Bedrock topography Configuration

In an earlier work, ^{Escamilla-Casas (2001} and ²⁰⁰³⁾ presented bedrock topography configurations of the Exeter and Hampton USGS 7.5-minute quadrangles based on depth to bedrock data estimated from 220 drill logs (USGS-WRD and NH DES Water Division) and outcrop elevations. Those configurations failed to identify the extension of the Merrimack Group, but showed the presence of minor intrusions similar to the Exeter Pluton, which occupies topographic highs. In this paper, a configuration of the bedrock topography covering a larger area and a larger data set is presented.

Figure 2a presents an ordinary Kriging prediction map which subdivides the bedrock topography into twelve contoured classes, ranging from -33.22 m to 61.6 m (-109 ft to 202 ft) relative to present sea level. The highest and lowest topographic values occur in the northwest and southeast portion of the map, respectively, accentuating a prominent contrast in elevation along a southwest - northeast trending sharp boundary.

Map contours delineate a southwest to northeast-trending fabric (Figure 2a), which stands out and coincides with the orientation of most of the bedrock geologic structures identified in the study area (Figure 1a). For instance, the sharp northeast-trending boundary between topographic highs and lows coincides with the trace of a strongly defined northeast-southwest lineament on the DEM that corresponds to the Calef Fault Zone (Figure 1b, ^2a, and ^2c). In the upper central section of the ordinary Kriging prediction map (Figure 2a) there is a topographic high surrounded by topographic lows suggesting the presence of a circular-shaped structure (Figure 1, ^2a, and ^2c). This circular-shaped topographic high, which is also observable in the present topography and the DEM, will be discussed and detailed in a separate section of this paper.

The two lowest topographic values in the ordinary Kriging prediction map are aligned along a northeast-southwest-trending line defining a trough (Figure 2a). The trough is separated by a local high (labeled A in Figure 2c). If the trough corresponds to a geologic structure, such a feature must be deeply imprinted in the bedrock topography. The trend of this deep linear structure probably extends to the southwest, as shown by the topographic low to the south. At present, there is no geologic structure mapped in that specific location, but the Nannie Island Fault Zone (Figure 1) crops out in two nearby locations along strike (^{Escamilla-Casas, 2003}). Furthermore, the presence of abundant Exeter Pluton-like satellitic bodies in the area could be responsible for the discontinuous nature of the inferred trough (^{Escamilla-Casas, 2001}).

Along the southern border of the ordinary Kriging prediction map, two topographic highs occur close to each other (labeled B in Figure 2c). The topographic high to the northwest has an elongated shape and is oriented parallel to the predominant structural grain in the area; the other has a round shape and thus, does not have a preferred orientation. These two features are separated by a narrow topographic low that can be connected along strike with more topographic lows to the northeast, thus defining the trace of a topographic lineament.

Characteristic of the south portion of the study area is the extensive cover of glacial sediments and swamps resulting in limited outcrop exposure, making field geological mapping difficult. Therefore, topographic lineaments and traces of geologic structures are poorly controlled on the surface of these areas due to a low topographic contrast. However, lineaments in the prediction map, although not prominent, assist in the fine-tuning of the traces of shear zones in this area, particularly, the Great Common and Portsmouth fault zones (Figure 1b). The prediction map shows a curved lineament (labeled C in Figure 2c) merging with a northeast-southwest trending lineament at its southern extent (southeast corner, labeled D in Figure 2c). Based on similarity of trends, it is suggested that the curved lineament corresponds to what is known as the Portsmouth Fault Zone, and the north-east-trending lineament is related to the Great Common Fault (Figure 1).

In the northeastern corner of the study area, the ordinary Kriging prediction map shows an incipient northwest-southeast trending lineament that is perpendicular to the predominant Northeast-Southwest fabric (Figure 1, ^2a, and ^2c). This lineament is not present in the Geologic State Map (^{Lyons, et al., 1997}), but it is consistent with previous interpretations that recognize a NW-SE trending structure in the area (e.g. ^{Ballard and Uchupi, 1972}; ^{Birch, 1979}; ^{Hussey and Pankiwsky, 1976}; ^{Stewart, 1971}; among others). McHone (1998) described it as the Burlington - Portsmouth Lineament (Figure 1b and ^2c) and interpreted it as being related to the opening of the Atlantic Ocean during the Mesozoic.

Topographic Lineaments

The topographic lineament map that is presented here (Figure 3) includes the analysis of lineaments observed on both, inland topography and high-resolution-bathymetry of the seacoast of New Hampshire. Onshore, the present topography shows a pervasive northwest - southeast structural grain that consists of glacial deposits characterized by discontinuous and elongated topographic highs, and a less persistent northeast - southwest grain. The lineament map shows a strong contrast between topographic highs and lows in the northwest edge of the study area (Figure 3), similar to the prediction map presented in the previous section (Figure 2a). Topographic profiles show that this topographic contrast corresponds to the Calef Fault Zone and consists of a lineament characterized by both, slopes gently dipping to the northwest and slopes steeply dipping to the southeast (Figures 3 and ⁵).

Figure 5 Mosaic of Digital Elevation Models of the study area showing the location of the topographic profiles (see text for explanation). Profiles A-A' and B B' show the difference in elevation along the Calef Fault Zone. Profile C-C' shows a deep topographic feature that corresponds to the Burlington-York topographic lineament. 

Sections A-A' and B-B' are northwest to southeast-trending topographic profiles perpendicular to the trace of the Calef Fault Zone (Figure 5). These topographic profiles show that the elevation to the west is higher than to the east, and that the topographic lineament defining the trace of the Calef Fault Zone has, locally, a difference in elevation of approximately 110 meters.

A single, well-defined, west-northwest to east-southeast trending prominent lineament crosscuts the uppermost portion of the study area (Figures 3 and ⁵). A topographic profile (Section C-C' on Figure 5) perpendicular to the trend of this lineament shows that it consists of a deep topographic low with a marked elevation contrast on both sides, where to the north of the lineament the elevation is higher than to the south. This lineament extends to the east offshore as it is observed on the bathymetry, proving that the lineament is imprinted on the seafloor topography (Figures 3, ⁵ and ⁶). Based on the lineaments location and trend, it is being suggested that this lineament very likely corresponds to a segment of the Burlington-Portsmouth Lineament described by McHone (1998). Since the trace crosses through York Harbor in Maine instead of Portsmouth, New Hampshire, the authors suggest that Burlington-York Lineament is a more appropriate name.

Figure 6 Traces of Euler poles to the long segments of the course of the Piscataqua and Cocheco rivers on a mosaic of merged Digital Elevation Models and bathymetry, converge at different points. However, both poles are close to the center of the Agamenticus Complex. For reference, the traces of the Portsmouth and Great Common fault zones and the Burlington-York lineament are indicated. 

The stepping up to the northwest en- echelon array of the course of the Piscataqua River can be estimated by Euler Pole analysis (Figure 6), by tracing perpendicular lines to the long segments of the steps. The resulting plot shows that the lines converge on a sector located close to the center of the topographic high where the Agamenticus Complex crops out. Nevertheless, perpendicular lines to the long segments of the course of Cocheco River define a different Euler Pole.

To the south and southeast regions of the study area, the low topographic relief obscures the identification and tracing of lineaments (Figure 3). Nevertheless, the illumination settings and shadow contrasts of the animation files aided with the identification of lineaments, which can be traced off-shore and connected with those shown on the bathymetry of Portsmouth Harbor (Figure 3). Based on descriptions of previous authors and the trends of the traces identified in Gerrish Island and Newcastle (e.g. ^{Hussey, 1980}; ^{Carrigan, 1984}), these lineaments likely correspond to the Portsmouth and Great Common faults zones (Figure 1, Figure 3). However, the inland continuation of these lineaments to the south suggests that these faults eventually merge onshore. This convergence of faults is also consistent with previous interpretation of the fault trends in the area (e.g. ^{Lyons et. al., 1997}). ^{Escamilla-Casas (2003}) published a map showing the traces of the Portsmouth and Great Common Fault Zones merging to the south, which is consistent with the lineament pattern presented in this paper.

A more recent geologic map of the area shows a new fault zone to the south of the Great Common Fault Zone (^{Bothner et al., 2004}). This new fault zone extends to the east and merges with both, the Portsmouth and Great Common fault zones in its southern extent. Lineaments corresponding to these faults continue offshore to the northeast, as shown on the bathymetry of Figure 3. The orientation and location of these lineaments, which are parallel to the trace of the Norumbega Fault System described by ^{Ludman and West (1999)}, indicate that the new fault, the Great Common, and Portsmouth fault zones, likely belong to the Norumbega Fault System. The authors propose the name of Hampton Fault Zone for the new fault, as Hampton is the main locality of the fault.

Sub-circular Structure

In plan view, a composite image of the merged digital terrain models and high-resolution bathymetric soundings suggests the presence of a circular structure bounded to the north by the Oyster River, to the south by the Lamprey River, and to the east by the Great Bay (Figures 2c and ⁷). The perimeter of the identified sub- circular structure is approximately 24 km long and covers an estimated area of 31 km². Its long and short diameters are 7.2 km and 5.5 km long, respectively.

Figure 7 Location of the sub-circular structure on the study area. (a) Radial drainage pattern delineating the periphery of the sub-circular structure. (b) Simplified geologic map of the sub-circular structure showing the joint sampling locations. For each field station, illustrations of the polar plots indicating the dip, dip direction, and relative percentage of occurrence of each joint set are shown. (c) Mosaic of merged Digital Elevation Models and bathymetry of Great Bay showing the location of the three topographic profiles. The topographic profiles delineate the boundaries of the sub-circular structure. A northwest- southeast-trending-normal fault (block down to the northeast) crosscuts the sub- circular structure. 

While a radial drainage system is present outside the sub-circular structure's boundary, a deranged pattern developed inside the structure (Figures 7a). The original drainage inside the structure was, very likely altered by fine-grained glacial deposits that formed wetlands and impounded streams resulting in the formation of small lakes. Both, wetlands and small lakes are common features present in the geomorphology of this area.

Three topographic profiles (A-A', B-B', and C-C'; Figure 7c) show that the sub-circular structure is elevated with respect to the surrounding rock topography and slightly tilted to the west (Figure 7c). These profiles depict a prominent topographic high close to its center and a deep channel-feature that bounds the structure to the east. The presence of this channel is also revealed by the high- resolution bathymetric soundings of Great Bay and by the Kriging analysis (Figures 8 and ^2a, respectively). Moreover, the soundings and Kriging analysis indicate that the channel is imprinted on the bedrock topography, discarding any possible association with soft sediment deposits (see DEM, Figure 3 and bathymetry in Figure 8).

Figure 8 Image generated from high-resolution bathymetric soundings of Great Bay to the east of the sub- circular structure. The image shows the presence of dioritic dikes, which are also traceable on the surface and illustrates the inferred fault-related channel that bounds the sub-circular structure to the east. 

Gravity surveys show that the Exeter Pluton presents a positive Bouger gravity anomaly in excess of 16 Mgals (^{Bothner, 1974}) in the location of the sub-circular structure. Two and three-dimensional models based on gravity surveys in this area also suggest the presence of a 1-3 km thick plutonic body with both, a southeastern steep contact and a shallower northwest contact with the host rock. Additionally, aerial magnetometric surveys revealed anomalies related to the Agamenticus Complex in this region (^{Bothner and Hussey, 1999}; ^{Brooks, 1990}).

Within the sub-circular structure, data sets of joints were grouped into two domains, each domain defined upon the dominant rock type where the joints occur (Figure 7b, Table 1). Domain 1 defined from the intrusive rocks from the Exeter Pluton and Domain 2 defined from the metasediments of the Merrimack Group (Kittery Formation). In both domains, the joint sets present a strong variation in the number of joint planes per meter (joint frequency). However, the orientation of the joints can be grouped into three main sets, regardless of the domain in which they occur. Table 1 summarizes the orientation (predominant strike and dip) of the joint sets observed within each rock domain.

Table 1 Summary of the Joint set orientations in each domain. 

Notes: • Values indicate the average dip direction (azimuth) and dip angle (degrees) of each joint set.

• Refer to Figure 6b for locations.

• n = sample population.

To analyze the variation of joint frequency and its spatial distribution through the study area, a polar plot of the vectors representing the relative percentage of occurrence and the average dip and dip direction of each joint set was created for each joint sampling location (Figure 7b, Table 1). The spatial distribution of the regions with higher joint frequency delineates the areas where prominent structures occur. Crosscutting relationships indicate that rocks of the Merrimack Group are older than the Exeter Pluton, and thus it is inferred that joints older than the age of emplacement of the Exeter Pluton might exist in the Merrimack Group. However, the orientation and crosscutting relationship of joints in each domain permitted the identification and establishment of the joints relative chronology (sequence of formation) to be made and identify the joints that are related to the mechanisms of the intrusion. A comparison of the attitude between joint sets of the two domains also aided with the identification of the overall relative chronology. Joint sets were investigated at eight field stations (Figure 7b, Table 1), four of them located within the Merrimack Group and four within the Exeter Pluton.

At least five distinct sets of joints are present in the rocks of the Exeter Pluton (Domain 1, Joint Sampling Locations 1, 6, 7, and 8, on Figure 7b; Table 1). At Location 1, the oldest set of joints dips 74° to the northwest and it is crosscut by a conjugate set of joints dipping 65° to the northeast and 68° to the south. A second set of joints is present dipping 68° to the southwest; its relative age with respect to the previously described sets is uncertain. A third set of joint planes (the youngest) dips 55° to the northwest and has the highest frequency. At location 6, two main sets of conjugate joints have been observed. Joints that belong to the older set dip 65° to the northeast and 70° to the southeast, while a younger joint set dips 67° to the northwest and 65° to the southeast. At this location, the third and youngest joint set dips 80° to the northwest, and has the highest joint frequency. At location 6, a few joint surfaces contain slickenfibers that indicate a south block down vertical displacement. At Location 7, the oldest joint set dips 80° to the northwest and is crosscut by two conjugate sets: one with planes dipping 65° to the northeast and the other dipping 68° to the southwest. The youngest joint set dips 78° to the northwest and 80° to the southeast. At Location 8, the oldest conjugate set of joints dips 60° to the northeast and 70° to the southwest. A younger conjugate set dips 75° to the northwest and 80° to the southeast. The set with highest frequency of planes is the youngest in this location and dips 65° to the northwest.

Analysis of joints from locations 1, 6, 7, and 8 (Figure 7b) indicates that the Exeter Pluton has two conjugate sets of joints: one set dipping to the northeast and to the southwest, and a younger set dipping to the northwest and to the southeast. Following the interpretation of ^{Dyer (1988)}, the geometry of these conjugate joint sets suggests that during their formation, the principal tectonic stress was vertically oriented. In locations 5, 6, and 8 (Figure 7b) joint planes with dip directions tangential to the circular structure have the highest frequency. Locations with high joint frequency around the elevated sub-circular structure indicate that this structure was significantly affected by brittle deformation, and shows the presence of other structures, for instance, a fault that bisects the sub-circular structure (Figure 7c). In all joint sets within the Exeter Pluton, the youngest set has the highest frequency of planes (Sample Location 1).

Each field station in the Merrimack Group contains at least four sets of joints (Joint Sampling Locations 2, 3, 4, and 5 of Figure 7b; Table 1). Rocks in field stations of the Merrimack Group are characterized by higher joint frequency than the rocks of the Exeter Pluton. At location 2, the oldest joint set dips 76° to the northeast and it is crosscut by a conjugate set of joints dipping 32° to the northeast and 55° to the southwest. Here, the youngest set has the highest frequency of joints, with planes steeply dipping to the southwest. On these joint planes, slickenfibers indicate a north block down vertical displacement. At location 3, the oldest joint set dips 16° to the northeast and with a conjugate set dipping 84° to the southwest. This conjugate set is crosscut by two sets of joints, one dipping 52° to the northwest and the younger set dips 65° to the southeast. The youngest joint set contain the highest frequency of joint planes, is characterized by planes dipping 66° to the west, and contain slickenfibers that indicate an east-block-up vertical displacement. At location 4, the oldest conjugate set of joints dip 46° to the northeast and 21° to the southwest and are crosscut by a joint set that dips gently to the southwest (12°). The highest frequency of joints corresponds to the youngest joint set with planes dipping 80° to the southeast. These planes also contain slickenfibers indicating vertical (north block up) and horizontal (to the west) displacements. At location 5, the oldest set consists of conjugate planes that dip 37° to the east and 80° to the west, and are crosscut by a younger conjugate joint set characterized by sub-vertical planes. The two previously described conjugate sets are crosscut by a set of joints with planes that dip gently (20°) to the south. The youngest joint set at location 5, dips steeply to the southwest and have the highest joint frequency. Slickenfibers are observed on the planes of this youngest set, but cannot be used as kinematic indicators at this location.

Analysis of the general orientation of joint sets from locations 2, 3, 4, and 5 (Figure 7b; Table 1) indicates that conjugate sets of joints have variable orientations in the Merrimack Group, and thus, the interpretation of the stress fields is uncertain in this unit. At each location 2, 3, and 4, (Figure 7b and Table 1) the oldest conjugate set of joints have planes that variably dip to the northeast and to the southwest. The variation in the orientation of these joint planes is likely influenced by the anisotropy inherent to bedding and the Acadian folding of the meta-sediments. Furthermore, it is likely that the Exeter Pluton fractured the host rocks during or after its emplacement, producing most of the joint sets. In all locations within the Merrimack Group, the strike of the younger set of joints is tangent to the presumed sub-circular structure and the slickenfibers on the joint planes indicate vertical displacement. This is consistent with the vertically oriented tectonic stress that is interpreted from the joint orientation present in the rocks of the Exeter Pluton. Hence, both, the orientations of joint planes and kinematic indications from slickenfibers of the youngest set of joints in both domains, support the hypothesis of the presence of a sub-circular structure bounded by a brittle fault with vertical motion. The variability of sense of vertical movement, inferred from kinematic indicators preserved on joint planes in both domains, suggests reactivation of the vertical displacement along the fault with respect to its host rock at different time.

Tectonic Model

The tectonic interpretation of the geological evolution of the area, takes into consideration that the emplacement of the pluton, which is presumably underneath the circular structure, occurred under an extensional regime that prevailed during the Mesozoic. Thus, such intrusion is responsible for influencing the evolution of Great Bay tidal estuary and the geometry of the course of the Piscataqua and Cocheco rivers in times of the opening of the North Atlantic Ocean. This innovative extensional model contains the following stages:

First, during the Mesozoic, the Agamenticus Complex intruded rocks of the Merrimack Group. Due to the intrusion, the surrounding crust was subjected to tensional stresses (Figure 9a). As a result, and according to the Euler Poles analysis, the breakup of the crust produced a set of fractures in an en-echelon array that later was occupied by the drainage system that led to the formation of the present course of the Piscataqua River. The en-echelon array implies breakup resolved by pull-apart and strike slip faulting, produced by the motion of a rigid segment of the crust on the curved surface of the Earth about an approximately rotation pole or the Euler pole. The presence of the preexisting Exeter Pluton may have refracted the intrusion-related fracture trend forming the branch that corresponds to the present course of the Cocheco River.

Figure 9 Tectonic model proposed in this study. (a) The initial stage in this model suggests that the emplacement of the Agamenticus Complex generated the extensional regime. The extension produced a set of fractures that defined the trace of the present course of the Piscataqua River. The presence of the preexisting Exeter Pluton refracted the trace of the extension fractures generating the trace of the present Cocheco River. (b) The emplacement of an intrusive body, probably associated with the Agamenticus Complex, did not have the buoyancy to completely remove the Exeter Pluton and Merrimack Group cap rock. During the emplacement of this intrusive body, a rising movement was followed by subsequent collapse after the cooling stage. As a result of this vertical movement, brittle deformation was imprinted on the host rocks. (c) Finally, glacial erosion of the softer metasedimentary Merrimack Group rocks shaped the present Great Bay. 

Second, it is probable that an Agamenticus- like igneous body was emplaced in the middle section of the Exeter Pluton (Figure 9b). Although this ascendant pluton did not have the buoyancy to remove the overlying rocks completely, its rising vertical movement imprinted brittle deformation on the Exeter Pluton and nearby rocks of the Merrimack Group. After cooling, the intrusive body very likely collapsed moving vertically again. This resulted in the ambiguous sense of vertical movement that is inferred from the slickenfibers observed on the joint planes. From the structural analysis and the lack of other structures, it is inferred that the concentration of brittle deformation related to each vertical movement occurred along the fault that surrounds the sub-circular structure. Based on gravity surveys, the 1-3 km thick intrusive body may be responsible for the positive Bouger anomaly of the Exeter Pluton; its correlation with the Agamenticus Complex is based on aerial magnetometry.

The formation of the topographic low near the Exeter Pluton, and subsequent events (intrusion of plutonic bodies and youngest imprint of brittle deformation), was very likely part of the initial stages of formation of the actual Great Bay during the lower to mid Paleozoic. The presence of northwest to southeast trending drumlins and several other distinctive glacial deposits indicate that glacial erosion was intense in the lower east portion of Great Bay during the Quaternary (Fig. 9c). The glacial erosion was particularly strong where the less competent metasedimentary rocks of the Merrimack Group are predominant. Consistent with a glacial origin, the lower east branch of Great Bay is oriented northwest to southeast paralleling the general trend of glacial deposits in the study area.