1) The document describes experiments using hollow aluminum and siliceous microspheres to simulate weaker layers in sandbox models of detachment folding. 2) Shear tests determined the microspheres have lower shear strengths than sand, making them suitable for simulating weaker decollement layers. 3) A series of sandbox experiments were conducted with varying ratios of sand and microspheres to observe the development of detachment folds and how deformation styles partition within thrust wedges.

1. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma DAVID ROSSI USE OF HOLLOW GRANULAR MATERIALS TO SIMULATE DETACHMENT FOLDING AND DEFORMATION PARTITIONING IN SANDBOX THRUST WEDGES

2. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma INTRODUCTION In analogue modelling the use of PURE SAND in normal gravity experimental condition approximates the upper crust as homogeneous brittle material obeying the Mohr-Coulomb failure criterion. The experimental simulation of sedimentary successions characterised by alternating weaker and stronger layers needs the use of granular materials OTHER THAN LOOSE SAND. For this reason we tested in laboratory the mechanical behaviour of a new materials: HOLLOW ALLUMINIUM and SILICEOUS MICROSPHERES. The aim of the present study is to simulate in laboratory the development of a multilayers asymmetric detachment folds.

3. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma MATERIAL Hollow aluminium hydroxid microspheres“MICROBALLS”and hollow siliceous microspheres“SI-CEL”are produced with different particle sizes and colours. Tested materials have grain size of respectively 25 mm (si-cel) and 40 mm for (microballs). MICROBALLS SI-CEL

4. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma DENSITY DETERMINATION Plot of data displaying the relationship between mass increase and height increase of the sediment column (h) for the investigated material. Density of MICROBALLS0.39 g/cm3± 0.65% Density of SI-CEL0.15 g/cm3± 1.41% 0 5

5. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma SHEAR TEST DATA Shear tests were carried out in a Casagrande direct shear box. MICROBALLS  = 0.46  = 24.70± 0.009 SI-CEL = 0.44  = 23.90± 0.008

6. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma COHESION The variability range of the extrapolated cohesion value has been calculated by estimating the interception probability () at 99% along the shear axis as (Sokal and James Rohlf, 1987): C’ - T*Es ≤  ≤ C’ + T*Es MICROBALLS C’ = 6 Pa - 1.6 ≤ C’ ≤ 9.8 SI-CEL C’ = 1.5 Pa  is virtually zero

7. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma EXPERIMENTAL SETUP The efficacy of the tested microspheres as decollement material in sandbox analogue modelling has been tested in numerous experiments with different configurations of the undeformed sand-microsphere multilayers. The undeformed multilayer of the models was constructed by sieving loose granular materials above a basal sheet of drafting film ( = 0.47). The total thickness of the undeformed sand-microsphere multilayer was comprise between 10.2 mm to 15.5 mm. The scale factor for lengths is 5*10-6 and therefore 1 cm in the box represent about 2 km in nature.

8. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma EXPERIMENT D01-12 sand = 100% microsphere = 0% mm

9. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma EXPERIMENT D01-06 sand = 66% microsphere = 34% Ratio between sand (s) and microsphere (m) thickness: Lower half s/m = 5 Upper half s/m = 0.9 mm

10. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma EXPERIMENT D01-10 sand = 38% microsphere = 62% Ratio between sand (s) and microsphere (m) thickness : Lower half s/m = 0.3 Upper half s/m = 0.5 mm

11. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma EXPERIMENT D01-08 sand = 35% microsphere = 65% Ratio between sand (s) and microsphere (m) thickness : Lower half s/m = 0.4 Upper half s/m = 0.9 mm

12. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma EXPERIMENT D03-05 sand = 22% microsphere = 78% Ratio between sand (s) and microsphere (m) thickness : Lower half s/m = 0.1 Upper half s/m = 0.6 mm

13. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma EXPERIMENT D01-07 sand = 18% microsphere = 82% Ratio between sand (s) and microsphere (m) thickness : Lower half s/m = 0.06 Upper half s/m = 0.6 mm

14. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma DISCUSSION Experimental results demonstrate that the lower shear strength ofMICROBALLSand SI-CELmicrospheres seems very useful to simulate weaker layers within the brittle crust. The advantage of using hollow microspheres instead of silicone putty to simulateweak decollement layersis that the former obey the Mohr-Coulomb failure criterion avoiding significant scaling problems like that in theSAND-SILICONEinterface. Thesinking trendof sand (or glass microspheres) into the viscous silicone putty produces mixed layer at the sand-silicone interface with a complex, non Newtonian rheology.

15. Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma CONCLUSIONS The model dynamic sequences allowed us to trace specific kinematic evolution. As a consequence, the contraction produces two different deformation processes in different regions of the multilayers:“Fault-bendandFold-propagation foldingcharacterizing thelower half sequencesof the deforming multilayers, whiledetachment foldingdominates in theupper half”. Results of the sandbox experiments emphasizes the primary role of the mechanical stratigraphy in the control of the deformations structural style within thrust wedges.