BH250-242
Title
BH250-242
Subject
Mylonite
Description
Major Minerals: calcite
BH250-242 is a mylonite. This mylonite from Death Valley developed from a carbonate protolith (limestone) and is composed predominantly of calcite, recording intense ductile deformation within a shear zone. The rock displays a strongly developed mylonitic foliation and lineation defined by a fine-grained, dynamically recrystallized calcite matrix, within which larger calcite porphyroblasts/porphyroclasts are preserved. These calcite grains are commonly elongated, rotated, and mantled by fine neoblasts, forming classic sigma- and delta-type structures that clearly indicate shear sense. A well-developed S–C fabric is present, with the S-surfaces representing the penetrative foliation and the C-surfaces marking discrete shear bands, together providing robust kinematic indicators of deformation. Deformation mechanisms are characteristic of carbonate systems, including dynamic recrystallization, mechanical twinning, and pressure solution, which facilitate significant grain-size reduction and strain localization, in some cases approaching ultramylonitic textures. In the broader tectonic context of Death Valley within the Basin and Range extensional province, this calcite-rich mylonite represents a zone of concentrated shear along major detachment faults, preserving a detailed microstructural and kinematic record of crustal extension (Mike Smith, Class of 1999, personal communication).
BH250-242 is a mylonite. This mylonite from Death Valley developed from a carbonate protolith (limestone) and is composed predominantly of calcite, recording intense ductile deformation within a shear zone. The rock displays a strongly developed mylonitic foliation and lineation defined by a fine-grained, dynamically recrystallized calcite matrix, within which larger calcite porphyroblasts/porphyroclasts are preserved. These calcite grains are commonly elongated, rotated, and mantled by fine neoblasts, forming classic sigma- and delta-type structures that clearly indicate shear sense. A well-developed S–C fabric is present, with the S-surfaces representing the penetrative foliation and the C-surfaces marking discrete shear bands, together providing robust kinematic indicators of deformation. Deformation mechanisms are characteristic of carbonate systems, including dynamic recrystallization, mechanical twinning, and pressure solution, which facilitate significant grain-size reduction and strain localization, in some cases approaching ultramylonitic textures. In the broader tectonic context of Death Valley within the Basin and Range extensional province, this calcite-rich mylonite represents a zone of concentrated shear along major detachment faults, preserving a detailed microstructural and kinematic record of crustal extension (Mike Smith, Class of 1999, personal communication).
Coverage
Location: Death Valley, California, USA
Nearby Geographic Feature: Death Valley
GPS Coordinates: 36.286156, -116.762936
Nearby Geographic Feature: Death Valley
GPS Coordinates: 36.286156, -116.762936
Date
2023
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 242. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-242,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/315.