The structure and petrology of the Johnny Lyon Hills area, Cochise County, Arizona

Cochise County, greywacke, slate, conglomerate, Pinal schist, Laramide orogeny

Silver, Leon Theodore ; Campbell, Ian

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Cochise County, greywacke, slate, conglomerate, Pinal schist, Laramide orogeny;   

Others  :  https://thesis.library.caltech.edu/8196/2/Silver_lt_1955.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

The Johnny Lyon Hills area is located in Cochise County insoutheastern Arizona. The rocks of the area include a central coreof Lower pre-Cambrian igneous and metamorphic rocks surrounded by acomplexly faulted and tilted section of Upper pre-Cambrian andPaleozoic strata. Limited exposures of Mesozoic and Tertiary sedimentaryand volcanic rocks are present at the north end of themap area. Late Tertiary and Quaternary alluvium almost completelysurrounds and overlaps upon the older rocks.

The older pre-Cambrian rocks include a section of more than9000 feet of generally moderately metamorphosed graywackes, slatesand conglomerates of the Pinal schist injected in zones by somewhatyounger rhyolite sheets. The original sediments were deposited ina geosyncline whose extent probably included large parts of Arizona,New Mexico and west Texas. During the Mazatzal Revolution the Pinalschist was deformed into northeast-trending, steeply dipping andplunging structures and the entire local section was overturnedsteeply toward the northwest. The pre-Cambrian Johnny Lyon granodioritewas emplaced as a large epi-tectonic pluton which modified themetamorphic character of part of the Pinal schist. Larsen methoddeterminations indicate an age of about 715 million years for thisrock, which is about the minimum age compatible with the geologicrelations.

The Laramide orogeny produced numerous major thrust faultsin the area involving all rocks older than and including the LowerCretaceous Bisbee group. Major compression from the southwest andsubsequent superimposed thrusting from the southeast and east areindicated. Minimum thrust displacements of more than a mile areclear and the probable displacements are of much greater magnitude.The crystalline core behaved as a single structural unit and probablycaused important local divergences from the regional patternof northeast-trending compressive forces. The massif was rotated asa unit 40 degrees or more about a northwest-trending axis overturningthe pre-Cambrian fold axes in the Pinal schist.

Swarms of Late Cretaceous(?) or Early Tertiary(?) lamprophyricdikes cross the Laramide structures and are probably related to thelarge Texas Canyon stock several miles southeast of the map area.Intermittent high angle faulting, both older and younger than thedikes, has continued since the Laramide orogeny and has been superimposedon the older structures. This steep faulting combined withthe fundamental northwesterly Laramide structural grain to producethe northwesterly trends characteristic of the mountain ridges andvalleys of the area.