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Quantitative Evaluation of Mechanical Heterogeneity of Sedimentary Sequences

Abstract

The terminology ‘mechanical stratigraphy’ was defined to depict the rock mechanical properties, layer thickness, and frictional properties of mechanical boundaries that constitute a rock sequence. Previous studies focusing on field outcrops have realized the important control of mechanical stratigraphy on fault nucleation and growth, fault zone processes, and fault population characteristics. Along-strike variability (e.g., fault throw distribution) and secondary faulting behaviours (e.g., diagenesis, compartmentalization, and fluid flow) can also be affected by the mechanical stratigraphy of a section. Geologists have realized the important control of mechanical stratigraphy on fault nucleation, fault linkage and fault compartmentalization. However, mechanical heterogeneity of stratigraphy has not been properly quantified in previous studies. Although previous studies suggested that the incompetent/competent ratios (i/c ratios) can help understand the control of mechanical stratigraphy on rates of fault propagation and displacement accumulation semi-quantitatively, we have to realize the possible variation of fault architecture in mechanical stratigraphy with identical i/c ratios. In this study, the average strength Save of a section is calculated by considering the strength and thickness of each individual layer. The strength difference SΔ and normalized strength difference SΔ/Save are defined for each individual layer in a section. The normalized strength difference SΔ/Save versus normalized height H/∑hi diagram can be employed to effectively evaluate the vertical mechanical heterogeneity of stratigraphy in a section. High frequency of fluctuation between negative and positive SΔ/Save values represents high mechanical heterogeneity, and vice versa. The scaled mechanical heterogeneity MHscaled can be used as an index to quantify the mechanical heterogeneity of stratigraphy in sections with different scales. However, it is important to realize that fault throw may promote or restrict the control of mechanical heterogeneity on faulting deformation. Mechanical heterogeneity may present limited control on fault architecture, particularly when the fault throw is several times larger than the scale of the section. Increasing of deformation rate may also minimise the control of mechanical heterogeneity on fault architecture.