Application of geometric models to inverted listric fault systems in sandbox experiments. Paper 2: insights for possible along strike migration of material during 3D hanging wall deformation Yasuhiro Yamada a, * , Ken McClay b a JAPEX Research Centre, 1-2-1, Hamada, Mihama, Chiba, 261-0025, Japan b Fault Dynamics Research Group, Geology Department, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK Received 30 December 2000; received in revised form 21 August 2002; accepted 23 September 2002 Abstract Fault geometry is a primary control on hanging wall deformation. In this study, a series of positive inversion analogue experiments was conducted using rigid fault surfaces of true 3D geometry, with consistent listric geometry along the transport direction. The deformed geometry of the top horizon of the syn-extension sequence on vertical serial sections was examined with a conventional 2D geometric restoration technique to calculate the inclined antithetic shear angle that best approximates the actual fault shape, and to estimate the amount of apparent horizontal shortening during contraction. The apparent shear inclination and the estimated apparent shortening show a systematic change along strike, corresponding to the plan geometry of the master detachment surface. At the hanging wall above embayments in the fault geometry, the uplift is highest, the apparent shear inclination is gentlest and the apparent horizontal shortening is greatest. In contrast, the apparent shear inclination is steepest and the estimated shortening is smallest above salients in the master detachment geometry, where the uplift is lowest. These changes suggest that the hanging wall displacement had an along-strike component during deformation; the hanging wall material moves from the regions above salients to those above embayments in the detachment surface during contraction. Average of the estimated apparent shortening was smaller than the actual amount of the experiments, probably due to tectonic compaction. This study shows that the geometry of the master detachment in plan view has the primary control on the lateral variation of the hanging wall deformation. The data presented in this paper help understand 3D geometric relations between the hanging wall deformation and the underlying detachment surfaces. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Geometric models; Listric fault systems; Sandbox experiments 1. Introduction The previous study (Yamada and McClay, 2003a) examined hanging wall deformation on a cross-section seen in an analogue experiment with a series of geometric models. The results illustrated that hanging wall deformation above inverted listric fault can be best approximated by the inclined simple shearing. Since the top horizon of the syn-extension sequence was horizontal at the beginning of contraction, the apparent shear inclination and the apparent shortening can be calculated from the deformed geometry of the horizon. In this paper, the inclined simple shear (ISS) method is applied to sandbox experiments carried out with 3D listric fault surfaces. The apparent shear angle which closely approximates to the actual geometry of the master detachment fault is then calculated for serial sections produced from 3D experimental models. The variation in the shear inclination, and the apparent shortening calculated from the shear inclination, as seen in the 3D models are also discussed. Finally, conceptual models are presented for displaying the along-strike displacement above 3D fault geometries. 2. Analogue experiments of positive inversion structures Analogue experiments above various listric fault geo- metries were analysed to investigate the hanging wall deformation (see Yamada (1999) and Yamada and McClay (2003b) for details), and those analysed in this paper had 0191-8141/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(02)00160-8 Journal of Structural Geology 25 (2003) 1331–1336 www.elsevier.com/locate/jsg * Corresponding author. E-mail address: yamada@rc.japex.co.jp (Y. Yamada).