Page 210 - Global Tectonics

196 CHAPTER 7

Deep Shallow
Marine Subaerial

Inner Landward flows
SDR
Outer high Sill

Normal Outer SDR/smooth
oceanic top basement
crust Subaqueous
Landward flows

Lava delta
Inner flows Vent

Sill

Figure 7.33 Interpretation of the main seismic facies of extrusive units at volcanic margins (modified from Planke
et al., 2000, by permission of the American Geophysical Union. Copyright © 2000 American Geophysical Union). Inset

shows enlargement of a region of landward subaqueous flows where lava deltas and inner flow units commonly occur.

Solid circles with vertical lines show locations of wells where drill holes have penetrated the various units. SDR, seaward

dipping reflectors (shaded). Bold black lines, sills.
landward and, typically, below the lava delta. Lava deltas is not necessarily accompanied by large-scale volcanism

form as flowing basalt spills outward in front of the and melting. Nonvolcanic margins lack the large volume
growing flood basalts. The emplacement of these fea- of extrusive and intrusive material that characterizes

tures is associated with the establishment of thicker their volcanic counterparts. Instead, the crust that char-
than normal ocean crust within the continent to ocean acterizes this type of margin may include highly faulted
transition zone (Planke et al., 2000). and extended continental lithosphere, oceanic litho-

The conditions and processes that form volcanic sphere formed by very slow sea floor spreading, or con-
rifted margins are the subject of much debate. In tinental crust intruded by magmatic bodies (Sayers et
general, the formation of the thick igneous crust al., 2001). In addition, these margins may contain areas
appears to require larger amounts of mantle melting up to 100 km wide that are composed of exhumed,
compared to that which occurs at normal mid-ocean serpentinized upper mantle (Fig. 7.34b,c) (Pickup et al.,
ridges. The origin of this enhanced igneous activity is 1996; Whitmarsh et al., 2001). Dipping refl ectors in
uncertain but may be related to asthenospheric tem- seismic profiles also occur within nonvolcanic margins.

peratures that are higher than those found at mid-ocean However, unlike in volcanic varieties, these refl ectors
ridges or to unusually high rates of upwelling mantle may be preferentially tilted continentward and do not
material (Nielson & Hopper, 2002, 2004). Both of these represent sequences of volcanic rock (Pickup et al.,
mechanisms could occur in association with mantle 1996). Some of these continentward-dipping refl ectors
plumes (Sections 5.5, 12.10), although this hypothesis represent detachment faults (Section 7.3) that formed
requires rigorous testing. during rifting (Boillot & Froitzheum, 2001).
Two end-member types of nonvolcanic margins

have been identified on the basis of relationships pre-
7.7.2 Nonvolcanic margins served in the North Atlantic region (Louden & Chian,
1999). The first case is derived from the southern Iberia

The occurrence of nonvolcanic margins (Fig. 7.34a) Abyssal Plain, Galicia Bank, and the west Greenland
shows that extreme thinning and stretching of the crust margins. In these margins rifting of the continent