Geology topics

T. L. Pratt

Publications and source records attributed to T. L. Pratt.

21 records · Page 2Linked to original sources

USGS1997

Seismic reflection images beneath Puget Sound, western Washington State: The Puget Lowland thrust sheet hypothesis

Seismic reflection data show that the densely populated Puget Lowland of western Washington state is underlain by subhorizontal Paleogene and Neogene sedimentary rocks deformed by west and northwest trending faults and folds. From south to north beneath the Lowland, features seen on the seismic data include: the horizontally-stratified, 3.5 km thick Tacoma sedimentary basin; the Seattle uplift with south dipping (∼20°) strata on its south flank and steeply (50° to 90°) north dipping strata and the west-trending Seattle fault on its north flank; the 7.5 km thick, northward-thinning Seattle sedimentary basin; the antiformal Kingston arch; and the northwest trending, transpressional Southern Whidbey Island fault zone (SWIF). Interpreting the uplifts as fault-bend and fault-propagation folds leads to the hypothesis that the Puget Lowland lies on a north directed thrust sheet. The base of the thrust sheet may lie at 14 to 20 km depth within or at the base of a thick block of basaltic Crescent Formation; its edges may be right-lateral strike-slip faults along the base of the Cascade Range on the east and the Olympic Mountains on the west. Our model suggests that the Seattle fault has a long-term slip rate of about 0.25 mm/year and is large enough to generate a M 7.6 to 7.7 earthquake.

Washington↗

USGS1997

High-resolution seismic reflection profiling of the Santa Monica Fault Zone, West Los Angeles, California

High-resolution seismic reflection data obtained across the Santa Monica fault in west Los Angeles reveal the near-surface geometry of this active, oblique-reverse-left-lateral fault. Although near-surface fault dips as great as 55° cannot be ruled out, we interpret the fault to dip northward at 30° to 35° in the upper few hundred meters, steepening to ≥65° at 1 to 2 km depth. A total of ∼180 m of near-field thrust separation (fault slip plus drag folding) has occurred on the fault since the development of a prominent erosional surface atop ∼1.2 Ma strata. In the upper 20 to 40 m strain is partitioned between the north-dipping main thrust strand and several closely spaced, near-vertical strike-slip faults observed in paleoseismologic trenches. The main thrust strand can be traced to within 20 m of the ground surface, suggesting that it breaks through to the surface in large earthquakes. Uplift of a ∼50,000-year-old alluvial fan surface indicates a short-term, dip-slip rate of ∼0.5 mm/yr, similar to the ∼0.6 mm/yr dip-slip rate derived from vertical separation of the oxygen isotope stage 5e marine terrace 3 km west of the study site. If the 0.6 mm/yr minimum, dip-slip-only rate characterizes the entire history of the fault, then the currently active strand of the Santa Monica fault probably began moving within the past ∼300,000 years.

Geophysical Research Letters↗

USGS1992

Widespread buried Precambrian layered sequences in the U.S. mid- continent: Evidence for large Proterozoic depositional basins

Large regions of the North American mid-continent are underlain by Precambrian layered rocks buried beneath Phanerozoic sedimentary strata. South of the Wichita Mountains, published seismic reflection profiles show a Precambrian layered assemblage extending for at least 40 km in both the north-south and east-west directions, and industry data show that it may continue 150 km to the southeast. Seismic reflection data in the Illinois region show a Precambrian layered assemblage extending 320 km in an east-west direction and 200 km in a north-south direction. In both cases, the layered rocks are as much as 12 km thick. Apparent sequence boundaries (onlap, downlap) within these assemblages suggest they are parts of large depositional basins with diffractions and dipping strat due to faulting. The layered sequences correlate with regions of relatively long-wavelength and low-amplitude magnetic anomalies; the extent of this magnetic signature suggests that about 200,000 km{2} of Illinois, Indiana, and western Ohio, about 50,000 km{2} of southernmost Oklahoma and north-central Texas, and about 32,000 km{2} of southern Missouri and northern Arkansas may be underlain by similar Precambrian strata. Drill holes indicate that the top of the mid-continent Precambrian "basement" is composed largely of silicic igneous rocks. Such material may comprise a large part of the layered sequences. Alternatively, these igneous rocks could be intermixed with, or underlain by, nonvolcanic (meta?)sedimentary strata. The strong reflectivity of some layers suggest that minor mafic flows and/or sills may also be present. Analysis of U/Pb and Nd/Sm isotopes within the granites and rhyolites imply that the layered sequences postdate crustal formation at 1.7-2.0 Ga and predate or are contemporaneous with the 1.3-1.5 Ga crystallization ages of the granites and rhyolites. Though these layered rocks have a spatial association with igneous rocks and thus have likely been metamorphosed, the possibility tha they contain Precambrian hydrocarbons that escaped heating is as yet untested.

Mid-continent region↗