Location: Stanford University
Anyone wishing to attend the lecture only is welcome at no cost.
This will be the 331st meeting since 1954.
(1) LREE-enriched arc tholeiites + alkaline basalts and distal turbidites of the North Fork + Eastern Hayfork terranes were deposited in a subsea environment during Permian(?), Triassic, and earliest Jurassic time. Landward, subduction resulted in production of the high-P Stuart Fork blueschist complex, then its exhumation at ~227 Ma. (2) Submarine eruption and sedimentation continued outboard during Early and Middle Jurassic time, producing the west-facing North Fork oceanic arc and adjacent, tectonically disrupted Eastern Hayfork mélange. These two terranes underwent low-T alteration by seawater at 100-200 °C and <100 MPa; alkali exchange and modest Mg-enrichment were accompanied by increases in greenstone bulk-rock 18O values from 6 to ~10, preceeding and during initial stages of island-arc formation at 175-200 Ma. By the end of this time, the outboard Western Hayfork calc-alkaline arc had accreted to the Eastern Hayfork terrane. (3) Suturing of the North Fork oceanic arc beneath the exhumed, landward Stuart Fork terrane at 165-170 Ma resulted in regional folding and ~sub- to greenschist-facies metamorphism. Pervasive recrystallization took place without substantial chemical or isotopic exchange under conditions of 300-425 °C, 300 ± 100 MPa, higher metamorphic grade in the north, lower in the south. (4) East-descending subduction or transpression continued seaward, and granitoid plutons were emplaced locally during 159-164 Ma, heating adjacent wall rocks to a maximum of ~500-600 °C at pressures of 200-300 MPa. The temperature increase caused devolatilization of metasediments and the exchange of high 18O fluids with intimately intercalated greenstones; 18O values in metavolcanic rocks locally increased to more than 15. Subsequent cooling yielded 150-164 Ma apparent mineral ages for the metamorphic aureoles. (5) Minor intrusion took place at the end of Jurassic time, when distinctive muscovite porphyry felsite dikes transected the Stuart Fork-North Fork thrust contact; formation of hydrothermal gold-bearing quartz veins, dated at 147 ± 3 Ma, may be associated with this event. Cenozoic exhumation resulted in range-front faulting and erosion. The documented interplay between Phanerozoic convergence/transpression and petrochemical evolution in a suprasubduction zone setting provides an illuminating model for crustal growth.
If you want to pay in advance:
Stanford faculty and students: Please make dinner reservations by Friday, Friday, November 3. Contact Dr. Juhn Liou via his mailbox (and leave check), Geological and Environmental Sciences Office, Mitchell Bldg. (Rm. 138). Make checks out to "PGS."
All others, including faculty and students from other Bay Area universities and colleges and USGS: Please make dinner reservations by Friday, November 3. Contact Willie Lee, at USGS, MS-977, 345 Middlefield Rd., Menlo Park, CA 94025, phone 650-329-4781. Send check made out to "PGS" to Willie.
Dinner is $25.00. Includes wine (5:30 to 6:15 PM.), dinner (6:15-7:30 PM.), tax, and tip. Note: PGS does not make revenue on this price.
For students from all universities and colleges, the dinner, including the social half-hour, is $5.00 and is partially subsidized thanks to the Associates of School of Earth Sciences, Stanford University (Note, no-show reservations owe the full price).
Dues for Academic Year 2000-2001 ($10.00) should be sent to Willie Lee, USGS, MS-977, 345 Middlefield Rd., Menlo Park, CA 94025. Willie's phone: 650-329-4781.
Officers: Gary Ernst, President; Mike Diggles, Vice President; Vicki Langenheim, Secretary; Willie Lee, Treasurer
Date created: 09/19/2000
Last modified: 02/19/2003
Created by: Mike Diggles, Vice President, PGS.