Scrocca et al., (2005)

Bill et al. 2001

Continental orogenic belts - the Western US Cordillera

Long-lasting subduction under the west coast of North America has contributed to several hundred million years of accretion and deformation at the continental margin. Bird (1998)'s map (below) shows the areas of Mesozoic - Cenozoic compressional deformation (Sevier: thin skinned; Laramide: thick skinned), zone of post-collisional extension (Basin and Range province) and the region of accreted terranes (Canadian Cordillera to Alaska). Although not shown on this map, accreted terranes contribute to the crust affected by deformation in the contractional and extensional belts as well.

Bird (1998)

A closer look at the thrust faults around the boundary between Sevier and Laramide deformation and the Colorado Plateau (Bird, 1998). The Sevier and Laramide deformational events (or Orogenies) actually overlap in space and time, but are distinguished broadly by the structural style. The Sevier Orogeny is associated with east-vergent, mostly thin-skinned fold and thrust belts. Across Utah, Nevada, and Arizona, it is possible to see Paleozoic marine sediments thrust over Mesozoic marine and terrestrial sediments along low-angle thrusts. The Laramide belt is characterized by thick-skinned deformation, with steep reverse faults cutting down into North American "basement" rocks (older continental crust on which the Phanerozoic sediments were deposited). Thus, the high Rockies in Colorado have Proterozoic gneisses exposed along the peak of the range due to deep exhumation.

Bird, 1998

The transition between Sevier-type and Laramide-type deformation is correlated to volcanism. In the figure from Bird (1984) below, the gray areas indicate the position of active volcanism at various times from late Cretaceous into the Tertiary. The subduction trench off the west coast shows where the Farallon Plate (which was east of the Pacific Plate) subducted under North America. In the latest Cretaceous (about 75-70 Ma), arc volcanism disappeared from a long section of the Farallon arc, moved somewhat east, and then resumed by about 40-30 Ma.

Bird, 1984

The gray areas show where the Farallon slab may have been subducting very shallowly ("flat-slab") similar to the central Andes today. The area of flat slab subduction is inferred from the intensity of upper plate deformation, as well as the pattern of volcanism.

Bird, 1989

At this shallow dip, there is little or no mantle above the dewatering source to melt and produce arc magmas, and the slab doesn't get deep enough to cause dewatering. No return flow occurs to bring fresh asthenospheric mantle melt source. However, the cold lithospheric mantle is negatively buoyant, and once collision stops it is no longer supported viscously. At some time interval after the end of contractional deformation, a short period of fast uplift and volcanism occurred. This has been attributed to the delamination or "cold drip" of old lithosphere detaching from the base of the thickened crust and falling away into the mantle. Since proposed for the SW US, this model has been applied to many modern and recent orogens around the world.

Bird, 1979

Bird (1979) Continental delamination and the Colorado Plateau
Bird (1984) Laramide crustal thickening event in the Rocky Mountain foreland and the Great Plains
Bird (1998) Kinematic history of the Laramide orogeny in latitudes 35-49N, western United States

*Please note - lots of other people have contributed to this field, I just picked Bird's papers because I found his graphics easy to understand.


Continental orogenic belts - an example from Mozambique

The map below (from Gray et al., 2008, Geological Society of London Special Publications) shows the continental fragments of the southern hemisphere as they were during the collisions that formed Gondwana about 650-550 Ma. On this map, the Archean cratons (continental blocks that were assembled in the Archean and have not been deformed or metamorphosed significantly) are shown with + symbols. The wavy lines show areas which were actively deformed/metamorphosed during the Neoproterozoic collisions. Dark gray areas and dotted areas were deposited and/or deformed after the assembly of Gondwana as a super-continent. From the outlines of the modern continents, you can see that when Gondwana broke up (Permo-Triassic), the rifting followed along some, but not all, of the Neoproterozoic orogenic belts.

The map below shows the geology of eastern Mozambique, which is essentially flat, but the geology displays the traces of Neoproterozoic collision. The northern half of the map shows a series of thrust-bounded terranes imbricated from west to east. These rocks are igneous and sedimentary rocks formed during a previous period of orogeny 1100 Mya and metamorphosed at granulite facies and thrusted during the 650 Ma collisions. To the south, the Nampula block is mostly made up of granite to tonalite plutonic rocks, also ~1100 in age, was metamorphosed at amphibolite facies during the construction of Gondwana. In between, the bright green belt contains rocks of both ~600 and ~1100 ages, intensively deformed, displaying a mixture of metamorphic grades. The deformation is called the Lurio Belt. The light pink blobs are post-orogenic granite plutons which intruded in the early Cambrian. These are not deformed at all.

Viola et al. suggested a tectonic story to explain the complex geological relationships. Below, the NW-SE cross section shows the Lurio Belt in Turquoise. Viola's scenario shows north-vergent low-angle thrusting early in the orogeny, followed by intense shortening and folding within a localized area (the Lurio Belt). About 500 Ma, granites intruded across the southern area in small plutons - presumably caused by mantle melting in the aftermath of the orogeny. Associated with this magmatism, the area was uplifted and eroded - speculatively associated with extension as the new mountains warmed up at their roots and collapsed under their own weight.