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Thornton Fault Low Angle Thrust- Generally Folded (Teeth on Upper Plate)

 I-93 Roadcuts – Study Site 1 

I-93 Roadcuts – Study Site 2

**Figure 1.** Bedrock geologic map for the area immediately surrounding the two study sites along the I-93 roadcuts in Woodstock, NH, USA. (After [20], sheet 1). Location map is shown in upper left. The study site is located in the Lower Rangeley Formation.

**Table 1.** Results of inversions for nine tectonic regimes based on the direct inversion method [4] with additional refining process [13]. Reg. = reference number of regime, also referred to in Table 2. MIFL = the control parameter, indicates the minimum individual fit level finally retained (see the term ω in [13] for detailed discussion of MIFL). NA = number of fault planes found acceptable at this level of fit. NR = number of fault planes rejected. The stress tensor obtained is characterized by the trends and plunges of the three principal stress axes, σ1, <sup>σ</sup>2, and <sup>σ</sup>3, and by Φ = (<sup>σ</sup>2 − <sup>σ</sup>3)/(<sup>σ</sup>1 − σ3), the ratio of the principal stress differences where 0 ≤ Φ13 [2]. υm = average value of the main estimator [4], τ\*m = ratio of the average shear stress to the maximum shear stress. αm = average value of the calculated shear-actual slip angle.


**Table 2.** Tectonic paleo-stress chronology including: tectonic events (compressional or extensional), relative chronological order, regime number, fault type, and orientation of the three principal stresses, as determined by fault-slip analysis in the present study. The time ranges are from the published literature where known tectonic events in the region have been dated as presented in Section 2 above. The orientation of the principal stresses for each tectonic event has one oriented vertical and two horizontal with azimuthal angles as shown.


Based upon the orientation of glacial striations, the last Wisconsin Ice sheet moved over the study site from WNW to the ESE [16]. At the last glacial maximum 14,000 years ago, the minimum thickness of the ice at the study site was approximately 1.6 km [27]. The glaciers swept away the thick loess, soils and vegetation that previously covered the bedrock. In some places, the advancing glacial ice plucked automobile-sized blocks from the leeward side of the ridges and small hills in the Hubbard Brook valley. Throughout the valley, glacial ice and water carved and polished the top of the bedrock to a smooth, undulating surface. Finally, as the ice sheet melted in place, the rock debris within the glacier, worked by rivers and streams on top of, within, and beneath the melting ice, was deposited as discontinuous layers on top of the bedrock surface with thickness increasing from 0 at and near the ridge crests and stream beds to as much as 50 m in the lower part of the valley [27]. Because of the glaciation-related erosion, the present-day rock condition of bedrock exposures is extremely fresh, which makes our study of brittle structures much easier than if it were weathered rock.

The geology and faults studied in this report are exposed in roadcuts along Interstate 93 (Figure 2) which were mapped at a scale of 1:200 by [16] sheet 2. All naturally occurring fractures greater than one meter in scale are shown on the map. Fracture orientation, trace length, aperture, surface roughness, and interconnectedness were measured and analyzed [16,28]. The compositional variability in the

schist persists to the millimeter scale. The schist has a well-defined foliation, which has been refolded at least twice, and the foliation can be highly variable at length scales less than a meter. At larger scales, the foliation strikes from 25 to 45 degrees east and dips steeply to the southeast, consistent with the regional tectonic fabric.

**Figure 2.** Photograph of a N5E striking roadcut exposure at the study site on I-93 containing NE striking, SE dipping faults included in this study. Note, most of the faults fractures exhibit dark planar surfaces. The rock type is primarily Concord granite (dark gray) with a Lower Rangeley schist block (light grey/white) exposed to the right of center above where the grass meets the bedrock and between the first and fourth drillhole from the left side of the photo. The subvertical lines are drill holes used in blasting the roadcut surface. Targets were used for rectification of photographs on which the geology and fractures were mapped by [16] (sheet 2).

The granitic rocks intrude the schist in the form of small tabular dikes and large anastomosing fingers ranging from 1000s of meters to the meter scale. The intrusion was prolific, and granitic rocks account for approximately 50% of the rock area mapped in the road cuts, Figure 2 [16] sheet 2. and in the 40 boreholes (totaling 4.6 km of wellbore) drilled in the Mirror Lake watershed, located at the eastern end of the Hubbard Brook valley [23]. Changes in lithology between granite and schist occurs every 5–9 m in the roadcuts [16] and the boreholes [23].

Bedrock fractures in the roadcuts, natural outcrops and the bedrock wells include joints (formed as Mode 1 fractures), faults (formed as Mode 2 fractures), and reactivated faults and joints. Fractures formed prior to the maximum burial and temperature (410–390 Ma) would have been destroyed by metamorphic recrystallization. We therefore assume that all the fractures that we observe in outcrop were formed after the peak metamorphic event at approximately 390 Ma. It is not possible to determine the age of fracture formation or reactivation using relative or radiometric dating. The brittle tectonic activity since 390 Ma could result from events during the Alleghenian orogeny (Permian, 299 to 251 Ma) and to younger tectonic events, such as the extension related to the opening of the northern Atlantic ocean approximately 200–175 Ma or to the glaciation-deglaciation cycles of the Quaternary (2.6 Ma to present) [27]. Little evidence for the age of brittle events can be obtained from stratigraphy or rock dating, although thin (~1 m) NE-SW striking diabase dikes occur in the

study site during the early Jurassic 200–146 Ma may be a brittle episode related to the opening of the Atlantic Ocean. Large numbers of fracture surfaces display syntectonic mineral infill or fiber growth. Because syntectonic minerals like quartz could not develop during the brittle events at very shallow depth, such mineralization indicates that most of the brittle tectonic activity that produced the fault slips took place at depths up to 15 km., and hence is related to tectonic episodes that predate the glaciation-deglaciation events

### **3. Data Collection and Stress Inversion Method of Analysis**

Two hundred and eight fault-slip data were collected at roadcuts in bedrock at two locations on Interstate-93 in Woodstock, New Hampshire as shown in Figure 1. The first location includes four sub-parallel vertical roadcut faces approximately 40 m apart, whose bedrock geology and fractures had been previously mapped [16]. The second location is a roadcut on the east side of the northbound lane of I-93, 1.3 km north of the first location. Figure 2 is a photograph of a section of a portion of roadcut at location 1 showing NE striking, SE dipping fractures in the Concord granite 370–365 Ma.

Faults were easily identifiable in the roadcuts, most of them bearing slickensides resulting from slip-parallel quartz growth. Numerous faults show minor (~1–2 mm) but clearly observable offsets of cleavage, schist-granite contacts, quartz-pegmatite veins, and along contacts of the diabase dikes and the schist and granite. Evidence of slip-parallel quartz growth was common. The strike and dip of the fault plane, the rake of the slickenside lineations, and the sense of relative offset were measured for each observable fault. The faults were numbered. All types of fault slips were found: dip-slip, strike-slip and oblique slip, with normal, reverse, right-lateral and left-lateral components of motion. This variety of fault slips indicates polyphase brittle tectonism, which was confirmed by differences in mineral fillings. The inferred tectonic regime/relative chronology (by number) and level of certainty, the roadcut location, and the fracture number on [16], were all noted. All the information recorded is listed in Appendix A Table A1.

Particular attention was paid to determination of the sense of motion on each fault. A variety of criteria were used, including: (1) offset of granite-schist and other metamorphic boundaries, (2) mineral growth along the slip direction, (3) presence of rough and polished facets along the fault surface, (4) asymmetrical striation markers, (5) striation-related micro-veins, (6) offsets of older fractures or veins, (7) presence of small Riedel's shear fractures, mainly R in type. Where possible, these criteria were cross-checked. As a result, three levels of certainty were considered concerning the senses of motion (see Appendix A, Table A1). The letter C refers to a slip sense that could be determined with certainty in the field, based on one or several unambiguous criteria. The letter P indicates that the slip sense is considered probable, which means that despite good observation some ambiguity could not be removed. The letter S refers to a poorly recorded sense of motion, in the absence of reliable criterion or with conflicting criteria. In that case an inferred "supposed" sense of motion was attributed, based on both the low-quality criterion (if any) and the behavior of the neighboring faults with well-recorded sense of motion and similar dip direction, attitude, and slip orientation.

Many faults were associated in conjugate or Riedel's type patterns with particular symmetries. Fault subsets were defined based on common geometry in terms of fault attitude, slip orientation and sense, fault dip direction, relation to other faults, and mechanical consistency. The relation between conjugate fault systems and stress has been highlighted by Daubrée's experiments [29] and Anderson's analysis [30]. In addition, Riedel's shears [11] often explain the relationships between faults at different scales.

Most fault slip data in the outcrops studied could not be interpreted in simple geometrical terms, because they resulted from reactivation of earlier faults or mechanical discontinuities (older faults, joints, veins, cleavage, contacts between rock types, etc.). Such inherited faults may have various attitudes oblique to all stress axes, contrary to the "newly formed" faults discussed above, which generally contain one principal stress axis and form symmetrical systems. For this reason, we undertook systematic inversion of the fault slip data to reconstruct the stress regimes. Such inversions are based

on consideration of the stress-slip relationships proposed by [31,32], which were used by [12], who first addressed the inverse problem in their pioneering work. Later studies demonstrated that the basic assumptions underlying the method were acceptable in the first approximation and well accounted for actual slip distribution (e.g., [2,3]), and numerical modeling experiments showed that deviations from the model are significant but remain statistically minor with regard to other sources of uncertainty [33].

The direct inversion method' used here is based on a least-square minimization, with a criterion called υ (upsilon) that depends on both the angle between the calculated shear and the actual slip, and the shear stress amplitude relative to maximum shear stress. For details, the reader is referred to the paper that describes this method [4]. We also use a robust refining process that was not described in the original formulation of the method but is presented in the use of another method especially designed for the stress inversion of earthquake focal mechanisms [13]. This additional process was facilitated by the negligible runtime of the inversion method, which involves analytical means instead of numerical search. A crucial parameter is the minimum fit level required for defining acceptable data. We use a scale from −100% (total misfit) to 100% (perfect fit). The lowest bound involves maximum shear stress acting in the direction opposite to slip. At the highest bound, the shear stress is also maximum but acts in the same direction and sense as the slip. A zero value indicates that slip occurs with shear stress perpendicular to slip, as the limit between consistent and inconsistent senses of motion. Note that this minimum fit level is linearly related to the RUP % estimator defined by [4], the values −100% and +100% corresponding to the values 200% and zero (respectively) in the RUP estimator and di ffers from the ω estimator defined by [13].

To determine a stress regime, υ is minimized as a function of the four unknowns that describe a reduced stress tensor: the orientations of the three principal stress axes and the ratio Φ = (<sup>σ</sup>2 − σ3)/ (<sup>σ</sup>1 − σ3). One obtains the smallest slip-shear angles and the largest possible shear stresses that can simultaneously exist for all the data taken together.

The real data dispersion, which depends on complex geological factors, is larger than the angular uncertainty of about 5◦ in our field data collection. To determine whether a stress inversion is significant or not, we use an iterative refining process that involves successive inversions with a progressively increasing demand for good individual fits. This process allows determination of the level of data rejection consistent with the data accuracy.
