The perception that orogens may be divided into eugeosynclines having abundant volcanic rocks and miogeosyncHnes lacking volcanics triggered the hypothesis that eugeosynclines are perhaps ancient island arcs. Subsequent geophysical and geological studies supported a concept of eugeosynclines as at least partly comprising ancient oceanic terranes as well as arcs. It has become increasingly evident that to understand the role of orogenesis in crustal evolution the extent of oceanic crust in the basement of eugeosynclinal belts must be determined.
A consistent feature of eugeosynclines is their composite nature as manifest in elongate tectono-strati- graphic units or tectonic elements. The stratigraphic, tectonic, and plutonic and metamorphic evolution of each element is distinct and yet is partly related to adjacent elements. Major tectonic elements are separated by long-lived faults. The lithological sequence of each element is correlated with its basement type and the nature and history of its boundaries. Ancient eugeosynclinal tectonic elements may be elucidated by comparison with modern tectonic elements clearly related to plate motions. The basement of such tectonic elements is highly varied, and thus all eugeosynclinal zones are not ensimatic.
Tectonic processes play a key role in determining preservation and mode of occurrence of oceanic lithosphere in orogens. To survive orogenesis, oceanic crust must have a thick low-density cap or be tectonically intercalated with thick lower density materials. Preservation of oceanic crust and mantle within orogens therefore requires some mechanism of crustal thickening, commonly by sedimentation (to form buried basement of sedimentary furrows), magmatism (to form basement of oceanic arcs), or tectonic imbrication (to form ophiolite sequences and melange belts). In the absence of such mechanisms, gravity and subduction can efficiently remove dense oceanic lithosphere from the crust. The crust of continental rifts, rhombochasms, sphenochasms, marginal basins, oceanic arcs, and remnant basins is susceptible to the crust-thickening mechanisms mentioned above and is, therefore, more abundantly preserved in orogenic belts than is normal ridge-generated oceanic crust.
The diagnosis of ensimatic tectonic elements within ancient orogens is difficult, but the composition of igneous rocks and other data can be related to basement composition. The composition of detritus can also indicate the nature and time of linkage of source blocks.
The sialic vs. simatic nature and extent of the initial basement of eugeosynclinal zones are highly varied and are dependent upon the evolution of the individual orogen in terms of geometry and nature of starting conditions, rifts, arcs, marginal and remnant basins, subduction zones, and strain history. The addition of oceanic and mantle materials to the continents by orogen accretion is complex due to the interaction and evolution of many processes. Single processes may be described but not single theories or finite models of orogenesis. Each orogen is a unique time-space collage of mappable elements, all generated, assembled, and rearranged by tectonic processes.