Victor E Camp | Geology Reports

USGS2017

Field-trip guide to the vents, dikes, stratigraphy, and structure of the Columbia River Basalt Group, eastern Oregon and southeastern Washington

The Columbia River Basalt Group covers an area of more than 210,000 km 2 with an estimated volume of 210,000 km 3 . As the youngest continental flood-basalt province on Earth (16.7–5.5 Ma), it is well preserved, with a coherent and detailed stratigraphy exposed in the deep canyonlands of eastern Oregon and southeastern Washington. The Columbia River flood-basalt province is often cited as a model for the study of similar provinces worldwide. This field-trip guide explores the main source region of the Columbia River Basalt Group and is written for trip participants attending the 2017 International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI) Scientific Assembly in Portland, Oregon, USA. The first part of the guide provides an overview of the geologic features common in the Columbia River flood-basalt province and the stratigraphic terminology used in the Columbia River Basalt Group. The accompanying road log examines the stratigraphic evolution, eruption history, and structure of the province through a field examination of the lavas, dikes, and pyroclastic rocks of the Columbia River Basalt Group.

Oregon, Washington↗

USGS2015

Yellowstone plume trigger for Basin and Range extension, and coeval emplacement of the Nevada–Columbia Basin magmatic belt

Widespread extension began across the northern and central Basin and Range Province at 17–16 Ma, contemporaneous with magmatism along the Nevada–Columbia Basin magmatic belt, a linear zone of dikes and volcanic centers that extends for >1000 km, from southern Nevada to the Columbia Basin of eastern Washington. This belt was generated above an elongated sublithospheric melt zone associated with arrival of the Yellowstone mantle plume, with a north-south tabular shape attributed to plume ascent through a propagating fracture in the Juan de Fuca slab. Dike orientation along the magmatic belt suggests an extension direction of 245°–250°, but this trend lies oblique to the regional extension direction of 280°–300° during coeval and younger Basin and Range faulting, an &sim;45° difference. Field relationships suggest that this magmatic trend was not controlled by regional stress in the upper crust, but rather by magma overpressure from below and forceful dike injection with an orientation inherited from a deeper process in the sublithospheric mantle. The southern half of the elongated zone of mantle upwelling was emplaced beneath a cratonic lithosphere with an elevated surface derived from Late Cretaceous to mid-Tertiary crustal thickening. This high Nevadaplano was primed for collapse with high gravitational potential energy under the influence of regional stress, partly derived from boundary forces due to Pacific–North American plate interaction. Plume arrival at 17–16 Ma resulted in advective thermal weakening of the lithosphere, mantle traction, delamination, and added buoyancy to the northern and central Basin and Range. It was not the sole cause of Basin and Range extension, but rather the catalyst for extension of the Nevadaplano, which was already on the verge of regional collapse.

Geosphere↗