Coupled Onshore Erosion and Offshore Sediment Loading As Causes of Lower Crust Flow on the Margins of South China Sea Peter D
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Clift Geosci. Lett. (2015) 2:13 DOI 10.1186/s40562-015-0029-9 REVIEW Open Access Coupled onshore erosion and offshore sediment loading as causes of lower crust flow on the margins of South China Sea Peter D. Clift1,2* Abstract Hot, thick continental crust is susceptible to ductile flow within the middle and lower crust where quartz controls mechanical behavior. Reconstruction of subsidence in several sedimentary basins around the South China Sea, most notably the Baiyun Sag, suggests that accelerated phases of basement subsidence are associated with phases of fast erosion onshore and deposition of thick sediments offshore. Working together these two processes induce pressure gradients that drive flow of the ductile crust from offshore towards the continental interior after the end of active extension, partly reversing the flow that occurs during continental breakup. This has the effect of thinning the continental crust under super-deep basins along these continental margins after active extension has finished. This is a newly recognized form of climate-tectonic coupling, similar to that recognized in orogenic belts, especially the Himalaya. Climatically modulated surface processes, especially involving the monsoon in Southeast Asia, affects the crustal structure offshore passive margins, resulting in these “load-flow basins”. This further suggests that reorganiza- tion of continental drainage systems may also have a role in governing margin structure. If some crustal thinning occurs after the end of active extension this has implications for the thermal history of hydrocarbon-bearing basins throughout the area where application of classical models results in over predictions of heatflow based on observed accommodation space. Introduction in a continental plate) to passive margins has resulted The process of continental crustal extension and the for- in the identification of subsidence anomalies, usually in mation of sedimentary basins have been quantified and the form of greater subsidence than would be expected described using a number of different approaches that from the degree of extension as measured on normal explain how strain is accommodated in the continen- faults identified within the upper crust in seismic profiles tal lithosphere (McKenzie 1978; Royden and Keen 1980; (Driscoll and Karner 1998; Davis and Kusznir 2004; Clift Tankard and Welsink 1987; Wernicke 1985). Although et al. 2002; Boillot et al. 1988; Lister et al. 1991). Not all it is possible to think of rifted, passive continental mar- brittle extension is identified on industrial style, deep- gins as being simply extreme examples of this type of penetrating seismic reflection profiles (Walsh et al. 1991). intra-continental deformation, culminating in seafloor Nonetheless, the anomalies found in passive margins are spreading as extension reaches a maximum (beta = infin- so large that even accounting for this uncertainty does ity), there is evidence to indicate that the style of strain not resolve the mismatch between the total amount of accommodation may be different when breakup occurs. subsidence recorded. The anomalies require high degrees Simple application of uniform extension models (i.e. of total crustal thinning and a higher degree of upper when the amount of extension is the same at all depths crust extension than actually observed (Clift et al. 2002). Early attempts to address this conundrum involved *Correspondence: [email protected] invoking simple shear type mechanisms, similar to those 1 Department of Geology and Geophysics, Louisiana State University, developed by Wernicke (1985) and colleagues in the Baton Rouge, LA 70803, USA Full list of author information is available at the end of the article Basin and Range province of the Western United States © 2015 Clift. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http:// creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Clift Geosci. Lett. (2015) 2:13 Page 2 of 11 (Lister et al. 1991; Boillot et al. 1988; Whitmarsh et al. prediction of local isostatic equilibrium. These workers 2001). In this scenario high degrees of subsidence could suggested that the reason for the lack of Moho topogra- be explained if the lower crust was extended more than phy was because of flow in the lower crust removing the the upper crust as a result of the passive margin being hills and troughs in the Moho after extension had ceased located in the upper plate of a simple shear system. This and because the ductile lower crust flowed away from the would imply contrasting reduced subsidence anomalies deepest crusts keels. on the facing conjugate margin. However, as noted by Ductile crustal flow is also thought to be important in Driscoll and Karner (1998), although many marine geolo- many orogenic settings. Clark and Royden (2000) pro- gists were keen to label the particular margin that they posed that the smooth, low-gradient eastern flanks of were studying as being in the upper plate position, very the Tibetan Plateau are largely caused by flow of the mid- few seem to consider the possibility of a lower plate set- dle crust away from the thickened crust in the center of ting. Driscoll and Karner (1998) labeled this phenom- Tibet towards the southeast. Such flow would be driven enon as the “upper plate paradox” because more often by the gravitational potential of the thick crust under than not conjugate margins both exhibited the “upper Tibet compared to the relatively thin crust under south- plate” characteristics. ern China and Indochina. At the same time, southward flow of Tibetan middle crust is inferred to play a role Lower crustal flow in the formation of the Greater Himalayan Crystalline One possible solution to the upper plate paradox is to Sequence (Beaumont et al. 2001). Focused erosion along understand the role of ductile flow in the ductile part the southern flank of the plateau has been proposed as of the crust. In regions where the crust was thick and/ the mechanism that causes the high degrees of denu- or the heatflow is high there is the possibility of large dation seen in the Greater Himalaya in which shallow thicknesses of middle and/or lower crust that are ductile rocks have been removed allowing the ductile mid crust (McKenzie et al. 2000) (Fig. 1). If this type of crust is com- to flow to the surface in a well-defined channel, which is mon then this allows for the possibility of non-uniform bounded by brittle faults at shallow levels (Harris 2007; strain accommodation (i.e., where the amount of exten- Hodges 2006; Godin et al. 2006). Although there are sion is not the same through the plate). There is certainly significant criticisms of the channel flow model for the abundant evidence that such thick, warm crust is sus- Himalaya (Harrison 2006; Webb et al. 2007) the con- ceptible to flow during the formation of wide rifts, such cept that thick, weak crust might be susceptible to flow as the Basin and Range (Buck 1991). Zuber et al. (1986) is common in both convergent and extensional plate set- recognized that although the topography of the Basin and tings. However, this concept has not yet been fully appre- Range was corrugated, the Moho underlying that area ciated in offshore zones. was remarkably flat, seemingly at odds with the normal South China Sea The South China Sea has become a natural laboratory -d -d -d for understanding the processes of continental breakup 2 GPa 2 GPa 2 GPa ab0 0 c 0 because of the ease by which conjugate margins might be compared. Furthermore, rifting was terminated during 10 10 10 active propagation as result of the collision of the south- Moho 20 20 20 ern Dangerous Grounds margin with Borneo (Hutch- 30 30 30 Moho Moho ison et al. 2000; Clift et al. 2008; Hinz et al. 1989). This 40 40 40 basin was formed as a result of extension starting in the 50 50 50 Late Cretaceous and accelerating during the Eocene, cul- 60 60 60 minating in breakup and the onset of seafloor spread- Depth (km) 70 70 70 ing likely around 30 Ma, at least offshore of southern 80 80 80 China (Su et al. 1989; Briais et al. 1993; Ru and Pigott Hainan Margin South China Margin South China Margin 1986; Franke et al. 2014; Barckhausen et al. 2014). Stud- 90 90 Pre-rift 90 Post-rift ies of extension in this system, based on industrial seis- 100 100 100 mic reflection profiles, suggested that the high degrees of Fig. 1 Schematic strength profiles for the different types of crust in the South China Sea. a Strong crust offshore southern Hainan, b subsidence seen, especially in the oceanward parts of the typical crust of southern China before rifting with thick weak middle rifted margins around this basin could be explained in crust, c crust under the Baiyun Sag at the time of low crustal flow. terms of depth-dependent extension (Davis and Kusznir Strength is shown as the stress difference during deformation (σ d). − 2004; Clift and Lin 2001). The mis-match between upper Modified from Burov and Watts (2006) crustal extension and total crust thinning inferred from Clift Geosci. Lett. (2015) 2:13 Page 3 of 11 subsidence increased towards the continent-ocean best estimates of the andesitic bulk crustal composition boundary (Clift et al. 2002; Davis and Kusznir 2004). the expectation is that crustal flow should have affected In this type of deformation the lower crust is consist- this region during the breakup. As extension proceeded ently more extended than the upper crust, which explains to high values thinning of the crust must eventually result the large amounts of subsidence that occur following the in the entire crust becoming brittle so that faults are able cessation of active extension.