STRAIN ACCUMULATION IN METAMORPHIC ROCKS OF THE LATE JURASSIC MARIPOSA FORMATION: A COMPARATIVE ANALYSIS OF MATRIX STRAIN USING FRY AND RF/FI METHODS

The purpose of this study is to distinguish the strain accumulated in the matrix and clasts using Fry analysis as opposed to Rf/Fi where only the matrix distribution is taken into account. The study are is located in the steeply dipping, 85 m thick and continuous section of the Late Jurassic Mariposa Formation in Sierra Nevada Foothills. The samples included interlayered slate and sandstone layers with approximately 5-10 m intervals. The results from the Fry analysis were twice as much as the Rf/Fi analysis showing how the whole rock accumulates the strain in comparison with the clasts. The clasts were mainly quartz, feldspar with matrix being cement with homogenous composition as the clasts. The methodology involved cutting the thin sections were cut into three perpendicular sections to produce a 3D ellipsoids of the strain ellipses per sample. EllipseFit software was used to measure the axial ratios and construct 3D ellipsoids along with necessary calculations for Rf/Fi and Fry analysis. The results of Fry analysis revealed 34.7% net shortening compared to Rf/Fi which was only 26%. This proves that matrix of the sample can accommodate at least 3% more than the clasts which can distort the otucomes. The final length of the measured outcrop was found to be 114.92 m as compared to the deformed length of 77.7 m. This value is only 105.8 m using Rf/Fi method which only estimates 26.7% shortening. The directions of the ellipsoids were NW trending with steep SE dipping. The orientations of the foliation planes (xy) were aligned with the strike and dip dataset. The conclusion of this study proves that the matrix is capable of accumulating strain which is most often left ignored while quantifying the strain.