EVOLUTION OF A LARGE MIOCENE GROWTH-STRUCTURE IN THE UPPER PLATE OF THE WHIPPLE DETACHMENT FAULT, NORTHEASTERN WHIPPLE MOUNTAINS, CALIFORNIA

Miocene sedimentary and volcanic rocks in the north-eastern Whipple Mo untains, California, and the north-western Aubrey Hills, Arizona, accu mulated in the upper plate of the Whipple detachment fault during regi onal extension and slip on the detachment. Miocene rocks in this area can be divided into three sequences: (1) pre-18.5-Ma dominantly volcan ic rocks; (2) the 18.5-Ma Peach Springs Tuff; and (3) post-18.5-Ma dom inantly sedimentary rocks. Important stratigraphic markers in sequence 3 include a 100- to 140-m-thick basalt unit and the voluminous War Ea gle landslide, both of which correlate across Lake Havasu from the nor th-east Whipple Mountains to the Aurbrey Hills. We divide elastic sedi mentary rocks of sequence 3 into three informal members: (3a) conglome rate and sandstone stratigraphically beneath the basalt; (3b) conglome rate and sandstone above the basalt and below the War Eagle landslide; and (3c) conglomerate and sandstone that overlie the War Eagle landsl ide. Detailed stratigraphic analysis and field mapping reveal dramatic south-westward thickening of member 3b strata, from about 50 m in the Aubrey Hills to over 1500 m in the north-east Whipple Mountains. In t he north-east Whipple Mountains, this thick dipping section is overlai n by the War Eagle landslide along a major angular unconformity; in th e Aubrey Hills the base of the War Eagle landslide is roughly parallel to bedding dips of underlying strata. The above stratigraphic relatio nships can be explained by syndepositional growth of a rollover monocl ine by progressive tilting of the hangingwall above a master listric n ormal fault (Whipple detachment fault). This phase of upper-plate defo rmation began shortly after deposition of the basalt and ended prior t o emplacement of the War Eagle landslide. Interbedded breccias low in member 3b, about 100 m above the basalt, record the first appearance o f mylonitic detritus in the section. Growth of this upper-plate rollov er was thus initiated at about the same time (shortly after deposition of the basalt) that the lower plate of the Whipple detachment fault w as first exposed at the earth's surface by tectonic denudation and lar ge-scale crustal uplift. These events are interpreted to record initia tion of a secondary breakaway fault on the north-east flank of the gro wing Whipple detachment dome shortly after deposition of the basalt at about 14.5 (+/-1.0)Ma.