Ultra-slow Spreading Ridge. The Gakkel Ridge in the Arctic Ocean is an
example of an ultra-slow spreading ridge segment (11 mm/yr half rate) (Michael
et al. 2003). Studies on other ultra-slow spreading ridges such as the southwest
Indian Ridge (SWIR) (Chu and Gordon 1999) led to the suggestion that volcanism
decreases as the spreading rate decreases (Karasik 1974). The SWIR near 41°S–
49°W and 34°S–69°W has a mean depth of 4,700 m and a thin crust of 4–5 km
(Muller and Jokat 2000; Meyzen et al. 2003). In 2001, scientists aboard the Healy,
a US Coast Guard icebreaker, and on a German research icebreaker, the R.V.
POLARSTERN, started the study of the ultra-slow spreading ridge located in the
Arctic Ocean. The Arctic Mid-Ocean Ridge Expedition (AMORE) expected the
Gakkel Ridge, where the spreading rate is only 1 cm (0.39 inches) per year, to
exhibit little, if any, volcanic activity (Michael et al. 2003). The spreading rate on
the Gakkel is about 20 times slower than that of the more frequently studied lower
latitude ocean-ridge systems. This ridge extends 1770 km (1100 miles) from north
of Greenland to Siberia. It is the deepest and most remote portion of the global
mid-ocean ridge system. Because the spreading rate decreases progressively
towards Siberia, it is expected that the amount of melting and magma production
would also decrease away from Greenland towards the east (Michael et al. 2003).
In fact, the central portions of the ridge showed virtually no volcanism and large
faults have exposed pieces of the Earth’s mantle directly on the sea floor.
Fast and ultra-fast Spreading Ridge. Very few areas of ultra fast spreading
(half spreading ([60 mm/year) ridge segments were found to be associated with
outcropping ultramafic rocks. This is because fast-spreading ocean ridges are
volcanically very active with a large magmatic budget and with more steady state
magmatic upwelling conditions than other slower spreading centers. However,
even in the Pacific, a colder lithosphere occurs near equatorial regions and continues south to 13°30
0 S (Garrett transform) where larger transform faults are
predominant (Fig. 4.5). Indeed, in the Equatorial Pacific the ultra-fast spreading
ridge segments located between 3 and 9°S reach a maximum depth of
3,200–4,700 m near 3–5°S (Lonsdale 1989). This area is compatible to a probable
exposition of lower crust and upper mantle peridotite.
A cold lithosphere in the south Pacific is also observed at the boundaries of the
Easter and Juan de Fuca microplates (25 and 30°S respectively), which are
characterized by transforms and magma-starved structures located at the ends of
the ridge propagators (Pito and Endeavor deeps) (Naar and Hey 1986; Francheteau
et al. 1988; Hekinian et al. 1995). The Terevaka transform fault, displacing the
West Rift branch of a spreading ridge near 24°N–116°W, and the Hess Deep
bordering the Cocos-Nazca Ridge in the Galapagos region near 2°14
0 N–101°33
0 W
are other areas with exposed peridotites. The difference between the Garrett
transform and the Hess Deep is that the Hess Deep peridotite is formed underneath
the EPR spreading center while the peridotites from Garrett were formed on the
EPR and then transported into the transform during the tectonics of spreading (see
Chap. 8).
Some of the conditions for the emplacement of exposed peridotite are listed
below. We may need:
96
4 Sea Floor Rocks
example of an ultra-slow spreading ridge segment (11 mm/yr half rate) (Michael
et al. 2003). Studies on other ultra-slow spreading ridges such as the southwest
Indian Ridge (SWIR) (Chu and Gordon 1999) led to the suggestion that volcanism
decreases as the spreading rate decreases (Karasik 1974). The SWIR near 41°S–
49°W and 34°S–69°W has a mean depth of 4,700 m and a thin crust of 4–5 km
(Muller and Jokat 2000; Meyzen et al. 2003). In 2001, scientists aboard the Healy,
a US Coast Guard icebreaker, and on a German research icebreaker, the R.V.
POLARSTERN, started the study of the ultra-slow spreading ridge located in the
Arctic Ocean. The Arctic Mid-Ocean Ridge Expedition (AMORE) expected the
Gakkel Ridge, where the spreading rate is only 1 cm (0.39 inches) per year, to
exhibit little, if any, volcanic activity (Michael et al. 2003). The spreading rate on
the Gakkel is about 20 times slower than that of the more frequently studied lower
latitude ocean-ridge systems. This ridge extends 1770 km (1100 miles) from north
of Greenland to Siberia. It is the deepest and most remote portion of the global
mid-ocean ridge system. Because the spreading rate decreases progressively
towards Siberia, it is expected that the amount of melting and magma production
would also decrease away from Greenland towards the east (Michael et al. 2003).
In fact, the central portions of the ridge showed virtually no volcanism and large
faults have exposed pieces of the Earth’s mantle directly on the sea floor.
Fast and ultra-fast Spreading Ridge. Very few areas of ultra fast spreading
(half spreading ([60 mm/year) ridge segments were found to be associated with
outcropping ultramafic rocks. This is because fast-spreading ocean ridges are
volcanically very active with a large magmatic budget and with more steady state
magmatic upwelling conditions than other slower spreading centers. However,
even in the Pacific, a colder lithosphere occurs near equatorial regions and continues south to 13°30
0 S (Garrett transform) where larger transform faults are
predominant (Fig. 4.5). Indeed, in the Equatorial Pacific the ultra-fast spreading
ridge segments located between 3 and 9°S reach a maximum depth of
3,200–4,700 m near 3–5°S (Lonsdale 1989). This area is compatible to a probable
exposition of lower crust and upper mantle peridotite.
A cold lithosphere in the south Pacific is also observed at the boundaries of the
Easter and Juan de Fuca microplates (25 and 30°S respectively), which are
characterized by transforms and magma-starved structures located at the ends of
the ridge propagators (Pito and Endeavor deeps) (Naar and Hey 1986; Francheteau
et al. 1988; Hekinian et al. 1995). The Terevaka transform fault, displacing the
West Rift branch of a spreading ridge near 24°N–116°W, and the Hess Deep
bordering the Cocos-Nazca Ridge in the Galapagos region near 2°14
0 N–101°33
0 W
are other areas with exposed peridotites. The difference between the Garrett
transform and the Hess Deep is that the Hess Deep peridotite is formed underneath
the EPR spreading center while the peridotites from Garrett were formed on the
EPR and then transported into the transform during the tectonics of spreading (see
Chap. 8).
Some of the conditions for the emplacement of exposed peridotite are listed
below. We may need:
96
4 Sea Floor Rocks