Page 114 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 101

an oceanic plateau under the sea. These remarkable ated with the rifting of continental areas, followed by

episodes of localized enhanced partial melting in the the initiation of sea floor spreading (Sections 7.7, 7.8).
mantle punctuate the geologic record and are collec- The Parana flood basalts of Uruguay and Brazil, and the

tively termed Large Igneous Provinces (LIPs) (Section Etendeka igneous province of Namibia, emplaced

7.4.1). It seems probable therefore that there are at least 130 Ma ago, were the first expression of the Tristan da
two types of hotspot and that those originating as LIPs Cunha hotspot, and precursors of the opening of the
are the most likely to be a result of plume heads rising South Atlantic. The Deccan Traps of western India
from deep within the mantle, probably from the thermal were extruded 65 Ma ago coinciding with the creation
boundary layer at the core–mantle boundary (Section of a new spreading center in the northwest Indian
12.10). Ocean. This hotspot would appear to be located at the

Courtillot et al. (2003) proposed five criteria for dis- present position of Reunion Island (Fig. 5.9). The fi rst
tinguishing such primary hotspots (Section 12.10). They igneous activity associated with the Iceland hotspot
suggest that, on the basis of existing knowledge, only would appear to have occurred 60 Ma ago giving rise to
seven present day hotspots satisfy these criteria, although the North Atlantic igneous province of Greenland and
ultimately 10–12 may be recognized. The seven are northwest Scotland, and heralding the initiation of sea

Iceland, Tristan da Cunha, Afar, Reunion, Hawaii, Lou- floor spreading in this area. The Afar hotspot fi rst
isville, and Easter (Fig. 5.8). The first four of these hot- appeared approximately 40 Ma ago with the outpouring

spots are within the “continental hemisphere,” which of flood basalts in the Ethiopian highlands, and igneous
consists of the Indian and Atlantic Oceans and the con- activity in the Yemen, precursors of rifting and spread-
tinents that surround them. All four were initially LIPs ing in the Red Sea and Gulf of Aden. The remaining

characterized by continental flood basalts, and associ- three primary hotspots of Courtillot et al. (2003) occur

WM
103 95–99 91 82–83 Corner Smts

30°N New England Smts
Great Meteor DT
group
Chagos - Lacc.Pl 58 80

63 55 48 77 82
0° 45
Walvis Ridge 113 EB Plat 31 58
PB Masc 34 32 62 Ninetyeast Ridge
44
107 88 78–79 79 Reunion 43 24.3
61–62 38 21.0
16.1
39–40 64 14.1
~50
30°S Rio Grande RIse 38–39 52 Broken 7.0
Ridge
30–31
Tristan da Cunha Tasm.
Smts
Kerguelen Plateau
60°S
60°W 30°W 0° 30°E 60°E 90°E 120°E 150°E
Figure 5.9 Hotspot tracks in the Atlantic and Indian Oceans. Large filled circles are present day hotspots. Small filled

circles define the modeled paths of hotspots at 5 Ma intervals. Triangles on hotspot tracks indicate radiometric ages.

WM, White Mountains; PB, Parana flood basalts; EB, Etendeka flood basalts; DT, Deccan Traps (modified and redrawn

from Müller et al., 1993, courtesy of the Geological Society of America).