Basal Décollement Splaying Induces Mid‐Crustal Tectonic Imbrication in an Intracontinental Orogen
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Abstract Mid‐to‐lower crustal rock exhumation is common in orogenic belts, but the deformation process exposing these rocks remains debated. Distributed deformation in low viscous crust extruding mid‐to‐lower crustal rocks as channel flow and localized deformation along shear zones imbricating rigid blocks are two end‐members that account for crustal thickening and unroofing. At the northwest of the Early Paleozoic orogenic belt in the South China Block, the Jiuling Massif includes orogenic root rocks exhumed from deep crustal level. Their structural pattern and exhumation history can improve our understanding on how continental mid‐to‐lower crust is deformed, thickened, and finally transported to the surface. Structural analysis reveals that two major mid‐crustal ductile shear zones and their splays are developed at temperatures of ∼350°C–550°C. Anisotropy of magnetic susceptibility (AMS) shows that the Southern Jiuling Batholith has a modified AMS pattern by syn‐orogenic compression, suggesting a gradually deformed rigid block. Combining surface geological evidence and deep structures by gravity modeling, we find shear zones rooted in basal décollement incrementally stacked the rigid granitic blocks. Along strike, the major shear zones evolved differently with more splays at their eastern portions. Thus, tectonic imbrication can evolve to pervasive flow‐like deformation as shear zones continue to splay and form an anastomosed shear zone system. The complexed structures by splayed shear zones segmenting and imbricating small rigid blocks may correspond to the geophysically low‐velocity zone in the crust, so shear zone splaying is a linking mechanism between tectonic imbrication and viscous flow deformation of the crust.Keywords:
Imbrication
Batholith
Mid-to-lower crustal rock exhumation is common in orogenic belts, but the deformation process exposing these rocks remains debated. Distributed deformation in low viscous crust extruding mid-to-lower crustal rocks as channel flow and localized deformation along shear zones imbricating rigid blocks are two end-members that account for crustal thickening and unroofing. At the northwest of the Early Paleozoic orogenic belt in the South China Block, the Jiuling Massif includes orogenic root rocks exhumed from deep crustal level. Their structural pattern and exhumation history can improve our understanding on how continental mid-to-lower crust is deformed, thickened, and finally transported to the surface. Structural analysis reveals that two major mid-crustal ductile shear zones and their splays are developed at temperatures of ∼350°C–550°C. Anisotropy of magnetic susceptibility (AMS) shows that the Southern Jiuling Batholith has a modified AMS pattern by syn-orogenic compression, suggesting a gradually deformed rigid block. Combining surface geological evidence and deep structures by gravity modeling, we find shear zones rooted in basal décollement incrementally stacked the rigid granitic blocks. Along strike, the major shear zones evolved differently with more splays at their eastern portions. Thus, tectonic imbrication gradually evolve to pervasive flow-like deformation as shear zones continue to splay and form an anastomosed shear zone system. The complexed structures by splayed shear zones segmenting and imbricating small rigid blocks may correspond to the geophysically low-velocity zone in the crust, so shear zone splaying is a linking mechanism between tectonic imbrication and viscous flow deformation of the crust, and reconcile these two end-members.
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Seismic data able to resolve the crustal structure are limited in quantity and quality with respect to the size and complexity of Tibet—Himalayas. They may be interpreted as indicating a strong heterogeneity: lack of continuity of even major interfaces across strike, defining different crustal blocks, but also lack of continuity of surface tectonic features down through the whole lithosphere. A thickening by imbrication of both the upper crustal and the lower crust-upper mantle levels is suggested. Indications from recent high-resolution surveys in other domains of thickened crust are also of a less smooth geometry of structures and depth than intuitively considered.
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The block structure of the Earth crust and tectonic zones are displayed at the dynamic seismic section in the amplitudes of the scattered waves. Tectonic zones are identified with thrusts.<br>The effective density model of the crust was created at the base on the complex interpretation of seismic and gravity data.<br>Revealed tectonic elements of the Earth crust can be use for prognosis of oil perspective areas.<br>
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Gold mineralisation in the Eastern Goldfields Province of the Yilgarn Block, Western Australia, is associated with shear zones within the greenstone supracrustal succession. Regional shear zones are imaged in seismic reflection sections as bands of strong reflections. Although individual wavelets within the bands of reflections can only be correlated over small distances, the bands of reflections can be correlated over tens of kilometres. The Bardoc Shear, adjacent to which considerable mineralisation has been found, dips west and links with the Ida Fault, which forms the boundary between the Eastern Goldfields Province and the Southern Cross Province farther west. The Ida Fault dips east at about 40°, and can be traced from the surface to about 27 km depth. Bands of reflections within the upper and middle crust have a similar seismic signature to the Ida Fault, Bardoc Shear Zone and the basal detachment of the greenstones, and are therefore Interpreted as shear zones. Interpreted shear zones in the upper crust under the greenstones mostly dip west. Shear zones in the lower crust dip east. The upper crust east of the Ida Fault and below the Bardoc Shear is an exception. There, east-dipping shear zones, including the Ida Fault, are interpreted to extend from the lower crust into the upper crust, thereby providing a simple plumbing system for mineralising fluids migrating from the lower crust, into the Bardoc Shear, and then to high levels in the greenstones.
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Abstract The Hoh Xil Basin (HXB) was located in the foreland of the proto‐Tibetan Plateau and has obtained a high elevation and thick crust since the Late Oligocene. However, crust thickening cannot be explained by the limited amount of upper crustal shortening and requires a further mechanism. Based on a linear dense nodal array, we use the receiver function method to study the detailed crustal structure beneath the HXB. Our images reveal that the HXB crust has been thickened to >70 km and is thicker than the protoplateau's crust. A series of north‐dipping interfaces imaged in the lower crust implies widespread imbrication structures. Considering the uplift history of the plateau, we propose that isostatic‐driven adjustments could equalize crustal thickness variations across the protoplateau margin in the Paleogene, and the HXB was elevated by extensive imbrications accommodated to a northward injection of the protoplateau lower crust in the Late Paleogene.
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Abstract Mid‐to‐lower crustal rock exhumation is common in orogenic belts, but the deformation process exposing these rocks remains debated. Distributed deformation in low viscous crust extruding mid‐to‐lower crustal rocks as channel flow and localized deformation along shear zones imbricating rigid blocks are two end‐members that account for crustal thickening and unroofing. At the northwest of the Early Paleozoic orogenic belt in the South China Block, the Jiuling Massif includes orogenic root rocks exhumed from deep crustal level. Their structural pattern and exhumation history can improve our understanding on how continental mid‐to‐lower crust is deformed, thickened, and finally transported to the surface. Structural analysis reveals that two major mid‐crustal ductile shear zones and their splays are developed at temperatures of ∼350°C–550°C. Anisotropy of magnetic susceptibility (AMS) shows that the Southern Jiuling Batholith has a modified AMS pattern by syn‐orogenic compression, suggesting a gradually deformed rigid block. Combining surface geological evidence and deep structures by gravity modeling, we find shear zones rooted in basal décollement incrementally stacked the rigid granitic blocks. Along strike, the major shear zones evolved differently with more splays at their eastern portions. Thus, tectonic imbrication can evolve to pervasive flow‐like deformation as shear zones continue to splay and form an anastomosed shear zone system. The complexed structures by splayed shear zones segmenting and imbricating small rigid blocks may correspond to the geophysically low‐velocity zone in the crust, so shear zone splaying is a linking mechanism between tectonic imbrication and viscous flow deformation of the crust.
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Batholith
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he Himalaya and adjacent Tibetan plateau, constituting Earth's largest region of elevated topography and anomalously thick crust, formed as a consequence of Cenozoic collision between india and Asia-itself considered the archetypal continent-continent collision. Here we rePOrt the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling. Our￣100km-long profile, acquired in the Tethyan Himalaya, shows a midcrustal reflection that probably marks the active thrust fault along which the indian plate is underthrusting southern Tibet; upper-crustal reflections with geometries suggestive of large-scale structural imbrication of the upper crust; and Moho reflections from the base of the double-normal-thickness crust underlying the region. These results lend substantial support to the view that crustal thickening beneath southernmost Tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the Tethyan Himalaya.
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