It's hard not to wince when you first look at the renderings of the Mormon Church's expanding kingdom at 16th and Vine Streets, unveiled last week by Mayor Nutter. The architectural chameleons at Robert Stern's office have paired a 1920s-style apartment tower with a teensy redbrick meetinghouse that looks as if it was dragged across town from colonial-era Society Hill.

As if that wasn't enough, those retro buildings will join a snow-white, double-spired, French classical Mormon temple by Perkins+Will that is already rising along the cliff edge of the Vine Street Expressway. The collection of architectural pastiches promises to be one of the weirder ensembles produced in 21st-century America outside of Las Vegas.

Weirder still, they could end up as one of the most civic-minded projects now being built in Philadelphia.

The Mormons' undertaking on Vine Street, which fills a block-size parking lot and promises to help Center City bridge the chasm of I-676, presents a difficult conundrum: How do you respond to a development where the architecture is awful, but the urbanism is terrific - especially in a city routinely shortchanged in both categories?

I get the Mormons' impulse to go with reassuring, historical styles. Like many religious groups, the Church of Jesus Christ of Latter-day Saints see themselves as a safe port in an uncertain world, and want buildings that suggest things will always stay the same. Stern's firm, based in New York, is only too happy to oblige.

This has been especially true in Philadelphia, where every year seems to bring the firm another plum commission. While the city is desperate for more architecture such as Norman Foster's planned Comcast tower, which communicates vitality and innovation, his office keeps giving us backward-looking designs such as the Museum of the American Revolution, a flattened version of Independence Hall.

What makes the copycats objectionable is the insistence that they can be realized on the cheap. To make the museum "fit in" with its Old City context, Stern's office would plop a windowless cupola on the roof like a cherry on a sundae, and face the bulky facade in factory-made brick panels. Fortunately, the Art Commission has demanded revisions.

The Mormons, to their credit, actually believe in building for the ages. Having chosen a traditional aesthetic for their Vine Street projects, they appear committed to doing them right.

So, both the meeting house and the 32-story tower will be faced in brick - the hand-laid kind, in the case of the meetinghouse. Unlike virtually every other developer working in the city, the Mormons are willing to pay a premium to bury the parking for the tower and the temple, ensuring that these two blocks of the city will be welcoming to pedestrians.

To persuade people to trek across I-676, the Mormons instructed Paul Whalen, their architect at Stern, to wrap the tower's entire base in shops and townhouse-size buildings. There are no blank walls. And they're not even asking for subsidies.

These decisions would be remarkable enough if the site were on a choice Center City corner, but it's next to a highway cloverleaf - the ultimate modern condition. It's separated from the vibrant Spring Garden neighborhood by Callowhill Street's Great Wall of parking garages, and the east side of 16th Street has no sidewalks. There is no there there, but the Mormons are determined to create one.

You can't tell any of this by looking at the renderings, so I reached out to Tom King, who runs the church's real estate investment arm. The church, which claims to be gaining 300,000 members a year worldwide, has become a successful developer, thanks in part to its members' willingness to tithe 10 percent of their incomes.

It's a business model that encourages a certain kind of quality. Since the Mormons finance their own projects, and don't flip them to big holding companies, they're willing to spend more up front to make their buildings last. If only more Philadelphia developers adopted that approach.

Because of the temple, the church decided the surrounding area needed upgrading. This isn't charity, King said. Philadelphia's rental market is strong and the tower will be a purely commercial project. Anyone can rent a unit.

Instead of creating a drive-in fortress, they understood the building would be more marketable if residents felt connected to Center City. Despite the presence of the cloverleaf, they noticed a steady parade of pedestrians on 16th Street. By lining the east side of the tower podium with retail, they'll encourage more.

King also sees Vine Street as a potentially walkable - and livable - street. It's not as noisy as you might think, given the expressway below. Those rowhouses wrap around three sides of the base: on Vine Street, on a new north-south street that cuts through the block, and on Wood Street. With doors and stoops, they will establish an instant streetscape in a part of the city that has none.

The meetinghouse should extend the walkable experience, although detailing along Vine Street will be crucial. Unlike the temple, it will be open to the public and will offer community rooms, a basketball court, and a landscaped plaza area at the corner of Franklin Town Boulevard, the fancy name for this stretch of 17th Street.

The meetinghouse's front door faces the temple's, which also will be heavily landscaped. Unlike the blocky Revolutionary War museum, the chapel portion of the meetinghouse actually telescopes up in a natural fashion, culminating in a 117-foot, colonial-inspired steeple.

Despite the old-timey architecture, it won't exactly be an old-time urban street, lined by continuous entrances. The temple has no door on Vine Street. Astonishingly, it doesn't even have one on its most public corner, where it addresses Logan Square. But with the development of the old Family Court as a hotel, a 23-story tower farther east at 10th and Vine, and two proposed mid-rises at Broad and Callowhill, this empty zone will be transformed into a participating part of the city again.

These developments are evidence that Philadelphia is emerging, after years of struggle, as a dynamic, modern urban center. Too bad the Mormons still believe the city's architecture belongs in the past.