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Antler orogeny and foreland basin: A model
R.C. Speed and N.H. Sleep GEOL 730 Emma McConville Mo#va#on: Understand the basic problems of the Antler orogeny (1)  The tectonics that iniIated it (2)  The mechanisms that permiKed oceanic strata to be thrust up some 4 km and laterally many tens of kms in the light of limits on nappe length imposed by rock strength (3)  The cause of foreland basin subsidence and apparently coupled upliO of a hinterland to the west Purpose: To present a general model, quanIfied where feasible, of the Antler orogeny that gives geologically and mechanically acceptable soluIons to the problems driving the moIvaIon of the study Methods
•  Review regional geological relaIonships •  Determine that extensive erosion and tectonic events have limited Antler orogeny exposure à best exposure are the Roberts Mountains •  Review past work and observaIons made on the Roberts Mountains Allochothon, Autochthon, and the Foreland Basin •  Constrain age of deformaIon (Late Devonian to mid-­‐Early Mississippian) •  Construct a tectonic model Post-­‐Antler tectonism
1.  Late Paleozoic deformaIon, upliO and removal of the Roberts Mountain allochthon between Winnemucca and Elko 2.  Golconda allochthon was emplaced above the Roberts Mountain allochthon 3.  Jurassic and Cretaceous deformaIon of Cordilleran thrust and fold belt 4.  Cenozoic Basin-­‐Range extension Conclusion: main disturbances of Antler orogenic features was by Mesozoic foreland thrusIng throughout the central cordillera except in the tectonic enclave in central Nevada à this is the enclave to study! Assump;on’s about the enclave (study area)
1.  Antler related rocks are insignificantly changed from their Mississippian posiIons relaIve to sub adjacent Paleozoic shelf rocks 2.  Eastern limit of lower Paleozoic oceanic rocks is the approximate trace of the Roberts Mountains thrust 3.  If a regional Mesozoic decollement extends from eastern to western Nevada, it lies deeply within or below the lower Paleozoic shelf succession Foreland basin
•  Maximum preserved thickness of the clasIc wedge are as great as 3.5 km •  Wedge thins eastwards ~200 km •  ClasIc wedge prograded eastward •  The basin broadened with Ime and subsidence rates increased westwards à subsidence rates are unclear due to poor Iming constraints on deposiIon 359.2-­‐348 Ma Da;ng of Events
Claim mid-­‐Paleozoic emplacement of the Roberts Mountains allochthon: 1. The Kinderhookian age of onset of subsidence of the Antler foreland basin and provenance is the Roberts Mountains allochthon 2. Coincidentally, the model correctly predicts where the toe of the allochthon is 3. No deformaIon of Triassic and late Paleozoic strata above allochthon Conceptual Model
The arc-­‐conInent collision model does the following: 1. Eliminates the classical mechanical problem of how to start the thrust 2. Explains the strong contrast in deformaIon of the allochthon and autochthon and the apparent lack of shortening in the autochthon 3. Eliminates the need for magmaIsm 4. Has a potenIal modern day analogue Suffers from the absence of a recognized Antler magma#c arc à provide thermal contrac#on and subsidence as a poten#al explana#on Roberts Mountains allochthon Model of Foreland Basin and Highlands
•  Infer: basin-­‐highland evoluIon was not a product of plate boundary tectonics but was simply a consequence of loading of the conInental shelf by the Roberts Mountains allochthon. •  The downward displacement of the lithosphere would be equal to or somewhat less than one-­‐half the isostaIc value at the toe of the prism and would diminish to zero at a point 100 to 200 km towards the conInent DeflecIon of thin elasIc plate: Water –filled early basin is 51 to 180 km. Later the basin deposits indicate that sedimentaIon rate exceeded the subsidence rate and that the basin ulImately became sediment filled. ConInental outboard is down flexed -­‐50 and 80 km. 80-­‐420 km the conInent is upwarping ~250 m Prism no longer migraIng aOer 80 km (130 km overlap). Foreland basin is 2.6 km, toe 110 km, upwarp 350 m @ 290m km -­‐560 km Foreland basin 3 km deep at toe and extends 140 km conInent ward from toe. Upwarp 140-­‐500 km and is over 300 m high @ 230 km from toe Breached prism has been reduced to sea level by erosion and foreland basin is full of sediment Conclusions
•  Antler orogeny iniIaIon was by arc-­‐conInental collision •  MoIons of highland-­‐foreland basin couple were due to verIcal loading of the conInental shelf by a beached accreIonary prism •  Model explains mechanical problems and predicts 1)  Antler magmaIc arc subsided deeply aOer collision by thermal contracIon 2)  IniIal Antler highlands were the subaerial part of the accreIonary prism 3)  Deepest part of the foreland basin was at the toe of the prism •  AnalyIcal models of elasIc deflecIons of the conInental slope predicts a flexural bulge as high as 350 m preceded the migraIng foreland basin