Electronic archive text to the manuscript Continental break-up mechanism; lessons from intermediate- and fast-extension settings 

Finite-element modeling of rifting processes

Numerical modeling, which was done by the finite-element program ANSYS® (Ansys Inc., Houston, USA), was focused on block movements, and stress and strain regimes in the rift setting for a hypothetical system including the following scenarios:

1)	three basic scenarios simulating different types of lithosphere (rheologically strong, intermediately strong and weak), defining initial crustal thicknesses as 30, 40, 50 km (keeping the upper/lower crust ratio at 1:1);
2)	each scenario from (1) undergoing an initial heat flow of 40, 60, 80 mWm-2;
3)	each individual scenario resulting from combinations of (1) and (2) undergoing extension at rates of 10, 30, 50 mmy-1.

All models were done for a 3-layer model made up of: upper crust, lower crust and the mechanically strong part of the lithospheric mantle. Mechanical behavior of the respective model layers was governed by the Mohr-Coulomb law and temperature- and strain rate-dependent creep laws. Pre-existing faults were defined using contact elements. 

Boundary conditions of thermal model included:

1)	heat transport by conduction and advection;
2)	fixed temperature value at Earth’s surface; and 
3)	fixed heat flow value at base of the lithosphere.

Thermal material parameters included:

1)	temperature-dependent thermal conductivity;
2)	specific heat;
3)	radiogenic heat production; and
4)	temperature-dependent density.

Boundary conditions for the mechanical model included:

1)	displacement-defined tectonic stresses;
2)	isostasy; and
3)	gravity forces.

Mechanical material parameters included:

1)	Young’s moduli;
2)	Poisson’s ratios;
3)	Stress/strain relationship governed by Byerlee’s law and creep law; and
4)	Fault friction coefficients.

    The upper crust was dissected by a single listric normal fault, which was pre-scribed to each scenario as detached at the top of the lower crust. The detachment was located along the interface between the upper and lower crust in all models.

Modeling outcomes included:

1)	total horizontal displacement;
2)	total vertical displacement;
3)	total strain;
4)	stress field characterized by principal stress vectors;
5)	vertical movement curve along the entire length of the model;

Gravity/magnetic modeling

Gravity and magnetic modeling was carried out to test the accuracy of interpretations of multiple data sets. Crustal layer depths provided by refraction data (Naini & Leyden, 1973; Chun & Yoshii, 1977; Rodgers et al., 1999; Saul et al., 2000; Kumar et al., 2001; Singh et al., 2004), thicknesses of lithostratigraphic horizons penetrated by wells, geometries of various lithostratigraphies interpreted from reflection seismic images have all been combined to produce geological cross sections, which were used as the input for gravity and magnetic modeling. Modeled cross sections have been constructed to interpret both basin and crustal architecture of the East Indian offshore. Modeled sedimentary rocks have been divided into layers and assigned densities that approximated a continuous density-depth function (Cordell, 1973; Sykes, 1996). 
    The modeled crust was divided into upper, middle and lower parts for continental, proto-oceanic and oceanic crusts (Meyers et al., 1996; Mooney et al., 1998; Rosendahl et al., 2005). For consistency, density values were held constant for each of the modeled layers for all models. In a similar fashion, various described rock units were assigned magnetic susceptibilities. For each gravity or magnetic model, a combination of densities or magnetic susceptibilities assigned to rock types constrained by geometries prescribed by geologic cross section resulted in a gravity or magnetic curve, which could be compared to the measured curve (Electronic archive Fig. 3). The degree of matching between the modeled and measured curve served as cross section quality grading. 
    Our intention was not to tune up the prescribed density or magnetic susceptibility patterns till the match is perfect but to choose among several alternative interpretation scenarios. We selected the ones where the match was reasonably good and rejected those where the match simply cannot work unless one prescribes truly unrealistic geological cross-section.

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