USE OF HOLLOW GRANULAR MATERIALS TO SIMULATE DETACHMENT FOLDING

AND DEFORMATION PARTITIONING

IN SANDBOX THRUST WEDGES

DAVID ROSSI

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

INTRODUCTIONIn 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.

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

MATERIALHollow 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

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

SI-CEL

MICROBALLS

0

2

4

6

8

10

12

0 5 10 15

height (cm)

0

1

2

3

4

0 5 1 1

height (cm)

y = 0.2978xR2 = 0.9971

y = 0.7297xR2 = 0.9992

DENSITY DETERMINATIONPlot of data displaying the relationship between mass increase and height increase of the sediment column (h) for the investigated material.

Density of MICROBALLS 0.39 g/cm3 ± 0.65%Density of SI-CEL 0.15 g/cm3 ± 1.41%

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

0 5

SHEAR TEST DATAShear 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

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

MICROBALLS

SI-CEL

n (kPa)

n (kPa)

o

o

0

100

200

250

150

50

0

100

200

250

150

50

y = 0.461x + 0.006R2 = 0.9958

y = 0.4448x + 0.0015R2 = 0.9966

0 100 200 300 400 500

0 100 200 300 400 500

COHESIONThe 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’ ≤ 9.8SI-CEL C’ = 1.5 Pa is virtually zero

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

Test SI-CEL MICROBALLS

T-student

coefficient (q)

STD error (q)

coefficient (m)

STD error (m)

F-ratio (1-39)

P

0.000

0.0015

2.103

0.4448

0.008

3215.205

> 0.0001

0.002

0.006

2.420

0.461

0.009

2608.043

> 0.0001

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.

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

EXPERIMENT D01-12

sand = 100%microsphere = 0%

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

mm

EXPERIMENT D01-06

sand = 66%microsphere = 34%

Ratio between sand (s) and microsphere (m) thickness:

Lower half s/m = 5Upper half s/m = 0.9

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

mm

EXPERIMENT D01-10

sand = 38%microsphere = 62%

Ratio between sand (s) and microsphere (m) thickness :

Lower half s/m = 0.3Upper half s/m = 0.5

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

mm

EXPERIMENT D01-08

sand = 35%microsphere = 65%

Ratio between sand (s) and microsphere (m) thickness :

Lower half s/m = 0.4Upper half s/m = 0.9

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

mm

EXPERIMENT D03-05

sand = 22%microsphere = 78%

Ratio between sand (s) and microsphere (m) thickness :

Lower half s/m = 0.1Upper half s/m = 0.6

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

mm

EXPERIMENT D01-07

sand = 18%microsphere = 82%

Ratio between sand (s) and microsphere (m) thickness :

Lower half s/m = 0.06Upper half s/m = 0.6

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

mm

DISCUSSIONExperimental results demonstrate that the lower shear strength of MICROBALLS and SI-CEL microspheres seems very useful to simulate weaker layers within the brittle crust.

The advantage of using hollow microspheres instead of silicone putty to simulate weak decollement layers is that the former obey the Mohr-Coulomb failure criterion avoiding significant scaling problems like that in the SAND-SILICONE interface.

The sinking trend of sand (or glass microspheres) into the viscous silicone putty produces mixed layer at the sand-silicone interface with a complex, non Newtonian rheology.

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma

CONCLUSIONSThe 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-bend and Fold-propagation folding characterizing the

lower half sequences of the deforming multilayers, while detachment folding

dominates in the upper 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.

Dipartimento di Scienze della Terra Universita’ “La Sapienza” di Roma