Margins A Research Initiative for Interdisciplinary Studies of the Processes Attending Lithospheric Extension and Convergence (1989) / Chapter Skim
Currently Skimming:
2 Results of the Workshop
2 Results of the Workshop
Pages 12-23
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 12... ...
To understand the causes and interactions that control fluid flow in post-rifting divergent margins. To understand the dynamics of short-term deformation at continental margins.
|
From page 13... ...
The gentle dip associated with subduction thrusts at convergent margins and with detachment faults at some continental rifts suggests low shear stresses across these types of fault. Heat flow and stress direction measurements at the well-studied San Andreas fault, a translational plate boundary, also indicate low shear stresses.
|
From page 14... ...
Apart from the effect of temperature, the presence of even a small percentage of melt drastically alters the rheological properties of the mantle. Magmatic Processes Magmatism at continental margins is the primary source of new continental crust e A long-standing problem is that the continents have an average composition of andesite, whereas melts from the mantle at convergent margin arcs and at continental 14
|
From page 15... ...
In continental rifts, a relationship has been recognized between the spacing of magmatic centers and structural segmentation, a relationship reminiscent of that for oceanic spreading centers. Many investigators studying the dynamics of oceanic crusty accretion believe that segmentation reflects the distribution of magma generation and ascent, so that magmatic processes actually control segmentation, but it is equally plausible that global stress patterns associated with plate motions govern segmentation, which in turn controls magma transport.
|
From page 16... ...
Arguably, the principal dif ferences between convergent and divergent margin magmatism are clue to clif ferences in the composition and rot e of associated fig aids. Fluids beneath continental rifts appear to be dominated by methane and carbon dioxide r whereas those beneath convergent margins are dominated by water released from dehydration of the subducting slab.
|
From page 17... ...
, whereas alteration resulting from fluid flow is best appreciated from observations of large, on-lane} outcrops Sedimentation Processes Sediments represent composite records of past climate, sea level fluctuations, ocean circul ation and chemistry, ant} variations in sediment supply. The margin depositional system responds to all these local, regional, and global geological processes, which transcend both shorelines and continent-ocean structural boundaries.
|
From page 18... ...
Along active margins , sorting out the geologic implications of commonly thick, of ten rapidly formed sedimentary sequences is critical for reconstructing plate-margin history and the associated development of continental crust. Major increases in sediment supply can be keyed to pulses of arc volcanism and margin uplift due to changes in the position of the underthrusting plate or to collision events e In summary, virtually all tectonic' oceanographic and climatological factors leave their combined imprint on margin stratigraphy.
|
From page 19... ...
The outlook, for instance, of the field geologist studying outcrops often differs markedly from that of the geophysicist studying seismic reflection profiles collected at sea. These disparate perspectives tend to inhibit understanding the cross ~ inked processes that operate at continental margins.
|
From page 20... ...
There is virtually no area in earth science today in which computational methods using high-speed computers have not been immensely valuable, and many in which advances in understanding simply would not have taken place at all were it not for the application of computer modeling. The development of subsurface images using seismic reflection techniques cannot be done without large high-speed computers, and the study of mantle convection would be extremely limited if computational facilities were not available.
|
From page 21... ...
Seismic acquisition carried out or proposed by universities or consortia that do not have suitable capabilities (the BIRPS program in the U.K., LITHOPROBE in Canada, COCORP and EDGE in the United States) employ seismic contractors who normally work for the petroleum exploration industry.
|
From page 22... ...
This statement applies equally to description of the geometry of mantle flow developed from tomographic inversion of earthquake data, the production of seismic-reflection images, and the determination of velocity structure of the crust. Field experiments aimed at defining fundamental processes of margin evolution also rely critically on insights gained from computational simulations, so that the critical location for these experiments is chosen properly, and is followed by optimum analysis of the resultant data.
|
From page 23... ...
the rheologica~ properties of the lithosphere under real earth conditions. Sustained support of theoretical and computational programs is an essential component of future margins research.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.