Emplacement and denudation history of the Lewis-Eldorado-Hoadley thrust slab in the northern Montana Cordillera, USA: implications for steady-state orogenic processes

Authors
Citation
Jw. Sears, Emplacement and denudation history of the Lewis-Eldorado-Hoadley thrust slab in the northern Montana Cordillera, USA: implications for steady-state orogenic processes, AM J SCI, 301(4-5), 2001, pp. 359-373
Citations number
62
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
AMERICAN JOURNAL OF SCIENCE
ISSN journal
0002-9599 → ACNP
Volume
301
Issue
4-5
Year of publication
2001
Pages
359 - 373
Database
ISI
SICI code
0002-9599(200104/05)301:4-5<359:EADHOT>2.0.ZU;2-C
Abstract
The northern Montana Cordilleran mountain chain provides unusual opportunit ies to evaluate long-term orogenic processes in a foreland thrust belt. The thick Lewis-Eldorado-Hoadley thrust slab and an imbricated thrust wedge in its footwall dominate the structure of the region. The slab is composed of the displaced Mesoproterozoic Belt-Purcell depositional basin and overlyin g Phanerozoic strata, tectonically inverted into the broad, deeply dissecte d Purcell anticlinorium. The geometric simplicity and stratigraphic integri ty of the structurally strong, thick slab constrain crustal-scale cross sec tions. Horizontal displacement of the slab increases from 40 km in central western Montana to 140 km at the United States-Canada border. Thermochronom eters from the exhumed thrust slab and its footwall coupled with erosional products in the neighboring foreland basin measure regional denudation duri ng Maastrichtian to middle Paleocene tectonic convergence. During the 15 my period of slab emplacement (74-59 Ma), regional denudation appears to have been minimal as the footwall subsided isostatically beneath the advancing tectonic load; erosion did not breach the Paleozoic cover of the slab. Simi larities in burial and denudation patterns from central Montana to southern Canada indicate that regional erosional denudation was steady-state with r espect to horizontal displacement and displacement rate. The steady-state c onfiguration of the slab was interrupted immediately upon cessation of thru sting at 59 Ma, when major uplift and denudation of the slab and its footwa ll began, yoked to isostatic rebound of an autochthonous mega-thrust ramp a t the depositional margin of the Belt-Purcell basin. The post-thrusting reb ound bent the slab into the Purcell anticlinorium, and the flexure was acco mpanied by extensional faulting of the slab. Although parameters of orogeni c relief are limited by its ancient nature, the northern Montana Cordillera is significant to evaluation of steady-state orogenic processes because it s history is clearly divided into a syn-orogenic phase with low denudation rates and an immediate post-orogenic rebound phase with high denudation rat es.