No abstract

No abstract

No abstract

The widespread occurrence of serpentinite along major strike slip, seismogenic faults warrants systematic investigation to determine how its frictional characteristics may affect slip along the fault. Different locations along the Motagua fault zone in Guatemala were sampled to investigate the sliding mode as a function of composition and texture, confining pressure, and displacement rate. Air‐dried, right circular cylinders, 7.1 cm in length and 3.3 cm in diameter, with a precut 35° to the cylinder and load axes were deformed at confining pressures up to 200 MPa, room temperature, and displacement rates of 10−4 and 10−7 cm/s. Compositional analyses of specimens cored from five blocks of serpentinite that were tested show that the serpentinite can be divided into two groups. One is a mesh‐textured serpentinite containing up to 70% serpentine, mostly lizardite, 11% enstatite phenocrysts, 19% oxides, and minor amounts of olivine and carbonate (undifferentiated). The other is a flare‐textured serpentinite containing up to 84% serpentinte, mostly antigorite, 10% oxides, 6% magnesite and dolomite, and almost no enstatite or olivine. The flare serpentinite undergoes the transition from stable sliding to stick slip sliding at confining pressures as low as 10 MPa. The mesh serpentinite results only in stable sliding up to confining pressures of 200 MPa and a displacement rate of 10−4 cm/s. At a displacement rate of 10−7 cm/s only one of the three mesh blocks exhibits stick slip. The shear stress required to initiate sliding for the flare‐textured serpentinites is given by τ = 0.77σn. For the mesh‐textured serpentinites at a displacement rate of 10−4 cm/s it is given by τ = 0.56σn, and at a displacement rate of 10−7 cm/s by τ = 0.50σn. Differences in fracture strength between the five specimen blocks correlate with differences in frictional strength. Differences in the sliding behavior and in the deformation observed along the sliding surfaces can be related to the mineralogical components of each serpentinite type. Extensive deformation occurs in the mesh serpentinites as a result of hard oxide and enstatite indenters ploughing through a soft lizardite substrate. Lizardite‐lizardite contacts prevail, and these deform by plastic flow. The stick slip behavior in the flare‐textured serpentinites is explained by brittle failure of antigorite‐antigorite contacts. The smaller grain size of the flare serpentinites, relative to the mesh serpentinites, may also contribute to the higher frictional strength of the flare serpentinites. We discuss the implications of the antigorite‐rich, flare‐textured serpentinite to seismogenic faulting. In particular, we consider the case of antigorite forming by shear‐heating‐induced progressive metamorphism of lizardite‐chrysotile serpentinites. If serpentine dehydration temperatures are approached, talc may form. Having a lower coefficient of friction than serpentine, talc may act as a ‘lubricant’ along the fault plane.

Fault wedges along the Polochic fault of northern Guatemala and Chiapas, Mexico record leftlateral slip of from 20 to 65 km that accompanied documented Neogene slip of 130 km across this North American--Caribbean plate boundary fault. Wedges can be correlated to source areas consistent with left-lateral displacement along the Polochic fault. They occur adjacent to restraining bends in three areas where changes in strike of the fault have been mapped. These wedges, which have been previously interpreted as allochthons emplaced along low-angle reverse faults, are bounded by high-angle faults. Folds that terminate against the Polochic fault are part of a regional fold belt that originated during the Campanian through Paleocene (Laramide) orogenic event. A previous contention that some of these major folds are secondary and related to fault movement is shown to be incorrect. The 130 km model of leftlateral offset across the Polochic fault is reviewed model is presented: (1) Late Cretaceous granitoid rocks from near Motozintla, Chiapas, Mexico (northern fault block), are correlated with granitic rocks of the Santa Maria Batholith in the region of Huehuetenango, Guatemala (southern block), (2) mineralization at the contacts of the granitoid intrusives of the two areas is correlated, (3) points of deflection of Laramide structural axes are correlated across the fault, (4) a major thrust sheet similar to that of the Sierra de Santa Cruz in eastern Guatemala is reassembled by reversing 130 km of left-lateral slip which has offset serpentinites north of the fault in western Guatemala and those south of the fault in central Guatemala. tation and associated Central