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THOMPSON FIELD FORUM ANNOUNCEMENT
Processes Controlling the Growth and Evolution of Continental
Batholiths, Coast Mountains, British Columbia, Canada
Terrace and Prince Rupert, British Columbia, Canada
12–17 August 2018
CONVENERS sources of magmas emplaced during HFE with those emplaced
during low flux periods.
Harold Stowell, Geological Sciences, University of Alabama,
Tuscaloosa, Alabama, USA This Thompson Field Forum will follow a transect across the
Coast Mountains at the latitude of Prince Rupert, Canada. This
Margaret Rusmore, Department of Geology, Occidental College, transect provides a window into the architecture of the batholith
Los Angeles, California, USA and the country rock, which preserve a record of batholith for-
mation and emplacement during terrane accretion, crustal con-
Glenn Woodsworth, Vancouver, British Columbia, Canada traction and thickening, and extensional collapse. The forum
will introduce researchers to this spectacular geology and focus
DESCRIPTION AND OBJECTIVES attention on areas where productive study can resolve outstand-
ing questions, which begin with the origin of the batholith.
This five-day field forum will explore the growth and evolution Regional maps coupled with large geochronological datasets
of continental batholiths. Although comprehensive datasets are delineate the broad framework of the batholith. Continuously
available for many batholiths (e.g., Sierra Nevada, California, exposed for >1500 km from south of Vancouver to north of
USA; Peninsular Ranges, Baja California, Mexico; and Fiordland, Juneau, Alaska, USA, the Coast Mountains Batholith consists of
New Zealand), the processes responsible for growth of large calc-alkaline, dioritic to granitic plutons ranging from early
Cordilleran batholiths are still poorly understood. The Coast Jurassic to Eocene. Overall, plutons become younger toward the
Mountains Batholith, British Columbia, is an ideal location for the continent. Magmatic patterns define two arcs or a doubled arc
field forum because it is the largest calc-alkaline plutonic complex prior to ca. 120–100 Ma. Built upon this broad framework,
on Earth. New data indicate that the batholith exhibits significant detailed investigations reveal a complex history of batholith
along-strike and cross-strike variations in structure, pluton com- growth that is not yet fully understood.
positions, and exhumation depths.
U-Pb zircon with complementary Hf and O isotope data com-
Magmatic arcs are the primary locations for production of con- bined with whole rock chemical and isotopic data identify HFE
tinental crust; therefore, continental batholiths should reflect these and indicate that magmas had larger crustal components during
processes. A growing body of data from upper and lower crustal some of these events documented for the Bella Coola to Ketchikan
exposure of batholiths around the globe indicate that most batho- segment of the batholith. Preliminary datasets for the batholith
liths are comprised of plutons that are emplaced during relatively south of Bella Coola indicate along-strike changes that likely
brief time periods (tens of millions of years) known as High Flux reflect fundamental variation in the magmatic system. Future
Events (HFE). These HFE punctuate the long history of magma- work will evaluate the roles of transpression and transtension dur-
tism in batholiths, resulting in HFE separated by low magmatic ing formation of the batholith.
fluxes that may last up to 100 million years. HFE may be trig-
gered by events external to the continental crust (e.g., subduction High-quality seismic profiles from the area of the Forum
angle, age of subducting crust) or events internal to the continen- Transect yield a precise location of the present Moho and average
tal crust (e.g., contraction or other crustal-scale rearrangement lithologies through the crust based on the ratio of compressional
within the arc). HFE have been documented in the northern Coast to shear wave velocities (Vp/Vs). These data constrain crustal
Mountains batholith, and associated isotopic data suggest an thickness to ~32 km beneath much of the batholith and clearly
increased signature from evolved continental crust during HFE. delineate a 5 km offset (shallower to the west) of the Moho at the
Recent research focuses on evaluating crustal contributions to Coast shear zone along the west side. Extensional collapse of the
batholith magmas in the central and southern Coast Mountains. batholith, leading to formation of Hecate Strait and Queen
One ongoing aspect of this research compares and contrasts the Charlotte basin, is evident in marine geophysical transects.
34 GSA Today | November 2017
