avalanche debris, which accumulate along steep slopes. Because of their faulted
nature, scientists believe that Fracture Zones are an ‘‘open window’’ into deeper
layers of the oceanic lithosphere. The depths measured in a fracture zone are
roughly equivalent to the amount of vertical displacement of the FZ area. Nevertheless, caution has to be exercised in interpreting any data acquired from the
apparent exposition of so called deep-seated material, which is believed to represent in situ exposed mantle material such as peridotite.
Francheteau et al. (1976) had questioned the assumption that deep-seated layers
(representing deep sections of the crust-lithosphere) are exposed during fracture.
Several factors need to be taken into consideration. For example, staircase faulting
along the wall of a fracture zone might never reach deep-seated material because
of its small vertical relief displacement. Indeed, at least two major processes must
be operating in order to expose Earth’s deeper layers. One process takes place
without appreciable vertical displacement but, instead, is due to plate extension
producing the cracks that expose Earth’s deeper sections. The other process
involves the forceful injection of deep-seated material during the tectonic activity
of faulting. For example, a diapir intrusion of altered (serpentinized) peridotite
could be squeezed through faulted blocks like ‘‘tooth paste’’.
In order to expose deep portions of the lithosphere, it would be necessary to
have a relatively large vertical motion of the fault plane surface. Most dives in the
FAMOUS area near 37°N on the Mid-Atlantic Ridge or in the Garrett transform of
the Pacific, have shown that the zones of faulting have small vertical scarps (\5 m
throw). However, the presence of numerous, small fault scarps would hardly be
able to expose layers from 2–3 km deep. In order to do this, it would be necessary
to be in a special situation with large vertical and quasi-uninterrupted uplift, which
is preferably associated with magma-starved regions (see Chap. 4). Areas that
have exposed deep-seated layers of the lower-crust or upper-mantle are found in
several locations such as the Vema FZ, the Saint Peter and St. Paul’s Rocks FZ, or
the Romanche and Terevaka (south Pacific) transform faults, which show uninterrupted sections of peridotite-gabbro-dyke complexes. These regions have
undergone the special tectonic mechanism known as thrust-folding giving rise to
uplifted slivers of transverse ridges (TR) and/or to the diapiric upwelling of
buoyant material (due to buoyancy forces) (i.e. serpentinization) (see Chap. 4).
Other fracture zones explored in the Atlantic Ocean (Campsie et al. 1973;
Fleming et al. 1970; Hekinian and Aumento 1973; Rusby 1993; Detrick et al.
1995; Lagabrielle et al. 1992a,b) have recovered basalt, plus mafic and ultramafic
intrusive rocks. The fracture zones displacing slow spreading ridge segments of the
Atlantic Ocean are the most privileged sites for exposing ultramafic rocks compared to the Pacific’s fast spreading ridge segments.
Transform faults are generally not privileged sites for oceanic crust creation.
Nevertheless, scientists have accumulated evidence that some transform faults are
also sites of magmatic upwelling producing volcanic activity. This happens when
the transform domain is influenced by the mechanism of extension during a change
in direction of the moving plates. In other words, a transform fault could become
the site of crustal extension, much like a spreading ridge segment, should it
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8 Fracture Zones and Transform Faults
nature, scientists believe that Fracture Zones are an ‘‘open window’’ into deeper
layers of the oceanic lithosphere. The depths measured in a fracture zone are
roughly equivalent to the amount of vertical displacement of the FZ area. Nevertheless, caution has to be exercised in interpreting any data acquired from the
apparent exposition of so called deep-seated material, which is believed to represent in situ exposed mantle material such as peridotite.
Francheteau et al. (1976) had questioned the assumption that deep-seated layers
(representing deep sections of the crust-lithosphere) are exposed during fracture.
Several factors need to be taken into consideration. For example, staircase faulting
along the wall of a fracture zone might never reach deep-seated material because
of its small vertical relief displacement. Indeed, at least two major processes must
be operating in order to expose Earth’s deeper layers. One process takes place
without appreciable vertical displacement but, instead, is due to plate extension
producing the cracks that expose Earth’s deeper sections. The other process
involves the forceful injection of deep-seated material during the tectonic activity
of faulting. For example, a diapir intrusion of altered (serpentinized) peridotite
could be squeezed through faulted blocks like ‘‘tooth paste’’.
In order to expose deep portions of the lithosphere, it would be necessary to
have a relatively large vertical motion of the fault plane surface. Most dives in the
FAMOUS area near 37°N on the Mid-Atlantic Ridge or in the Garrett transform of
the Pacific, have shown that the zones of faulting have small vertical scarps (\5 m
throw). However, the presence of numerous, small fault scarps would hardly be
able to expose layers from 2–3 km deep. In order to do this, it would be necessary
to be in a special situation with large vertical and quasi-uninterrupted uplift, which
is preferably associated with magma-starved regions (see Chap. 4). Areas that
have exposed deep-seated layers of the lower-crust or upper-mantle are found in
several locations such as the Vema FZ, the Saint Peter and St. Paul’s Rocks FZ, or
the Romanche and Terevaka (south Pacific) transform faults, which show uninterrupted sections of peridotite-gabbro-dyke complexes. These regions have
undergone the special tectonic mechanism known as thrust-folding giving rise to
uplifted slivers of transverse ridges (TR) and/or to the diapiric upwelling of
buoyant material (due to buoyancy forces) (i.e. serpentinization) (see Chap. 4).
Other fracture zones explored in the Atlantic Ocean (Campsie et al. 1973;
Fleming et al. 1970; Hekinian and Aumento 1973; Rusby 1993; Detrick et al.
1995; Lagabrielle et al. 1992a,b) have recovered basalt, plus mafic and ultramafic
intrusive rocks. The fracture zones displacing slow spreading ridge segments of the
Atlantic Ocean are the most privileged sites for exposing ultramafic rocks compared to the Pacific’s fast spreading ridge segments.
Transform faults are generally not privileged sites for oceanic crust creation.
Nevertheless, scientists have accumulated evidence that some transform faults are
also sites of magmatic upwelling producing volcanic activity. This happens when
the transform domain is influenced by the mechanism of extension during a change
in direction of the moving plates. In other words, a transform fault could become
the site of crustal extension, much like a spreading ridge segment, should it
256
8 Fracture Zones and Transform Faults