Page 185 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 171

basalts. These eruptions represent one major subcate- prior to or after the onset of extension (Menzies et al.,
gory of a broad group of rocks known as Large Igneous 2002).
Provinces (LIPs). Estimation of the total volumes of lava in LIPs is
Large Igneous Provinces are massive crustal emplace- complicated by erosion, dismemberment by sea fl oor

ments of mostly mafic extrusive and intrusive rock that spreading, and other tectonic processes that postdate
originated from processes different from normal sea their eruption. The Hawaiian Islands are well studied in
floor spreading. LIPs may cover areas of up to several this respect. Seismic data have shown that beneath the

2
million km and occur in a wide range of settings. crust is a zone of rocks with a particularly high seismic
Within oceanic plates, LIPs form oceanic plateaux such velocity, which is probably derived from the same
as Kerguelen and Ontong Java (Fig. 7.15). This latter mantle source as the surface volcanic rocks. For Hawaii
example occupies an area two-thirds that of Australia. a basic relationship exists between velocity structure
The Siberian and Columbia River basalts are examples and the total volume of igneous rock (Coffi n &
that have erupted in the interior of continental plates. Eldholm, 1994). This relationship has been applied to
In East Africa, the Ethiopian and Kenyan fl ood basalts other LIPs to determine their volumes. For example,
are associated with active continental rifting and the the Columbia River basalts are composed of 1.3 million
3
Deccan Traps in India and the Karoo basalts in southern km , whereas the Ontong Java Plateau is composed of
3
Africa were emplaced near rifted continental margins. at least 27 million km of volcanic rock and possibly
This diversity indicates that not all LIPs are associated twice this amount (Section 5.5). These values are much
with zones of extension. Within rifts their eruption can higher than the continental flood basalts of East Africa.

occur synchronously with rifting or million of years In Kenya the total volume of flood basalts has been

SIBE
NAIP
COLR
DECC

ETHI

ONTO

KERG

LIPs
Rifted volcanic margins

Figure 7.15 Map showing global distribution of Large Igneous Provinces (modified from Coffin & Edholm, 1994, by

permission of the American Geophysical Union. Copyright © 1994 American Geophysical Union). Rifted volcanic
margins are from Menzies et al. (2002). Labeled LIPs: Ethiopian flood basalts (ETHI); Deccan traps (DECC); Siberian

basalts (SIBE); Kerguelen plateau (KERG); Columbia River basalts (COLR); Ontong Java (ONTO); North Atlantic igneous
province (NAIP).