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Orientations were collected on flow foliations of volcanic units and bedding of sedimentary units throughout the field area. Flow banding is defined in silicic lavas by color banding and flow alignment of phenocrysts, and rocks often develop partings along the foliation which could be directly measured. In mafic lavas, flow foliation was sometimes expressed as platy partings in the interior of lava flows, and sometimes as a vesicle flattening foliation. In all types of volcanic rock, flow foliations can be quite chaotic, and surfaces were chosen that were a good representation of most of the foliation in a given outcrop.

Nevertheless, flow foliations are often poor indicators of paleohorizontal, particularly in silicic lava flows and domes, and our orientations should be viewed as only giving an “average” attitude at best. Sedimentary strata within the volcanic succession provide more direct proxies for paleohorizontal. Though some sedimentary beds were likely originally deposited horizontally, many, particularly those interleaved with Stage 1 and 2 silicic lavas represent pyroclastic apron deposits and were likely emplaced with steep original dips. Such deposits can also be easily modified and deformed by later lava emplacement as well. As such, no one measurement alone is ever taken to represent the original orientation of a flow, but rather the average of many orientations across a wide area gives an estimate of

paleohorizontal. Despite the complexity of possible original dips for both flow foliation and bedding, there is a remarkable overall consistency in the data and strong agreement between stratified epiclastic deposits and flow banding in lavas (Fig. 12).

Structural domains I and II are divided based on differences in average dip of Stage 2 and 3 lavas. Domain I includes all of the western Whipple Mountains and the easternmost extent of the Mopah Range, while Domain II includes the topographically higher interior of the Mopah Range on the western side of the field area (Fig. 15). Below, we describe the orientation of units in each of these two domains. Because the low relief and exceedingly rare and poor exposure of fault surfaces makes a structural analysis based on fault lineations impossible, orientations of units and strike of faults provide the only reliable constraints on how extension direction varies through time within the field area.

Domain I:

Stage 1 lavas outcrop only in Domain I, and have an average strike of 128° and dip of 76°. Stage 2a and 2b lavas are homoclinal in Domain I (Fig. 12) with average strike and

dip to be 122° and 60° respectively, based on averaging the mean vector of bedding

orientations and the center of the densest cluster of foliation measurements (mean vector of foliations does not approximate paleohorizontal because some foliation data collected from the cores of lava domes likely represent something closer to paleovertical, throwing off the mean of the data set. See Fig. 12). The variation between the average orientation of Stage 1 and 2 lavas may be explained in a few different ways. First, it is important to note that Stage 1 lavas display less well developed flow foliations than Stage 2 lavas, and also contain few interbedded sedimentary horizons. Stage 1 lavas also cover a much smaller areal extent than Stage 2 lavas, and are exposed only in the far eastern side of the field area in the Whipple Mountains. Because of this, few orientation data for Stage 1 (n=11) were collected relative to Stage 2 (n-498).

It is possible that the ~16° difference of average dip between Stage 1 and Stage 2 lava represents differential tilting between the two volcanic stages, implying that extensional normal faulting and block rotation began before Stage 2 volcanism (between ~20.6-20 Ma). However, because of the small number of Stage 1 orientations and limited exposure, the variation is not statistically significant. The similar orientations of 2a and 2b units preclude growth faulting during their deposition, so the steeper dips of Stage 1 are unlikely to

represent the steepest units in a longer episode of growth faulting. It is also possible that the few locations where Stage 1 orientations were discernable actually reflect variation in the degree of tilting across different fault blocks in Domain I.

The orientations of Stage 3 sedimentary rocks and lavas in Domain I are different than the older lavas they overlie (Fig. 12). The most remarkable characteristic of the Stage 3 accumulations is that they display fanning dips up section, ranging in some exposures from 67° to 14° dip from base to top (Figs. 12, 13). Our interpretation for up-section shallowing

dips of Tc and Tocba in the Mopah Range is that they were deposited in at least one actively subsiding half-graben basin. Fault A (Figs. 15, 13) is one fault that accommodated

accumulation of growth strata, and was subsequently buried when accommodation space in the subsiding basin was filled and Tocba lavas overtopped the fault scarp, spilling over the footwall block. The average strike of Stage 3 units is ~152° (about 30° from the ~122° average strike of Stage 1 and 2 units)

Stage 4 lavas, which occur only in Domain I are largely untilted. The only

discernably tilted Stage 4 units are exposed in the NE corner of the mapped area, where Tpst and Tob are cut by a few small offset faults, resulting in local tilting of ~8° (the 4 bedding measurements in Fig. 15 are from this location). All other exposures of Stage 4 volcanics show no evidence of tilting, and together have a horizontal average orientation.

Domain II:

All units in Domain II are considerably less tilted than their counterparts in Domain I. In addition to this another difference between Domains I and II is that in Domain II, 2a and 2b lavas are not homoclinal as they are in Domain I. Here 2b lavas lie in ~20° angular

unconformity above older 2a lavas (Fig. 12). The average strike and dip of 2a lavas and sedimentary rocks is 144° and 30° respectively, while the average of 2b lavas and pyroclastic rocks is strikes 125 and dips only 9°. Though the mean vectors of these relatively small data sets overlap, field observations confirm that this difference is real (Fig. 3C). This implies that some faulting and tilting event occurred between the eruption of 2a and 2b volcanics (between 19.6-19.5 Ma) in the interior of the Mopah Range, slightly before the inception of tilting just to the east.

The average orientation of Stage 3 lavas in Domain II is horizontal. This suggests that the normal faulting which resulted in tilting of Stage 3 lavas in Domain I was limited to the eastern 2/3 of the field area and did not affect the interior of the Mopah Range. Tilting of the Mopah interior had largely ceased by 19.5 Ma.