The thermochronology for several suites of Mesozoic metamorphic and plutonic rocks collected throughout the northern Peninsular Ranges batholith (PRB) was studied as part of a collaborative isotopic study to further our understanding of the magmatic and tectonic history of southern California. These sample suites include: a traverse through the plutonic rocks across the northern PRB ( N = 29), a traverse across a central structural and metamorphic transition zone of mainly metasedimentary rocks at Searl ridge ( N = 20), plutonic samples from several drill cores ( N = 7) and surface samples ( N = 2) from the Los Angeles Basin, a traverse across the Eastern Peninsular Ranges mylonite zone ( N = 6), and a suite of plutonic samples collected across the northern PRB ( N = 13) from which only biotite 40 Ar/ 39 Ar ages were obtained. These geochronologic data help to characterize five major petrologic, geochemical, and isotopic zonations of the PRB (western zone, WZ; western transition zone, WTZ; eastern transition zone, ETZ; eastern zone, EZ; and upper-plate zone, UPZ). Apparent cooling rates were calculated using U-Pb zircon (zr) and titanite (sphene) ages; 40 Ar/ 39 Ar ages from hornblende (hbl), biotite (bi), and K-feldspar (Kf); and apatite fission-track (AFT) ages from the same samples. The apparent cooling rates across the northern PRB vary from relatively rapid in the west (zr-hbl ~210 °C/m.y.; zr-bio ~160 °C/m.y.; zr-Kf ~80 °C/m.y.) to less rapid in the central (zr-hb ~280 °C/m.y.; zr-bio ~90 °C/m.y.; zr-Kf ~60 °C/m.y.) and eastern (zr-hbl ~185 °C/m.y.; zr-bio ~180 °C/m.y.; zr-Kf ~60 °C/m.y.) zones. An exception in the eastern zone, the massive San Jacinto pluton, appears to have cooled very rapidly (zr-bio ~385 °C/m.y.). Apparent cooling rates for the UPZ samples are consistently slower in comparison (~25–45 °C/m.y.), regardless of which geochronometers are used. Notable characteristics of the various ages from different dating methods include: (1) Zircon ages indicate a progressive younging of magmatic activity from west to east between ca. 125 and 90 Ma. (2) Various geochronometers were apparently affected by emplacement of the voluminous (ETZ and EZ) La Posta–type plutons emplaced between 99 and 91 Ma. Those minerals affected include K-feldspar in the western zone rocks, biotite and K-feldspar in the WTZ rocks, and white mica and K-feldspar in rocks from Searl ridge. (3) The AFT ages record the time the rocks cooled through the AFT closure temperature (~100 °C in these rocks), likely due to exhumation. Throughout most of the northern traverse, the apatite data indicate the rocks cooled relatively quickly through the apatite partial annealing zone (PAZ; from ~110 °C to 60 °C) and remained at temperatures less than 60 °C as continued exhumation cooled them to present-day surface temperatures. The ages indicate that the western “arc” terrane of the WZ was being uplifted and cooled at ca. 91 Ma, during or shortly after intrusion of the 99–91 Ma La Posta–type plutons to the east. Uplift and cooling occurred later, between ca. 70 Ma and ca. 55 Ma, in the central WTZ, ETZ, and EZ rocks, possibly as upwarping in response to events in the UPZ. The UPZ experienced differential exhumation at ca. 50–35 Ma: Cooling on the western edge was taking place at about the same time or shortly after cooling in the younger samples in the ETZ and EZ, whereas on the east side of the UPZ, the rocks cooled later (ca. 35 Ma) and spent a prolonged time in the apatite PAZ compared to most northern traverse samples. Apparent cooling rates from Los Angeles Basin drill core samples of plutonic rocks show that four are similar to the WTZ thermal histories, and two are similar to the WTZ histories, indicating that the eastern part of the Los Angeles Basin area is underlain by mainly western zone PRB rocks. Thermal histories revealed by samples from Searl ridge indicate that the WTZ magmatism intruded the metasedimentary rocks prior to their deformation and metamorphism at ca. 97 Ma. Both low-grade schists and metasandstones of the western side of the ridge and high-grade gneisses of the eastern side of the ridge have thermal histories consistent with eastern zone rocks—suggesting a temporal/thermal relationship between the western transition zone and the eastern zones. Limited ages from six samples across the Eastern Peninsular Ranges mylonite zone (EPRMZ) indicate that this zone underwent cooling after emplacement of the youngest UPZ rocks at 85 Ma, suggesting that thrusting along the EPRMZ was either coeval with emplacement of the UPZ plutonic rocks or occurred shortly afterwards (~10–15 m.y.). Alternatively, the EPRMZ thrusting may have occurred at temperatures under ~180 °C at yet a later date. The geochronology presented here differs slightly from previous studies for similar rocks exposed across the middle and southern portions of the PRB, in that our data define a relatively smooth progression of magmatism from west to east, and the transition from western, oceanic-arc plutonism to eastern, continental arc plutonism is interpreted to have occurred at ca. 99–97 Ma and not at ca. 105 Ma.

Abstract In the Powder River Basin of southeast Montana and northeast Wyoming, coal beds exposed by regional erosion have burned naturally from as early as the Pliocene to the present. Layers of reddish clinker, formed by baking, welding, and melting of sediments above burned coal beds, cover over 4000 km 2 and cap ridges and escarpments throughout the dissected landscape of the Powder River Basin. Fission-track (ZFT) and (U-Th)/He (ZHe) ages of zircon grains from baked sandstones in clinker provide new insights about rates of regional erosion as well as episodic advance of coal fires into hillsides. Older, resistant clinker layers up to 60 m thick, formed by the burning of thick coal beds, cap summits and broad benches. Younger clinker rims, from thinner coals, form ledges on valley sides. ZHe ages of clinker, mainly from the Wyodak-anderson coal zone of the fort Union formation in the Rochelle Hills east of Wright, Wyoming, and from the Wyodak-anderson and Knobloch coal zones in the Tongue River valley near Ashland and Birney, Montana, range from 1.1 Ma to 10 ka. These dates generally agree with ZFT ages of clinker analyzed in the early 1980s, but they are a more precise record of ancient coal fires in the region. Our data indicate 0.2–0.4 km of vertical erosion in the past 1 m.y. Spatial-temporal patterns of clinker ages may prove to be useful in deciphering the patterns of fluvial incision and basin excavation in the Powder River Basin during the late Cenozoic and in weighing the relative importance of uplift, variations in climate, and base-level change.

Abstract This field trip highlights the current understanding of the tectonic assemblage of the rocks of the Central Appalachians, which include the Coastal Plain, Piedmont, and Blue Ridge provinces. The age and origin of the rocks, the timing of regional deformation and metamorphism, and the significance of the major faults, provide the framework of the tectonic history which includes the Mesoproterozoic Grenvillian, Ordovician Taconian, Devonian to Mississippian Neoacadian, and Mississippian to Permian Alleghanian orogenies.

Abstract The field trip will traverse the highly asymmetrical Front Range to examine the west flank's major Laramide thrusts, subsequent volcanic rocks and normal faults, as well as the east flank's higher-angle thrust and reverse faults and their associated fault-propagation folds. Laramide to Holocene tectonics will be debated on the outcrop and during our evening soak at Hot Sulphur Springs.

Abstract A wide variety of dating methods are used in Quaternary research, and each method has many applications and limitations. Because of this variety, we cannot discuss the applications and limitations of all methods here. The more versatile and widely used methods, including 14 C, K/Ar, fission-track, U-series, paleomagnetism, thermoluminescence, and amino acid dating are treated more comprehensively in this chapter than other methods that are shown on the summary chart. The summary chart is provided here to give an overview of dating work and research for the Quaternary. This summary consists mainly of a table (Plate 2) that is modified and updated from Colman and Pierce (1977, Plate 1, ref. 66). The table is intended as an overview and concise guide to Quaternary dating methods. It contains many subjective judgments and should not be considered definitive; the entries for applicability, age range, and optimum resolution are particularly interpretive. Details concerning assumptions, analytical techniques, uncertainties, and interpretations should be obtained from specialized references using the key references in Plate 2 as a guide. The dating methods described range from well-known and established techniques to experimental procedures whose results are subject to considerable interpretation. Key references included on Plate 2 are intended as an entry into the vast literature on dating methods; space prohibits a more complete listing. We have emphasized recent review papers and notable examples of applications as sources of additional references and information. Dating methods discussed in other sections of this chapter are indicated by asterisks in.

Fission tracks are zones of intense damage that result when fission fragments pass through a solid. 238 U is the only naturally occurring isotope whose decay rate results in a significant number of tracks over geologic time. Spontaneous fission of 238 U occurs at a known rate, and by determining the number of fission tracks and the amount of uranium present in a mineral or glass, its age may be determined. Many geologic materials contain trace amounts of uranium, but because of such factors as uranium abundance and track retention, zircon and glass are the only materials routinely dated in Quaternary samples. Applications of fission-track dating to Quaternary studies include the dating of volcanic ash and archaeological material. The method has also been used to determine the rate of landform development in the Powder River Basin of Wyoming through the dating of clinker formed by the natural burning of coal beds. In the Himalayas of northern Pakistan, fission-track dating of zircon and apatite has shown that uplift rates during the Quaternary were as high as 1 cm/yr, which accounts for the incredible mountainous relief.