Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1995
Description:
The global mid-ocean ridge system is one of the most striking geological features on
the surface of the Earth. In this system, the East Pacific Rise (EPR) is the fastest
spreading ridge and is thus considered as the most active magmatically among the plate
boundaries. In January and February of 1988, an extensive survey by the Massachusetts
Institute of Technology and the Woods Hole Oceanographic Institution was conducted
along the EPR between 9°05' and 9°55'N to study the crustal structure of the axial region.
This thesis, the result of that cruise, comprises four main topics: (1) characterization of
normal faulting from Sea Beam bathymetric data, (2) application of mechanical models to
explore the hypothesis that buoyancy arising from crustal magma chambers and
gravitational spreading of the upper crust are the principal processes leading to the initiation
and development of normal faults, (3) investigation of seafloor magnetization anomalies to
constrain upper crustal structure, and (4) analysis of gravity anomalies to examine possible
correlations between observed variations in seafloor manifestations of volcanism and
deformation and underlying structure. Thus, each topic focuses on different levels of the
mid-ocean ridge. Together with the results of seismic and other observations, the findings
are woven into a better understanding of the tectonic processes and structure of fastspreading
mid-ocean ridges.
First, to understand the characteristics of normal faults at fast-spreading ridges, we
utilized swaths of Sea Beam bathymetry and estimated the distribution and geometry of
normal fault zones using the slope of the seafloor as the criterion for a faulted surface. In our survey area, nonnal fault activity begins 2-8 km off-axis and continues at least to 30-40
km from the axis, as indicated by an increase in the total and average throws of normal
fault zones versus distance from the axis. There appears to be no significant difference in
the plan-view area of inward- and outward-facing nonnal fault zones. The distance from
the rise axis to the nearest large-offset fault zone (throw 〉 20 m) on either side of the axis
is approximately symmetric to the north of 9°23'N, but the midpoint between nearest largeoffset
fault zones is offset 2-3 km to the west of the bathymetric axis to the south of
9°23'N. The continued growth of nonnal fault zones suggests that significant extensional
stress persists to greater distances from the axis than previously thought and that the rise
axis possesses a finite strength. The argument that the rise axis has finite strength is
consistent with recent evidence for solidified axial dikes along magmatically active portions
of the EPR from near-bottom seismic refraction experiments, which suggests that, while
eruption of magma at the rise axis weakens the axis, the persistence of such weak zones is
short-lived and the emplacement zones at any given time are localized along the axis.
We examined how the presence of a low-density, low-strength magma chamber within
the crust and gravitational spreading of a mechanically strong upper crust over an
underlying substrate contribute to the fonnation of faults at a fast spreading mid-ocean
ridge by comparing the predicted stress field with the observed pattern of normal fault
zones. We employed boundary element methods to incorporate buoyancy and
gravitational spreading as body forces in an elastic medium, and we detennined stress and
strain fields for a variety of rise axis conditions and a range of possible sets of material
properties for different parts of the mid-ocean ridge.
Our results show that the strength of the rise axis is one of the most crucial factors
governing the near-axis stress field. If the rise axis is mechanically weak, the maximum
extensional stress from buoyancy occurs at shallow depth off the rise axis. A weak rise
axis may result from recent magmatism such as the intrusion of dikes into the upper crust.
On the other hand, if the rise axis is mechanically strong, which may result after
solidification and cooling of the dike zone, the maximum surface extensional stress occurs
on the rise axis. However, the reduction in size of a magma chamber that would
accompany cessation of dike injection would lead to less buoyancy and thus a lower
likelihood of stress levels sufficient for faulting. For a given set of material strengths and a
given magnitude of buoyancy force, the flexural rigidity of the upper crust plays an
important role in detennining if a zone of extension will develop off axis and, if so, the
position and horizontal extent of that zone. A thin or mechanically weak upper crust is
more likely to develop a zone of extension than one that is thick or mechanically strong.
The stress field resulting from gravitational spreading is similarly affected by the strength
of the rise axis. While buoyancy can explain a consistent distance at which normal faults
initiate off-axis, gravitational spreading can account for continued activity on normal faults
to a greater distance from the axis than can buoyancy. The existence of a magma lens can
play an important role in reducing the magnitude of the stress field for a weak rise axis, as
the crust above the magma lens can slide and thus relieve the thickness-averaged
extensional stress.
Next, we inverted surface ship measurements of the scalar magnetic field along the
EPR between 9°10' and 9°50'N. We examined whether the axial magnetization high,
which increases in amplitude to the south in our area, can best be explained by variations in
the thickness or in the magnetization intensity of the source layer. The variation in axial
magnetization is too large to be explained solely by the variations in the depth to the top of
the axial magma chamber indicated by reflection seismology. For a magnetic source layer
that is 500 or 750 m thick, the observed along-axis variations in FeO and Ti02 explain
only 36 and 60%, respectively, of the total variance of axial magnetization anomalies.
Therefore, a combination of variations in magnetic layer thickness and in intensity of
magnetization (by variations in the FeO and Ti02 contents of the source rock or by other
mechanisms) is needed to explain the along-axis variation of axial magnetization. In
addition to the increase in amplitude to the south, the axial magnetization high exhibits at
least three marked changes in magnitude and offsets in its along-axis linearity ('magnetic
devals') (at 9°25', 9°37', and 9°45'N) which appear to be related to boundaries or offsets
between the segments of the axial summit caldera (ASC). Because the amplitudes of the
axial magnetization anomalies are highest at the midpoints of the ASC segments, we
speculate that midpoints of the ASC segments are the loci of more frequent lava eruptions,
and the seafloor basalts at the midpoints are thus younger and more magnetic, than at the
segment ends.
The magnetization shows distinct short-wavelength (~ 5 km) banding to the north of
9°25'N over a region that does not appear to have been affected by an overlapping
spreading center. Among the possible explanations for these off-axis magnetization
anomalies are short geomagnetic reversal events within the Brunhes epoch, variations in
the paleointensity of the Earth's field, variations in the magnetization intensity of the source
rock due to variability in the magmatic supply, and variations in the degree of hydrothermal
alteration at the rise axis. On the basis of comparisons of forward models and
observations, short geomagnetic reversal events appear to be the most likely explanation of
these anomalies.
The analysis of sea-surface gravity field measurements shows an axial residual mantle
Bouguer gravity anomaly too large to be explained by the anomalous temperature of the
mantle or by changes in the thickness of the crust. The broad axial residual gravity low is
interpreted as a signal arising largely from the upper mantle, presumably by presence of
partial melt along the rise axis. A northward increase in the width of the low implies a
greater melt fraction in the region to the north than to the south, especially on the Pacific
plate side. The residual gravity anomaly also shows several short-wavelength local lows
along the axis (e.g., 9°21', 9°32', and 9°42'N) which correlate with along-axis variations in
axial magnetization and tomographic images of mid-crustal seismic velocities. Along axis
the local lows have an amplitude of 1.5-3 mGal and appear at a nearly regular spacing (10-
15 km). Across the axis, however, the local lows show a greater variation (3-5 mGal),
suggesting that there is an additional gravity anomaly signal arising from a low-density
structure that is approximately continuous along the axis. The anomalous masses
producing the local lows are interpreted as zones of relatively high melt concentration,
formed within the crust by recent replenishment of magma from the upper mantle, that are
surrounded by a region of lesser melt concentration corresponding to the low-velocity
volume imaged by seismic tomography. If the zone of high melt concentration are
modeled as circular rods of radius 1 km, along-axis length 10 km, and center of mass 2.25
km below the seafloor, density contrasts of 200-350 kg/m3 are needed to match the
observed anomalies. For larger anomalous mass volumes, the density contrasts would be
lower. The findings of this study support the hypothesis that the axis of the EPR can be
divided into segments 10-15 km in length, with each segment defmed by the locus and
timing of most recent emplacement of magma in the axial crust. The segments in the
study area appear to be in different phases of a magmatic cycle, but the period of such a
magmatic cycle is not known. By this view, the discrete emplacement of magma bodies
gives rise to along-axis variations in crustal structure manifested as short-wavelength
residual gravity anomalies and magnetic devals. Another consequence of a rise axis at
which magma is emplaced at discrete locations is that the mechanical strength of the axial
upper crust varies with position along the axis and over time. During active magmatism,
the rise axis acts as a weak zone and the buoyancy of the axial magma chamber and
surrounding low-velocity volume can lead to initiation of off-axis normal faulting.
However, for a long segment of the rise bounded by transform faults, the axis will have
sections with a solidified rucial injection zone as well as sections undergoing active
magmatism, and thus the rise overall may appear to have finite strength. If such a finitestrength
ridge axis is subject to significant extensional stress as a result of gravitational
spreading, mantle convective tractions, or differential cooling, then continued normal fault
activity would extend over a broad region to distances of at least several tens of kilometers
from the spreading axis.
Description:
The work in this thesis was supported by the National Science Foundation under grants
OCE-8615797, OCE-8615892, and OCE-9000177.
Keywords:
Geology
;
Plate tectonics
;
Faults
;
Thomas Washington (Ship) Cruise
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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