Approximately 100 heat flow measurements in the San Andreas fault zone indicate (1) there is no evidence for local factional heating of the main fault trace at any latitude over a 1000‐km length from Cape Mendocino to San Bernardino, (2) average heat flow is high (∼2 HFU, ∼80 mW m−2) throughout the 550‐km segment of the Coast Ranges that encloses the San Andreas fault zone in central California; this broad anomaly falls off rapidly toward the Great Valley to the east, and over a 200‐km distance toward the Mendocino Triple Junction to the northwest. As others have pointed out, a local conductive heat flow anomaly would be detectable unless the frictional resistance allocated to heat production on the main trace were ≲100 bars. Frictional work allocated to surface energy of new fractures is probably unimportant, and hydrologic convection is not likely to invalidate the conduction assumption, since the heat discharge by thermal springs near the fault is negligible. Explanations for the low dynamic friction fall into two intergradational classes: those in which the fault is weak all of the time and those in which it is weak only during earthquakes (possibly just large ones). The first class includes faults containing anomalously weak gouge materials and faults containing materials with normal frictional properties under near‐lithostatic steady state fluid pressures. In the second class, weakening is caused by the event (for example, a thermally induced increase in fluid pressure, dehydration of clay minerals, or acoustic fluidization). In this class, unlike the first, the average strength and ambient tectonic shear stress may be large, ∼1 kbar, but the stress allocated to elastic radiation (the apparent stress) must be of similar magnitude, an apparent contradiction with seismic estimates. Unless seismic radiation is underestimated for large earthquakes, it is difficult to justify average tectonic stresses on the main trace of the San Andreas fault in excess of ∼200 bars. The development of the broad Coast Range heat flow anomaly southward from Cape Mendocino suggests that heat flow increases by a factor of 2 within 4 m.y. after the passage of the Mendocino Triple Junction. This passage leaves the San Andreas transform fault zone in its wake; the depth of the anomalous sources cannot be much greater than the depth of the seismogenic layer. Some of the anomalous heat may be supplied by conduction from the warmer mantle that must occur south of the Mendocino transform (where there is no subducting slab), and some might be supplied by shear heating in the fault zone. With no contribution from shear heating, extreme mantle upwelling would be required, and asthenosphere conditions should exist today at depths of only ∼20 km in the northernmost Coast Ranges. If there is an appreciable contribution from shear heating, the heat flow constraint implies that the seismogenic layer is partially decoupled at its base and that the basal traction is in the sense that resists right lateral motion on the fault(s). As a result of thes...
Permanent slides for microscopic study are indispensable in the teaching of a basic course in botany and also in specialized advanced courses. In some advanced courses, the students prepare many of the slides used in the course, but in elementary courses the slides are furnished. In the latter case, the slides either are purchased from commercial somxes or made in the departmental laboratory. Biological supply houses make excellent slides of the subjects commonh used in elementary teaching, but the quality is likely to be variable. 1. Use fresh, normal material. 2. Remove pieces having the desired features and oriented so as to establish planes in which microtome sections are to be cut. 3. Cut into suitable pieces, with minimum bruising, compression, or desiccation. 4. Immerse the pieces promptly into the killing (fixing) fluid (Chap. 3) , and promote quick penetration of the fluid by removing air with an aspirator (Fig. 3.1). 5. Record the necessary data concerning species, location, date, parts selected, and killing fluid used.
Reduced heat flow in the Basin and Range province is characteristically greater by 50% to 100% than that in stable regions; in the hotter subprovinces like the Battle Mountain High, it is greater by 300%. Evidence for distributed tectonic extension and magmatism throughout the province suggests that much of the anomalous heat is transferred from the asthenosphere by convection in the lithosphere, in the solid state by stretching, and in the magmatic state by intrusion. Simple steady-state thermomechanical models of these processes yield relations among reduced heat flow, asthenosphere flux, lithosphere thickness, extension rate, and basalt production by the asthenosphere. Thermal effects in an extending lithosphere lead to decreased estimates of temperature and increased estimates of lithosphere thickness in the Basin and Range province. Moderate extension rates can account for high heat flow in the province without calling on anomalous conductive flux from the asthenosphere. The heat and mass budgets of bimodal volcanic centers suggest that they occur at points where the lithosphere is pulling apart rapidly, drawing up basalt to fill the void. Intrusion can probably facilitate lithosphere extension at low stress levels either by brittle " hydrofracturing" by basaltic dikes or by warming and thinning caused by basaltic underplating. Whether lithosphere extension occurs in the distributed mode or in the plate-tectonic mode might depend largely upon whether the lateral divergence of mass can be supplied by asthenosphere basalt, or whether it must be supplied by the ascent of very viscous ultramafic material which requires wide conduits separated by large distances. For a range of plausible models of distributed extension, the anomalous heat flow increases roughly 1 HFU (10~6 cal cm 2 -s~') for every 1% to 2% /m .y. increase in extension rate; the relation suggests extension rates in the Great Basin consistent with estimates from structural evidence. It also suggests much more rapid local extension in the hotter subprovinces, an inference supported by limited evidence from other sources.Where equation 1 applies throughout large provinces, the reduced heat flow qr is uniform, and it is reasonable to assume that the more local effects expected from hydrologic and magmatic convection are unimportant and that qr represents uniform conductive flux from on June 30, 2015 memoirs.gsapubs.org Downloaded from
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