Ramp Region Research Articles

Deformation mechanisms in the southeastern ramp region of the Pine Mountain block, Tennessee

A field and petrofabric study of the southwestern part of the Pine Mountain thrust sheet shows that most deformation features may be attributed to one of two processes. The first, layer-parallel compression, is responsible for (a) a layer-parallel maximum shortening direction in the mechanically twinned fabric of the limestone and dolomite country rock, (b) transport- parallel extension fractures and joints, (c) transport-normal stylolite surfaces, and (d) some small thrusts. The second, bending at thrust-ramp hinges, is responsible for (a) non-bedding-parallel maximum shortening directions in mechanically twinned calcite from fracture fillings, (b)transport-normal extension fractures and joints, and (c) small normal faults. Possible explanations for the failure of calcite in the country rock to twin in response to bending at the ramp include both (1) an early imposed, pre-thrusting, calcite twin strain hardening and (2) deformation of the country rock by competing mechanisms such as pressure solution. Small normal faults are parallel to the strike of the thrust sheet and seem to have evolved from strike-parallel extension fractures in response to continued extension. Both small thrusts and stylolite surfaces are rare in outcrop. There seems to be no major change in deformation of rocks which have traveled through different fractions of the total ramp length, so long as the rocks have passed onto the ramp.

Self-consistent study of a perpendicular collisionless and nonresistive shock

Particle dynamics and field behavior associated with a perpendicular collisionless supercritical and viscous shock are investigated by use of numerical simulation. A one-dimensional, relativistic, fully electromagnetic and nonperiodic particle simulation code (for both electrons and ions) is used where self-consistent space-charge effects and induced effects are totally included. The principal field patterns of the shock (trailing wave train, ramp, and foot region) are studied in detail and are shown to have scale lengths mainly dictated by ion dynamics; the behavior of the corresponding plasma currents associated with the different field components is also presented. Ions are shown to suffer successive ‘‘acceleration–trapping–detrapping’’ at the shock front, and locally in the trailing wave train of the downstream region through combined effects of the electrostatic and magnetic fields. While detrapped, the reflected ions describe very large Larmor orbits and cause a ring distribution; a large rapid nonstochastic ion heating results from this ion gyration. This heating (resistivity-free) is the main source of dissipation and is responsible for large field damping. Competitive effects such as particle stochasticity, particle trapping, wave damping, wave overtaking, and dispersion effects are shown to interact with each other and to affect the overall dissipation mechanism. Comparison with previous works is also discussed. Various Mach number situations are considered, leading to the definition of a transitory regime between subcritical and supercritical regimes and of a corresponding critical threshold of the electrostatic field. In contrast with the supercritical regime, the subcritical regime is characterized by a low density of trapped-reflected ions, a broad ion distribution function with a weak tail, and a weak adiabatic bulk ion heating.

Effect of electron thermal anisotropy on the kinetic cross-field streaming instability

The investigation of the kinetic cross-field streaming instability, motivated by the research of collisionless shock waves and previously studied by Wu et al., is discussed more fully in the present work. Since, in the ramp region of a quasi-perpendicular shock, electrons can be preferentially heated in the direction transverse to the ambient magnetic field, it is both desirable and necessary to include the effect of the thermal anisotropy on the instability associated with a shock. The present study has found that Te⊥ > Te‖ can significantly enhance the peak growth rate of the cross-field streaming instability when the electron beta is sufficiently high. Furthermore, the present analysis also improves the analytical and numerical solutions previously obtained.