Geoscience Reference
In-Depth Information
Figure 10.51.
A schematic
illustration of a submarine
landslide on a continental
margin slipping on the
bottom-simulating
reflector (BSR). The
sediment is transported to
deep water in a turbidity
current. Significant
volumes of gas would be
released in such a failure.
hydrate layers are identifiable seismically as bottom-simulating reflectors (Figs.
4.43 and 4.44) and they may act as planes of failure in landslips (Fig. 10.51).
During glaciation, sediment is bulldozed onto the continental shelf by ice, or
accumulates there as a result of down cutting at times of low sea level (125 m at
the peak of the most recent ice age). When the ice age ends, this sediment can be
unstable. Isostatic uplift of deglaciated land can induce earthquakes, triggering
landslips. About 6200 B.C., a large earthquake (magnitude
8.2), with a 160-km
fault break and uplift of 5-15 m, may have shaken Norway. Offshore, the Storegga
slide occurred, spreading 3300 km
3
of debris across the floor of the Norwegian
Sea and producing a 20-m high tsunami on Shetland. It is likely that methane
hydrates were involved in the failure. Since the Storegga slide, new gas has seeped
up from older rocks deeper in the sequence, and has created the Ormen Lange
gas field, one of Europe's largest.
∼
10.4 Continental rift zones
10.4.1 Introduction
Some of the continental rift zones which are active today have not yet, and perhaps
may never, become active mid-ocean ridges (refer to Section 10.3.6). However,
some features are common to all continental rift zones:
1. a rift or graben structure with a rift valley flanked by normal faults;
2. negative Bouguer gravity anomalies;
3. higher than normal surface heat flow;
4. shallow, tensional seismicity; and
5. thinning of the crust beneath the rift valley
These features are in agreement with those expected for the early stages of exten-
sional rifting.
