Wide Orogen Research Articles

Effects of Tectonic Quiescence Between Orogeny and Rifting

AbstractTectonic quiescence is a period of nearly null relative plate motion. During the Atlantic Ocean opening, rifting cut across continental segments of supercontinent Pangea that had been through different periods of quiescence, which varied from tens to hundreds of Myr. Using numerical models, we explored the effects of quiescence between orogeny and rifting on the pre‐rift lithosphere and subsequent conjugate rifted margin configuration. Our model results showed that quiescence of ∼30–60 Myr caused wide orogenic wedges (>300 km) to be heated beneath their centers, which led to the breakup of the lithospheric mantle before the breakup of the continental crust during rifting, resulting in hyperextended conjugate rifted margins. In these scenarios, continental lithosphere breakup away from the suture contributed to the preservation of previously subducted lower crust along the subduction zone. Long periods of tectonic quiescence (∼100 Myr) experienced by a wide orogen permitted efficient orogenic collapse and lateral spreading, with pronounced lithospheric thermal weakening. Consequently, post‐quiescence rifting reactivated the suture and effectively drove plate eduction, exhuming most of the subducted continental lower crust and mantle lithosphere. Ultimately, ultra‐wide domains developed into asymmetric conjugate margins. Contrastingly, in narrow orogenic wedges (<200 km), different quiescence durations had minor effects on the margin architecture. In these cases, a longer quiescence time contributed to a larger preservation of subducted crust in the fossil subduction zone after rifting. Our models agree with conjugate rifted margins in the North and South Atlantic, which probably developed after the collapse of wide orogens and subsequent periods of tectonic quiescence.

The causes of sinuous crustal-scale deformation patterns in hot orogens: Evidence from scaled analogue experiments and the southern Central Andes

The cause of upper-crustal segmentation into rhomb-shaped, shear zone-bound domains associated with contractional sedimentary basins in hot, wide orogens is not well understood. Here we use scaled multilayered analogue experiments to investigate the role of an orogen-parallel crustal-strength gradient on the formation of such structures. We show that the aspect ratio and size of domains, the sinuous character and abundance of transpressional shear zones vary with the integrated mechanical strength of crust. Upper-crustal deformation patterns and the degree of strain localization in the experiments are controlled by the ratio between the brittle and ductile strength in the model crust as well as gradients in tectonic and buoyancy forces. The experimental results match the first-order kinematic and structural characteristics of the southern Central Andes and provide insight on the dynamics of underlying deformation patterns in hot, wide orogens.

Basin width control of faulting in the Naryn Basin, south‐central Kyrgyzstan

In Central Asia's Tien Shan, deformation is distributed across the wide orogen, a characteristic of intracontinental mountain building. Active faults are commonly found within intramontane basins that separate its constituent ranges. In order to explore the controls on this intramontane basin deformation, we study the Naryn Basin of south‐central Kyrgyzstan. A series of five balanced cross‐sections reveals a transition in patterns of faulting from faults confined to basin margins to faults focused within the basin center. The 20‐km‐wide eastern Naryn Basin displays deformation attributed to low‐angle splays of the northern, basin‐bounding fault. In the 40‐km‐wide western Naryn Basin, the pattern of deformation linked to the northern range remains, but is accompanied by steeper faults that dip both south and north without being directly linked to the basin‐bounding fault. We compare these cross‐sections to synthetic aperture radar interferometry (InSAR) measurements of surface deformation. Profiles of InSAR‐derived surface deformation rates across the Naryn Basin reveal that in the west, deformation is distributed across the broad basin interior, whereas in the east, rapid uplift is concentrated at the margin of the narrower basin. From the geodetic and structural data, we infer that in the western Naryn Basin, deformation has migrated away from the northern basin margin and into the interior. Deformation of the eastern basin interior, however, remains linked to the basin‐bounding fault. A simple mechanical model demonstrates that basin width may control basin deformation whereby basin‐interior faulting in the narrow, eastern Naryn Basin is inhibited by the overburden of adjacent ranges.

Ordovician continental margin terranes in the Lachlan Orogen, Australia: Implications for tectonics in an accretionary orogen along the east Gondwana margin

Four continental margin turbidite ± black shale terranes of the Lachlan Orogen in the southern Tasmanides of eastern Australia formed in two major systems along the east Gondwana margin and constrain the Ordovician assembly of this accretionary orogen. Key features are the dissimilar stratigraphies of the adjacent Bendigo and Melbourne terranes in the western system; the dissimilar stratigraphies of the adjacent Melbourne and Albury‐Bega terranes that reflect juxtaposition of the two systems during the Middle Devonian, and the presence of the Albury‐Bega Terrane both west and east of the Macquarie Arc in the eastern system that also includes the ocean floor Narooma Terrane and igneous ocean crust terrane(s). Repetition of the Albury‐Bega Terrane either side of the arc requires either rifting or orogen‐parallel, strike‐slip duplication of a once contiguous package. Terrane interactions began in the earliest Gisbornian with early docking, uplift, deformation, and exchange of detritus. Amalgamation occurred in the earliest Silurian Benambran Orogeny with accretion in the Middle Devonian. Over 40 Myr, discrete turbidite terranes aligned along the Gondwana margin in two systems were converted into a very wide orogen characterized by the along‐strike juxtaposition of superficially similar terranes.

Tectonic Evolution of the Western Margin of Gondwana Inferred from Syntectonic Emplacement of Paleozoic Granitoid Plutons in Northwest Argentina

The Cerro Durazno, Brealito, and Cobres granitoid plutons belong to the eastern Ordovician magmatic belt of northwest Argentina and are associated with a network of NW‐ and N‐striking ductile shear zones. A structural analysis of these plutons and their metamorphic host rocks was conducted to elucidate mid‐ to upper‐crustal magmatism and tectonism at the western margin of Gondwana. Pluton emplacement took place at 480–455 Ma, toward the end of a ca. 75‐Ma‐long episode of regional deformation and high‐temperature metamorphism. The Cerro Durazno and Brealito plutons were probably emplaced as NE‐SW‐striking, vertical magma sheets that coalesced into large magma bodies under overall E‐W shortening that generated SW‐dipping metamorphic foliations at a later stage of pluton formation. These fabrics were superimposed to the magmatic structures in the granite plutons and to the high‐grade metamorphic structures of the host rocks. At the northern pluton margins, ductile high‐strain zones, notably the Agua Rosada Shear Zone north of the Cerro Durazno pluton, further localized ascent of granitoid magma. Similarly, emplacement of the Brealito and Cobres plutons is genetically related to prominent shear zones. Geometry and metamorphic character of the shear zone network in the eastern Ordovician magmatic belt point to orogen‐parallel extension and transverse shortening of the belt under variable metamorphic conditions. The width as well as protracted deformation, high‐temperature metamorphism, anatexis, and granitoid magmatism of the magmatic belt are key characteristics of a hot, wide orogen that evolved during the lower Paleozoic at the western margin of Gondwana. Remarkable geotectonic similarities with respect to the modern Andes point to comparable plate‐tectonic conditions at the lower Paleozoic and the Cenozoic convergent plate margins.

Paleoproterozoic tectonic evolution of the North China Craton

The Archean North China Craton consists of two major blocks, separated by the Central Orogenic Belt. The age of collision of the two blocks along the Central Orogenic Belt is controversial. Some models suggest that the Archean blocks collided at 1.8 Ga, during the Luliang Orogeny (1.7–1.9 Ga). In this model, high-pressure granulite facies metamorphism accompanied collision at 1.8 Ga. Other models have suggested that the Eastern and Western Blocks collided at 2.5 Ga, soon after 2.6–2.5 Ga ophiolitic and arc rocks throughout the orogen were formed. We synthesize the geology, geochronology, and tectonics of the Neoarchean through Mesoproterozoic evolution of the North China Craton. We suggest that the Eastern and Western Blocks collided at 2.5 Ga during an arc/continent collision, forming a foreland basin on the Eastern Block, a granulite facies belt on the western block, and a wide orogen between the two blocks. This collision was followed rapidly by post-orogenic extension and rifting that formed mafic dike swarms and extensional basins along the Central Orogenic Belt, and led to the development of a major ocean along the north margin of the craton. An arc terrane developed in this ocean, and collided with the north margin of the craton by 2.3 Ga, forming a 1400 km long orogen known as the Inner Mongolia–Northern Hebei Orogen. A 1600 km long granulite-facies terrain formed on the southern margin of this orogen, representing a 200 km wide uplifted plateau formed by crustal thickening. The orogen was converted to an Andean-style convergent margin between 2.20 and 1.85 Ga, recorded by belts of plutonic rocks, accreted metasedimentary rocks, and a possible back-arc basin. A pulse of convergent deformation is recorded at 1.9–1.85 Ga across the northern margin of the craton, perhaps related to a collision outboard of the Inner Mongolia–Northern Hebei Orogen, and closure of the back arc basin. This event caused widespread deposition of conglomerate and sandstone of the basel Changcheng Series in a foreland basin along the north margin of the craton. At 1.85 Ga the tectonics of the North China Craton became extensional, and a series of aulacogens and rifts propagated across the craton, along with the intrusion of mafic dike swarms. The northern granulite facies belt underwent retrograde metamorphism, and was uplifted during extensional faulting. High pressure granulites are now found in the areas where rocks were metamorphosed to granulite facies and exhumed two times, at 2.5 and 1.8 Ga, exposing rocks that were once at lower crustal levels. Rifting led to the development of a major ocean along the southwest margin of the craton, where oceanic records continue until 1.5 Ga.

Present-day deformation of the Qaidam basin with implications for intra-continental tectonics

The Qaidam basin is a plateau of low relief, about 3 km in elevation and covering a large area of 1.2×10 5 km 2. Physiographically, the basin marks a transitional region between the northeastern Tibetan plateau (5±0.5 km in elevation) and the surrounding low-lying areas. The northeastern and southwestern edges of the basin are bounded by the Qilian Shan–Nan Shan and Qimantag–Eastern Kunlun Shan mountain belts, respectively. At present, the entire region is under northeast–southwest compression. Based on a synthesis of recent geological and geophysical results, we propose that each flanking mountain belt consists of a sequence of crustal nappes, originating from ramp thrusts at mid-crustal levels (>15 km deep). Over a distance range of 700 km, these two broad fold-and-thrust belts, operating in tandem and with some uniform shortening in the intervening Qaidam basin, successively raised the surface elevation from below 2 km to 5 km. At depth, the crust is correspondingly thickened from 40 km beneath the foreland of the Qilian Shan to approximately 55 km under the interior of the Qaidam basin, and then to nearly 70 km beneath Tibet. In contrast to the Himalayan front where earthquake faulting occurs down to a depth of 50 km in the foreland but the zone of active thrust faulting is less than 200 km in width, plateau building in the Qaidam region illustrates how a wide orogen is developing along northeastern Tibet in the interior of Eurasia.

Coupling and decoupling of crust and mantle in convergent orogens: Implications for strain partitioning in the crust

Topographic relief, strain partitioning and crust‐mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust. This result is obtained by solving analytically for basally driven three‐dimensional flow within an idealized crust with a laterally invariant, depth‐dependent Newtonian viscosity. Two end‐member flow states are shown to occur. In the first, crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust. In the second, upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust. This model is applied to deformation of continental crust above “plate‐like” mantle that undergoes oblique convergence across a narrow “suture.” If no significant low‐viscosity zone develops within the crust, crustal deformation is coupled to motion of the underlying mantle, and a triangular mountain range develops. Surface shortening occurs at the foot of the mountains, normal to the trend of the range, while strike slip and extension are partitioned along the axis of the orogen. If a low‐viscosity zone is initially absent but develops during crustal thickening, a steep‐sided, flat‐topped plateau forms. Crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau. Surface shortening normal to the plateau occurs at the foot of the plateau, while surface extension normal to the plateau occurs at the plateau edge. If a low‐viscosity zone is present in the lower crust prior to convergence, a wide orogen with low topographic relief develops. Crustal deformation is decoupled from the mantle except at the edges of the deforming region, and surface shortening is partitioned uniformly across the belt. This study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen. Similar morphology and strain partitioning can be observed in many real orogenic belts, including: the Eastern Alps, the Tibetan plateau, and the Lachlan (Tasman) fold belt.