Low‐Viscosity Crustal Layer Controls the Crustal Architecture and Thermal Distribution at Hyperextended Margins: Modeling Insight and Application to the Northern South China Sea Margin
Abstract:A low‐viscosity crustal layer (LVCL) due to partial melt and/or hydration has been detected within the continental crust (e.g., in South China block), but its role in the development of hyperextended margins is still not entirely understood. Using 2‐D thermomechanical modeling, we simulate the lithospheric extension with a LVCL embedded within the continental crust. Results show that the ductile layer determines the width of highly thinned crust and the crustal thermal distribution. Detailed effects are found … Show more
“…Such differences possibly stem from that the slab in Model 8 has been subducted to greater depth, thus increasing the interplate contact area and exhibiting a strong plate coupling during the rifting stage. As extension progresses, the continent extends over a large scale and develops a typical necking in the lithospheric mantle, similar to those in previous studies (e.g., Chenin et al, 2017; Duretz et al, 2016; Li, Sun, et al, 2019). At the end of the system's development, breakup initiates in the continental interior, which is approximately 200 km away from the trench, separating a microcontinent from the main continent body.…”
The dynamics of continental breakup at convergent margins has been described as the results of backarc opening caused by slab rollback or drag force induced by subduction direction reversal. Although the rollback hypothesis has been intensively studied, our understanding of the consequence of subduction direction reversal remains limited. Using thermo-mechanical modeling based on constraints from the South China Sea (SCS) region, we investigate how subduction direction reversal controls the breakup of convergent margins. The numerical results show that two distinct breakup modes, namely, continental interior and edge breakup ("edge" refers to continent above the plate boundary interface), may develop depending on the "maturity" of the convergent margin and the age of the oceanic lithosphere. For a slab age of~15 to~45 Ma, increasing the duration of subduction promotes the continental interior breakup mode, where a large block of the continental material is separated from the overriding plate. In contrast, the continental edge breakup mode develops when the subduction is a short-duration event, and in this mode, a wide zone of less continuous continental fragments and tearing of the subducted slab occur. These two modes are consistent with the interior (relic late Mesozoic arc) and edge (relic forearc) rifting characteristics in the western and eastern SCS margin, suggesting that variation in the northwest-directed subduction duration of the Proto-SCS might be a reason for the differential breakup locus along the strike of the SCS margin. Besides, a two-segment trench associated with the northwest-directed subduction is implied in the present-day SCS region.
“…Such differences possibly stem from that the slab in Model 8 has been subducted to greater depth, thus increasing the interplate contact area and exhibiting a strong plate coupling during the rifting stage. As extension progresses, the continent extends over a large scale and develops a typical necking in the lithospheric mantle, similar to those in previous studies (e.g., Chenin et al, 2017; Duretz et al, 2016; Li, Sun, et al, 2019). At the end of the system's development, breakup initiates in the continental interior, which is approximately 200 km away from the trench, separating a microcontinent from the main continent body.…”
The dynamics of continental breakup at convergent margins has been described as the results of backarc opening caused by slab rollback or drag force induced by subduction direction reversal. Although the rollback hypothesis has been intensively studied, our understanding of the consequence of subduction direction reversal remains limited. Using thermo-mechanical modeling based on constraints from the South China Sea (SCS) region, we investigate how subduction direction reversal controls the breakup of convergent margins. The numerical results show that two distinct breakup modes, namely, continental interior and edge breakup ("edge" refers to continent above the plate boundary interface), may develop depending on the "maturity" of the convergent margin and the age of the oceanic lithosphere. For a slab age of~15 to~45 Ma, increasing the duration of subduction promotes the continental interior breakup mode, where a large block of the continental material is separated from the overriding plate. In contrast, the continental edge breakup mode develops when the subduction is a short-duration event, and in this mode, a wide zone of less continuous continental fragments and tearing of the subducted slab occur. These two modes are consistent with the interior (relic late Mesozoic arc) and edge (relic forearc) rifting characteristics in the western and eastern SCS margin, suggesting that variation in the northwest-directed subduction duration of the Proto-SCS might be a reason for the differential breakup locus along the strike of the SCS margin. Besides, a two-segment trench associated with the northwest-directed subduction is implied in the present-day SCS region.
“…The central part exhibits transitional characteristics (Figure 9d and 9g). According J o u r n a l P r e -p r o o f to the numerical analysis by Li et al (2019), the rheologically weak LVLs in the mid-crust could control the width of the highly thinned crust (thickness smaller than 15 km) and the crustal thermal distribution by accommodating deformation during extension. They therefore proposed that thicker LVLs result in wider range of ultra-thinned crust in the eastern NSM, while thinner LVLs are responsible for narrower range of ultra-thinned crust in the western NSM.…”
Section: Implications For Continental Riftmentioning
confidence: 99%
“…Tending to form weak domains, inherited LVLs in continental crust can also impact later tectonic events, such as continental rifting. Pre-existing weak zones play an important role in controlling the location and duration of extension, rifting styles and post-rift evolution of continental margins (Sutra and Manatschal, 2012;Manatschal et al, 2015;Festa et al, 2019;Li et al, 2019;Zhao et al, 2020). Furthermore, structurally inherited weaknesses can control present-day intraplate deformation and seismicity by adjusting strain localization on a local scale (Tarayoun et al, 2019).…”
mentioning
confidence: 99%
“…Through numerical simulation, Li et al, (2019) argues that LVLs could determine the width of the hyper-extended crust in the northern margin of the South China Sea (SCS). The LVLs may also be responsible for the present-day seismicity and historical earthquakes by providing an unstable crustal structure (Zou, 1998;Xu et al, 2006).…”
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“…1 and Supplementary Fig. 1 ) and its southern counterpart (up to 500 km) 7 , 32 – 34 , fringed by a stripe of highly extended (<15 km) continental ribbons tapering into the oceanic lithosphere 35 – 37 . Fig.…”
During extension, the continental lithosphere thins and breaks up, forming either wide or narrow rifts depending on the thermo-mechanical state of the extending lithosphere. Wide continental rifts, which can reach 1,000 km across, have been extensively studied in the North American Cordillera and in the Aegean domain. Yet, the evolutionary process from wide continental rift to continental breakup remains enigmatic due to the lack of seismically resolvable data on the distal passive margin and an absence of onshore natural exposures. Here, we show that Eocene extension across the northern margin of the South China Sea records the transition between a wide continental rift and highly extended (<15 km) continental margin. On the basis of high-resolution seismic data, we document the presence of dome structures, a corrugated and grooved detachment fault, and subdetachment deformation involving crustal-scale nappe folds and magmatic intrusions, which are coeval with supradetachment basins. The thermal and mechanical weakening of this broad continental domain allowed for the formation of metamorphic core complexes, boudinage of the upper crust and exhumation of middle/lower crust through detachment faulting. The structural architecture of the northern South China Sea continental margin is strikingly similar to the broad continental rifts in the North American Cordillera and in the Aegean domain, and reflects the transition from wide rift to continental breakup.
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