of faulting. Such a hypothesis is consistent with an apparent lack of seismicity beneath the hydrothermal field, the location of hypocenters around the low velocity zone (Chapter 4), attenuation of P wave energy to instruments atop the high (Chapter 4), the higher b-values associated with the along-axis high region, and the occurrence of high-angle (or very low angle) normal faulting and reverse faulting, as well as the variability in nodal plane orientations, associated with inner floor events beneath the along-axis high and the volcano. In Chapter 4, we report results from the explosive refraction line and from the tomographic inversion of P wave travel time residuals for seismic velocity structure in the vicinity of the hydrothermal field. The twcrdimensional along-axis P wave structure beneath the inner floor indicates that young oceanic crust cannot be adequately characterized by a simple, laterally homogeneous velocity structure, but that one-dimensional StruGtures are at least locally valid (at 5-10 km length scales). The shallowmost crust (upper 1-2 km) beneath an axial volcano and the along-axis high is characterized by significantly higher velocities (by more than 1 km/s) than are associated with the upper crust in the deepest portions of the median valley. The variation is inferred to be a consequence of more recent magmatic and volcanic activity in the along-axis high region, as compared with the alongaxis deep where tectonic fissuring has created a highly porous crust characterized by lower seafloor velocities. The crust beneath the along-axis deep appears to be typical of normal young oceanic crust, with a mantle velocity of 8.25 krn/s observed at 5 k:m depth. A low velocity zone centered beneath the along-axis high and extending under an axial volcano is imaged from 3 to 5 km depth (7.2 km/s to 6.0 km/s); the velocity decrease is required to satisfy the travel time residual data and to explain the severe attenuation in compressional wave energy to instruments atop the along-axis high. The presence of an active high-temperature hydrothermal field atop the along-axis high, together with the observations of lower P wave velocities, the absence of microearthquake activity greater than 4 km in depth, and the propagation of S waves through the crust beneath the volcano and along-axis high (Chapter 3), suggest that the volume corresponds to a region of hot rock with no seismically-resolvable pockets of partial melt. The shallow velocity gradients describing the low velocity volume(<0.6 s-l) appear to be a corrunon characteristic of inferred zones of magmatic intrusion on the MAR. Comparison of the depth to the velocity inversion with the depths determined in other seismic studies at locally high regions along the MAR, the Juan de Fuca Ridge, and the East Pacific Rise reveals a correlation between lid thickness and spreading rate, suggesting that the amount of magma available at each location is spatially variable, or that the differences in lid thickness are describing the temporal evolution of magmatic intrusions beneath mid-ocean ridges. In Chapter 5, the first direct measurement of upper mantle P- and S-wave delay times beneath an oceanic spreading center is presented. Two independent estimates of the epicenters and origin times are made for each of two earthquakes in a 1985 earthquake swarm near 25°50'N on the Mid-Atlantic Ridge using local and teleseismic arrival time data. Comparison indicates a 14-20 km northward bias in the epicenters teleseismically located using a Herrin [1968] Earth model. The bias is due to departures of the actual velocity structure from that implicit in the travel time tables used for the locations, combined with unbalanced station distribution. The comparison of origin times for the best-located event, after correction for the epicentral bias and for an oceanic crustal thickness, shows there to be only slightly lower velocities than a Herrin [1968] upper mantle; the P-wave delay is +0.3 ± 0.9 s (+0.2 ± 0.9 sand -2.4 ± 0.9 s relative to the isotropic Preliminary Earth Reference Model (PREM) and the Jeffreys-Bullen [1940] (JB) travel time tables, respectively). The lack of a resolvable P-wave delay suggests that the Herrin [1968] model is a good approximation to the average upper mantle velocity beneath this segment of the MAR. Measurement of the S-wave delay for the same MAR swarm event shows there to be a positive delay (+3.1 ± 2.0 s), or larger travel times and slower velocities compared to the JB S-wave tables (+ 3.9 ± 2.0 s relative to the isotropic PREM S-wave model). In contrast to the larger P-wave delays found in other MAR studies, the lack of a significant seismic anomaly near 26°N indicates that sizeable regions of low velocity material do not presently exist in the upper few hundred kilometers of mantle beneath this section of the ridge. This evidence argues for substantial along-axis variations in the active upwel~ng of mantle material along the slowly-spreading Mid-Atlantic Ridge. In order to explain the observation of a smaller than expected P wave delay in a region where the S delay suggests significant temperature anomalies (low velocities), we propose a model for mantle upwelling in which the decrease in travel time is due to an anisotropic P wave structure (fast direction vertical); the anisotropy results from the reorientation of olivine crystals parallel to the ascending flow and balances the travel time delay due to a region of low velocities.