Syn-extensional basin inversion over detachment faults: An example from the Raft River Basin, U.S.A.

Abstract

Several elements of the petroleum systems hosted in passive margins are intimately linked to the structural evolution of the margin. Understanding this structural evolution helps exploration geologists calculate the basal heat flow and the subsidence history, and evaluate the stratigraphic architecture, the timing of source and reservoir deposition. All of these are essential inputs or boundary conditions to basin models that are designed to predict source maturity and generation timing. Recent models of the formation of passive indicate that mantle exhumation along extensional detachment faults plays a fundamental role in continental break-up. The understanding of all the aspects of detachment faulting in hyperextended margins is challenging because almost all of them are in deep-water settings, and hard to study in detail, especially the features at the outcrop-scale.

A global compilation of all of the studied detachment fault systems indicates that 76% of these are continental (metamorphic core complexes), 19% are along oceanic spreading centers (oceanic core complexes) and 5% represent ocean-continent transition detachments. This distribution is strictly defined by accessibility, and it highlights that the data collected from well studied metamorphic core complexes may be used to understand some aspects of the detachment faults that control hyper-extension. An aspect that is captured in many young metamorphic core complexes is the inversion of the syn-extensional basin and the uplift of the stratigraphic package eposited in the basin. These events are common in continental core complexes and may be common during the evolution of the continental margins and would be represented by unconformities in the stratigraphic record.

The Raft River Basin is a syn-extensional basin that formed as a response to the exhumation of the Albion – Raft-River – Grouse-Creek metamorphic core complex in the northern Basin and Range Province. Integrating new field mapping, well data, a reinterpretation of geophysical data and geochronology constraints, allows for the documentation of the complex normal fault geometry of the Raft River Basin and understanding of its structural evolution and the inversion of its stratigraphy. Three generations of normal faults can be identified and restored that indicate that the Raft River Basin records an approximate horizontal strain of 1.2-1.3 (ß-factor of 2.1).

The structural evolution of the basin can be defined by four periods. From 14 to 9.5 Ma, the Albion Mountains were exhumed along a high-angle normal fault (Albion fault) that rooted into a brittle-ductile transition zone epresented by the mylonitic fabric in the Raft River detachment. During the exhumation of the complex a deep basin formed that was filled by ∼2.5 km of alluvial fan, fluvial and lacustrine deposits. From 9.5 to 8.2 Ma, the eruption of Snake River Plain - type rhyolite lavas was accompanied by very little extensional strain within the basin. After 8.2 to ∼5 Ma, the rotation of the Albion fault to shallow angles occurred and the deposits of the Raft River Basin were cut, rotated and uplifted by a younger generation of intra-basin normal faults. All of the stratigraphy is unconformably overlain by <5 Ma flat-lying strata, consistent with passive filling of the basin due to thermal subsidence. This entire history is consistent with the rolling-hinge model proposed by Buck (1988).

We propose that the 9.5-8.2 Ma volcanism provides first-order evidence for the high heat flow, that would enhance lower crust mobility and vertical flow due to focused density reduction (density instability). This heat and density instability was partly responsible for the uplift of the Raft River Mountains, initiation of the migration of the hinge of the Raft River detachment, and ultimately the inversion of the 14-8 Ma strata in the Raft River basin.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015