A geometric model of faulted detachment folding with pure shear and its application in the Tarim Basin, NW China

doi:10.1007/s11707-016-0591-2

Front. Earth Sci.

2017, Vol. 11

Issue (2) : 416-426 https://doi.org/10.1007/s11707-016-0591-2

RESEARCH ARTICLE

A geometric model of faulted detachment folding with pure shear and its application in the Tarim Basin, NW China

Zewei YAO¹, Guangyu HE¹(

), Xiaoli ZHENG¹, Chuanwan DONG¹, Zicheng CAO², Suju YANG², Yi GU³

¹. School of Earth Sciences, Zhejiang University, Hangzhou 310027, China
². SINOPEC Northwest Oilfield Company, Urumqi 830011, China
³. Wuxi Research Institute of Petroleum Geology, SINOPEC, Wuxi 214126, China

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Abstract

We present an improved geometric model of faulted detachment folding with pure shear that is characterized by core thickening and a ramp-discordant backlimb. The model includes a two-stage evolution: 1) detachment folding involving pure shear with fixed hinges, and 2) faulted detachment folding, in which the core of anticline thrusts above a break-through fault in forelimb by limb rotation. The growth strata patterns of the model are also discussed with respect to factors such as limb rotation, tectonic uplift rate, and sedimentation rate. A thrust-related fold, called a TBE thrust fold, in the Tarim Basin in NW China, is analyzed as an example of the theoretical model. The result indicates that the TBE thrust fold has undergone a two-stage evolution with shortening of a few hundred meters. Both the theoretical model and the actual example indicate that the shortening in the detachment folding stage takes up a large proportion of the total shortening. The structural restoration of the TBE thrust fold also provides new evidence that the formation of a series of thin-skinned structures in the SE Tarim Basin initiated in the Late Ordovician. The model may be applicable to low-amplitude faulted detachment folds.

Keywords faulted detachment folding geometric model pure shear growth strata Tarim Basin shortening

Corresponding Author(s): Guangyu HE

Just Accepted Date: 19 October 2016 Online First Date: 17 November 2016 Issue Date: 19 May 2017

Cite this article:

Zewei YAO,Guangyu HE,Xiaoli ZHENG, et al. A geometric model of faulted detachment folding with pure shear and its application in the Tarim Basin, NW China[J]. Front. Earth Sci., 2017, 11(2): 416-426.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-016-0591-2
https://academic.hep.com.cn/fesci/EN/Y2017/V11/I2/416

Fig.1 Geometric models of four types of fault-related folding. (a) Break-thrust folding (modified from Fischer et al., 1992). (b) Classical fault-propagation folding (Suppe and Medwedeff, 1990). (c) Faulted detachment folding (Mitra, 2002). (d) Double-edge fault-propagation folding (Tavani et al., 2006). d_b is backlimb dip and q is ramp dip.

Fig.2 Stepwise evolution of faulted detachment folding with pure shear. (a) Initial geometry of faulted detachment fold, where initial limb length L₀ is approximately 6 units and the thickness of layer is 1 unit. (b) Fixed-hinge, low-amplitude, symmetric detachment fold formed with pure shear (modified from Epard and Groshong, 1995). (c) Break-through folding with a fault in forelimb of detachment fold with increased shortening. The strata of grey in the base are the basal detachment layers.

Fig.3 Numerical model of growth sedimentation and progressive limb rotation (Hardy and Poblet, 1994).

Fig.4 Relations of limb rotation (a), (c) and uplift (b), (d) and limb length (d) versus shortening for different models. (a), (b) corresponding to the model of progressive limb rotation (Hardy and Poblet, 1994). (c), (d) corresponding to the detachment fold under given conditions (see text for detail). The shortening unit in (a), (b) is the limb length and in (c), (d) is the thickness of single layer.

Fig.5 Forward modeling of growth faulted detachment folding with certain simplifications for uplift rate larger than sedimentation rate (a), (b) and for uplift rate less than sedimentation rate (c), (d). U=uplift rate, S=sedimentation rate. Note the growth strata at detachment folding stage are thicker than that at break-through stage.

Fig.6 A detachment fold showing contrasting measures of shortening. The initial limb length is L₀. S is horizontal shortening. A_b is the area of the triangle with grey color (modified from Suppe, 2011)

Fig.7 Relations of total shortening and pure shear shortening in detachment fold with pure shear in the case where q<= 60°. The distance between curve and dotted line is the bed-length shortening. The unit of S and S_ε is the undeformed limb length L₀.

Fig.8 Geometric elements used in the derivation of relationships of shortening, uplift, and displacement. See text for detail.

Fig.9 Graph of relationships between shortening d and displacement d_f verse backlimb dip d_b with certain ramp dip q based on Eqs. (2) and (3).

Fig.10 (a) Sketch map of tectonic units of the Tarim Basin. (b) Location of the TBE thrust fold in the Tanggubasi depression of the Central uplift of the Tarim Basin.

Fig.11 A schematic diagram showing stratigraphic units in the Tanggubasi depression, Tarim Basin.

Fig.12 (a) Interpreted seismic profile of TBE thrust fold (for location see Fig. 10). Vertical exaggeration is approximately 1. (b) Line drawings of time-depth converted seismic profile of the TBE thrust fold. The seismic reflection interfaces T1, T2, T3, T4, T5 are the stratigraphic boundaries of certain units (see Fig. 11 for detail). TWT: two-way travel time.

Fig.13 Displacement–distance diagram showing that displacement decreases slightly with increasing distance. The three dots from left to right correspond to T4, T3, and T2, respectively. The decrease of displacement may be due to drag effect of the reverse fault.

Fig.14 (a) Areas of relief are plotted versus height above reference level and the slope is the total shortening. (b) Bed length shortening and total shortening. Note that reference level is the base of Cambrian. Three points is the shortening of T4, T3, and T2, respectively.

Fig.15 Restoration cross-sections of the TBE thrust fold. (a) Faulted detachment folding during deposition of upper Sangtamu formation. (b) Detachment folding during deposition of the Qiaerbake formation to the Sangtamu formation. (c) Pre-folding stage at the end of the Middle Ordovician.

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