The Rocks of Paradise!

The Pinto Peak intrusion is unusual as the country rock bedding contacts are not deflected toward or into parallelism with the intrusion margins and the country rocks are not folded due to emplacement of the intrusion. Hacker et al. [2002] argued that the intrusion likely intruded from the north into the Cretaceous Iron Springs Formation and then stepped up section into the Tertiary Claron Formation to the south (Figure 2). Based on the AMS data reported here, this scenario is difficult to reconcile if the magnetic fabrics reflect magma flow into and/or away from the vent area. We envision an alternative scenario that adequately explains the observed fabric data from the main intrusive phase. Furthermore, we speculate on the relation between the fabrics in the vent facies to the main intrusive phase of the Pinto Peak intrusion. We propose that the fabric data from the northern part of the intrusion reflects magma flowing away from the vent area after it became blocked. The steep fabric data from the southern part of the intrusion reflects the likely ascent site of magma from depth that fed the intrusive phase as well as delivered magma to the vent (Figure 7a). We note that this is our preferred interpretation of the AMS data and alternative interpretations are possible.

[27] In the north, magnetic lineations generally plunge steeply away from the vent area and become shallower to the north. Following an eruption, the magma drained away from the vent conduit as magma pressure in the igneous body at depth decreased allowing shallow magma flow to the north (Figure 7b). Just east of the vent, three sites yield steep magnetic foliations that likely experienced a strong degree of magmatic strain as the magma drained from the vent conduit. This is reflected in the strongly oblate susceptibility ellipsoids (Table 1). Once clear of the vent area the magma experienced a reduced flattening strain reflected by the progressive decrease in T away from the vent (Table 1, sites PP4, PP5, and PP6) yet the fabrics remain oblate. The oblate fabrics in these sites remain steep reflecting draining of the conduit back into the main body of the intrusion at depth (Figure 7b). Moving north, the magnetic fabrics are dominantly prolate reflecting a degree of magmatic strain dominated by stretching and little flattening associated with emplacement. The magma was thus more likely to flow unrestricted allowing the linear elements of the fabric to develop with orientations that are generally consistent across the northern part of the intrusion. We interpret the flow pattern in the north to reflect subhorizontal flow of magma filling a tabular sheet associated with the propagation of the intrusion to the north. It is important to note that the AMS fabrics record the last increment of magmatic strain (finite strain). Therefore, only a minor amount of sagging (magmatic flow) within the intrusion is required to generate the observed fabrics. It is therefore reasonable to interpret the consistent AMS fabric data as recording the last increment of strain related to magma flow and in this case reflecting flow away from the Pinto Peak vent area.

[28] In the southern part of the intrusion, the magnetic fabrics plunges/dips are steep to subvertical with no preferred strike/trend. We argue that the southern part of the intrusion represents the ascent site of the magma from depth (Figure 7a). In this scenario, magma ascended from depth along a steep feeder system to shallow crustal levels. Once at shallow depths, the magma lifted the overlying county rocks leading to a weakened roof and the formation of a focused network of fractures. Eventually, the magma forced its way upward through the network of fractures with an incipient vent developing. At or shortly after the magma reached the incipient vent, the overlying country rocks became unstable resulting in gravitational collapse of the roof releasing a gravity slide mass (Figure 7b). The rapid decrease in pressure led to explosive decompression of the magma chamber resulting in a pyroclastic eruption that deposited the Rock of Paradise ash flow tuff. Following the main pyroclastic eruption, effusive eruptions spilled from the vent flooding lava to the east covering part of the gravity slide mass.

[29] The magnetic fabrics in the conduit rocks are difficult to interpret largely because we have only data from seven sites in these rocks and the timing of fabric formation is difficult to constrain. Nonetheless, we offer a tentative interpretation of the vent facies data. The oblate magnetic fabrics in the vent area likely represent flattening against the conduit walls. This is further supported by the weak planar flow foliation observed in the vent conduit rocks. The variation in the strike and dip of the magmatic fabric likely records the site’s proximity to the wall of the vent. Near the central part of the conduit, subvertical AMS fabrics and weak magmatic flow banding are interpreted to reflect draining of the magma out of the vent while sites away from the central vent edifice likely preserve a local flow regime associated with flattening near the margins or roof of the vent.