Donald F. Palmer

Cataclastic rocks of the San Gabriel fault—an expression of deformation at deeper crustal levels in the San Andreas fault zone

The San Gabriel fault, a deeply eroded late Oligocene to middle Pliocene precursor to the San Andreas, was chosen for petrologic study to provide information regarding intrafault material representative of deeper crustal levels. Cataclastic rocks exposed along the present trace of the San Andreas in this area are exclusively a variety of fault gouge that is essentially a rock flour with a quartz, feldspar, biotite, chlorite, amphibole, epidote, and Fe-Ti oxide mineralogy representing the milled-down equivalent of the original rock (Anderson and Osborne, 1979; Anderson et al., 1980). Likewise, fault gouge and associated breccia are common along the San Gabriel fault, but only where the zone of cataclasis is several tens of meters wide. At several localities, the zone is extremely narrow (several centimeters), and the cataclastic rock type is cataclasite, a dark, aphanitic, and highly comminuted and indurated rock. The cataclastic rocks along the San Gabriel fault exhibit more comminution than that observed for gouge along the San Andreas. The average grain diameter for the San Andreas gouge ranges from 0.01 to 0.06 mm. For the San Gabriel cataclastic rocks, it ranges from 0.0001 to 0.007 mm. Whereas the San Andreas gouge remains particulate to the smallest grain-size, the ultra-fine grain matrix of the San Gabriel cataclasite is composed of a mosaic of equidimensional, interlocking grains. The cataclastic rocks along the San Gabriel fault also show more mineralogiec changes compared to gouge from the San Andreas fault. At the expense of biotite, amphibole, and feldspar, there is some growth of new albite, chlorite, sericite, laumontite, analcime, mordenite (?), and calcite. The highest grade of metamorphism is laumontite-chlorite zone (zeolite facies). Mineral assemblages and constrained uplift rates allow temperature and depth estimates of 200 ± 30°C and 2–5 km, thus suggesting an approximate geothermal gradient of ~50°C/km. Such elevated temperatures imply a moderate to high stress regime for the San Andreas, which is consistent with experimental rock failure studies. Moreover, these results suggest that the previously observed lack of heat flow coaxial with the fault zone may be the result of dissipation rather than low stress. Much of the mineralogy of the cataclastic rocks is still relict from the earlier igneous or metamorphic history of the protolith; porphyroclasts, even in the most deformed rocks, consist of relict plagioclase (oligoclase to andesine), alkali feldspar, quartz, biotite, amphibole, epidote, allanite, and Fe-Ti oxides (ilmenite and magnetite). We have found no significant development of any clay minerals (illite, kaolinite, or montmorillonite). For many sites, the compositions of these minerals directly correspond to the mineral compositions in rock types on one or both sides of the fault. Whole rock major and trace element chemistry coupled with mineral compositions show that mixing within the zone of cataclasis is not uniform, and that originally micaceous foliated, or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material. As previously found for the gouge along the San Andreas, chemical mobility is not a major factor in the formation of cataclastic rocks of the San Gabriel fault. We see only minor changes for Si and alkalies; however, there is a marked mobility of Li, which is a probable result of the alteration and formation of new mica minerals. The gouge of the San Andreas and San Gabriel faults probably formed by cataclastic flow. There is some indication, presently not well constrained, that the fine-grained matrix of the cataclasite of from the San Gabriel fault formed in response to superplastic flow.

Petrogenesis of cataclastic rocks within the San Andreas fault zone of Southern California U.S.A.

Abstract This paper petrologically characterizes cataclastic rocks derived from four sites within the San Andreas fault zone of southern California. In this area, the fault traverses an extensive plutonic and metamorphic terrane and the principal cataclastic rock formed at these upper crustal levels is unindurated gouge derived from a range of crystalline rocks including diorite, tonalite, granite, aplite, and pegmatite. The mineralogical nature of this gouge is decidedly different from the “clay gouge” reported by Wu (1975) for central California and is essentially a rock flour with a quartz, feldspar, biotite, chlorite, amphibole, epidote and oxide mineralogy representing the milled-down equivalent of the original rock. Clay development is minor (less than 4 wt. %) to nonexistent and is exclusively kaolinite. Alterations involve hematitic oxidation, chlorite alteration on biotite and amphibole, and local introduction of calcite. Electron microprobe analysis showed that in general the major minerals were not reequilibrated with the pressure—temperature regime imposed during cataclasis. Petrochemically, the form of cataclasis that we have investigated is largely an isochemical process. Some hydration occurs but the maximum amount is less than 2.2% added H 2 O. Study of a 375 m deep core from a tonalite pluton adjacent to the fault showed that for Si, Al, Ti, Fe, Mg, Mn, K, Na, Li, Rb, and Ba, no leaching and/or enrichment occurred. Several samples experienced a depletion in Sr during cataclasis while lesser number had an enrichment of Ca (result of calcite veining). Texturally, the fault gouge is not dominated by clay-size material but consists largely of silt and fine sand-sized particles. An intriguing aspect of our work on the drill core is a general decrease in particulate size with depth (and confining pressure) with the predominate shifting sequentially from fine sand to silt-size material. The original fabric of these rocks is commonly not disrupted during the cataclasis. It is evident that the gouge development in these primarily igneous crystalline terranes is largely an in situ process with minimal mixing of rock types. Fabric analyses reveal that brecciation (shattering), not shearing, is the major deformational mechanism at these upper crustal levels.

Paleomagnetic and sedimentological studies at Lake Tahoe, California-Nevada

Abstract Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis. Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of 14 C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P. X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction. Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined. Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion.

The Biogeography and Physicochemical Characteristics of Aquatic Habitats of Freshwater Ostracods in Canada and the United States

Abstract A North American Combined Ostracode Database (NACODe) is introduced, comprising ostracod occurrence, environmental and climatic data from Canada (Delorme Database, 6719 sites) and the United States (North American Non-Marine Ostracode Database, 609 sites). NACODes ostracod data are binary (present/not present) and include 108 species with three or more occurrences, from 34 genera. Biogeographic maps based on NACODe show the northern and southern distributional limits of several species, likely reflecting thermal (e.g. mean annual air temperature) boundaries. For others only northern (thermal) limits are indicated or their distributions extend beyond the confines of the data set. Hydrochemical limits are verified for some species, such as Limnocythere sappaensis and Limnocythere staplini which respectively prefer waters enriched in either bicarbonate or calcium ions. The combined data sets capture more fully the range of physicochemical characteristics of ostracod species’ host water than either of the component databases, facilitating improved palaeohydrochemical and palaeoclimatic reconstructions.

Chapter 11 – The Versatility of Quaternary Ostracods as Palaeoclimate Proxies: Comparative Testing of Geochemical, Ecological and Biogeographical Approaches

Abstract As important palaeoenvironmental proxies, non-marine ostracods provide several independent lines of evidence of palaeoclimatic and hydrological change in Quaternary records. Species diversity, biogeography, ecology and shell geochemistry (δ 18 O, δ 13 C, Mg/Ca, Sr/Ca and 87 Sr/ 86 Sr) are all potential sources of information concerning past environmental conditions. However, often there is not a strong covariance between changes in species assemblages and their corresponding suite of geochemical measurements; that is, in some records, there is little change in the shell geochemistry, whereas the species assemblages show significant change, and vice versa. Thus, multivariate statistical analyses used to develop transfer functions or modern analogues, based upon species assemblages, may or may not correspond to changes in the shell geochemistry. However, this lack of covariance is in itself informative and by examining the observed changes in ostracod shell geochemical measurements and species assemblages, it is possible to identify and track process dynamics in palaeolimnological records.

A 6000 year water level history of Europe Lake, Wisconsin, USA

Europe Lake occupies a small, closed, basin that would have been an embayment in Lake Michigan during the high water level events in the larger lake. Cores recovered from the lake reveal late Holocene water level fluctuations in the basin that are inferred from changes in taxa and abundance of molluscs, ostracodes, magnetic susceptibility, organic carbon, and oxygen isotopes.Non-glacial, Holocene lacustrine/paludal sedimentation in this portion of the Europe Lake basin started after 6600 RCYBP and was probably initiated by a rise in the water table of the deep bedrock aquifer, during the Nipissing transgression in Lake Michigan. Isotopically light ground water from this source was probably a major contributor during this phase to the negative δ18O spikes in Valvata tricarinata and Amnicola limosa.The start of stable lacustrine conditions is marked by maximum diversity of ostracode and mollusc taxa and a shift toward much more positive δ18O values. The Europe Lake basin at this time became an embayment of Lake Michigan. This event was probably coeval with the peak of the Nipissing transgression, when the water plane reached an altitude of about 183 m.The isolation of Europe Lake from Lake Michigan started at about 2390 RCYBP and is probably due to a drop in water level in Lake Michigan and/or to isostatic uplift of the Door Peninsula. Since isolation from Lake Michigan, water levels in Europe lake have been controlled primarily by fluctuations in local precipitation, evaporation and ground water discharge.

Middle Holocene Hydrologic Change and Hypolimnion Formation in Lake Erie

Data from a nearshore sediment core and a deep-water sediment core from the central basin of Lake Erie reveal shifts in sediment properties and stable isotope composition of shell carbonate between ca. 4,600 and 3,500 14C yrs BP. Radiocarbon dates are corrected for the hardwater effect by subtracting 670 years based on a modern calibration for the central basin. Silt content increased in the deep water core at 4,600 and again, slightly, at 3,900 14C yrs BP. Sand size increased in the nearshore core at 3,500 14C yrs BP. δ18O of shell carbonate increased and δ13C decreased in both cores between about 4,200 and 3,500 14C years BP. Magnetic susceptibility and percent calcite decreased sharply and percent organic carbon increased slightly in the deep water core beginning at 4,000 14C yrs BP. Most of the changes in sediment properties and stable isotope composition of shell carbonate occurred between 4,200 and 3,900 14C yrs BP. They coincide in time with the Nipissing II highstand of Lake Nipissing, in the Huron and Michigan basins, and with evidence for higher-than-present lake levels in Lake Erie. The changes in proxy data are interpreted as evidence for an influx of surface water as drainage from the Upper Great Lakes was rerouted through Lake Erie. There is little evidence in the sediment proxy record for changes in Lake Erie during the earlier Nipissing I highstand or the middle Holocene transition in regional climate. A 9,000-year composite stable isotope record for the central basin shows that the sediment cores document a transformation in drainage that established the modern hydrologic system for the lake. High lake level induced a seasonal hypolimnion, setting the stage for the low pH, oxygen-depleted bottom waters of today.