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Analog Modeling of Penetrative Strain Around Laramide Structures: Similarities and Differences Between Thick and Thin-Skinned Styles of Deformation


Normal 0 false false false EN-US X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin-top:0in; mso-para-margin-right:0in; mso-para-margin-bottom:8.0pt; mso-para-margin-left:0in; line-height:107%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri",sans-serif; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Penetrative strain or distributed deformation can be a significant reducer of reservoir quality, acting to reduce porosity and permeability by grain impingement, grain rotation and close packing, or by cataclasis of grains. It is therefore important to have a theoretical framework to understand the magnitude and distribution of penetrative strain in both thin and thick-skinned settings. Analog models can give such a theoretical framework because the initial model conditions are constrained, allowing both the tectonic shortening and the shortening due to penetrative strain to be deconvolved. A series of analog models simulating thick-skinned (Laramide) structures was run in this study. The rigid basement was approximated by potters’ clay and the overlying sedimentary cover by fine grained sand. A preexisting fault was cut into the basement before the model was shortened and the fault was lubricated with gear oil to allow slip to occur. In each model the angle of the fault dip was varied, with fault dips ranging from 40 to 80 degrees. The resulting penetrative strain magnitudes and distributions were compared to a previously run series showing the magnitude and distribution of penetrative strain in a thin-skinned setting. Results indicate that penetrative strain decreases with depth in a Laramide setting, compared to the increase with depth observed in a thin-skinned setting. The amount of deformation accommodated in each setting is similar, although the magnitudes vary in a Laramide setting with the dip of the fault plane. Finally, a zone of deformation featuring significant (up to 20%) penetrative strain is observed in the models which stretches into the foreland for a distance equal to the width of the thrust belt or the width of the Laramide uplift. Volumetric calculations for fields in Laramide basins or fold-thrust belts should therefore take into account porosity and permeability reduction due to distributed deformation as well as porosity generated by initial sedimentary process and diagenesis.