IGCP-574 : Bending and Bent Orogens, and Continental Ribbons


 

 

IGCP-574 : Background

Background

There is perhaps no more significant continental geological feature than mountains. Mountain systems, or Orogens, are the factories in which stable continental crust is manufactured; they exert a first order control on local and global climate; and they are host to the bulk of Earth’s economic mineral deposits. Much of human history, including the development of distinct populations and the related construction of political boundaries, revolves around our interactions with and migrations along and across mountain belts. Therefore, understanding the origin and evolution of mountain systems is of great geological, economic, and social significance. The primary goal of IGCP Project 574 is to develop an improved understanding of the plate tectonic interactions and processes that initially give rise to and which subsequently modify great mountain systems through the study of map- or plan-view bends of mountain belts.


Orogens extend hundreds to thousands of kilometers across Earth’s surface, and while they are roughly linear in plan, all are to some degree curved or bent when observed in map view. The question is are these bends tectonically significant features? If bends were restricted to minor deflections both in terms of scale and magnitude, we would ascribe little significance to them. This is, however, not the case. For example, The western end of the Paleozoic Variscan Orogen of Europe is characterized by a 180° hairpin bend that affects a 500 km wide mountain system (Suess, 1909). This Iberian bend of the Variscan mountain system formed in the Permian (Weil et al., 2001; Weil et al., 2000) is temporally associated with a massive thermal and magmatic event that metamorphosed much of the crust of central Pangea (Gutiérrez-Alonso et al., 2004), and may be the single largest structure ever mapped on Earth (Gutiérrez-Alonso et al., 2008). Modern bends are equally impressive and of equivalent geological significance. Some of the greatest topographic relief on Earth is to be found associated with the still evolving tight bends (called syntaxes) that adorn the eastern and western ends of the Himalayas (Burg and Podladchikov, 2000). These syntaxes are characterized by elevated heat flow and are the sites of exhumation of large tracts of highly metamorphosed lower crustal rocks (Zeitler et al., 2001). Likewise, Earth’s second largest and highest plateau, the Altiplano of South America, sits astride, and reached its present elevation during formation of, the great Bolivian bend of the Andes (Allmendinger et al., 1997; Isacks, 1988).


Despite the scale of these structures and their spatial and genetic association with crustal-scale exhumational, magmatic, and thermal events, there remains little consensus regarding the processes responsible for producing bends of orogens (Sussman and Weil, 2004)(and references therein). Hence the question remains, are they tectonically significant? Thomas, in his groundbreaking papers on the Appalachians (Thomas, 1977, 2006), established a basic assumption in the interpretation of bent mountain belts in which the map-view geometry of an orogen is a reflection of the primary shape of the pre-collisional continental rifted margin. Hence the salients and recesses that characterize the Appalachian mountains are commonly interpreted to reflect the geometry of the reentrants and promontories that characterized the Iapetan passive margins of Laurentia. A number of observations, however, are inconsistent with such an endogenic interpretation of the bends of the Appalachians. For example, paleomagnetic data from the region require that at least parts of the orogen began as more linear features that were subsequently bent (Stamatakos et al., 1996), a conclusion supported by recent studies constraining the orientation of the stress field responsible for Appalachian orogeny (Ong et al., 2007). We are then faced with two conflicting paradigms concerning the origin and significance of bends of mountain systems: either the bends are primary features specific to each mountain belt, or they are the result of tectonic processes that are responsible for some of the largest documented structures on the face of the Earth.


The Iberian bend of the Variscan Orocline provides further insight into this debate. The arcuate geometry of the Variscan mountain system, which is inferred to have developed in response to the collision of Gondwana with Laurasia forming Pangea, has been variously interpreted as reflecting the original (pre-collisional) geometry of the continental margin (Brun and Burg, 1982; Lefort, 1979), the result of indentation during collision (Matte and Ribeiro, 1975), or as a structural artifact (Martinez-Catalan, 1990; Perez-Estaun et al., 1988). These interpretations of the bend as being a primary paleogeographic or structural feature cannot be reconciled with paleomagnetic data showing that the bend resulted from buckling of an originally much more linear mountain system (Van der Voo et al., 1997; Weil, 2006; Weil et al., 2001; Weil et al., 2000). This secondary or tectonic interpretation of the Iberian bend requires that the Variscan belt of northern Iberia formed a linear ribbon continent that buckled prior to being added to the north margin of Gondwana, and seems in conflict with geological data that ties Iberia to Gondwana. Alternatively, the ribbon continent may have formed an elongate archipelago that rooted into Gondwana to the southeast and extended north across the Rheic ocean that separated Gondwana and Laurentia which is consistent with the strong stratigraphic ties to Gondwana. Resolution of this debate is central to determining the causes of the Permian magmatic and thermal event that affected much of Iberia (Gutiérrez-Alonso et al., 2004), and is therefore of significant local interest; exploration strategies remain dependent upon having a broad understanding of the processes responsible for thermally driven fluid flow. The more fundamental issue in understanding the paleogeographic and tectonic evolution that led to the formation of Pangea is dependent upon our successfully resolving the origin of the Iberian bend (Gutiérrez-Alonso et al., 2008).


Therefore, understanding how bends of mountain systems develop is a fundamental first order Earth System problem whose resolution is central to understanding thepaleogeographic and tectonic evolution of Earth; this is the aim of our proposed IGCP project.


References Cited :


Allmendinger, R.W., Jordan, T.E., Kay, S.M., and Isacks, B.L., 1997, The evolution of the Altiplano-Puna plateau of the central Andes: Annual Review of Earth and Planetary Sciences, v. 25, p. 139-174.

Brun, J.P., and Burg, J.P., 1982, Combined thrusting and wrenching in the Ibero-Armorican arc: a corner effect during continental collision: Earth and Planetary Science Letters, v. 61, p. 319-332.

Burg, J.-P., and Podladchikov, Y., 2000, From buckling to asymmetric folding of the continental lithosphere: numerical modeling and application of the Himalayan syntaxes, in Khan, M.A., Treloar, P.J., Searle, M.P., and Jan, M.Q., eds., Tectonics of the Nanga Parbat Syntaxis and the Western Himalaya, Volume 170: Geological Society Special Publication: London, The Geological Society, p. 219-236.

Gutiérrez-Alonso, G., Fernández-Suárez, J., and Weil, A.B., 2004, Orocline triggered lithospheric delamination, in Sussman, A.J., and Weil, A.B., eds., Orogenic curvature: Integrating paleomagnetic and structural analyses: Special Paper 383: Boulder, Colorado, The Geological Society of America, p. 121-130.

Gutiérrez-Alonso, G., Fernández-Suárez, J., Weil, A.B., Murphy, J.B., Nance, R.D., Corfu, F., and Johnston, S.T., 2008, Self-subduction of the Pangean global plate: Nature Geoscience.

Isacks, B.L., 1988, Uplift of the central Andean plateau and bending of the Bolivian orocline: Journal of Geophysical Research, v. 93, p. 3211-3231.

Lefort, L.P., 1979, Iberian-Armorican arc and Hercynian Orogeny in Western Europe: Geology, v. 7, p. 384-388.

Martinez-Catalan, J.R., 1990, A non-cylindrical model for the northwest Iberian allochthonous terranes and their equivalents in the Hercynian belt of western Europe: Tectonophysics, v. 179, p. 253-272.

Matte, P., and Ribeiro, A., 1975, Forme et orientation de l'ellipsoide de deformation dans la viration Hercynienne de Galicia: relation avec le plissement et hypotheses sur la genese de l'arc Iberio-Armoricain: Comptes Rendus de l'Academie des Sciences, v. 280, p. 2825-2828.

Ong, P.F., Van der Pluijm, B.A., and Van der Voo, R., 2007, Early rotation and late folding in the Pennsylvania salient (U.S. Appalachians): Evidence from calcite-twinning analysis of Paleozoic carbonates: Geological Society of America Bulletin, v. 119, p. 796-804.

Perez-Estaun, A., Bastida, F., Alonso, J.L., Marquinez, J., Aller, J., Alvarez-Marron, J., Marcos, A., and Pulgar, J.A., 1988, A thin-skinned tectonics model for an arcuate fold and thrust belt: the Cantabrian zone (Variscan-Armorican arc): Tectonics, v. 7, p. 517-537.

Stamatakos, J., Hirt, A.M., and Lowrie, W., 1996, The age and timing of folding in the central Appalachians from paleomagnetic results: Geological Society of America Bulletin, v. 108, p. 815-829.

Suess, E., 1909, The Face of the Earth (translated from German by Sollas, H.B.C., and Sollas, W.J.): Oxford, Clarendon, p. 672.

Sussman, A.J., and Weil, A.B., 2004, Preface, in Sussman, A.J., and Weil, A.B., eds., Orogenic Curvature: Integrating paleomagnetic and structural analyses, Volume Special Paper 383: Denver CO, Geological Society of America, p. v-vii.

Thomas, W.A., 1977, Evolution of Appalachian-Ouachita salients and recesses from reentrants and promontories in the continental margin: American Journal of Science, v. 277, p. 1233-1278.

—, 2006, Tectonic inheritance at a continental margin: GSA Today, v. 16, p. 4-11.

Van der Voo, R., Stamatakos, J.A., and Pares, J.M., 1997, Kinematic constraints on thrust-belt curvature from syndeformational magnetizations in the Lagos del Valle Syncline in the Cantabrian arc, Spain: Journal of Geophyiscal Research, v. 102, p. 10,105-10,119.

Weil, A.B., 2006, Kinematics of orocline tightening in the core of an arc: Paleomagnetic analysis of the Ponga Unit, Cantabrian Arc, northern Spain: Tectonics, v. 25, p. TC3012, doi:10.1029/2005TC001861.

Weil, A.B., Van der Voo, R., and van der Pluijm, B.A., 2001, Oroclinal bending and evidence against the Pangea megashear: The Cantabria-Asturias arc (northern Spain): Geology, v. 29, p. 991-994.

Weil, A.B., Van der Voo, R., van der Pluijm, B.A., and Pares, J.M., 2000, The formation of an orocline by multiphase deformation: a paleomagnetic investigation of the Cantabria-Asturias Arc (northern Spain): Journal of Structural Geology, v. 22, p. 735-756.

Zeitler, P.K., Meltzer, A.S., Koons, P.O., Craw, D., Hallet, B., Chamberlain, C.P., Kidd, W.S.F., Park, S.K., Seeber, L., Bishop, M., and Shroder, J., 2001, Erosion, Himalayan geodynamics, and the geomorphology of metamorphism: GSA Today, v. 11, p. 4-9.
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