STRESS FIELD ROTATION OR BLOCK ROTATION: AN EXAMPLE FROM THE LAKE MEAD FAULT SYSTEM

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however that faults, when arranged in sets, must generally rotate as they slip.Nut et al., (1986) showed how sufficiently large rotations require the development of new sets of faults which are more favorably oriented to the principal direction of stress.This leads to the appearance of multiple fault sets in which older faults are offset by younger ones, both having the same sense of slip.Consequently correct paleostress analysis must include the possible effect of fault and material rotation, in addition to stress field rotation.
The combined effects of stress field rotation and material rotation were investigated in the Lake Mead Fault System (LMFS) especially in the Hoover Dam area..  (Anderson, 1973;Bohannon, 1983) tho total slip across this system is approximately 65 kin.Geological evidence shows that strike slip faulting has begun 13.5 to 11.3 Ma ago and continued through Miocene to early Pliocene and possibly to Pleistocene times (Anderson, 1973;Bohannon, 1983).(1983) suggests specifically that most of the left lateral faulting occurred during late Miocene time.This phase of intense faulting was followed by decreasing volcanic and plutonic activity and by normal faulting that formed broadly spaced basins and ranges (Anderson et al., 1972;Angelier et al., 1985).(

DISCUSSION
The results reviewed above can be summarized as follows: ( (4) As shown in figure 4 the structural paleostress indicators from the Lake Mead fault system yield, assuming no material rotation, a 60" clockwise stress field rotation since mid-Miocene (Angelier et al., 1985).In contrast, paleostress indicators throughout the Basin and Range suggest only a 30" clockwise stress field rotation (Anderson and Ekxen, 1977;Zoback et a., 1981).
(5) The current maximum horizontal stress as inferred from earthquakes fault plane solutions, is oriented N30" E. This direction is in good agreement with stress orientation derived from post mid-Miocene structures in other parts of the Great Basin.

SUMMARY
We suggest that the Lake Mead -Hoover Dam area experienced the same 30" CW tectonic stress field rotation that presumably affected the entire basin and range province (Zoback et al., 1981).In addition this region has also experienced a 30" CCW material rotation of blocks and faults (Figure 4) giving rise to the apparent discrepancy between the stress history of the LMFS and the rest of the Basin and Range province.
There must exist other regions where the tectonic history involved both paleostress field rotation and material rotation.(1986).
fault slip takes place along short north-south trending right-lateral strike slip faults (set #3) (Rogers and Lee, 1976; Rogers et al., 1984) and not along the NW older trending ones, or the major NE faults.(3) The paleomagnetic data reveals that the NW trending right-lateral strike slip faults (set #2) were most probably involved in a 30" counterclockwise material rotation of blocks and faults (Figure 2).This has led us to suggest (Ron et.al., 1986) that the original orientation of these faults was approximately north-south as shown in figure 3. Presumably, as a consequence of their rotation they locked up, and a new, NS trending set of currently seismically active faults has developed.

Figure 3
Figure 3 presents our proposed model for the fault geometry, the sense of

Figure
Figure 3. Structural model for the development of multiple faults due to the material

Figure
Figure 4. Combined material rotation and stress field rotation in the Lake Mead Fault

of crustal blocks bounded by slipping faults. On the basis of
simple 2-D kinematic analysis Freund showed that rotations are not only possible, but generally unavoidable.Furthermore he showed that the sense of these rotations is directly controlled by the orientation of the faults (arranged in sets) relative to the direction of the principle tectonic shortening, and that the magnitude of the rotation of crustal blocks is controlled by the magnitude of the crustal shortening.Significant evidence has accumulated by now to suggest that block and fault rotations due to crustal shortening and extension are widespread.For example Ron et al., (1984) showed that adjacent conjugate fault domains experienced both clockwise and counterclockwise rotations in a single tectonic setting of northern Israel.

As a result, domains of multiple sets are formed in which the younger faults systemati- cally offset the older ones. This relatively simple process of crustal deformation by multiple sets of rotating faults and blocks thus results in complex fault patterns which do not require stress field rotations. Consequently tectonic and structural analysis of complex fault systems, using fault plane inversion methods which exclude fault rotation, can lead to unduly complicated and often erroneously inferred paleostress histories. Although it is most probable that in many cases stress field rotations actually take place over geological time, these rotations are
probably slow and gradual.As a result, much of the observed complexity of the fault pattern observed insitu could very well be due to multiple fault sets formed by material rotation of blocks and their bounding faults, not by stress field rotation.To determine the relative importance of the two rotations -stress field rotation vs. material rotation they have to be determined independently.Stress rotations must be inferred indirectly from regional structural and tectonic features (e.g., Zoback et.ai.,1981).In contrast material rotations can be determined directly from paleomagnetic declination and inclination anomaly measurements.The purpose of this short paper is to report on a probable case of combined material rotation and stress field rotation in the Lake Mead fault system, Nevada.The evidence is based on three types of information: Structural evidence (Zoback et a., 1981; Angelier et al., 1985), paleomagnetic data (Ron et al., 1986; Geissman, 1986), and seismicity (Rogers and Lee, 1976; Rogers et al., 1984).DATA Structures.The 30 by 80 km Lake Mead fault system (figure 1) includes a few long northeast-trending left-lateral strike slip faults (set # 1 in figure I & 2).Based on offsets of Late Neogene volcanic rocks

of the Hoover Dam area relative to the unrotated region to southeast. This rotation -is close both in sense and magnitude to Ron's et al., (1986) results for the Hamblin-Cleopatra area.
(Ekren, 1977;ery consistent with the sense of stress rotation proposed for the entire Basin and Range(Ekren, 1977; Zoback et al., 1981), the magnitude of this stress rotation is in quakes in the Lake Mead region that many faults are right handed strike slip in nature,