Laboratory friction experiments are important for understanding fault restrengthening (healing) between failure events. To date, studies have focused mainly on time and velocity dependence of friction for small perturbations about conditions for steady state sliding. To investigate healing under a wider range of conditions, as appropriate for the interseismic period and dynamic rupture on seismogenic faults, we vary shear load for holds &tgr;hold, hold time th, load point velocity V, and initial gouge layer thickness T0. We shear layers of granular quartz in a biaxial testing apparatus at room temperature and humidity. In addition to conventional slid-hold-slide (CSHS) healing tests, we perform tests in which shear stress is rapidly reduced prior to each hold. Identical slip histories are used in all experiments. Our CSHS tests show time-dependent healing, where Δμ is the difference between peak static friction and prehold sliding friction, consistent with previous work. For a given th we find a systematic increase in peak static strength and Δμ with decreasing &tgr;hold (for th=100 s, Δμ=0.007 for CSHS tests and 0.05 for &tgr;hold=0 tests). Significantly, healing tests at zero shear stress show decreasing static frictional yield strength with increasing th; thus we observe time-dependent weakening in this case. We vary initial layer thickness (0.5--3 mm) and find greater healing for thicker layers. Numerical simulations using rate and state friction laws show that neither the Dieterich nor Ruina evolution laws predict our experimentally observed healing rates for the full range of conditions studied. Our results have significant implications for the mechanics of deformation within granular media. We present a micromechanical model based on stress chains, jamming, and time-dependent unjamming of sheared granular layers. As applied to earthquakes, our data indicate that coseismic stress drop is expected to have an important effect on fault healing rates and static yield strength. ¿ 2001 American Geophysical Union